CN116178953A - Polyimide film - Google Patents

Abstract

The present invention relates to a polyimide film. A polyimide film comprising a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit derived from a diamine(B) A polyimide resin having a storage elastic modulus at 280 ℃ of less than 3X 10 ⁸ Pa, and a glass transition temperature of 200 to 290 ℃.

Description

Polyimide film

Technical Field

The present invention relates to a polyimide-based film which can be used as a substrate material such as a printed circuit board or an antenna board capable of handling high frequency bands.

Background Art

Flexible printed circuit boards (hereinafter sometimes referred to as FPCs) are thin, lightweight, and flexible, so they can be mounted in a three-dimensional, high-density manner. They are used in many electronic devices such as mobile phones and hard disks, and contribute to their miniaturization and lightness. In the past, polyimide resins with excellent heat resistance, mechanical properties, and electrical insulation were widely used in FPCs.

In recent years, the fifth generation mobile communication system, known as 5G, is becoming increasingly popular. In the polyimide materials used in the past, when high-frequency signals used for 5G communications are transmitted, the transmission loss is large, resulting in disadvantages such as loss of electrical signals and longer signal delay time. Therefore, polyimide films with low dielectric loss tangent (hereinafter, sometimes recorded as Df) and relative dielectric constant (hereinafter, sometimes recorded as Dk) are studied for the purpose of reducing transmission loss.

For example, in Patent Document 1, a polyimide resin precursor obtained by reacting a tetracarboxylic anhydride component containing tetracarboxylic anhydride and biphenyltetracarboxylic anhydride containing ester with a diamine component containing 75 mol % or more of p-phenylenediamine, and a polyimide resin obtained by curing the aforementioned polyimide resin precursor are disclosed. In addition, in Patent Document 2, a polyimide film is disclosed, which has a non-thermoplastic polyimide layer containing a non-thermoplastic polyimide and a thermoplastic polyimide layer containing a thermoplastic polyimide, wherein the aforementioned non-thermoplastic polyimide contains a tetracarboxylic acid residue and a diamine residue derived from a specific diamine compound, wherein the tetracarboxylic acid residue contains at least one of a tetracarboxylic acid residue (BPDA residue) derived from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and a tetracarboxylic acid residue (TAHQ residue) derived from 1,4-phenylenebis(trimellitic acid monoester) dianhydride (TAHQ).

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2018-150544

Patent Document 2: International Publication No. 2018/061727

Summary of the invention

As a metal-clad laminate for FPC, a copper-clad laminate (hereinafter sometimes referred to as CCL) is widely used, which has a copper foil layer on one or both sides of a single-layer or multi-layer polyimide resin. CCL is sometimes manufactured by casting a polyimide resin precursor solution on a copper foil and thermally imidizing the film of the polyimide resin precursor, and the thermal imidization is usually performed by heating at a high temperature of, for example, about 360°C.

In the transmission of high-frequency current, the phenomenon called skin effect, in which the current flows densely near the surface of the conductor, becomes significant. Therefore, when metal-clad laminates such as CCL are used in high-frequency circuits, the roughness and oxidation of the metal foil surface are likely to cause an increase in transmission loss. For example, for copper foil, if it is heated at a high temperature above 350°C, there is a tendency for the surface of the copper foil to become rough, the crystal grain size to increase, oxidation, etc., and the interface is prone to become rough. Therefore, in the process of thermal imidization, metal foils such as copper foil are exposed to high temperatures together with polyimide resins, which will lead to a decrease in transmission loss.

However, in the conventional manufacture of polyimide resins and polyimide films, Df and Dk cannot be sufficiently reduced when thermal imidization is performed at a low temperature below 350° C. If thermal imidization is performed at a high temperature of 350° C. or higher to produce CCL in order to sufficiently reduce Df and Dk of polyimide resins and polyimide films, roughness of the copper foil surface is generated as described above, and therefore, it is difficult to simultaneously suppress roughness and oxidation of the copper foil surface and reduce Df of the polyimide layer to form a CCL with low transmission loss when used in a high-frequency circuit.

Therefore, an object of the present invention is to provide a polyimide film having a low Df that can suppress roughness on the surface of a metal foil such as a copper foil and form a metal-clad laminate such as a CCL having a low transmission loss in a high frequency band.

The inventors of the present application have conducted intensive studies to solve the above-mentioned problems and have completed the present invention. That is, the present invention provides the following preferred embodiments.

[1] A polyimide film comprising a polyimide resin containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine, wherein the polyimide resin has a storage elastic modulus of less than 3×10 ⁸ Pa at 280° C. and a glass transition temperature of 200 to 290° C.

[2] The polyimide film according to [1], wherein the structural unit (A) comprises a structural unit (A1) derived from a tetracarboxylic anhydride containing an ester bond.

[3] The polyimide film according to [2], wherein the structural unit (A) further includes a structural unit (A2) derived from a tetracarboxylic anhydride having a biphenyl skeleton.

[4] The polyimide film according to [3], wherein the structural unit (A) satisfies the relationship of formula (X).

(Content of the structural unit (A3) derived from tetracarboxylic anhydride excluding the structural unit (A1) and the structural unit (A2))/(Total amount of the structural unit (A1) and the structural unit (A2))<1.1(X)

[5] The polyimide film according to any one of [2] to [4], wherein the structural unit (A1) is a structural unit (a1) derived from a tetracarboxylic anhydride represented by the formula (a1).

[In formula (a1), Z represents a divalent organic group,

R ^a1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

s independently represents an integer from 0 to 3]

[6] The polyimide film according to any one of [3] to [5], wherein the structural unit (A2) is a structural unit (a2) derived from a tetracarboxylic anhydride represented by the formula (a2).

[In formula (a2), R ^a2 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

t independently represents an integer from 0 to 3]

[7] The polyimide film according to any one of [1] to [6], wherein the structural unit (B) comprises a structural unit (B1) derived from a diamine having a biphenyl skeleton.

[8] The polyimide film according to [7], wherein the structural unit (B1) is a structural unit (b1) derived from a diamine represented by the formula (b1).

[In formula (b1), R ^b1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

p represents an integer from 0 to 4]

[9] The polyimide film according to [7] or [8], wherein the content of the structural unit (B1) is greater than 30 mol % based on the total amount of the structural unit (B).

[10] The polyimide film according to any one of [1] to [9], wherein the structural unit (B) comprises a structural unit (b2) derived from a diamine represented by the formula (b2).

[In formula (b2), R ^b2 each independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

W independently represents -O-, _-CH2- , -CH2 _{- CH2-} , -CH( _CH3 )-, -C( _CH3 ) _2- , -C( _CF3 ) _2- , -COO-, -OOC-, _-SO2- , -S-, -CO- or -N( ^Rc )-, ^Rc represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom, m represents an integer of 1 to 4,

q independently represents an integer from 0 to 4]

[11] The polyimide film according to [10], wherein in the structural unit (b2), m is 3, and W each independently represents -O- or -C(CH ₃ ) ₂ -.

[12] A laminated film comprising a metal foil layer on one or both sides of the polyimide film according to any one of [1] to [11].

[13] A flexible printed circuit board comprising the polyimide film according to any one of [1] to [11].

[14] A method for producing a laminated film, comprising the following steps:

A step of applying a polyimide resin precursor solution containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine onto a substrate; and

A step of imidizing a polyimide resin precursor by heat treatment at 200° C. or higher and lower than 350° C. to form the polyimide film according to any one of [1] to [11] on a substrate.

[15] The method for producing a laminated film according to [14], wherein the substrate is a metal foil.

Effects of the Invention

According to the present invention, it is possible to provide a polyimide film having a low Df, which can suppress roughness on the surface of a metal foil such as a copper foil and form a metal-clad laminate such as a CCL having a low transmission loss in a high frequency band.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described in detail. It should be noted that the scope of the present invention is not limited to the embodiments described herein, and various modifications can be made without departing from the gist of the present invention.

〔Polyimide film〕

The polyimide film of the present invention comprises a polyimide resin containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine. In this specification, polyimide is sometimes described as PI. In the present invention, the so-called "structural unit derived from..." refers to "structural unit derived from...", for example, "structural unit (A) derived from tetracarboxylic anhydride" refers to "structural unit (A) derived from tetracarboxylic anhydride".

<Polyimide resin>

The storage elastic modulus (hereinafter sometimes described as E') of the PI-based resin at 280°C is less than 3×10 ⁸ Pa, and the glass transition temperature (hereinafter sometimes described as Tg) is 200-290°C. The inventors of the present application have found that if a PI-based resin having an E' of less than 3×10 ⁸ Pa at 280°C and a Tg of 200-290°C is used, a PI-based film with a low Df can be obtained even at a low thermal imidization temperature. It is inferred that this is because if the E' and Tg of the PI-based resin at 280°C are within the above range, an ideal higher-order structure that suppresses the rotational motion of the PI-based resin is easily formed, thereby suppressing the rotation of the polar groups in the PI-based resin and reducing the loss of electrical energy in the form of thermal motion.

The polyamic acid, which is a precursor of the PI-based resin, starts to imidize at about 200°C. Generally, the molecular structure of polyamic acid has a high degree of freedom, but after imidization, it becomes relatively rigid and the degree of freedom of the molecular structure decreases. It is believed that if the Tg of the PI-based resin is 200-290°C, during the imidization process, even if the thermal imidization temperature is low, the thermal imidization temperature exceeds the Tg of the PI-based resin, so the amic acid site and the imide site are simultaneously active to form a higher-order structure, so the resin as a whole is easy to form an ideal higher-order structure that suppresses rotational motion. In addition, it is believed that if E' at 280°C is less than 3×10 ⁸ Pa, the following effect can be obtained: when forming a higher-order structure, the imide site can move sufficiently flexibly, so the resin as a whole is particularly easy to form an ideal higher-order structure that suppresses rotational motion.

From the viewpoint of easily reducing the Df of the PI film, the E' of the PI resin at 280°C is less than 3×10 ⁸ Pa, preferably 2×10 ⁸ Pa or less, more preferably 1.5×10 ⁸ Pa or less, further preferably 1×10 ⁸ Pa or less, and particularly preferably 0.8×10 ⁸ Pa or less. From the viewpoint of easily suppressing deformation during processing of the PI film, the E' of the PI resin at 280°C is preferably 1×10 ⁴ Pa or more, more preferably 1×10 ⁵ Pa or more, and further preferably 1×10 ⁶ Pa or more. The E' of the PI resin can be measured by dynamic viscoelasticity measurement, for example, by the method described in the Examples.

The E' of the PI-based resin at 280°C can be adjusted by appropriately adjusting the types of structural units constituting the PI-based resin and their composition, as well as the molecular weight and production method of the PI-based resin, especially the imidization conditions, etc. For example, it can be adjusted to the above range by adjusting it to the range described as a preferred embodiment in the description described later.

From the viewpoint that the Df of the obtained PI film can be easily reduced even at a low temperature of the thermal imidization temperature, the Tg of the PI resin is 290°C or less, preferably lower than 290°C, more preferably 280°C or less, further preferably 275°C or less, further more preferably 260°C or less, particularly preferably 250°C or less, and particularly more preferably 240°C or less. In addition, from the viewpoint that the Df of the PI film can be easily reduced and the heat resistance of the PI film can be easily improved, the Tg of the PI resin is 200°C or more, more preferably 202°C or more, and further preferably 205°C or more. The Tg of the PI resin can be measured by dynamic viscoelasticity measurement, for example, it can be measured by the method described in the Examples.

The Tg of the PI-based resin can be adjusted by appropriately adjusting the types of structural units constituting the PI-based resin and their composition, as well as the molecular weight and production method of the PI-based resin, especially the imidization conditions, etc. For example, it can be adjusted to the above range by adjusting it to the range described as a preferred method in the description described later.

(Structural unit (A) derived from tetracarboxylic anhydride)

The PI-based resin contains a structural unit (A) derived from tetracarboxylic anhydride (hereinafter, sometimes simply referred to as structural unit (A)). There are no particular restrictions on the structural unit (A) as long as the E' and Tg of the PI-based resin at 280°C are within the above ranges. For example, a structural unit derived from a tetracarboxylic anhydride represented by formula (1) is preferred.

[In formula (1), Y represents a tetravalent organic group]

In formula (1), Y independently represents a tetravalent organic group, preferably a tetravalent organic group having 4 to 40 carbon atoms, and more preferably a tetravalent organic group having 4 to 40 carbon atoms having a cyclic structure. Examples of the cyclic structure include alicyclic rings, aromatic rings, and heterocyclic structures. For the aforementioned organic group, the hydrogen atoms in the organic group may be substituted by a halogen atom, a hydrocarbon group, an alkoxy group, or a halogenated hydrocarbon group, in which case the carbon atoms of these groups are preferably 1 to 8. The PI-based resin of the present invention may contain a plurality of Ys, and the plurality of Ys may be the same or different. Examples of Y include: groups or structures represented by formulas (31) to (40); groups obtained by replacing the hydrogen atoms in the groups represented by formulas (31) to (40) with methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, fluoro, chloro, or trifluoromethyl groups; and tetravalent chain hydrocarbon groups having 1 to 8 carbon atoms.

[In formulae (31) to (40), R ¹⁹ to R ²⁶ and R ^23' to R ^26' each independently represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms, and the hydrogen atoms contained in R ¹⁹ to R ²⁶ and R ^23' to R ^26' each independently may be substituted with a halogen atom,

^V1 and ^V2 independently represent a single bond (excluding the case where e+d=1), -O-, _-CH2- , -CH2 _-CH2- , _-CH ( _CH3 )-, -C( _CH3 ) _2- , -C( _CF3 ) _2- , _-SO2- , -S-, -CO-, -N( ^Rj )-, formula (a),

(In formula (a), R ²⁷ to R ³⁰ are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,

D independently represents a single bond, -C(CH ₃ ) ₂ - or -C(CF ₃ ) ₂ -,

i represents an integer from 1 to 3,

＊ indicates a connection key)

^Rj represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted by a halogen atom,

e and d independently represent an integer from 0 to 2 (wherein e+d is not 0),

f represents an integer from 0 to 3,

g and h independently represent an integer from 0 to 4,

In formula (39), Z represents a divalent organic group,

R ^a1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

s independently represent an integer from 0 to 3,

In formula (40), ^Ra2 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

t independently represents an integer from 0 to 3,

＊ indicates a connection key].

From the viewpoint of easily improving the mechanical and thermal properties of the PI film, in the PI resin of the present invention, as Y in formula (1), it is preferred that at least one structure selected from the group consisting of structures represented by formula (31), formula (32), formula (33), formula (39) and formula (40) be included, more preferably at least one structure selected from the group consisting of structures containing ester bonds and structures containing biphenyl skeletons, and further preferably at least one structure selected from the group consisting of structures represented by formula (39) and formula (40). It should be noted that in this specification, the so-called mechanical properties refer to mechanical properties including bending resistance, folding resistance, and elastic modulus. The so-called improvement of mechanical properties, for example, means that bending resistance and/or elastic modulus become higher. In addition, the so-called thermal properties refer to thermal properties including Tg, linear expansion coefficient (hereinafter, sometimes recorded as CTE), thermal properties with less degeneration and degradation caused by heat, and thermal properties with less deformation after heating. The so-called improvement of thermal properties, for example, means that Tg becomes higher and/or CTE becomes lower.

In formulae (31) to (33), R ¹⁹ to R ²⁶ and R ^23' to R ^26' independently represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms.

Examples of the alkyl group having 1 to 6 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, 2-ethylpropyl, and n-hexyl.

Examples of the alkoxy group having 1 to 6 carbon atoms include methoxy, ethoxy, propyloxy, isopropyloxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy, hexyloxy and cyclohexyloxy.

As the aryl group having 6 to 12 carbon atoms, phenyl, tolyl, xylyl, naphthyl and biphenyl can be mentioned. The hydrogen atoms contained in R ¹⁹ to R ²⁶ and R ^23' to R ^26' can be substituted by halogen atoms independently of each other, and examples of the halogen atoms include fluorine atoms, chlorine atoms, bromine atoms and iodine atoms. Among them, from the viewpoint of easily improving the mechanical and thermal properties of the PI film, R ¹⁹ to R ²⁶ and R ^23' to R ^26' are preferably hydrogen atoms or alkyl groups having 1 to 6 carbon atoms independently of each other, more preferably hydrogen atoms or alkyl groups having 1 to 3 carbon atoms, and further preferably hydrogen atoms.

In formula (31), ^V1 and ^V2 each independently represent a single bond (excluding the case where e+d=1), -O-, _-CH2- , -CH2-CH2-, -CH( _{CH3 )} -, -C( _CH3 ) _{2- ,} -C( _CF3 ) _2- , _-SO2- , -S-, -CO-, -N( ^Rj )- or formula (a). From the viewpoint of easily improving the mechanical and thermal properties of the PI film, preferably they represent a single bond (excluding the case where e+d=1), -O-, _-CH2- , -C(CH3) _2- , -C( _CF3 ) _2- or -CO-, and more preferably they represent a single bond (excluding the case where e+d=1), -O-, _-C ( _CH3 ) _2- or -C( _CF3 ) _2- . ^Rj represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted by a halogen atom. Examples of the monovalent hydrocarbon group having 1 to 12 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, 2-ethylpropyl, n-hexyl, n-heptyl, n-octyl, tert-octyl, n-nonyl and n-decyl, which may be substituted by a halogen atom. Examples of the halogen atom include the same halogen atoms as described above.

In formula (31), e and d each independently represent an integer of 0 to 2 (wherein e+d is not 0), and preferably represent 0 or 1 from the viewpoint of being able to easily reduce the Df of the PI film even when the imidization temperature is low. In addition, e+d preferably represents 1. It should be noted that in formula (31), when e is 0, it means that the two benzene rings are not bonded by ^V1 , and when d is 0, it means that the two benzene rings are not bonded by ^V2 .

In formula (32) and formula (33), f represents an integer of 0 to 3, and preferably represents 0 or 1, and more preferably represents 0, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature.

In formula (33), g and h are each independently an integer of 0 to 4, and are preferably an integer of 0 to 2, and more preferably 0 or 1, from the viewpoint of easily improving the mechanical and thermal properties of the PI film. In addition, g+h is preferably an integer of 0 to 2. It should be noted that when f is 1 or more, a plurality of g and h may be the same or different from each other independently.

In formula (a), R ²⁷ to R ³⁰ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

Examples of the alkyl group having 1 to 6 carbon atoms include the alkyl groups having 1 to 6 carbon atoms exemplified above. Among them, R ²⁷ to R ³⁰ each independently represent preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and more preferably a hydrogen atom, from the viewpoint of easily improving the mechanical and thermal properties of the PI film.

In formula (a), D represents a single bond, -C(CH ₃ ) ₂ - or -C(CF ₃ ) ₂ -. When D has such a structure, the mechanical and thermal properties of the PI film are easily improved. i represents an integer of 1 to 3, and is preferably 1 or 2 from the viewpoint of easily improving the mechanical and thermal properties of the PI film. When i is 2 or more, a plurality of Ds and R ²⁷ to R ³⁰ may be the same or different from each other independently.

In formula (39), Z represents a divalent organic group. From the viewpoint of easily improving the mechanical and thermal properties of the PI film, it is preferably a divalent organic group having 4 to 40 carbon atoms, more preferably a divalent organic group having 4 to 40 carbon atoms and having a cyclic structure, further preferably a divalent organic group having 4 to 40 carbon atoms and having an aromatic ring, particularly preferably a divalent organic group represented by formula (z1), (z2) or (z3), and particularly more preferably a divalent organic group represented by formula (z1).

[In formula (z1) to formula (z3), ^Rz11 to ^Rz14 each independently represent a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom,

R ^z2 each independently represents a monovalent hydrocarbon group which may have a halogen atom,

n represents an integer from 1 to 4,

j independently represents an integer from 0 to 3,

＊ indicates a connection key]

In the formula (z1), ^Rz11 to ^Rz14 each independently represent a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom.

Examples of the monovalent hydrocarbon group include an aromatic hydrocarbon group, an alicyclic hydrocarbon group, and an aliphatic hydrocarbon group.

Examples of the aromatic hydrocarbon group include aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenyl.

Examples of the alicyclic hydrocarbon group include cycloalkyl groups such as cyclopentyl and cyclohexyl.

Examples of the aliphatic hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, 2-ethylpropyl, n-hexyl, n-heptyl, n-octyl, tert-octyl, n-nonyl and n-decyl.

Examples of the halogen atom include the halogen atoms described above.

From the viewpoint of easily improving the mechanical and thermal properties of the PI film, ^Rz11 to ^Rz14 independently represent preferably a hydrogen atom or an alkyl group which may have a halogen atom, more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, further preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms which may have a halogen atom, and particularly preferably a hydrogen atom.

In formula (z1), n represents an integer of 1 to 4, preferably an integer of 1 to 3, more preferably 1 or 2, and particularly preferably 2, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature.

In formula (z2), R ^z2 each independently represents a monovalent hydrocarbon group which may have a halogen atom, and examples of the monovalent hydrocarbon group include the monovalent hydrocarbon groups exemplified above. From the viewpoint of easily improving the mechanical and thermal properties of the PI film, R ^z2 each independently preferably represents an alkyl group which may have a halogen atom, more preferably represents an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, and further preferably represents an alkyl group having 1 to 3 carbon atoms which may have a halogen atom.

In formula (z2), j each independently represents an integer of 0 to 3, and is preferably 0 or 1, more preferably 0, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature.

In formula (39), ^Ra1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom. From the viewpoint of easily improving the mechanical and thermal properties of the PI film, it is preferred that they independently represent an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms or an aryl group having 6 to 12 carbon atoms.

Examples of the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms include the groups exemplified above.

The hydrogen atoms contained in R ^a1 may be independently substituted by halogen atoms, and examples of the halogen atoms include the halogen atoms exemplified above. Among them, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature, R ^a1 independently preferably represents an alkyl group having 1 to 6 carbon atoms, and more preferably represents an alkyl group having 1 to 3 carbon atoms.

In formula (39), s each independently represents an integer of 0 to 3, preferably an integer of 0 to 2, and more preferably 0 or 1, from the viewpoint of easily improving the mechanical and thermal properties of the PI film.

In formula (40), ^Ra2 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom. From the viewpoint of easily improving the mechanical and thermal properties of the PI film, it is preferred that they independently represent an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms or an aryl group having 6 to 12 carbon atoms.

Examples of the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms include the groups exemplified above.

The hydrogen atoms contained in ^Ra2 may be independently substituted by halogen atoms, and examples of the halogen atoms include the halogen atoms exemplified above. Among them, from the viewpoint of easily improving the mechanical and thermal properties of the PI film, ^Ra2 may be independently preferably an alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms.

In formula (40), t independently represents an integer of 0 to 3, and preferably represents an integer of 0 to 2, and more preferably represents 0 or 1, from the viewpoint of easily improving the mechanical and thermal properties of the PI film.

Specific examples of the structures represented by Formulae (31) to (33), (39) and (40) include structures represented by Formulae (41) to (56). In these formulae, * represents a connecting bond.

In one embodiment of the present invention, when at least one selected from the group consisting of structures represented by formula (31) to (33), formula (39) and formula (40) is included as Y in formula (1), the ratio of the structural unit derived from tetracarboxylic anhydride represented by at least one selected from the group consisting of structures represented by formula (31) to (33), formula (39) and formula (40), especially the ratio of the structural unit derived from tetracarboxylic anhydride represented by at least one selected from the group consisting of structures represented by formula (39) and formula (40), relative to the total molar amount of the structural unit (A), is preferably 30 mol% or more, more preferably 50 mol% or more, further preferably 70 mol% or more, particularly preferably 90 mol% or more, and preferably 100 mol% or less. If the above ratio is within the above range, it is easy to reduce the Df of the PI film. The ratio of the above structural unit can be measured using, for example, ¹ H-NMR, or can be calculated from the input ratio of the raw materials.

(Structural unit (A1) derived from tetracarboxylic anhydride containing an ester bond)

In one embodiment of the present invention, the structural unit (A) preferably includes a structural unit (A1) derived from a tetracarboxylic anhydride containing an ester bond (hereinafter, sometimes simply referred to as structural unit (A1)). If the structural unit (A) includes the aforementioned structural unit (A1), an ester bond having molecular orientation is introduced into the PI-based resin, and therefore, it is easy to orient in the process of applying the PI-based resin precursor solution to a substrate and imidizing the coating film, and it is easy to reduce Df even if the imidization temperature is low. In addition, for the same reason, it is easy to reduce CTE and improve the dimensional stability of the PI-based film.

In one embodiment of the present invention, the structural unit (A1) is not particularly limited as long as it contains an ester bond. The ester bond contained in the structural unit (A1) may be one or more. From the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature, the structural unit (a1) derived from a tetracarboxylic anhydride represented by formula (a1) is preferred.

[In formula (a1), Z represents a divalent organic group,

R ^a1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

s independently represents an integer from 0 to 3]

^Ra1 in formula (a1) independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom. From the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature, Ra1 independently represents an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms or an aryl group having 6 to 12 carbon atoms.

Examples of the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms include the groups exemplified above.

The hydrogen atoms included in R ^a1 may be independently substituted with a halogen atom, and examples of the halogen atom include the halogen atoms exemplified above.

Among them, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature, ^Ra1 is preferably an alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms, each independently of the other.

In addition, s in formula (a1) each independently represents an integer of 0 to 3, preferably represents 0 or 1, and more preferably represents 0.

Z in formula (a1) represents a divalent organic group, and as the divalent organic group, the divalent organic groups exemplified above as the divalent organic groups in formula (39) can be cited. Among them, from the viewpoint that the Df of the PI film can be easily reduced even at a low imidization temperature, Z is preferably a divalent organic group represented by formula (z1), formula (z2), or formula (z3), and more preferably a divalent organic group represented by formula (z1).

[In formulae (z1) to (z3), ^Rz11 to ^Rz14 each independently represent a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom,

R ^z2 each independently represents a monovalent hydrocarbon group which may have a halogen atom,

n represents an integer from 1 to 4,

j independently represents an integer from 0 to 3,

＊ indicates a connection key]

In one embodiment of the present invention, ^Rz11 to ^Rz14 in the formula (z1) each independently represent a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom.

Examples of the monovalent hydrocarbon group include the monovalent hydrocarbon groups exemplified above.

From the viewpoint of easily lowering the Df of the PI film even at a low imidization temperature, ^Rz11 to ^Rz14 each independently represent preferably a hydrogen atom or an alkyl group which may have a halogen atom, more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, further preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms which may have a halogen atom, and particularly preferably a hydrogen atom.

In one embodiment of the present invention, R ^z2 in formula (z2) independently represents a monovalent hydrocarbon group which may have a halogen atom, and examples of the monovalent hydrocarbon group include the monovalent hydrocarbon groups exemplified above. From the viewpoint of easily improving the mechanical and thermal properties of the PI film, R ^z2 independently represents preferably an alkyl group which may have a halogen atom, more preferably an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, and further preferably an alkyl group having 1 to 3 carbon atoms which may have a halogen atom.

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature, in the benzene ring having ^Rz11 to ^Rz14 in formula (z1), at least one of ^Rz11 to ^Rz14 may be a monovalent hydrocarbon group which may have a halogen atom, but preferably all of ^Rz11 to ^Rz14 are hydrogen atoms.

j in formula (z2) independently represents 0 to 3. In one embodiment of the present invention, j independently represents preferably 0 or 1, more preferably 0, and even more preferably all j are 0, from the viewpoint of easily reducing Df of the PI film even at a low imidization temperature.

In formula (z1), n represents an integer of 1 to 4, preferably an integer of 1 to 3, more preferably 1 or 2, and particularly preferably 2, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature.

In a preferred embodiment of the present invention, formula (a1) is preferably represented by formula (a1') or formula (a1").

If the PI-based resin contains a structural unit derived from a tetracarboxylic anhydride represented by formula (a1), especially formula (a1') or formula (a1") as the structural unit (A1), the Df of the obtained PI-based film can be easily reduced even if the imidization temperature is low.

In one embodiment of the present invention, relative to the total amount of structural unit (A), the content of structural unit (A1) is preferably 10 mol% or more, more preferably 15 mol% or more, further preferably 20 mol% or more, further more preferably 30 mol% or more, particularly preferably 35 mol% or more, particularly preferably 40 mol% or more. In addition, relative to the total amount of structural unit (A), the content of structural unit (A1) is preferably 75 mol% or less, more preferably 70 mol% or less, further preferably 65 mol% or less, particularly preferably 60 mol% or less. If the content of structural unit (A1) is within the above range, it is easy to produce the orientation of PI resin, and the Df of PI film is easy to reduce. The ratio of the above structural unit can be measured using, for example, ¹ H-NMR, or it can be calculated by the input ratio of raw materials.

(Structural unit (A2) derived from tetracarboxylic anhydride containing a biphenyl skeleton)

In one embodiment of the present invention, the structural unit (A) preferably includes a structural unit (A2) derived from a tetracarboxylic anhydride containing a biphenyl skeleton (hereinafter, sometimes simply referred to as structural unit (A2)). If the structural unit (A) includes the structural unit (A2), the Df of the obtained PI film can be easily reduced even if the imidization temperature is low.

In one embodiment of the present invention, the structural unit (A2) is not particularly limited as long as it contains a biphenyl skeleton, and the biphenyl skeleton contained in the structural unit (A2) may be one or more. In addition, in one embodiment of the present invention, the structural unit (A2) is preferably a structural unit containing a biphenyl skeleton but not an ester bond. In this specification, the structural unit derived from a tetracarboxylic anhydride containing both an ester bond and a biphenyl skeleton is not the structural unit (A2), but is classified as a structural unit (A1) derived from a tetracarboxylic anhydride containing an ester bond.

In one embodiment of the present invention, the structural unit (A2) is preferably a structural unit (a2) derived from a tetracarboxylic anhydride represented by the formula (a2).

[In formula (a2), R ^a2 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

t independently represents an integer from 0 to 3]

^Ra2 in formula (a2) independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom. From the viewpoint that the Df of the PI film can be easily reduced even at a low imidization temperature, it is preferred that they independently represent an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms or an aryl group having 6 to 12 carbon atoms. As the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms and the aryl group having 6 to 12 carbon atoms, the groups exemplified above can be cited. The hydrogen atoms contained in ^Ra2 independently may be substituted with a halogen atom, and as the halogen atom, the halogen atom exemplified above can be cited. Among them, from the viewpoint that the Df of the PI film can be easily reduced even at a low imidization temperature, ^Ra2 independently represents preferably an alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms.

In addition, t in formula (a2) each independently represents an integer of 0 to 3, preferably represents 0 or 1, and more preferably represents 0.

There is no particular restriction on the bonding positions of the two carboxylic anhydrides bonded to the benzene ring constituting the biphenyl skeleton in formula (a2). Based on the single bond bonding the two benzene rings, they can be independently 3,4-, or 2,3-. From the perspective of easily reducing the Df of the PI film even at a low imidization temperature, 3,4- is preferred.

In a preferred embodiment of the present invention, formula (a2) is preferably represented by formula (a2').

When the PI-based resin contains a structural unit derived from a tetracarboxylic anhydride represented by formula (a2), especially formula (a2'), as the structural unit (A2), the Df of the obtained PI-based film can be easily reduced even at a low imidization temperature.

In one embodiment of the present invention, relative to the total amount of structural unit (A), the content of structural unit (A2) is preferably 25 mol% or more, more preferably 30 mol% or more, further preferably 35 mol% or more, particularly preferably 40 mol% or more. In addition, relative to the total amount of structural unit (A), the content of structural unit (A2) is preferably 90 mol% or less, more preferably 85 mol% or less, further preferably 80 mol% or less, further preferably 70 mol% or less, particularly preferably 60 mol% or less. If the content of structural unit (A1) is within the above range, it is easy to produce the orientation of PI resin, and the Df of PI film is easy to reduce. The ratio of the above structural unit can be measured using, for example, ¹ H-NMR, or it can be calculated by the input ratio of raw materials.

In one embodiment of the present invention, the total amount of structural units (A1) and structural units (A2) relative to the total amount of structural units (A) is preferably 50 mol% or more, more preferably 60 mol% or more, further preferably 70 mol% or more, further more preferably 80 mol% or more, and particularly preferably 90 mol% or more relative to the total amount of structural units (A). In addition, the upper limit of the total amount of structural units (A1) and structural units (A2) is not particularly limited, for example, it can be 100 mol% or less. The ratio of the aforementioned structural units can be measured using, for example, ¹ H-NMR, or it can be calculated by the input ratio of the raw materials.

(Structural unit (A3))

In one embodiment of the present invention, the structural unit (A) may include a structural unit (A3) derived from a tetracarboxylic anhydride other than the structural unit (A1) and the structural unit (A2) (hereinafter, sometimes simply referred to as structural unit (A3)).

It should be noted that, in the present specification, the “structural unit (A3) derived from tetracarboxylic anhydride other than the structural unit (A1) and the structural unit (A2)” refers to a structural unit derived from tetracarboxylic anhydride that does not belong to either the structural unit (A1) or the structural unit (A2), and the “content of the structural unit (A3)” refers to the total amount of the structural unit (A3) when a plurality of structural units (A3) are present.

In one embodiment of the present invention, the structural unit (A3) includes a structural unit derived from a tetracarboxylic anhydride containing neither an ester bond nor a biphenyl skeleton, for example, a structural unit derived from a tetracarboxylic anhydride wherein Y in the formula (1) is represented by the formula (31) to the formula (38). From the viewpoint of being able to easily reduce the Df of the PI film even at a low imidization temperature, a structural unit derived from a tetracarboxylic anhydride wherein Y in the formula (1) is represented by the formula (42) to the formula (49) or the formula (53) is preferred, and a structural unit derived from a tetracarboxylic anhydride wherein Y in the formula (1) is represented by the formula (42), the formula (46), the formula (49) or the formula (53) is more preferred.

In one embodiment of the present invention, when the structural unit (A3) is included, its content can be, for example, 0.01 to 55 mol% or 0.01 to 40 mol% relative to the total amount of the structural unit (A), preferably 40 mol% or less, more preferably 35 mol% or less, further preferably 30 mol% or less, particularly preferably 25 mol% or less, and usually 0.01 mol% or more, preferably 10 mol% or more. If the content of the structural unit (A3) is within the above range, it is easy to produce the orientation of the PI-based resin, and the Df of the PI-based film is easy to reduce. The ratio of the aforementioned structural unit can be measured using, for example, ¹ H-NMR, or it can be calculated from the input ratio of the raw materials.

In one embodiment of the present invention, when the structural unit (A3) is included, preferably a structural unit of a tetracarboxylic anhydride in which Y in formula (1) is represented by formula (32), and more preferably a structural unit of a tetracarboxylic anhydride in which Y in formula (1) is represented by formula (32) with f=0, its content is preferably 10 mol% or more, more preferably 20 mol% or more, further preferably 30 mol% or more, particularly preferably 40 mol% or more, preferably 90 mol% or less, more preferably 80 mol% or less, further preferably 70 mol% or less, and particularly preferably 60 mol% or less. If the content is within the above range, the Df of the PI film is easily reduced. The ratio of the above structural units can be measured using, for example, ¹ H-NMR, or can be calculated from the input ratio of the raw materials.

In one embodiment of the present invention, when the structural unit (A) includes the structural units (A1) and (A2), the structural unit (A) preferably satisfies the relationship of formula (X).

(Content of structural unit (A3))/(Total amount of structural unit (A1) and structural unit (A2))<1.1(X)

When the structural unit (A) satisfies the relationship of formula (X), that is, when the value of the left side of formula (X) is less than 1.1, especially less than 0.67, the Df of the obtained PI film is easily reduced. As a result, even if the imidization temperature of the PI resin is low, the Df of the obtained PI film is easily reduced, and a PI film with excellent balance with mechanical properties can be obtained.

In one embodiment of the present invention, from the viewpoint that the Df of the PI film is easily reduced even if the imidization temperature is low, and the mechanical properties are easily improved, the value on the left side of formula (X) is preferably 0.6 or less, more preferably 0.5 or less, further preferably 0.4 or less, further more preferably 0.3 or less, and particularly preferably 0.20 or less. In addition, the lower limit of the value on the left side of formula (X) is not particularly limited and may be 0 or more.

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film, the structural unit (A) preferably includes a structural unit derived from a tetracarboxylic anhydride represented by formula (a2) and/or a structural unit derived from a tetracarboxylic anhydride represented by formula (32) in which Y in formula (1) is f=0. In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film, the structural unit (A) preferably includes at least one selected from the group consisting of a structural unit derived from a tetracarboxylic anhydride represented by formula (a1), a structural unit derived from a tetracarboxylic anhydride represented by formula (a2), and a structural unit derived from a tetracarboxylic anhydride represented by formula (32) in which Y in formula (1) is f=0, and more preferably includes a structural unit derived from a tetracarboxylic anhydride represented by formula (a2) and/or a structural unit derived from a tetracarboxylic anhydride represented by formula (32) in which Y in formula (1) is f=0 in addition to the structural unit derived from the tetracarboxylic anhydride represented by formula (a1).

(Structural unit (B) derived from diamine)

The PI-based resin contains a structural unit (B) derived from a diamine (hereinafter, sometimes simply referred to as structural unit (B)). There are no particular restrictions on the structural unit (B) as long as the E' and Tg of the PI-based resin at 280°C are within the above ranges, and for example, a structural unit derived from a diamine represented by formula (2) is preferred.

_H2NX - _NH2 (2)

[In formula (2), X represents a divalent organic group]

In formula (2), X represents a divalent organic group, preferably a divalent organic group having 2 to 100 carbon atoms. Examples of the divalent organic group include a divalent aromatic group, a divalent aliphatic group, and the like. Examples of the divalent aliphatic group include a divalent non-cyclic aliphatic group or a divalent cyclic aliphatic group. Among them, from the viewpoint of easily improving the mechanical and thermal properties of the PI film, a divalent cyclic aliphatic group and a divalent aromatic group are preferred, and a divalent aromatic group is more preferred. For the divalent organic group, the hydrogen atoms in the organic group may be substituted by a halogen atom, a hydrocarbon group, an alkoxy group or a halogenated hydrocarbon group, in which case the carbon atoms of these groups are preferably 1 to 8. It should be noted that in the present specification, a divalent aromatic group is a divalent organic group having an aromatic group, and may contain an aliphatic group or other substituent in a part of its structure. The divalent aliphatic group is a divalent organic group having an aliphatic group, and may contain other substituents in a part of its structure, but does not contain an aromatic group.

In one embodiment of the present invention, the PI-based resin may contain a plurality of Xs, and the plurality of Xs may be the same or different from each other. Examples of X in formula (2) include: groups (structures) represented by formulas (60) to (65); groups in which hydrogen atoms in the groups represented by formulas (60) to (65) are replaced by methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, fluoro, chloro or trifluoromethyl groups; and the like.

[In formula (60) and formula (61), ^Ra and ^Rb each independently represent a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

W independently represents a single bond, -O-, _-CH2- , _-CH2 - _CH2- , -CH( _CH3 )-, -C( _CH3 ) _2- , -C( _CF3 ) _2- , -COO-, -OOC-, _-SO2- , -S-, -CO- or -N( ^Rc )-; ^Rc represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom;

t represents an integer from 0 to 4, u represents an integer from 0 to 4, n represents an integer from 0 to 4,

In formula (62), ring A represents a cycloalkane ring having 3 to 8 carbon atoms,

R ^d represents an alkyl group having 1 to 20 carbon atoms,

r represents an integer of 0 or more and (the number of carbon atoms in Ring A - 2) or less,

S1 and S2 independently represent an integer from 0 to 20,

In formula (60) to formula (65), * represents a connecting bond.]

Other examples of X in formula (2) include divalent noncyclic aliphatic groups such as ethylene, 1,3-propylene, 1,4-butylene (tramethylene group), 1,5-pentylene, 1,6-hexylene, 1,2-propylene, 1,2-butanediyl, 1,3-butanediyl, 1,12-dodecanediyl, 2-methyl-1,2-propanediyl, and 2-methyl-1,3-propanediyl. Hydrogen atoms in the divalent noncyclic aliphatic group may be substituted with halogen atoms, and carbon atoms may be substituted with heteroatoms such as oxygen atoms and nitrogen atoms.

Among them, from the viewpoint of easily improving the mechanical and thermal properties of the PI film, the PI resin in the present invention preferably includes the structures represented by formula (60) and formula (61) as X in formula (2), and more preferably includes the structure represented by formula (60).

In formula (60) and formula (61), the connecting bond of each benzene ring or each cyclohexane ring can be bonded to any position of the ortho, meta or para position, or the α, β or γ position based on -W- or a single bond connecting each benzene ring or each cyclohexane ring. From the viewpoint of easily reducing the Df of the PI film even when the imidization temperature is low, and from the viewpoint of easily improving the dimensional stability, it can be preferably bonded to the meta or para position, or the β or γ position, and it can be more preferably bonded to the para or γ position.

^Ra and ^Rb independently represent a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom, preferably a halogen atom, or an alkyl group, alkoxy group or aryl group which may have a halogen atom, more preferably a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms. Examples of the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms include the groups exemplified above. The hydrogen atoms contained in ^Ra and ^Rb independently may be substituted with a halogen atom, and examples of the halogen atom include the halogen atoms exemplified above. From the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability, ^Ra and ^Rb are each independently preferably an alkyl group having 1 to 6 carbon atoms or a fluorinated alkyl group having 1 to 6 carbon atoms, and from the viewpoint of easily improving the adhesion to a substrate such as a copper foil, an alkyl group having 1 to 6 carbon atoms not containing fluorine is more preferred, an alkyl group having 1 to 3 carbon atoms not containing fluorine is further preferred, and a methyl group is particularly preferred.

In formula (60) and formula (61), t and u are each independently an integer of 0 to 4, preferably an integer of 0 to 2, more preferably 0 or 1, from the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability.

In formula (60) and formula (61), W independently represents a single bond, -O-, _-CH2- , _-CH2 - _CH2- , -CH( _CH3 )-, -C( _CH3 ) _2- , -C(CF3) _2- , -COO-, -OOC-, _-SO2- , -S-, -CO-, or -N( ^Rc )-. From the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability, it preferably represents -O-, _-CH2- , -C( _CH3 ) _2- , -C( _CF3 )2-, -COO-, -OOC-, or -CO-. From the viewpoint of easily improving the adhesion to a substrate such as copper foil, it more preferably represents _a single bond, -O-, _-CH2- , or -C( _CH3 ) _2- . It is even more preferably -O- or -C( _CH3 ) _2- . R ^c represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom. Examples of the monovalent hydrocarbon group having 1 to 12 carbon atoms include the monovalent hydrocarbon groups having 1 to 12 carbon atoms exemplified above.

In formula (60) and formula (61), n is an integer of 0 to 4, and is preferably an integer of 0 to 3, and more preferably 1 to 3, from the viewpoint of facilitating the reduction of the Df of the PI film and improving the dimensional stability. When n is 2 or more, a plurality of W, ^Ra , and t may be the same or different, and the positions of the bonds of the benzene rings based on -W- may be the same or different.

When the PI-based resin in the present invention contains two or more structures represented by formula (60) and formula (61) as X in formula (2), W, n, ^Ra , ^Rb , t and u in one formula (60) and formula (61) may be the same as or different from W, n, ^Ra , ^Rb , t and u in another formula (60) and formula (61) independently.

In formula (62), ring A represents a cycloalkane ring having 3 to 8 carbon atoms. Examples of the cycloalkane ring include cyclopropane ring, cyclobutane ring, cyclopentane ring, cyclohexane ring, cycloheptane ring, and cyclooctane ring, and preferably, a cycloalkane ring having 4 to 6 carbon atoms. In ring A, each connecting bond may be adjacent to each other or may not be adjacent to each other. For example, when ring A is a cyclohexane ring, the two connecting bonds may be in a positional relationship of α, β, or γ, and may preferably be in a positional relationship of β or γ.

^Rd in formula (62) represents an alkyl group having 1 to 20 carbon atoms, preferably an alkyl group having 1 to 10 carbon atoms, and examples thereof include the groups exemplified above. r in formula (62) represents an integer of 0 or more and up to "the number of carbon atoms in ring A - 2". r is preferably 0 or more and preferably 4 or less. S1 and S2 in formula (62) each independently represent an integer of 0 to 20. S1 and S2 each independently represent preferably 0 or more, more preferably 2 or more and preferably 15 or less.

Specific examples of the structures represented by formula (60) to formula (62) include structures represented by formula (71) to formula (92). In these formulas, * represents a connecting bond.

In a preferred embodiment of the present invention, when at least one structural unit derived from a diamine represented by formula (60) and formula (61) is included as X in formula (2), the ratio of the structural unit derived from the diamine represented by formula (60) and formula (61), especially the ratio of the structural unit derived from the diamine in which n is 1 and W represents a single bond in formula (60), relative to the total molar amount of the structural unit (B), is preferably 25 mol% or more, more preferably greater than 30 mol%, further preferably 50 mol% or more, further more preferably 70 mol% or more, particularly preferably 90 mol% or more, and preferably 100 mol% or less. If the ratio of the structural unit derived from the diamine represented by formula (60) and formula (61), especially the ratio of the structural unit derived from the diamine in which at least one W represents a single bond in formula (60), in formula (2) is within the above range, it is easy to reduce the Df of the PI film and to improve the dimensional stability. The ratio of the structural unit can be measured using, for example, ¹ H-NMR, or can be calculated from the input ratio of the raw materials.

(Structural unit (B1) derived from diamine containing biphenyl skeleton)

In one embodiment of the present invention, the structural unit (B) preferably includes a structural unit (B1) derived from a diamine containing a biphenyl skeleton (hereinafter, sometimes simply referred to as structural unit (B1)). For a PI-based resin in which the structural unit (B) includes the aforementioned structural unit (B1), the Df of the resulting PI-based film is easily reduced even if the imidization temperature is low, so that even if the imidization temperature is low, the circuit including the resulting PI-based film is easily reduced in transmission loss.

In one embodiment of the present invention, the structural unit (B1) is not particularly limited as long as it contains a biphenyl skeleton, and the biphenyl skeleton contained in the structural unit (B1) may be one or more. In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film even if the imidization temperature is low, the structural unit (B1) is preferably a structural unit (b1) derived from a diamine represented by formula (b1).

[In formula (b1), R ^b1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

p independently represents an integer from 0 to 4]

In formula (b1), R ^b1 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom, preferably represents a halogen atom, or an alkyl group, alkoxy group or aryl group which may have a halogen atom, and more preferably represents a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms. Examples of the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms include the groups exemplified above. The hydrogen atoms included in R ^b1 independently may be substituted with a halogen atom, and examples of the halogen atom include the halogen atoms exemplified above. From the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability, R ^b1 is preferably independently an alkyl group having 1 to 6 carbon atoms or a fluorinated alkyl group having 1 to 6 carbon atoms, and from the viewpoint of easily improving the adhesion to a substrate such as a copper foil, an alkyl group having 1 to 6 carbon atoms not containing fluorine is more preferably an alkyl group having 1 to 3 carbon atoms not containing fluorine, and a methyl group is particularly preferably.

In formula (b1), p each independently represents an integer of 0 to 4, and is preferably an integer of 0 to 2, more preferably 0 or 1, from the viewpoint of easily reducing the Df of the PI-based film and easily improving the dimensional stability.

In formula (b1), the _-NH2 group bonded to each benzene ring can be bonded to any position of the ortho, meta or para position, or the α, β or γ position based on the single bond connecting each benzene ring. From the viewpoint of easily reducing the Df of the PI film even when the imidization temperature is low, and from the viewpoint of easily improving the dimensional stability, it can be preferably bonded to the meta or para position, or the β or γ position, and can be more preferably bonded to the para or γ position.

In a preferred embodiment of the present invention, formula (b1) is preferably represented by formula (b1').

When the PI-based resin contains a structural unit derived from a diamine represented by formula (b1), especially formula (b1'), as the structural unit (B1), the Df of the obtained PI-based film is easily reduced even at a low imidization temperature.

In one embodiment of the present invention, relative to the total amount of structural unit (B), the content of structural unit (B1) (preferably structural unit (b1)) is preferably 25 mol% or more, more preferably greater than 30 mol%, further preferably 35 mol% or more, further more preferably 40 mol% or more, especially preferably 60 mol% or more, particularly preferably 70 mol% or more, particularly preferably 80 mol% or more, further particularly preferably 90 mol% or more. If the content of structural unit (B1) (preferably structural unit (b1)) is above the above lower limit, even if the imidization temperature is low temperature, it is easy to reduce Df. In addition, the upper limit of the content of structural unit (B1) (preferably structural unit (b1)) is not particularly limited, and can be 100 mol% or less relative to the total amount of structural unit (B). The ratio of the aforementioned structural unit can be measured using, for example, ¹ H-NMR, or it can be calculated from the input ratio of the raw materials.

(Structural unit (B2))

In one embodiment of the present invention, the structural unit (B) preferably includes a structural unit (B2) derived from a diamine having two or more aromatic rings and each aromatic ring is bonded via a divalent organic group (hereinafter, sometimes simply referred to as structural unit (B2)). Examples of the divalent organic group in the structural unit (B2) include an alkylene group which may have a halogen atom, -O-, -COO-, -OOC-, -SO ₂ -, -S-, -CO-, or -N(R ^c )-, wherein R ^c represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom. Among them, the divalent organic group in the structural unit (B2) is preferably _-O- , _-CH2- , -CH2 _-CH2- , -CH( _CH3 )-, -C( _CH3 ) _2- , -C( _CF3 )2-, -COO-, -OOC-, _-SO2- , _-S- , -CO- or -N( ^Rc )-.

In one embodiment of the present invention, the structural unit (B) preferably includes a structural unit (b2) derived from a diamine represented by the formula (b2) (hereinafter, sometimes simply referred to as structural unit (b2)) as the structural unit (B2).

[In formula (b2), R ^b2 each independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom,

W independently represents -O-, _-CH2- , -CH2 _{- CH2-} , -CH( _CH3 )-, -C( _CH3 ) _2- , -C( _CF3 ) _2- , -COO-, -OOC-, _-SO2- , -S-, -CO- or -N( ^Rc )-, ^Rc represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom, m represents an integer of 1 to 4,

q independently represents an integer from 0 to 4]

If the structural unit (B) contains the structural unit (B2), especially the structural unit (b2), the Df of the obtained PI film is easily reduced even if the imidization temperature is low. As a result, the transmission loss of the electronic circuit formed by containing the obtained PI film is easily reduced even if the imidization temperature is low.

In formula (b2), R ^b2 independently represents a halogen atom, or an alkyl group, alkoxy group, aryl group or aryloxy group which may have a halogen atom, preferably a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms. As the alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms, the groups exemplified above can be mentioned. The hydrogen atoms contained in R ^b2 independently may be substituted by a halogen atom, and as the halogen atom, the halogen atom exemplified above can be mentioned. From the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability, R ^b2 is preferably independently an alkyl group having 1 to 6 carbon atoms or a fluorinated alkyl group having 1 to 6 carbon atoms. From the viewpoint of easily improving the adhesion to a substrate such as a copper foil, an alkyl group having 1 to 6 carbon atoms not containing fluorine is more preferably an alkyl group having 1 to 3 carbon atoms not containing fluorine, and a methyl group is particularly preferably.

In formula (b2), q independently represents an integer of 0 to 4, and is preferably an integer of 0 to 2, and more preferably 0 or 1, from the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability.

In formula (b2), W independently represents -O-, _-CH2- , -CH2 _-CH2- , _-CH (CH3)-, -C( _CH3 ) _2- , -C( _CF3 ) _2- , -COO-, -OOC-, _-SO2- , -S-, -CO- or -N( _Rc )-. From the viewpoint of easily reducing the Df of the PI film and easily improving the dimensional stability, it preferably _represents -O-, _-CH2- , -C( _CH3 ) _2- , -C( ^CF3 ) _2- , -COO-, -OOC- or -CO-. From the viewpoint of easily improving the adhesion to a substrate such as copper foil, it more preferably represents -O-, _-CH2- or -C( _CH3 ) _2- . It is further preferably -O- or -C( _CH3 ) _2- . R ^c represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted by a halogen atom. Examples of the monovalent hydrocarbon group having 1 to 12 carbon atoms include the monovalent hydrocarbon groups having 1 to 12 carbon atoms exemplified above, which may be substituted by a halogen atom. Examples of the halogen atom include the same halogen atoms as described above.

In formula (b2), m is an integer of 1 to 4, and is preferably an integer of 1 to 3, and more preferably 2 or 3, from the viewpoint of facilitating reduction of Df of the PI film and facilitating improvement of dimensional stability. In formula (b2), a plurality of W, R ^b2 , and q may be the same or different, and the position of -W- based on -NH ₂ of each benzene ring may be the same or different.

In formula (b2), -W- can be bonded to any position of the ortho, meta or para position, or the α, β or γ position based on _{the -NH2} of each benzene ring. From the viewpoint of easily reducing the Df of the PI film even when the imidization temperature is low, and from the viewpoint of easily improving the dimensional stability, it can be preferably bonded to the meta or para position, or the β or γ position, and can be more preferably bonded to the para or γ position.

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film even at a low imidization temperature and easily improving the adhesion between the PI film and a substrate such as copper foil, it is preferred that m in formula (b2) is 3 and W each independently represents -O- or -C(CH ₃ ) ₂ -, and it is more preferred that formula (b2) is represented by formula (b2').

If the PI-based resin contains a structural unit (b2), especially a structural unit derived from a diamine represented by formula (b2'), E' at 280°C is easily reduced. As a result, there is a tendency that the Df of the PI-based resin is easily reduced even by low-temperature thermal imidization. Therefore, even if the imidization temperature is low, it is easy to obtain a PI-based film with low Df and excellent adhesion to copper foil.

In one embodiment of the present invention, the structural unit (b2) may include a structural unit derived from a diamine represented by formula (b2') in which m is 1 and W represents -O-, in addition to or in place of the structural unit derived from the diamine represented by formula (b2').

In one embodiment of the present invention, relative to the total amount of structural unit (B), the content of structural unit (B2) is preferably 0 mol% or more, more preferably 0.3 mol% or more, further preferably 0.5 mol% or more, further more preferably 0.8 mol% or more, particularly preferably 1 mol% or more, particularly preferably 5 mol% or more, further particularly preferably 8 mol% or more. If the content of structural unit (B2) is above the lower limit, it is easy to improve the adhesion of the obtained PI film to substrates such as copper foil. In addition, relative to the total amount of structural unit (B), the upper limit of the content of structural unit (B2) is preferably 75 mol% or less, more preferably 60 mol% or less, further preferably 40 mol% or less, further more preferably 30 mol% or less, particularly preferably 20 mol% or less. If the content of structural unit (B2) is below the upper limit, there is a tendency that mechanical properties such as CTE are easily improved. The ratio of the aforementioned structural unit can be measured using, for example, ¹ H-NMR, or it can also be calculated from the input ratio of the raw materials.

(Structural unit (B3))

The PI-based resin may contain a diamine-derived structural unit (B3) other than the structural unit (B1) and the structural unit (B2) (hereinafter, sometimes simply referred to as the structural unit (B3)). Examples of the structural unit (B3) include a diamine-derived structural unit wherein m in the formula (b2) is 0, a diamine-derived structural unit wherein X in the formula (2) is represented by the formulas (61) to (64), and the like. Among these, a diamine-derived structural unit wherein X in the formula (2) is represented by the formulas (71), (74), (77), (78), (89), and (90) is preferred, and a diamine-derived structural unit (derived from p-phenylenediamine) wherein X in the formula (2) is represented by the formula (74) is more preferred. In the present specification, the "diamine-derived structural unit (B3) other than the structural unit (B1) and the structural unit (B2)" refers to a diamine-derived structural unit different from both the structural unit (B1) and the structural unit (B2).

In one embodiment of the present invention, when the structural unit (B) contains the structural unit (B3), the content of the structural unit (B3) is preferably 25 mol% or less, more preferably 20 mol% or less, further preferably 10 mol% or less, and preferably 0.01 mol% or more, relative to the total amount of the structural unit (B).

In one embodiment of the present invention, the PI resin may contain a halogen atom, preferably a fluorine atom, which can be introduced by using the above-mentioned halogen atom-containing substituent. When the PI resin contains fluorine atoms, the relative dielectric constant of the obtained PI film is easily reduced. As a fluorine-containing substituent preferably used to make the PI resin contain fluorine atoms, for example, a fluorine group and a trifluoromethyl group can be mentioned.

In another embodiment of the present invention, from the viewpoint of easily improving the adhesion between the obtained PI-based film and a substrate such as copper foil, it is preferred that the PI-based resin does not contain fluorine atoms. In addition, if the PI-based resin contains fluorine, there is a tendency to weaken the interaction between molecular chains. Therefore, if the PI-based resin does not contain fluorine atoms, there is a tendency to easily form a high-order structure that suppresses the molecular rotation of the PI-based resin, and as a result, it is easy to obtain the effect of the present invention.

When the PI resin contains halogen atoms, the content of halogen atoms, especially fluorine atoms, in the PI resin is preferably 0.1 to 35% by mass, more preferably 0.1 to 30% by mass, further preferably 0.1 to 20% by mass, and particularly preferably 0.1 to 10% by mass, based on the mass of the PI resin. If the content of halogen atoms is above the above lower limit, it is easy to improve the heat resistance and dielectric properties of the obtained PI film. If the content of halogen atoms is below the above upper limit, it is advantageous in terms of cost, it is easy to reduce the CTE of the PI film, and it is easy to synthesize the PI resin. The so-called dielectric properties refer to dielectric-related properties including relative dielectric constant and dielectric loss tangent. The so-called increase or improvement of dielectric properties indicates that the relative dielectric constant and/or dielectric loss tangent are reduced.

In one embodiment of the present invention, the imidization rate of the PI resin is preferably 90% or more, more preferably 93% or more, further preferably 95% or more, and usually 100% or less. From the viewpoint of easily improving mechanical properties, thermal properties, and dielectric properties, the imidization rate is preferably above the above-mentioned lower limit. The imidization rate represents the ratio of the molar amount of the imide bond in the PI resin to the value of 2 times the molar amount of the structural unit from the tetracarboxylic acid compound in the PI resin. It should be noted that, when the PI resin contains a tricarboxylic acid compound, the imidization rate represents the ratio of the molar amount of the imide bond in the PI resin to the value of 2 times the molar amount of the structural unit from the tetracarboxylic acid compound in the PI resin to the total molar amount of the structural unit from the tricarboxylic acid compound. In addition, the imidization rate can be obtained using IR method, NMR method, etc.

In one embodiment of the present invention, the weight average molecular weight (hereinafter, sometimes the weight average molecular weight is recorded as Mw) of the PI resin in terms of polystyrene is preferably 100,000 or more, more preferably greater than 100,000, further more preferably 110,000 or more, further more preferably 120,000 or more, particularly preferably 130,000 or more, preferably 1,000,000 or less, more preferably 700,000 or less, further preferably 500,000 or less, particularly preferably 300,000 or less. If Mw is above the lower limit, it is easy to improve mechanical properties such as folding resistance. If Mw is below the upper limit, it is advantageous from the viewpoint of processability during film making.

In one embodiment of the present invention, from the viewpoint of easily improving bending resistance, the ratio (Mw/Mn) of the Mw of the PI resin to the number average molecular weight (hereinafter, the number average molecular weight is sometimes recorded as Mn) is preferably 3.5 or more, more preferably 4.0 or more, further preferably 4.2 or more, further preferably 4.5 or more, particularly preferably 4.7 or more, preferably 8.0 or less, more preferably 7.0 or less, further preferably 6.0 or less, and particularly preferably 5.5 or less. It should be noted that for Mw and Mn, gel permeation chromatography (hereinafter, sometimes recorded as GPC) can be performed to determine the results by converting to standard polystyrene.

In one embodiment of the present invention, the content of the PI resin in the PI film is preferably 60% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and particularly preferably 90% by mass or more, relative to the mass of the PI film of the present invention. In addition, the upper limit of the content of the PI resin is not particularly limited, and is, for example, 100% by mass or less, preferably 99% by mass or less, and more preferably 95% by mass or less, relative to the mass of the PI film. If the content of the PI resin is within the above range, it is easy to improve mechanical properties and thermal properties.

The PI membrane of the present invention may include fillers as needed. As fillers, metal oxide particles such as silica, alumina, inorganic salts such as calcium carbonate, fluororesins, polymer particles such as cycloolefin polymers, etc. can be cited. The filler can be used alone or in combination of two or more. In the case of including a filler, its content is preferably 50% by mass or less, more preferably 40% by mass or less, further preferably 30% by mass or less, and preferably 0.01% by mass or more relative to the total mass of the PI membrane.

In addition, in one embodiment of the present invention, the PI film of the present invention may include additives as needed. As additives, for example, antioxidants, flame retardants, crosslinking agents, surfactants, compatibilizers, imidization catalysts, weathering agents, lubricants, anti-blocking agents, antistatic agents, anti-corona agents, drip-free agents, pigments, etc. can be cited. Additives can be used alone or in combination of two or more. The content of various additives can be appropriately selected within the scope of not damaging the effect of the present invention. In the case of including various additives, the total content is preferably 7% by mass or less relative to the total mass of the PI film, more preferably 5% by mass or less, further preferably 4% by mass or less, preferably 0.001% by mass or more.

<Polyimide Film>

The PI film of the present invention comprises a PI resin containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine, and the E' of the PI resin at 280°C is less than 3×10 ⁸ Pa, and the Tg is 200 to 290°C, so that even if the thermal imidization temperature is low, the Df can be reduced, and thus it can be suitably used for metal-clad laminates such as CCLs that can cope with high-frequency bands. In addition, the present invention also includes the following PI film, which comprises a PI resin obtained by imidizing a PI resin precursor by heat treatment at 200°C or more and less than 350°C.

In one embodiment of the present invention, the CTE of the PI film is preferably 50ppm/K or less, more preferably 40ppm/K or less, further preferably 30ppm/K or less, further more preferably 25ppm/K or less, preferably 0ppm/K or more, more preferably 5ppm/K or more, further preferably 8ppm/K or more, further more preferably 12ppm/K or more. By setting the above range, the CTE of the copper foil and the PI layer are close, so that the peeling of the laminated film can be suppressed. It should be noted that the CTE can be measured, for example, using a thermomechanical analyzer (sometimes described as "TMA"), and can be obtained using the method described in the examples.

Printed circuits are required to reduce transmission loss. Transmission loss is represented by the sum of dielectric loss, which is loss caused by the electric field generated by the dielectric, and conductor loss, which is loss caused by the current flowing through the conductor. In addition, it is known that dielectric loss is approximately proportional to the index E represented by formula (i).

E＝Df×(Dk) ^1/2 (i)

[In formula (i), Df represents dielectric loss tangent, and Dk represents relative dielectric constant]

In the high-frequency region used for 5G FPC, there is a tendency for dielectric loss to increase, and therefore, a material having a small value of the aforementioned index E and capable of suppressing dielectric loss is particularly required.

On the other hand, the current of high-frequency signals is concentrated on the outermost surface of the conductor. Therefore, the conductor loss is related to the dielectric properties of the dielectric in contact, and it is known that it is approximately proportional to (Dk) ^1/2 .

As described above, the PI-based film of the present invention includes a PI-based resin containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine. The PI-based resin has an E' of less than 3×10 ⁸ Pa at 280° C. and a Tg of 200 to 290° C., so that Df and Dk are reduced. As a result, the dielectric loss index E and the conductor loss are also reduced, and transmission loss can be reduced in a circuit including the PI-based film.

In one embodiment of the present invention, the dielectric loss index E of the PI film at 10 GHz is preferably 0.009 or less, more preferably 0.008 or less, further preferably 0.007 or less, and particularly preferably 0.006 or less. The smaller the index E, the lower the transmission loss of the electronic circuit including the PI film, and therefore, the lower limit of the index E is not particularly limited, and for example, it can be 0 or more.

In one embodiment of the present invention, from the viewpoint of easily reducing the transmission loss of the electronic circuit formed by the PI film, the Df of the PI film at 10 GHz is preferably less than 0.004, more preferably 0.0038 or less, further preferably 0.0035 or less, further preferably 0.0033 or less, particularly preferably 0.003 or less, particularly preferably 0.0027 or less, and particularly preferably 0.0024 or less. The smaller the Df, the lower the transmission loss of the electronic circuit formed by the PI film, and therefore, the lower limit of the Df is not particularly limited, and for example, it may be 0 or more.

In one embodiment of the present invention, the Dk of the PI film at 10 GHz is preferably less than 3.50, more preferably 3.45 or less, further preferably 3.40 or less, further more preferably 3.38 or less, particularly preferably 3.36 or less, particularly preferably 3.33 or less, further particularly preferably 3.30 or less, particularly preferably 3.27 or less, and particularly more preferably 3.22 or less.

Df and Dk of the PI-based film can be measured using a vector network analyzer and a resonator, for example, by the method described in the Examples.

In one embodiment of the present invention, the number of bends until the PI film of the present invention breaks in the MIT folding fatigue test according to ASTM standard D2176-16 is more than 15,000 times, preferably more than 20,000 times, more preferably more than 50,000 times, further preferably more than 100,000 times, further more preferably more than 150,000 times, and particularly preferably more than 200,000 times. If the aforementioned number of bends is above the above lower limit, even if the bends are repeated, the generation of cracks, ruptures, creases, etc. can be effectively suppressed. In addition, the upper limit of the aforementioned number of bends is not particularly limited, for example, it can also be less than 10,000,000 times. It should be noted that the MIT folding fatigue test can be measured using a MIT folding fatigue tester, for example, it can be measured using the method described in the embodiment.

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film and easily improving the mechanical properties, E' of the PI resin at 280°C and the CTE of the PI film containing the PI resin preferably satisfy the relationship of formula (Y).

(CTE of PI film) × (E' of PI resin at 280°C) ^1/2 ≥ 30,000

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the PI film and easily improving the bending resistance, the value of the left side of the formula (Y) is preferably 40,000 or more, more preferably 50,000 or more, further preferably 100,000 or more, further more preferably 120,000 or more, particularly preferably 125,000 or more, preferably 1,500,000 or less, more preferably 1,000,000 or less, further preferably 850,000 or less, further more preferably 700,000 or less, particularly preferably 500,000 or less, and particularly preferably 450,000 or less.

The thickness of the PI film of the present invention can be appropriately selected according to the application, preferably 5 μm or more, more preferably 10 μm or more, further preferably 20 μm or more, preferably 500 μm or less, more preferably 300 μm or less, further preferably 100 μm or less, particularly preferably 80 μm or less, and particularly preferably 50 μm or less. The thickness of the film can be measured using a film thickness meter or the like. It should be noted that when the film of the present invention is a multilayer film, the above thickness represents the thickness of the single layer portion.

The PI-based film of the present invention may be subjected to surface treatment such as corona discharge treatment, plasma treatment, ozone treatment, etc. by a method generally used in industry.

The PI film of the present invention has a low Df, and therefore can be suitably used in substrate materials for printed circuit boards and antenna substrates that can cope with high-frequency bands. Metal-clad laminates such as CCL used for FPC widely use laminates having a metal foil such as a copper foil layer on one or both sides of a single-layer or multi-layer PI-based resin. When the PI film of the present invention is used as a resin layer, since the Df can be reduced even if the imidization temperature is low for the PI film of the present invention, even if the CCL or other metal-clad laminate is manufactured by thermally imidizing the PI-based resin precursor coating on a metal foil such as copper foil, the degradation of the copper foil surface can be suppressed, so that a metal-clad laminate such as CCL with excellent high-frequency characteristics can be obtained.

[Method for producing polyimide film]

The method for producing the PI-based film of the present invention is not particularly limited, and the film can be produced, for example, by a method including the following steps.

A step of applying a PI-based resin precursor solution containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine onto a substrate; and

A step of imidizing a PI-based resin precursor by heat treatment at 200° C. or higher and lower than 350° C.

<Polyimide Resin Precursor Solution Coating Step>

(Preparation of PI-based resin precursor solution)

The PI-based resin precursor solution contains a PI-based resin precursor including a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine, and a solvent, and can be prepared by mixing the aforementioned PI-based resin precursor with the solvent. In addition, in one embodiment of the present invention, the reaction liquid containing the PI-based resin precursor obtained by synthesizing the aforementioned PI-based resin precursor can also be appropriately diluted with a solvent as needed and used in the form of a PI-based resin precursor solution.

The PI-based resin precursor in the present invention can be obtained by reacting tetracarboxylic anhydride with diamine. In addition to the tetracarboxylic acid compound, a dicarboxylic acid compound or a tricarboxylic acid compound may be reacted.

As tetracarboxylic anhydride used in the synthesis of PI-based resin precursors, for example, aromatic tetracarboxylic acid compounds such as aromatic tetracarboxylic dianhydride and aliphatic tetracarboxylic acid compounds such as aliphatic tetracarboxylic dianhydride can be mentioned. The tetracarboxylic acid compound can be used alone or in combination of two or more. In addition to the dianhydride, the tetracarboxylic acid compound can also be a tetracarboxylic acid compound analog such as an acyl chloride compound.

Examples of the tetracarboxylic acid compound include tetracarboxylic anhydride represented by the above formula (1), preferably tetracarboxylic anhydride represented by formula (a1), tetracarboxylic anhydride represented by formula (a2), and tetracarboxylic anhydride wherein Y in formula (1) is represented by formula (32).

Specific examples of the tetracarboxylic acid compound include pyromellitic dianhydride (hereinafter, sometimes referred to as PMDA), 4,4'-(4,4'-isopropylidene diphenoxy) diphthalic anhydride (hereinafter, sometimes referred to as BPADA), 1,4,5,8-naphthalenetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter, sometimes referred to as BPDA), 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (hereinafter, sometimes referred to as 6FDA), 4,4'-oxydiphthalic acid dianhydride (hereinafter, sometimes referred to as ODPA), 2,2',3,3'-, 2,3,3',4'- or 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, p-phenylenebis(trimellitic acid monoester acid dianhydride) (hereinafter, sometimes referred to as TAHQ), trimellitic acid anhydride and 2,2',3,3 ',5,5'-hexamethyl-4,4'-biphenol ester (hereinafter, sometimes referred to as TMPBP), 4,4'-bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-ylcarbonyloxy)biphenyl (hereinafter, sometimes referred to as BP-TME), 2,3',3,4'-diphenyl ether tetracarboxylic dianhydride, bis(2,3-dicarboxyphenyl) ether dianhydride, 3,3",4,4"-terphenyl tetracarboxylic dianhydride, 2,3,3",4" -terphenyltetracarboxylic dianhydride, 2,2", 3,3"-terphenyltetracarboxylic dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-propane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,2,7,8 -, 1,2,6,7-phenanthrenetetracarboxylic dianhydride, 1,2,9,10-phenanthrenetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)tetrafluoropropane dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride (hereinafter sometimes referred to as HPMDA), 2,3,5,6-cyclohexanetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, 4,4'- Bis(2,3-dicarboxyphenoxy)diphenylmethane dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (hereinafter, sometimes referred to as CBDA), norbornane-2-spiro-α'-spiro-2"-norbornane-5,5',6,6'-tetracarboxylic dianhydride, p-phenylenebis(trimellitic anhydride), 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 4,8-dimethyl-1,2,3,5,6,7- Hexahydronaphthalene-1,2,5,6-tetracarboxylic acid dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,3,6,7-tetrachloronaphthalene-2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 1,4,5,8-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 1,4,5,8-tetrachloronaphthalene-2,3,6, 7-tetracarboxylic acid dianhydride, 2,3,8,9-perylenetetracarboxylic acid dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, 4,5,10,11-perylenetetracarboxylic acid dianhydride, 5,6,11,12-perylenetetracarboxylic acid dianhydride, pyrazine-2,3,5,6-tetracarboxylic acid dianhydride, pyrrolidine-2,3,4,5-tetracarboxylic acid dianhydride, thiophene-2,3,4,5-tetracarboxylic acid dianhydride, bis(2,3-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, etc. Among them, even if the imidization temperature is low temperature, it is easy to reduce the Df of the obtained PI system membrane. It is preferably BPDA, PMDA, TAHQ, BP-TME, and more preferably BPDA, TAHQ, BP-TME. These tetracarboxylic acid compounds can be used alone or in combination of two or more.

Examples of the diamine compound used for synthesis of the PI-based resin precursor include aliphatic diamines, aromatic diamines, and mixtures thereof.

As said diamine compound, the diamine compound represented by said formula (2) is mentioned, For example, the diamine compound represented by formula (b1) and the diamine compound represented by formula (b2) are mentioned preferably.

It should be noted that, in the present embodiment, the so-called "aromatic diamine" means a diamine having an aromatic ring, which may contain an aliphatic group or other substituents in a part of its structure. The aromatic ring may be a monocyclic ring or a condensed ring, and examples thereof include a benzene ring, a naphthalene ring, an anthracene ring, and a fluorene ring, but are not limited thereto. Among them, a benzene ring is preferred. In addition, the so-called "aliphatic diamine" means a diamine having an aliphatic group, which may contain other substituents in a part of its structure, but does not have an aromatic ring.

Specific examples of the diamine compound include 1,4-diaminocyclohexane, 4,4'-diamino-2,2'-dimethylbiphenyl (hereinafter, sometimes referred to as m-Tb), 4,4'-diamino-3,3'-dimethylbiphenyl, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (hereinafter, sometimes referred to as TFMB), 4,4'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene (hereinafter, sometimes referred to as 1,3-APB), 1,4-bis(4-aminophenoxy)benzene (hereinafter, sometimes referred to as TPE-Q), 1,3-bis(4-aminophenoxy)benzene (hereinafter, sometimes referred to as TPE-Q), and 1,3-bis(4-aminophenoxy)benzene. benzene (hereinafter sometimes referred to as TPE-R), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (hereinafter sometimes referred to as BAPP), 2,2'-dimethyl-4,4'-diaminobiphenyl, 3,3'-dihydroxy-4,4'-diaminobiphenyl, 2,2-bis-[4-(3-aminophenoxy)phenyl]propane, bis[4-(4-aminophenoxy)]biphenyl, bis[4-(3-aminophenoxy)biphenyl, bis[1-(4-aminophenoxy)]biphenyl, bis[1-(3-aminophenoxy)]biphenyl, bis[4-(4-aminophenoxy)phenyl]methane, bis[4-(3-aminophenoxy)] bis[4-(4-aminophenoxy)phenyl]methane, bis[4-(4-aminophenoxy)phenyl]ether, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)]benzophenone, bis[4-(3-aminophenoxy)]benzophenone, 2,2-bis-[4-(4-aminophenoxy)phenyl]hexafluoropropane, 2,2-bis-[4-(3-aminophenoxy)phenyl]hexafluoropropane, 4,4'-methylenebis-o-toluidine, 4,4'-methylenebis-2,6-dimethylaniline, 4,4'-methylenebis-2,6-diethylaniline, 4,4'-methylenedianiline, 3,3'-methylene diphenylamine, 4,4'-diaminodiphenylpropane, 3,3'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, 3,3'-diaminodiphenylethane, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 3,3-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, benzidine, 3,3'-diaminobiphenyl, 3,3'-dimethoxybenzidine, 4,4"-diamino-p-terphenyl, 3,3"-diamino-p-terphenyl, m-phenylenediamine, p-phenylenediamine (hereinafter sometimes referred to as p-PDA), resorcinol-bis(3-aminophenyl) ether, 4 ,4'-[1,4-phenylenebis(1-methylethylidene)]dianiline, 4,4'-[1,3-phenylenebis(1-methylethylidene)]dianiline, bis(p-aminocyclohexyl)methane, bis(p-β-amino-tert-butylphenyl) ether, bis(p-β-methyl-δ-aminopentyl)benzene, p-bis(2-methyl-4-aminopentyl)benzene, p-bis(1,1-dimethyl-5-aminopentyl)benzene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 2,4-bis(β-amino-tert-butyl)toluene, 2,4-diaminotoluene, m-xylene-2,5-diamine, p-xylene-2,5-diamine amine, m-phenylenediamine, p-phenylenediamine, piperazine, 4,4'-diamino-2,2'-bis(trifluoromethyl)bicyclohexane, 4,4'-diaminodicyclohexylmethane, 4,4"-diamino-p-terphenyl, terephthalic acid bis(4-aminophenyl) ester, 1,4-bis(4-aminophenoxy)-2,5-di-tert-butylbenzene, 4,4'-(1,3-phenylene diisopropylidene) dianiline, 1,4-bis[2-(4-aminophenyl)-2-propyl]benzene, 2,4-diamino-3,5-diethyltoluene, 2,6-diamino-3,5-diethyltoluene, 4,4'-bis(3-aminophenoxy) biphenyl, 4,4'-(hexafluoropropylene) diphenylamine, 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, 1,2-diaminopropane, 1,2-diaminobutane, 1,3-diaminobutane, 2-methyl-1,2-diaminopropane, 2-methyl-1,3-diaminopropane, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, norbornanediamine, 2'-methoxy-4,4'-diaminobenzanilide, 4,4'-diaminobenzanilide, bis[4-(4-amino 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 9,9-bis[4-(3-aminophenoxy)phenyl]fluorene, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 2,5-diamino-1,3,4-oxadiazole, bis[4,4'-(4-aminophenoxy)]benzanilide, bis[4,4'-(3-aminophenoxy)]benzanilide, 2,6-diaminopyridine, 2,5-diaminopyridine, and the like. Among them, from the viewpoint that the Df of the obtained PI film can be easily reduced even at a low imidization temperature, m-Tb, BAPP, TPE-Q, TPE-R, etc. are preferred, and m-Tb, BAPP, etc. are more preferred. The diamine compound can be used alone or in combination of two or more.

The PI-based resin precursor may be a product obtained by reacting other tetracarboxylic acids, dicarboxylic acids, tricarboxylic acids, and anhydrides and derivatives thereof in addition to the tetracarboxylic acid compound used in the synthesis of the PI-based resin precursor, within a range that does not impair the various physical properties of the PI-based film.

As other tetracarboxylic acids, water addition products of anhydrides of the above-mentioned tetracarboxylic acid compounds can be mentioned.

Examples of the dicarboxylic acid compound include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, and similar acid chloride compounds and acid anhydrides thereof, and two or more thereof may be used in combination. Specific examples include terephthalic acid; isophthalic acid; naphthalene dicarboxylic acid; 4,4'-biphenyl dicarboxylic acid; 3,3'-biphenyl dicarboxylic acid; dicarboxylic acid compounds of chain hydrocarbons having 8 or less carbon atoms, and compounds in which two benzoic acids are linked by a single bond, -O-, _-CH2- , -C( _CH3 ) _2- , -C( _CF3 ) _2- , _-SO2- , or phenylene, and acid chloride compounds thereof.

Examples of the tricarboxylic acid compound include aromatic tricarboxylic acids, aliphatic tricarboxylic acids, and similar acid chloride compounds and anhydrides thereof, and two or more thereof may be used in combination. Specific examples include 1,2,4-benzenetricarboxylic acid anhydride; 2,3,6-naphthalenetricarboxylic acid-2,3-anhydride; and a compound in which phthalic anhydride and benzoic acid are linked via a single bond, -O-, _-CH2- , -C( _CH3 ) _2- , -C( _CF3 ) _2- , _-SO2- , or a phenylene group.

In the production of the PI-based resin precursor, the usage-amounts of the diamine compound, the tetracarboxylic acid compound, the dicarboxylic acid compound, and the tricarboxylic acid compound can be appropriately selected according to the ratio of each structural unit of the desired PI-based resin.

In the present invention, the total number of moles of diamine compounds used relative to 1 mole of the total amount of tetracarboxylic acid compounds is defined as the amine ratio. In a preferred embodiment of the present invention, the amine ratio is preferably 0.90 moles or more, preferably 0.999 moles or less, relative to 1 mole of the total amount of tetracarboxylic acid compounds. In another embodiment, the amine ratio is preferably 1.001 moles or more, preferably 1.10 moles or less, relative to 1 mole of the total amount of tetracarboxylic acid compounds.

In one embodiment of the present invention, when the amine ratio is 1 or less, the amine ratio is preferably 0.90 mol to 0.999 mol, more preferably 0.95 mol to 0.997 mol, and even more preferably 0.97 mol to 0.995 mol.

In one embodiment of the present invention, when the amine ratio is 1 or more, the amine ratio is preferably 1.001 mol to 1.1 mol, more preferably 1.002 mol to 1.05 mol, and further preferably 1.003 mol to 1.03 mol.

If the amine ratio is close to 1.0 mol, the molecular weight tends to increase sharply during synthesis, and if the amine ratio is greatly different from 1.0 mol, the molecular weight of the obtained PI-based resin tends to decrease easily. If the molecular weight increases sharply, it tends to grow unevenly in the synthetic material, and the physical properties of the PI-based resin tend to be difficult to stabilize. On the other hand, if the molecular weight is too low, the mechanical properties tend to decrease.

The reaction temperature of the diamine compound and the tetracarboxylic acid compound is preferably 50°C or less, more preferably 40°C or less, and further preferably 30°C or less. If the reaction temperature is below the above upper limit, the Df of the obtained PI film is easily reduced, and this tendency is particularly significant in a PI film containing a PI resin containing an ester bond, especially a PI resin containing a structural unit (A1). In addition, the reaction temperature of the diamine compound and the tetracarboxylic acid compound is preferably 5°C or more, more preferably 10°C or more, and further preferably 15°C or more. If the reaction temperature is above the above lower limit, there is a tendency to easily increase the reaction rate and shorten the polymerization time.

The reaction time is not particularly limited, and is, for example, about 0.5 to 72 hours, and preferably 3 to 24 hours. When the reaction time is within the above range, the Df of the obtained PI film can be easily reduced even at a low imidization temperature.

The reaction of the diamine compound and the tetracarboxylic acid compound is preferably carried out in a solvent. As a solvent, there is no particular limitation as long as it does not affect the reaction, and examples thereof include: alcohol solvents such as water, methanol, ethanol, ethylene glycol, isopropanol, propylene glycol, ethylene glycol methyl ether, ethylene glycol butyl ether, 1-methoxy-2-propanol, 2-butoxyethanol, and propylene glycol monomethyl ether; phenol solvents such as phenol and cresol; ester solvents such as ethyl acetate, butyl acetate, ethylene glycol methyl ether acetate, propylene glycol methyl ether acetate, and ethyl lactate; lactone solvents such as γ-butyrolactone (hereinafter, sometimes recorded as GBL) and γ-valerolactone; ketone solvents such as acetone, methyl ethyl ketone, cyclopentanone, cyclohexanone, 2-heptanone, and methyl isobutyl ketone; Aliphatic hydrocarbon solvents such as pentane, hexane, and heptane; alicyclic hydrocarbon solvents such as ethylcyclohexane; aromatic hydrocarbon solvents such as toluene and xylene; nitrile solvents such as acetonitrile; ether solvents such as tetrahydrofuran and dimethoxyethane; chlorinated solvents such as chloroform and chlorobenzene; amide solvents such as N,N-dimethylacetamide (hereinafter, sometimes described as DMAc) and N,N-dimethylformamide (hereinafter, sometimes described as DMF); sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, and cyclopentane; carbonate solvents such as ethylene carbonate and propylene carbonate; pyrrolidone solvents such as N-methylpyrrolidone (hereinafter, sometimes described as NMP); and combinations thereof. Among them, from the viewpoint of solubility, phenol solvents, lactone solvents, amide solvents, and pyrrolidone solvents can be preferably used appropriately, and amide solvents are more preferred.

In one embodiment of the present invention, from the viewpoint of easily reducing the Df of the obtained PI film even at a low imidization temperature, the boiling point of the solvent used in the reaction of the diamine compound and the tetracarboxylic acid compound is preferably 230° C. or less, more preferably 200° C. or less, and further preferably 180° C. or less. In addition, from the viewpoint of easily reducing the Df of the obtained PI film, the boiling point of the above-mentioned solvent is preferably 100° C. or more, more preferably 120° C. or more.

The reaction of the diamine compound and the tetracarboxylic acid compound can be carried out in an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere or under reduced pressure as necessary, and is preferably carried out in an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere while stirring in a strictly controlled dehydrated solvent.

The solvent contained in the PI-based resin precursor solution can include the solvents exemplified as the solvents used in the reaction of the diamine compound and the tetracarboxylic acid compound, preferably a lactone solvent, an amide solvent, a pyrrolidone solvent, and more preferably an amide solvent. In addition, in one embodiment of the present invention, from the viewpoint that the Df of the obtained PI film can be easily reduced even at a low imidization temperature, the boiling point of the solvent contained in the PI-based resin precursor solution is preferably 230°C or less, more preferably 200°C or less, further preferably 180°C or less, and particularly preferably 170°C or less. In addition, from the viewpoint that the Df of the obtained PI film can be easily reduced, the boiling point of the aforementioned solvent is preferably 100°C or more, more preferably 120°C or more.

The content of the PI-based resin precursor contained in the PI-based resin precursor solution is preferably 8% by mass or more, more preferably 10% by mass or more, further preferably 12% by mass or more, particularly preferably 13% by mass or more, relative to the total amount of the PI-based resin precursor solution, and preferably 30% by mass or less, more preferably 25% by mass or less, further preferably 23% by mass or less, particularly preferably 20% by mass or less. If the content of the PI-based resin precursor is within the above range, the processability during film formation is excellent.

(Application of polyimide resin precursor solution)

The step of applying the PI-based resin precursor solution is a step of applying the PI-based resin precursor solution on a substrate to form a coating film.

In the coating process, a composition is applied to a substrate using a known coating method or coating method to form a coating film. As known coating methods, for example, wire rod coating, reverse coating, gravure coating, roller coating, die coating, comma coating, lip coating, spin coating, screen printing, injection blade coating, dipping, spraying, curtain coating, slit coating, cast film, etc. can be cited. When a solution of a PI-based resin precursor is applied or coated on a substrate, a single layer of a PI-based resin precursor can be applied to the substrate, or a multilayer of a PI-based resin precursor can be applied to the substrate. In the case of applying a multilayer of a PI-based resin precursor on a substrate, the coating and drying can be performed in multiple times, or multiple layers can be applied simultaneously.

As examples of substrates, metal plates (such as copper plates, etc.) such as metal foils (such as copper foils), SUS plates such as SUS foils and SUS tapes, glass substrates, PET films, PEN films, other PI resin films other than the PI film of the present invention, polyamide resin films, etc. can be cited. Among them, from the viewpoint of excellent heat resistance, preferably copper plates, SUS plates, glass substrates, PET films, PEN films, etc. can be cited, and from the viewpoint of adhesion to the film and cost, more preferably copper plates, SUS plates, glass substrates or PET films, etc. can be cited.

<Imidization step>

The imidization step is a step of imidizing the PI-based resin precursor applied on the substrate by heat treatment at 200° C. or higher and lower than 350° C.

In one embodiment of the present invention, the imidization process is preferably the following process: before the imidization of the PI-based resin precursor, the PI-based resin precursor solution applied to the substrate is heated and dried at a relatively low temperature, for example, below 200°C, and the obtained dry film of the PI-based resin precursor is imidized by heat treatment at a temperature above 200°C and below 350°C.

In one embodiment of the present invention, the dry film of the PI resin precursor on the substrate may be imidized to obtain the PI film, or the dry film of the PI resin precursor may be peeled off from the substrate and the dry film peeled off from the substrate may be imidized to obtain the PI film.

In one embodiment of the present invention, there is no particular restriction on the drying temperature of the PI-based resin precursor applied to the substrate as long as it is within the temperature range at which the solvent can be dried and solidified. From the viewpoint of avoiding surface roughness due to rapid drying and suppressing wrinkles, kinks, etc. generated during processing, the drying temperature is preferably lower than 300°C, more preferably lower than 260°C, further preferably lower than 200°C, and further more preferably lower than 180°C. In addition, from the viewpoint of productivity, it is preferably higher than 50°C, more preferably higher than 80°C, and further preferably higher than 100°C.

For the PI-based resin precursor in the present invention, even if imidization is performed at a low temperature, the Df of the obtained PI-based film can be reduced. The heat treatment temperature in the imidization process, that is, the imidization temperature, is preferably lower than 350°C, more preferably below 340°C, further preferably below 330°C, further more preferably below 310°C, and particularly preferably below 300°C. If the imidization temperature is below the above-mentioned upper limit, even when a metal foil such as copper foil is used as a substrate, thermal degradation of the metal foil, especially the copper foil, can be suppressed, so it is easy to obtain a CCL with excellent high-frequency characteristics. In addition, from the perspective of easily fully increasing the imidization rate, the imidization temperature is preferably above 200°C, more preferably above 210°C, and further preferably above 220°C. In addition, from the perspective of easily obtaining a smooth film, it is preferably heated in stages. For example, after heating at a relatively low temperature of 50 to 150°C to remove the solvent, it can be heated in stages to a temperature in the range of 200°C or more and less than 350°C for imidization.

In one embodiment of the present invention, the reaction time in the imidization is preferably 0.5 to 24 hours, more preferably 1 to 12 hours. In addition, in one embodiment of the present invention, the time for maintaining a temperature of 200°C or more is preferably 10 minutes to 90 minutes, more preferably 15 minutes to 70 minutes, and further preferably 20 minutes to 50 minutes. If the reaction time of 200°C or more in the imidization is within the above range, it is easy to fully increase the imidization rate, it is easy to prevent oxidative degradation of the resin, and it is easy to improve the dielectric properties and bending resistance of the obtained PI film.

After imidization, the coating film formed on the substrate is peeled off from the substrate, thereby obtaining a PI film. In one embodiment of the present invention, when the substrate is a metal foil such as copper foil, the PI film can be formed without peeling the coating film from the metal foil such as copper foil, and the obtained laminated film having the PI film laminated on the metal foil such as copper foil is used for metal-clad laminates such as CCL.

When the film of the present invention is a multilayer film, it can be produced by a multilayer film forming method such as a coextrusion processing method, an extrusion lamination method, a heat lamination method, or a dry lamination method.

〔Laminated film〕

The PI film of the present invention has a low Df and can therefore be suitably used in the formation of a metal-clad laminate for FPC. Therefore, the following laminated film is included, wherein the laminated film uses the PI film of the present invention as a PI layer and comprises a PI layer and a metal foil layer. In one embodiment of the present invention, the laminated film of the present invention may include a metal foil layer only on one side of the PI layer, or may include a metal foil layer on both sides of the PI layer.

In one embodiment of the present invention, examples of the metal foil include copper foil, SUS foil, and aluminum foil. From the viewpoint of electrical conductivity and metal workability, copper foil is preferred.

For the PI film of the present invention, even if the thermal imidization temperature is low, Df is low, and it can be suitably used to form a CCL with excellent high-frequency characteristics. Therefore, in a preferred embodiment of the present invention, the laminated film of the present invention is preferably a laminated film including a copper foil layer on one or both sides of the PI film of the present invention.

In one embodiment of the present invention, the thickness of the metal foil layer, especially the copper foil layer, is preferably 1 μm or more, more preferably 5 μm or more, and from the viewpoint of easy miniaturization of the circuit and easy improvement of folding resistance, it is preferably 100 μm or less, more preferably 50 μm or less, further preferably 30 μm or less, and particularly preferably 20 μm or less. The thickness of the metal foil layer, especially the copper foil layer, can be measured using a film thickness meter or the like. It should be noted that when the two sides of the PI film contain metal foil layers, especially copper foil layers, the thickness of each metal foil layer, especially each copper foil layer, may be the same as or different from each other.

In one embodiment of the present invention, the thickness of the laminated film is preferably 5 μm or more, more preferably 10 μm or more, further preferably 15 μm or more, and preferably 100 μm or less, more preferably 80 μm or less, further preferably 60 μm or less. The thickness of the laminated film can be measured using a film thickness meter or the like.

The laminated film of the present invention may include other layers such as a functional layer in addition to the PI film and the metal foil layer, especially the copper foil layer. As the functional layer, the layers exemplified above may be cited, for example, a thermoplastic PI resin layer including a thermoplastic PI resin, an adhesive layer, etc. The functional layer may be used alone or in combination of two or more.

In one embodiment of the present invention, the laminated film of the present invention can be a double-layer metal-clad laminated plate composed of a metal foil layer and a PI layer, or a three-layer metal-clad laminated plate composed of a metal foil layer, a PI layer and an adhesive layer. From the viewpoint of heat resistance, dimensional stability and lightweight, it is preferably a double-layer metal-clad laminated plate that does not include an adhesive layer.

In addition, for the PI-based film of the present invention, even if the imidization temperature is low, Df is low, so even if a laminated film in which the metal foil is copper foil is produced by thermal imidization of the PI-based resin precursor coating film on the copper foil, degradation of the copper foil surface can be suppressed. Therefore, the laminated film of the present invention has excellent high-frequency characteristics even if it does not include an adhesive layer.

In addition, in one embodiment of the present invention, the PI film of the present invention and the metal foil layer, especially the copper foil layer, can be directly connected, or a functional layer can be inserted between the PI film and the metal foil layer, especially the copper foil layer so that they are connected via the functional layer. From the perspective of easily improving mechanical properties and thermal properties, it is preferred that the PI film and the metal foil layer, especially the copper foil layer, are directly connected.

The functional layer that can be inserted between the PI film of the present invention and the metal foil layer can be a thermoplastic PI layer. From the perspective of easily improving mechanical and thermal properties, the layer directly in contact with the metal foil layer, especially the copper foil layer, is preferably the PI film of the present invention or a thermoplastic PI layer as a functional layer.

[Method for producing laminated film]

The present invention also includes a method for manufacturing a laminated film, the method comprising the following steps:

A step of applying a PI-based resin precursor solution containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine onto a substrate; and

A step of imidizing a PI-based resin precursor by heat treatment at 200° C. or higher and lower than 350° C. to form the PI-based film of the present invention on a substrate.

With regard to the "step of applying a PI-based resin precursor solution containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine onto a substrate" and the "step of imidizing the PI-based resin precursor by heat treatment at a temperature of not less than 200°C and not more than 350°C to form the PI-based film of the present invention on the substrate" in the method for producing a laminated film of the present invention, the descriptions related to each step recorded in the item [Method for producing a polyimide-based film] also apply.

In one embodiment of the present invention, the substrate is preferably a metal foil, particularly preferably a copper foil. The descriptions regarding the metal foil, particularly the copper foil, are the same as those regarding the metal foil described in the section [Laminated film].

The laminated film of the present invention can also be manufactured by a method other than the above method, for example, the following method: on other substrates other than the metal foil contained in the laminated film, a PI-based resin precursor solution containing a structural unit (A) from a tetracarboxylic anhydride and a structural unit (B) from a diamine is applied and dried, and the dry film of the PI-based resin precursor thus obtained is peeled off from the aforementioned substrate, and the dry film of the aforementioned PI-based resin precursor after peeling is bonded to the metal foil. As a method for bonding the dry film of the PI-based resin precursor to the metal foil, a pressurization-based method, a lamination method using a hot roller, etc. can be adopted, and the imidization of the PI-based resin precursor can also be performed simultaneously in the bonding process. However, for the PI-based film of the present invention, even if the imidization temperature is low, Df is low, so even if the laminated film in which the metal foil is a copper foil is manufactured by, for example, thermal imidization of the PI-based resin precursor coating on the copper foil, the degradation of the copper foil surface can be suppressed. Therefore, the laminated film of the present invention has excellent high-frequency characteristics even if it is manufactured without the bonding process described above.

〔Flexible printed circuit board〕

The PI film of the present invention has a low Df and can reduce the transmission loss of a circuit including the PI film, and thus can be suitably used as an FPC substrate material. The present invention also includes an FPC substrate including the above-mentioned PI film.

[Example]

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

The abbreviations used in Examples, Comparative Examples and Reference Examples represent the following compounds.

BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride

TAHQ: p-phenylene bis(trimellitic acid monoester dianhydride)

PMDA: pyromellitic dianhydride

BP-TME: 4,4'-bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-ylcarbonyloxy)biphenyl

ODPA: 4,4'-oxydiphthalic anhydride

BPADA: 4,4'-(4,4'-isopropylidene diphenoxy) diphthalic anhydride

m-Tb: 4,4'-diamino-2,2'-dimethylbiphenyl

BAPP: 2,2-bis[4-(4-aminophenoxy)phenyl]propane

TPE-R: 1,3-bis(4-aminophenoxy)benzene

TPE-Q: 1,4-bis(4-aminophenoxy)benzene

PDA: p-phenylenediamine

[Synthesis of polyimide resin precursor]

(Example 1)

17.93 g (84.4 mmol) of m-Tb and 0.35 g (0.9 mmol) of BAPP were dissolved in 284 g of DMAc, and then 19.35 g (42.2 mmol) of TAHQ was added, and the mixture was stirred at 20° C. for 1 hour under a nitrogen atmosphere. Then 12.42 g (42.2 mmol) of BPDA was added, and the mixture was stirred at 20° C. for 24 hours under a nitrogen atmosphere to obtain a PI resin precursor composition. The molar ratio of the diamine monomer to the acid dianhydride monomer used was 1.01.

(Examples 2 to 16 and Comparative Examples 1 to 5)

The PI resin precursor composition was obtained in the same manner as in Example 1 except that the types of monomers used and the monomer composition were changed as shown in Table 1. The order of adding the monomers was in the order of diamine and acid dianhydride unless otherwise specified, with the diamine being added in the order of deriving the structural units (B1), (B2), and (B3) and the acid dianhydride being added in the order of deriving the structural units (A1), (A2), and (A3).

[Manufacturing of polyimide film]

The PI resin precursor compositions obtained in Examples 1 to 16 and Comparative Examples 1 to 5 were appropriately diluted with the solvent used in the synthesis of the PI resin precursor so that the content of the PI resin precursor was 10% by mass or more, and the viscosity was adjusted to 40,000 cps or less, thereby preparing a PI resin precursor solution. The PI resin precursor solution was film-formed under any of the following film-forming conditions 1 to 4 as shown in Table 1, to obtain a PI film formed of a PI resin.

<Film Forming Conditions 1>

The PI resin precursor solution was cast on a glass substrate, and a coating of the PI resin precursor solution was formed using an applicator at a line speed of 0.4 m/min. The coating was heated at 120°C for 30 minutes, and the obtained film was peeled off from the glass substrate and fixed to a metal frame. For the film fixed to the metal frame, the temperature was raised from 30°C to 270°C in an atmosphere of 7% oxygen concentration for 19 minutes, and then cooled to 200°C for 35 minutes to prepare a PI film. The temperature above 220°C was maintained for 23 minutes. In addition, the temperature above 200°C was maintained for 34 minutes.

<Film Forming Conditions 2>

The PI resin precursor solution was cast on the roughened side (surface roughness; Rz = 1.3 μm) of an electrolytic copper foil (manufactured by JX Metal Co., Ltd., JXEFL-BHM, thickness of 12 μm), and a coating of the PI resin precursor solution was formed using an applicator at a line speed of 0.4 m/min. The coating was heated at 120°C for 30 minutes to dry it. Then, the laminated film of the copper foil and the precursor was fixed to a metal frame, and in an atmosphere with an oxygen concentration of 1%, the temperature was raised from 30°C to 320°C over 9 minutes, then heated at 320°C for 6 minutes, and cooled to 200°C over 15 minutes, thereby producing a laminated film of the PI film and the copper foil. The temperature above 220°C was maintained for 21 minutes. In addition, the temperature above 200°C was maintained for 25 minutes.

The obtained laminated film of the PI film and the copper foil was immersed in a large volume 40 mass % ferric chloride aqueous solution at room temperature for 10 minutes, and after visually confirming that no copper remained, it was dried at 80° C. for 1 hour to obtain a single PI film.

<Film Forming Conditions 3>

The PI resin precursor solution was cast on a glass substrate, and a coating of the resin precursor solution was formed using an applicator at a line speed of 0.4 m/min. The coating was heated at 120°C for 30 minutes, and the obtained film was peeled off from the glass substrate and fixed to a metal frame. For the film fixed to the metal frame, the temperature was raised from 30°C to 320°C in an atmosphere with an oxygen concentration of 1% for 9 minutes, then heated at 320°C for 6 minutes, and cooled to 200°C for 15 minutes to produce a PI film. The temperature above 220°C was maintained for 21 minutes. In addition, the temperature above 200°C was maintained for 25 minutes.

<Film Forming Conditions 4>

The PI resin precursor solution was cast on a glass substrate, and a coating of the PI resin precursor solution was formed using an applicator at a line speed of 0.4 m/min. The coating was heated at 120°C for 30 minutes, and the obtained film was peeled off from the glass substrate and fixed to a metal frame. For the film fixed to the metal frame, the temperature was raised from 30°C to 320°C in an atmosphere with an oxygen concentration of 1% over 5 minutes, then heated at 320°C for 5 minutes, and cooled to 200°C over 15 minutes to produce a PI film. The temperature above 220°C was maintained for 17 minutes. In addition, the temperature above 200°C was maintained for 21 minutes.

[Synthesis of polyimide resin precursor and production of polyimide film]

(Comparative Example 6)

Dissolve 70.33 g (331 mmol) of m-Tb in 720 g of NMP, then add 73.06 g (248 mmol) of BPDA and 37.94 g (83 mmol) of TAHQ, and stir at room temperature for 1 hour under a nitrogen atmosphere. Then, stir at 60°C for 20 hours to obtain a PI resin precursor composition. In addition, the Mw of the PI resin precursor is 91,000 and the Mn is 27,000 in terms of polystyrene. The PI resin precursor composition is appropriately diluted with NMP to adjust the viscosity, thereby preparing a PI resin precursor solution, and the obtained PI resin precursor solution is subjected to film formation under the aforementioned film forming condition 1 to obtain a PI film.

The obtained PI film had a thickness of 30 μm, a Dk of 3.45, a Df of 0.0040, an index E of 0.0074, and a CTE of 38.2 ppm. The Tg of the PI resin was 255° C., and E′ at 280° C. was 5.53×10 ⁸ Pa.

In addition, Tg obtained from the storage elastic modulus curve using the tangent method was 230°C.

The PI films obtained in the examples and comparative examples were subjected to various measurements and evaluations. The measurement and evaluation methods are described below.

<Measurement of glass transition temperature (Tg)>

The Tg of the PI resin obtained in the examples and comparative examples was determined by measuring the PI film as follows.

Using a dynamic viscoelasticity measuring apparatus (manufactured by IT Keisoku Seigyo Co., Ltd., DVA-220), the measurement was performed under the following sample and conditions to obtain a tanδ curve which is the ratio of the storage modulus (Storage modulus, E') to the loss modulus (Loss modulus, E") value. The top of the peak of the tanδ curve was taken as Tg.

Test piece: a rectangular parallelepiped with a length of 40 mm, a width of 5 mm, and a thickness of 50 μm (the thickness varies depending on the film used)

Experimental mode: single frequency, constant heating rate

Experimental method: stretching

Sample clamping interval length: 15mm

Determination starting temperature: room temperature ~ 342℃

Heating rate: 5℃/min

Frequency: 10Hz

Static/dynamic stress ratio: 1.8

Main collected data:

(1) Storage modulus (E’)

(2) Loss modulus (E”)

(3)tanδ(E”/E’)

<Measurement of storage elastic modulus (E')>

E' of the PI resins obtained in Examples and Comparative Examples at 280°C was determined by measuring the dynamic viscoelasticity in the same manner as in the measurement of Tg.

<Measurement of Weight Average Molecular Weight Mw and Number Average Molecular Weight Mn>

The polystyrene-equivalent Mw and Mn of the PI resin precursor obtained by synthesis were measured using GPC. The GPC measurement was performed under the following conditions.

(1) Pretreatment method

The sample was diluted with DMF and filtered through a 0.45 μm membrane filter, and the obtained solution was used as a measurement solution.

(2) Measurement conditions

Column: Connect two TSKgel SuperAWM-H (inner diameter 6.0 mm, length 150 mm)

Eluent: DMF (added with 10 mmol/L lithium bromide and 30 mmol/L phosphoric acid)

Flow rate: 0.6mL/min

Detector: RI detector

Column temperature: 40°C

Injection volume: 20 μL

Molecular weight standard: Standard polystyrene

<Determination of Coefficient of Thermal Expansion (CTE)>

The CTE of the PI films obtained in Examples and Comparative Examples was measured using TMA under the following conditions, and the CTE at 50° C. to 100° C. was calculated.

Equipment: TMA/SS7100 manufactured by Hitachi High-Tech Science Corporation

Load: 50.0mN

Temperature program: from 20°C to 130°C at a rate of 5°C/min

Test piece: a rectangular parallelepiped with a length of 40 mm, a width of 5 mm, and a thickness of 50 μm (the thickness varies depending on the film used).

<Evaluation of Dielectric Loss Index E>

The dielectric loss index E of the film is calculated by the following formula.

E＝Df×(Dk) ^1/2 (i)

Df: Dielectric loss tangent

Dk: relative dielectric constant

(Determination of Df and Dk)

A 50 mm×50 mm measurement sample was cut out from the PI film obtained in the examples and comparative examples, and Df and Dk were measured under the following conditions: The sample was humidified at 25° C./55% RH for 24 hours and then measured.

Device: Compact USB vector network analyzer manufactured by Anritsu Corporation (product name: MS46122B)

Cavity resonator manufactured by AET Inc. (TE mode 10 GHz type)

Measurement frequency: 10 GHz

Measurement atmosphere: 23℃/50%RH

<Evaluation of bending resistance>

The bending resistance of the PI film obtained in Examples 4, 10, 13, 14 and 15 was evaluated by measuring the number of times the film was bent under the following conditions. The film was cut into a rectangle with a length of 100 mm and a width of 10 mm using a dumbbell cutter. The cut film was placed in an MIT folding fatigue tester (manufactured by Toyo Seiki Seisaku-sho, MIT-DA) in accordance with ASTM standard D2176-16. Under the conditions of a test speed of 175 cpm, a bending angle of 135°, a load of 750 g, and a bending fixture of R=1.0 mm, the film was alternately bent in both the front and back directions, and the number of bends until breakage occurred was measured. The more times the film is bent, the better the bending resistance.

The number of bending times of the PI films obtained in Examples 4, 10, 13, 14 and 15 were 240,000, 15,000, 160,000, 130,000 and 20,000 respectively. In addition, for the PI films obtained in Examples 4, 10, 13, 14 and 15, the values of (CTE of PI film)×(E' of PI resin at 280°C) ^1/2 were 136141, 115683, 141751, 121951 and 116579 respectively.

Table 1 shows the measurement and evaluation results of the PI films obtained in Examples and Comparative Examples, and the amine ratios.

[Table 1]

As shown in Table 1, it was confirmed that, for the PI films obtained in Examples 1 to 16, even if the maximum temperature of the imidization treatment was a low temperature such as 270°C or 320°C, Df was lower than that of the comparative example, and the dielectric loss index E was lower. Therefore, for the PI film of the present invention, even if it is manufactured by casting a PI resin precursor solution on a copper foil and thermally imidizing the coating film of the PI resin precursor solution on a metal foil such as a copper foil, thermal degradation such as roughness of the surface of a metal foil such as a copper foil can be suppressed, and therefore, it can be suitably used for metal-clad laminates such as CCLs that can cope with high frequency bands and have low transmission loss.

Claims (15)

1. A polyimide film comprising a polyimide resin containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine, the polyimide resin having a storage elastic modulus at 280 ℃ of less than 3X 10 ⁸ Pa, and a glass transition temperature of 200 to 290 ℃.

2. The polyimide-based film according to claim 1, wherein the structural unit (A) comprises a structural unit (A1) derived from a tetracarboxylic anhydride containing an ester bond.

3. The polyimide-based film according to claim 2, wherein the structural unit (A) further comprises a structural unit (A2) derived from a tetracarboxylic anhydride having a biphenyl skeleton.

4. The polyimide-based film according to claim 3, wherein the structural unit (A) satisfies the relationship of the formula (X),

(content of structural unit (A3) derived from tetracarboxylic anhydride excluding the structural unit (A1) and the structural unit (A2)/(total amount of the structural unit (A1) and the structural unit (A2)) <1.1 (X).

5. The polyimide-based film according to any one of claims 2 to 4, wherein the structural unit (A1) is a structural unit (A1) derived from a tetracarboxylic anhydride represented by the formula (A1),

in the formula (a 1), Z represents a 2-valent organic group,

R ^a1 independently of one another, represents a halogen atom or an alkyl, alkoxy, aryl or aryloxy group which may have a halogen atom,

s independently of one another represent an integer from 0 to 3.

6. The polyimide-based film according to any one of claims 3 to 5, wherein the structural unit (A2) is a structural unit (A2) derived from a tetracarboxylic anhydride represented by the formula (A2),

In the formula (a 2), R ^a2 Independently of one another, represents a halogen atom or an alkyl, alkoxy, aryl or aryloxy group which may have a halogen atom,

t independently of one another represents an integer from 0 to 3.

7. The polyimide-based film according to any one of claims 1 to 6, wherein the structural unit (B) comprises a structural unit (B1) derived from a diamine having a biphenyl skeleton.

8. The polyimide-based film according to claim 7, wherein the structural unit (B1) is a structural unit (B1) derived from a diamine represented by the formula (B1),

in the formula (b 1), R ^b1 Independently of one another, represents a halogen atom or an alkyl, alkoxy, aryl or aryloxy group which may have a halogen atom,

p represents an integer of 0 to 4.

9. The polyimide-based film according to claim 7 or 8, wherein the content of the structural unit (B1) is more than 30 mol% with respect to the total amount of the structural units (B).

10. The polyimide-based film according to any one of claims 1 to 9, wherein the structural unit (B) comprises a structural unit (B2) derived from a diamine represented by the formula (B2),

in the formula (b 2), R ^b2 Independently of one another, represents a halogen atom or an alkyl, alkoxy, aryl or aryloxy group which may have a halogen atom,

W independently of one another represents-O-, -CH ₂ -、-CH ₂ -CH ₂ -、-CH(CH ₃ )-、-C(CH ₃ ) ₂ -、-C(CF ₃ ) ₂ -、-COO-、-OOC-、-SO ₂ -, -S-, -CO-or-N (R) ^c )-，R ^c Represents a monovalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom,

m represents an integer of 1 to 4,

q independently of one another represents an integer from 0 to 4.

11. The polyimide-based film according to claim 10, wherein m in the structural unit (b 2) is 3,W, independently of each other, represents-O-or-C (CH ₃ ) ₂ -。

12. A laminated film comprising a metal foil layer on one or both surfaces of the polyimide film according to any one of claims 1 to 11.

13. A flexible printed circuit board comprising the polyimide film according to any one of claims 1 to 11.

14. A method for producing a laminated film, comprising the steps of:

a step of applying a polyimide resin precursor solution containing a structural unit (A) derived from a tetracarboxylic anhydride and a structural unit (B) derived from a diamine to a substrate; and

a step of imidizing a polyimide resin precursor by heat treatment at 200 ℃ or higher and lower than 350 ℃ to form the polyimide film according to any one of claims 1 to 11 on a substrate.

15. The method for producing a laminated film according to claim 14, wherein the substrate is a metal foil.