TWI888563B - Polyimide film and method for producing the same - Google Patents

Abstract

The subject of the present invention is to provide a colorless polyimide film having high tensile strength and tensile elasticity, large elongation at break and low linear expansion coefficient and a method for manufacturing the same. The solution of the present invention is a multi-layer film using high-strength polyimide as (a) the outer layer and polyimide with excellent optical properties as (b) the inner layer. A polyimide solution or a polyimide precursor solution for forming the (a) layer is coated on a temporary support, and after drying the support until the solvent content reaches 5 to 40% by mass, a polyimide solution or a polyimide precursor solution for forming the (b) layer is coated, and the same coating is repeated as needed, and finally heat-treated to obtain a multi-layer polyimide film.

Description

Polyimide film and method for producing the same

The present invention relates to a colorless polyimide film having a low linear expansion coefficient and good mechanical properties and a method for producing the same.

Polyimide film has excellent heat resistance, good mechanical properties, and is widely used in the electrical and electronic fields as a flexible material. However, general polyimide film cannot be used in display devices and other parts that require light to penetrate because of its yellow-brown color. On the other hand, display devices are becoming thinner and lighter, and are required to be more flexible. Therefore, although attempts are being made to develop substrate materials that replace glass substrates with flexible polymer film substrates, colored polyimide films cannot be used as substrate materials for liquid crystal displays that display by transmitting light ON/OFF. They can only be used in peripheral circuits such as TAB and COF that carry drive circuits in display devices, and in small parts such as the back side of reflective display methods or self-luminous display devices.

Based on the above background, the development of colorless and transparent polyimide films is being carried out. As a representative example, there are attempts to develop colorless and transparent polyimide films using fluorinated polyimide resins, semi-aliphatic cyclic or fully-aliphatic cyclic polyimide resins, etc. (Patent Documents 1 to 3). Although these films have little color and are transparent, their mechanical properties are not as good as those of colored polyimide films. In addition, in industrial production and in the case of use where exposure to high temperatures is expected, thermal decomposition or oxidation reactions may occur, so colorlessness and transparency may not be maintained. From this point of view, a method of performing a heat treatment while spraying a gas with a predetermined oxygen content has been proposed (Patent Document 4). However, in an environment with an oxygen concentration of less than 18%, the manufacturing cost is high and industrial production is extremely difficult. [Prior Technical Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 11-106508 [Patent Document 2] Japanese Patent Publication No. 2002-146021 [Patent Document 3] Japanese Patent Publication No. 2002-348374 [Patent Document 4] WO2008/146637

That is, there is a trade-off between practical properties such as heat resistance and mechanical properties and colorlessness and transparency, and it is very difficult to produce a colorless and transparent polyimide film that satisfies all the requirements. The subject of the present invention is to provide a polyimide film having excellent mechanical properties and colorlessness and transparency.

The inventors of the present invention attempted to achieve a balanced polyimide film by combining multiple polyimide resins. Generally speaking, when combining multiple resin components for compounding, blending, or copolymerization, the result of combining only the advantages of each component may not be obtained. On the contrary, there are many cases where the disadvantages are multiplied. However, as a result of the inventors' continued and dedicated research, they found that by combining polyimide resins in a manner to form a specific structure and forming a film, the advantages of each component can be fully extracted, thereby completing the present invention. That is, the present invention is composed as follows. [1] A multilayer polyimide film, characterized in that it has a multilayer polyimide layer, which is formed by laminating at least two polyimide layers of different compositions in the thickness direction, and a transition layer, which exists between the (a) layer constituting the multilayer polyimide layer and the (b) layer adjacent to the (a) layer, and the chemical composition gradually changes; the overall thickness of the film is greater than 3 μm and less than 120 μm, the overall yellowness index of the film is less than 5, and the overall light transmittance of the film is greater than 86%. [2] The multilayer polyimide film as described in [1], characterized in that the lower limit of the thickness of the transition layer is 0.01 μm, and the upper limit is either 3% of the total thickness of the film or 1 μm. [3] The multilayer polyimide film as described in [1] or [2], characterized in that: The (a) layer is mainly composed of a polyimide having a yellowness index of 10 or less and a total transmittance of 85% or more when the film is formed into a single film with a thickness of 25±2 μm, and The (b) layer is mainly composed of a polyimide having a yellowness index of 5 or less and a total transmittance of 90% or more when the film is formed into a single film with a thickness of 25±2 μm. [4] A multilayer polyimide film as described in any one of [1] to [3], characterized in that the (a) layer is present on both sides of the (b) layer, one side and the other side, and the transition layer is present between the (a) layer and the (b) layer on one side of the (b) layer and between the (a) layer and the (b) layer on the other side of the (b) layer, and has a layer structure in which the (a) layer, the transition layer, the (b) layer, the transition layer, and the (a) layer are sequentially stacked.

[5] A multilayer polyimide film as described in any one of [1] to [4], characterized in that the polyimide of the aforementioned (a) layer is a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic acid anhydride containing 70% by mass or more of alicyclic tetracarboxylic acid anhydride and a diamine containing 70% by mass or more of a diamine having an amide bond in the molecule; or a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic acid anhydride containing 70% by mass or more of alicyclic tetracarboxylic acid anhydride and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule.

[6] A multilayer polyimide film as described in any one of [1] to [5], characterized in that the polyimide of the aforementioned (b) layer is a polyimide having a chemical structure obtained by tetracarboxylic anhydride containing 70% by mass or more of aromatic tetracarboxylic anhydride and diamine containing 70% by mass or more of diamine having a sulfur atom in the molecule; or a polyimide having a chemical structure obtained by polycondensation of tetracarboxylic anhydride containing 30% by mass or more of tetracarboxylic acid containing trifluoromethyl in the molecule and diamine containing 70% by mass or more of diamine having trifluoromethyl in the molecule.

[7] A method for producing a multilayer polyimide film as described in [1], [2], [3], [5] or [6], which at least comprises: 1: applying a polyimide solution or a polyimide precursor solution for forming layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1, 3: applying the polyimide solution or polyimide precursor solution for forming the (b) layer on the coating a2 to obtain the coating ab1; 4: heating the entire layer to obtain a layered product with a residual solvent content of less than 0.5% by mass based on the entire layer.

[8] A method for producing a multilayer polyimide film as described in [1], [2], [3], [5] or [6], comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming layer (b) on a temporary support to obtain a coating a1; The polyimide precursor solution is applied to the coating a2 to obtain the coating ab1; 4: the whole layer is heated to obtain a layered body with a residual solvent content of more than 5% by mass and less than 40% by mass based on the whole layer, and then it is peeled off from the temporary support to obtain a self-supporting film; 5: the two ends of the aforementioned self-supporting film are held to further obtain a film with a residual solvent content of less than 0.5% by mass based on the whole layer.

[9] A method for producing a multilayer polyimide film as described in any one of [1] to [6], comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming layer (b) on a temporary support to obtain a coating a1; 4: applying a polyimide solution or a polyimide precursor solution for forming layer (b) to a substrate to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 4: drying the coating ab1 to obtain a coating ab2 with a residual solvent content of 5-40% by mass based on the whole layer; 5: applying the polyimide solution or polyimide precursor solution for forming the (a) layer to the coating ab2 to obtain a coating ab1; 6: heating the whole layer to obtain a layered product with a residual solvent content of less than 0.5% by mass based on the whole layer.

[10] A method for producing a multilayer polyimide film as described in any one of [1] to [6], comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming layer (b) on the coating a2 to obtain a coating ab1; 4: drying the coating ab1 to obtain a residual solvent content of the entire layer. 5: applying the polyimide solution or polyimide precursor solution for forming the (a) layer on the coating ab2 to obtain the coating ab1; 6: heating the entire layer to obtain a layered product with a residual solvent content of 8% to 40% by mass, and then peeling it from a temporary support to obtain a self-supporting film; 7: holding both ends of the aforementioned self-supporting film to further obtain a film with a residual solvent content of 0.5% to 0.5% by mass.

The present invention may further include the following structures. [11] A method for manufacturing a multilayer polyimide film, characterized in that 1 and 2 in the above-mentioned [8] are repeated to form an odd number of layers of 5 or more. [12] A multilayer polyimide film as described in [1] to [6], characterized in that the thickness of the (a) layer is less than 25% of the total thickness of the film. However, the multilayer polyimide film as described in [1] to [5] is characterized in that, when there are multiple (a) layers, the total thickness of the (a) layers is more than 1% of the total thickness of the film, preferably more than 2%, more preferably more than 4%, and less than 25%, preferably less than 13%, and more preferably less than 7%.

The multilayer polyimide film of the present invention has excellent mechanical properties and colorless transparency because it is made by laminating at least two polyimide layers with different compositions in the thickness direction and having a transition layer with a gradually changing chemical composition between the two polyimide layers.

[Form used to implement the invention]

The polyimide of the (a) layer in the present invention is preferably: a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having an amide bond in the molecule, or a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule; the above polyimide has good mechanical properties, high elongation at break, and has excellent properties of showing low CTE, but is easier to color.

On the other hand, the polyimide of the (b) layer is preferably a polyimide having a chemical structure composed of a tetracarboxylic anhydride containing 70% by mass or more of an aromatic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a sulfur atom in the molecule, or a polyimide having a chemical structure composed of a tetracarboxylic anhydride containing 30% by mass or more of a tetracarboxylic anhydride containing a trifluoromethyl group in the molecule and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule; it has high colorless transparency, but is hard and brittle as a resin, and it is difficult to exert sufficient elongation at break when it is made into a film, so it is difficult to say that it is necessarily good for flexible applications, and it is also difficult to produce as a continuous film.

If the two are blended or copolymerized, only a film with properties intermediate or below those of the two can be obtained. Furthermore, the colorless and transparent properties tend to be lowered by the properties of the (a) layer, which is easily colored.

However, as in the present invention, by forming these two components of polyimide into independent layers to divide their functions, and then applying a specific manufacturing method, a balanced film can be obtained, that is, a film having colorless transparency, sufficient film strength for practical use, high elongation at break, and low linear expansion coefficient.

The polyimide film is obtained by coating a polyimide solution or a solution of a polyimide precursor on a support, drying it, and performing a chemical reaction as required. In the present invention, a solution of multiple components is coated and dried for each component until the fluidity is lost and the solution becomes semi-solid, and the coating and drying are repeated to form a multi-layer structure. After the necessary layers are formed, the solution is dried by final heating and a chemical reaction is performed as required to obtain a solid film. Polyimide is chemically stable. Therefore, even if a second polyimide solution or polyimide precursor solution of a different composition (or the same chemical composition) is coated on a first polyimide film and a solid polyimide film is obtained by heat drying or catalysis, the first polyimide and the second polyimide will not be chemically bonded. Therefore, the interface bonding strength is weak, and only a film that is easily peeled off between layers can be obtained. However, if the coating and semi-drying operations are repeated as in the present invention, the solvent concentration of the first coated part is low, and the solvent concentration of the later coated part is high, so the concentration gradient causes the diffusion of the solvent across the interface. At the same time, the dissolved polymer will also follow the solvent and want to move, so microscopic flow mixing will occur near the interface, forming an extremely thin transition layer with a gradual chemical composition change. This transition layer can buffer the mismatch such as stress generated between layers with different physical properties, and at the same time, it shows a strong bonding strength between the layers, so as to obtain a stable multi-layer film with good property balance.

The multilayer polyimide film of the present invention has a thickness of 3 μm or more and 120 μm or less. Since the mechanical properties will be good, it is preferably 4 μm or more, more preferably 5 μm or more, and even more preferably 8 μm or more. Moreover, since the transparency will be good, it is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less.

The multilayer polyimide film of the present invention has a yellow index of 5 or less. Since transparency is improved, it is preferably 4 or less, more preferably 3.5 or less, and most preferably 3 or less. The lower limit of the yellow index is not particularly limited because it is better to be lower, but it is industrially sufficient as long as it is 0.1 or more, and it may be 0.2 or more.

The multilayer polyimide film of the present invention has a total light transmittance of 86% or more. Since the transparency becomes good, it is preferably 87% or more, more preferably 88% or more, and even more preferably 89% or more. The upper limit is specifically limited, but in industry, it only needs to be 99% or less, and it does not matter even if it is 98% or less.

In the present invention, at least two types of polyimide with different compositions are used and layered in the thickness direction. Polyimide is generally a polymer obtained by the polycondensation reaction of tetracarboxylic anhydride and diamine. The at least two types of polyimide layers include (a) layer and (b) layer, and the (a) layer and (b) layer are preferably each mainly composed of polyimide with the following characteristics. Here, the polyimide with the following characteristics is preferably contained in each layer in an amount of 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 100% by mass. Furthermore, the so-called different compositions must at least mean that the resin compositions of the polyimides are different, for example, different from the same resin compositions except for the presence or absence of a lubricant or the amount of the lubricant mixed therein.

The polyimide mainly used in the (a) layer (hereinafter, "mainly" may be omitted and simply recorded as "polyimide used in the (a) layer" or "polyimide used as the (a) layer", etc.) is preferably a polyimide having a yellow index of 10 or less and a total transmittance of 85% or more when formed into a single film with a thickness of 25±2μm. Since the transparency will be good, the yellow index is preferably 9 or less, more preferably 8 or less, and even more preferably 7 or less. The lower limit of the yellow index is not particularly limited, but in industry it only needs to be 0.1 or more, and it does not matter even if it is 0.2 or more. The total transmittance is preferably 86% or more, more preferably 87% or more, and even more preferably 88% or more. The upper limit is not specifically set, but in industry, it is sufficient as long as it is below 99%, and even below 98% is fine.

The thickness of the (a) layer in the multilayer polyimide film is preferably greater than 1 μm, more preferably greater than 1.5 μm, more preferably greater than 2 μm, and particularly preferably greater than 3 μm, because the mechanical strength will be good. In addition, because the transparency will be good, it is preferably less than 119 μm, more preferably less than 100 μm, more preferably less than 50 μm, and particularly preferably less than 20 μm.

The polyimide mainly used in the (a) layer is preferably a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride when the total acid component is 100% by mass and a diamine containing 70% by mass or more of a diamine having an amide bond in the molecule when the total amine component is 100% by mass, or a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule.

The polyimide mainly used in the (b) layer (hereinafter, "mainly" may be omitted and simply described as "polyimide used in the (b) layer" or "polyimide used as the (b) layer", etc.) is preferably a polyimide having a yellow index of 5 or less and a total light transmittance of 90% or more when a single film having a thickness of 25±2μm is formed. Since the transparency is good, the yellow index is preferably 4 or less, and more preferably 3 or less. The lower limit of the yellow index is not particularly limited, but industrially it can be 0.1 or more, and even 0.2 or more is not a problem. The total light transmittance is preferably 91% or more, and more preferably 92% or more. The upper limit is not particularly limited, but industrially it can be 99% or less, and even 98% or less is not a problem. The yellowness index of the polyimide used in the (b) layer is preferably less than the yellowness index of the polyimide used in the (a) layer. In addition, the total light transmittance of the polyimide used in the (b) layer is preferably greater than the total light transmittance of the polyimide used in the (a) layer.

The thickness of the (b) layer in the multilayer polyimide film is preferably greater than 1 μm, more preferably greater than 2 μm, more preferably greater than 3 μm, and particularly preferably greater than 4 μm, because the mechanical strength will be good. In addition, because the transparency will be good, it is preferably less than 119 μm, more preferably less than 100 μm, more preferably less than 80 μm, and particularly preferably less than 50 μm.

The polyimide mainly used in the (b) layer is preferably a polyimide having a chemical structure composed of a tetracarboxylic anhydride containing 70% by mass or more of aromatic tetracarboxylic anhydride when the total acid component is set to 100% by mass and a diamine containing at least 70% by mass or more of a diamine having a sulfur atom in the molecule when the total amine component is set to 100% by mass, or a polyimide having a chemical structure composed of a polycondensation of a tetracarboxylic anhydride containing at least 30% by mass or more of a tetracarboxylic acid containing a trifluoromethyl group in the molecule and a diamine containing at least 70% by mass or more of a diamine having a trifluoromethyl group in the molecule.

Examples of the alicyclic tetracarboxylic anhydride in the present invention include 1,2,3,4-cyclobutanetetracarboxylic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, 1,2,3,4-cyclohexanetetracarboxylic acid, 1,2,4,5-cyclohexanetetracarboxylic acid, 3,3',4,4'-bicyclohexanetetracarboxylic acid, bicyclo[2,2,1]heptane-2,3,5,6-tetracarboxylic acid, bicyclo[2,2,2]octane-2,3,5,6-tetracarboxylic acid, bicyclo[2,2,2]oct-7-enyl-2,3,5,6-tetracarboxylic acid, tetrahydroanthracene-2,3,6,7-tetracarboxylic acid, tetradecahydro-1,4:5,8:9,10 -trimethanoanthracene-2,3,6,7-tetracarboxylic acid, decahydronaphthalene-2,3,6,7-tetracarboxylic acid, decahydro-1,4-ethano-5,8-methanonaphthalene-2,3,6,7-tetracarboxylic acid, acid), norbornane-2-spiro-α-cyclopentanone-α’-spiro-2’’-norbornane-5,5’’,6,6’’-tetracarboxylic acid (also known as “norbornane-2-spiro-2’-cyclopentanone-5’-spiro-2’’-norbornane-5,5’’,6,6’’-tetracarboxylic acid”), methylnorbornane-2-spiro-α-cyclopentanone-α’-spiro-2’’-(methylnorbornane)-5,5’’,6,6’’-tetracarboxylic acid, norbornane-2-spiro-α-cyclohexanone-α’-spiro-2’’-norbornane-5,5’’,6,6’’-tetracarboxylic acid (also known as “norbornane-2-spiro-2’-cyclohexanone-6’-spiro-2’’- Norbornane-5,5'',6,6''-tetracarboxylic acid"), methylnorbornane-2-spiro-α-cyclohexanone-α'-spiro-2''-(methylnorbornane)-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclopropanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclobutanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cycloheptanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclooctanone-α'-spiro -2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclononanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclodecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cycloundecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclododecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclododecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclotridecanone-α'- Tetracarboxylic acids such as spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclotetradecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-cyclopentadecanone-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, norbornane-2-spiro-α-(methylcyclopentanone)-α'-spiro-2''-norbornane-5,5'',6,6''-tetracarboxylic acid, and anhydrides thereof. Among these, dianhydrides having two anhydride structures are preferred, and in particular, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclohexanetetracarboxylic dianhydride, and 1,2,4,5-cyclohexanetetracarboxylic dianhydride are preferred, 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,4,5-cyclohexanetetracarboxylic dianhydride are more preferred, and 1,2,3,4-cyclobutanetetracarboxylic dianhydride is further preferred. In addition, these may be used alone or in combination of two or more.

As the aromatic tetracarboxylic anhydride in the present invention, there can be mentioned 4,4'-(2,2-hexafluoroisopropylidene)diphthalic acid, 4,4'-oxydiphthalic acid, bis(1,3-dihydroxy-1,3-dihydro-2-benzofuran-5-carboxylic acid) 1,4-phenylene ester, bis(1,3-dihydroxy-1,3-dihydro-2-benzofuran-5-carboxylic acid) 4,4'-[4,4'-(3-oxo-1,3-dihydro-2-benzofuran-1,1-diyl)bis(benzene-1,4-diyloxy)]dibenzene-1,2-dicarboxylic acid, 3,3',4,4'-diphenylketonetetracarboxylic acid, 4,4'-[(3-oxo-1,3-dihydro-2-benzofuran-1,1-diyl)bis(benzene-1,4-diyloxy)]dibenzene-1,2-dicarboxylic acid dihydro-2-benzofuran-1,1-diyl)bis(toluene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-[(3-oxo-1,3-dihydro-2-benzofuran-1,1-diyl)bis(1,4-xylene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-[4,4'-(3-oxo-1,3-dihydro-2-benzofuran-1,1-diyl)bis(4-isopropyl-toluene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-[4,4'-(3-oxo-1,3-dihydro-2-benzofuran-1,1-diyl)bis(naphthalene-1,4-diyloxy)]dibenzene -1,2-dicarboxylic acid, 4,4'-[4,4'-(3H-2,1-benzoxathiol-1,1-dioxo-3,3-diyl)bis(benzene-1,4-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-diphenylketonetetracarboxylic acid, 4,4'-[(3H-2,1-benzoxathiol-1,1-dioxo-3,3-diyl)bis(toluene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-[(3H-2,1-benzoxathiol-1,1-dioxo-3,3-diyl)bis(1,4-xylene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'- [4,4'-(3H-2,1-benzothiol-1,1-dioxo-3,3-diyl)bis(4-isopropyl-toluene-2,5-diyloxy)]dibenzene-1,2-dicarboxylic acid, 4,4'-[4,4'-(3H-2,1-benzothiol-1,1-dioxo-3,3-diyl)bis(naphthalene-1,4-diyloxy)]dibenzene-1,2-dicarboxylic acid, 3,3',4,4'-diphenyl ketone tetracarboxylic acid, 3,3',4,4'-diphenyl ketone tetracarboxylic acid, 3,3',4,4'-diphenyl sulfone tetracarboxylic acid, 3,3',4,4'-biphenyl tetracarboxylic acid, 2,3,3',4'-biphenyl tetracarboxylic acid, pyromelitic acid, 4,4'-[spiro(𠮿 -9,9'-fluorinated)-2,6-diylbis(oxycarbonyl)]diphthalic acid, 4,4'-[spiro(𠮿 [0063] Aromatic tetracarboxylic acids may be used alone or in combination of two or more.

In the present invention, in addition to tetracarboxylic anhydride, tricarboxylic acids and dicarboxylic acids can also be used. As tricarboxylic acids, there can be listed aromatic tricarboxylic acids such as trimellitic acid, 1,2,5-naphthalenetricarboxylic acid, diphenyl ether-3,3',4'-tricarboxylic acid, and diphenylsulfone-3,3',4'-tricarboxylic acid, or hydrogenated products of the above aromatic tricarboxylic acids such as hexahydrotricrimellitic acid, ethylene glycol bistrimellitate, propylene glycol bistrimellitate, 1,4-butanediol bistrimellitate, polyethylene glycol bistrimellitate, and alkylene glycol bistrimellitate, and these monoanhydrides and esters. Among these, monoanhydrides having one acid anhydride structure are preferred, and trimellitic anhydride and hexahydrotricrimellitic anhydride are particularly preferred. Furthermore, these may be used alone or in combination.

As dicarboxylic acids, aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, and 4,4'-oxydiphenylcarboxylic acid, or hydrogenated products of the above aromatic dicarboxylic acids such as 1,6-cyclohexane dicarboxylic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, 2-methylsuccinic acid, and acyl chlorides or esters thereof can be listed. Among them, aromatic dicarboxylic acids and hydrogenated products thereof are preferred, and terephthalic acid, 1,6-cyclohexane dicarboxylic acid, and 4,4'-oxydiphenylcarboxylic acid are particularly preferred. In addition, dicarboxylic acids can be used alone or in combination of a plurality of them.

As the diamine having an amide bond in the molecule of the present invention, aromatic diamines and alicyclic amines can be mainly used. Examples of aromatic diamines include 2,2'-dimethyl-4,4'-diaminobiphenyl ester, 1,4-bis[2-(4-aminophenyl)-2-propyl]benzene, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2'-bistrifluoromethyl-4,4'-diaminobiphenyl ester, 4,4'-bis(4-aminophenoxy)biphenyl ester, 4,4'-bis(3-aminophenoxy)biphenyl ester, bis[4-(3-aminophenoxy)biphenyl ester, bis[4-(3-aminophenoxy)phenyl]ketone, bis[4-(3-aminophenoxy)phenyl]sulfide, bis[4-(3-aminophenoxy)phenyl]sulfide, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, m-aminobenzylamine, p-aminobenzylamine, 4-amino 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 2,2'-trifluoromethyl-4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 4, 4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl ketone, 3,4'-diaminodiphenyl ketone, 4,4'-diaminodiphenyl ketone, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, bis[4-(4-aminophenoxy)phenyl]methane, 1,1-Bis[4-(4-aminophenoxy)phenyl]ethane, 1,2-Bis[4-(4-aminophenoxy)phenyl]ethane, 1,1-Bis[4-(4-aminophenoxy)phenyl]propane, 1,2-Bis[4-(4-aminophenoxy)phenyl]propane, 1,3-Bis[4-(4-aminophenoxy)phenyl]propane, 2,2-Bis[4-(4-aminophenoxy)phenyl]propane, 1,1-Bis[4-(4-aminophenoxy)phenyl]propane 1,3-Bis[4-(4-aminophenoxy)phenyl]butane, 1,4-Bis[4-(4-aminophenoxy)phenyl]butane, 2,2-Bis[4-(4-aminophenoxy)phenyl]butane, 2,3-Bis[4-(4-aminophenoxy)phenyl]butane, 2-[4-(4-aminophenoxy)phenyl]-2-[4-(4-aminophenoxy)-3-methylphenyl]propane, 2,2-Bis[ 4-(4-aminophenoxy)-3-methylphenyl]propane, 2-[4-(4-aminophenoxy)phenyl]-2-[4-(4-aminophenoxy)-3,5-dimethylphenyl]propane, 2,2-bis[4-(4-aminophenoxy)-3,5-dimethylphenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 1,4-bis(3-aminophenoxy) Benzene, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl]ketone, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)phenyl]sulfide 1,3-Bis[4-(4-aminophenoxy)benzyl]benzene, 1,3-Bis[4-(3-aminophenoxy)benzyl]benzene, 1,4-Bis[4-(3-aminophenoxy)benzyl]benzene, 4,4'-Bis[(3-aminophenoxy)benzyl]benzene, 1,1-Bis[4-(3-aminophenoxy)phenyl]propane, 1,3-Bis[4-(3-aminophenoxy)phenyl]propane , 3,4'-diaminodiphenyl sulfide, 2,2-bis[3-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, bis[4-(3-aminophenoxy)phenyl]methane, 1,1-bis[4-(3-aminophenoxy)phenyl]ethane, 1,2-bis[4-(3-aminophenoxy)phenyl]ethane, bis[4-(3-aminophenoxy)phenyl]sulfone, 4,4'-bis[3-(4-aminophenoxy)phenyl] phenoxy) benzoyl] diphenyl ether, 4,4'-bis[3-(3-aminophenoxy) benzoyl] diphenyl ether, 4,4'-bis[4-(4-amino-α,α-dimethylbenzyl) phenoxy] diphenyl ketone, 4,4'-bis[4-(4-amino-α,α-dimethylbenzyl) phenoxy] diphenyl sulfone, bis[4-{4-(4-aminophenoxy) phenoxy} phenyl] sulfone, 1,4-bis[4-(4-aminophenoxy) phenoxy] )phenoxy-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-aminophenoxy)phenoxy-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-trifluoromethylphenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-fluorophenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-methylphenoxy)-α,α-dimethyl benzyl]benzene, 1,3-bis[4-(4-amino-6-cyanophenoxy)-α,α-dimethylbenzyl]benzene, 3,3'-diamino-4,4'-diphenoxydiphenyl ketone, 4,4'-diamino-5,5'-diphenoxydiphenyl ketone, 3,4'-diamino-4,5'-diphenoxydiphenyl ketone, 3,3'-diamino-4-phenoxydiphenyl ketone, 4,4'-diamino-5-phenoxydiphenyl ketone, 3,4' -Diamino-4-phenoxydiphenyl ketone, 3,4'-diamino-5'-phenoxydiphenyl ketone, 3,3'-diamino-4,4'-biphenyloxydiphenyl ketone, 4,4'-diamino-5,5'-biphenyloxydiphenyl ketone, 3,4'-diamino-4,5'-biphenyloxydiphenyl ketone, 3,3'-diamino-4-biphenyloxydiphenyl ketone, 4,4'-diamino-5-biphenyloxydiphenyl ketone, 3,4' -Diamino-4-biphenyloxydiphenyl ketone, 3,4'-diamino-5'-biphenyloxydiphenyl ketone, 1,3-bis(3-amino-4-phenoxybenzyl)benzene, 1,4-bis(3-amino-4-phenoxybenzyl)benzene, 1,3-bis(4-amino-5-phenoxybenzyl)benzene, 1,4-bis(4-amino-5-phenoxybenzyl)benzene, 1,3-bis(3-amino-4-biphenyloxybenzyl) Benzene, 1,4-bis(3-amino-4-biphenyloxybenzyl)benzene, 1,3-bis(4-amino-5-biphenyloxybenzyl)benzene, 1,4-bis(4-amino-5-biphenyloxybenzyl)benzene, 2,6-bis[4-(4-amino-α,α-dimethylbenzyl)phenoxy]benzonitrile, 4,4'-[9H-fluoren-9,9-diyl]bisaniline (also known as "9,9-bis(4-aminophenyl)fluoren"), spiro(𠮿) -9,9'-fluorinated)-2,6-diylbis(oxycarbonyl)]bisaniline, 4,4'-[spiro(𠮿 -9,9'-fluorinated)-2,6-diylbis(oxycarbonyl)]bisaniline, 4,4'-[spiro( -9,9'-fluorenyl)-3,6-diylbis(oxycarbonyl)]bisaniline, 5-amino-2-(p-aminophenyl)benzo oxazole, 6-amino-2-(p-aminophenyl)benzo oxazole, 5-amino-2-(m-aminophenyl)benzo oxazole, 6-amino-2-(m-aminophenyl)benzo oxadiazole, 2,2'-p-phenylenebis(5-aminobenzoic acid oxadiazole), 2,2'-p-phenylenebis(6-aminobenzoic acid azole), 1-(5-aminobenzoic acid oxadiazole)-4-(6-aminobenzo oxadiazole)benzene, 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bis(2,6-diaminodiphenyl)benzo[1,2-d:5,4-d'] oxadiazole, 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:4,5-d']bis(2,6 ... oxadiazole, 2,6-(3,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bis(2,6 ... oxadiazole, 2,6-(3,4'-diaminodiphenyl)benzo[1,2-d:4,5-d']bis(2,6 ... oxadiazole, 2,6-(3,3'-diaminodiphenyl)benzo[1,2-d:5,4-d']bis(2,6 ... oxadiazole, 2,6-(3,3'-diaminodiphenyl)benzo[1,2-d:4,5-d']bis(2,6 ... Furthermore, part or all of the hydrogen atoms on the aromatic ring of the aromatic diamine may be substituted with a halogen atom, an alkyl or alkoxy group having 1 to 3 carbon atoms, or a cyano group, and part or all of the hydrogen atoms of the alkyl or alkoxy group having 1 to 3 carbon atoms may be substituted with a halogen atom.

Examples of the alicyclic diamines include 1,4-diaminocyclohexane, 1,4-diamino-2-methylcyclohexane, 1,4-diamino-2-ethylcyclohexane, 1,4-diamino-2-n-propylcyclohexane, 1,4-diamino-2-isopropylcyclohexane, 1,4-diamino-2-n-butylcyclohexane, 1,4-diamino-2 -isobutylcyclohexane, 1,4-diamino-2-sec-butylcyclohexane, 1,4-diamino-2-tert-butylcyclohexane, 4,4'-methylenebis(2,6-dimethylcyclohexylamine), 9,10-bis(4-aminophenyl)adenine, 2,4-bis(4-aminophenyl)cyclobutane-1,3-dicarboxylic acid dimethyl ester, and the like.

In the present invention, it is preferred that the (a) layer exists on both sides of the (b) layer on one side and the other side, and the transition layer exists between the (a) layer and the (b) layer on one side of the (b) layer and between the (a) layer and the (b) layer on the other side of the (b) layer, and that the (a) layer, the transition layer, the (b) layer, the transition layer, and the (a) layer are sequentially stacked. Hereinafter, the layer structure in which the (a) layer, the transition layer, the (b) layer, the transition layer, and the (a) layer are sequentially stacked is also referred to as "(a)/(b)/(a)". Similarly, a layer structure formed by sequentially stacking (a) layer, transition layer, and (b) layer may be referred to as "(a)/(b)", and a layer structure formed by sequentially stacking (a) layer, transition layer, (b) layer, transition layer, (a) layer, transition layer, (b) layer, transition layer, and (a) layer may be referred to as "(a)/(b)/(a)/(b)/(a)". In the present invention, the (a) layer and the (b) layer may be a two-layer structure of (a)/(b), or a three-layer structure of (a)/(b)/(a), and preferably a five-layer structure of (a)/(b)/(a)/(b)/(a), and may also be a thin film with seven, nine or more odd-numbered layers. In the case of an odd-numbered layer, it is preferred to arrange the film in such a way that the (a) layer is located at the outermost layer. By setting the (a) layer, which has superior mechanical properties and a smaller linear expansion coefficient than the (b) layer, as the outermost layer, the linear expansion coefficient of the entire film can be suppressed to a low side, and the surface layer with excellent mechanical strength can be attached, thereby improving the handling of the film and maximizing the excellent optical properties of the polyimide (b) layer, which becomes the inner layer. The (b) layer is preferably thicker than the (a) layer. The ratio of the thickness of the (b) layer to the thickness of the (a) layer is preferably (b) layer/(a) layer = greater than 1, more preferably 1.5 or more, and more preferably 2 or more. In addition, it is preferably 20 or less, more preferably 15 or less, and more preferably 12 or less.

In the present invention, when the thickness of the (a) layer is a plurality of layers, the total thickness of the (a) layer is preferably less than 34% of the total thickness of the film, more preferably less than 26%, more preferably less than 13%, more preferably less than 7%. The thickness of the (a) layer is more than 1% of the total thickness of the film, more preferably more than 2%, more preferably more than 4%. By limiting the thickness of the (a) layer to this range, a film in which the mechanical properties of the (a) layer and the optical properties of the (b) layer are balanced can be obtained. When the thicknesses of the (a) layer and the (b) layer are shown, from the center of the transition layer in the thickness direction, the (a) layer side is included in the (a) layer, and the (b) layer side is included in the (b) layer.

In the present invention, it is preferred that there is a transition layer (mixed layer) between the (a) layer and the (b) layer, in which the composition changes continuously from the polyimide of the (a) layer to the polyimide of the (b) layer. The lower limit of the thickness of the transition layer is preferably 0.01 μm or more. It is more preferably 0.02 μm or more, and even more preferably 0.05 μm or more. In addition, the upper limit of the thickness of the transition layer is preferably 3% or less of the total thickness of the film or 1 μm or less. As a preferred range of the upper limit, it is preferably 2.8% or 0.9 μm of the total thickness of the film, and more preferably 2.5% or 0.8 μm of the total thickness of the film. By setting the transition layer within the above range, both transparency and mechanical strength can be taken into account. In addition, the thickness of the transition layer refers to the thickness of the region where the composition of the polyimide (a) layer and the polyimide (b) layer is gradually changed from one side to the other by mixing, and the composition ratio (mass ratio) of the polyimide (a) layer/polyimide (b) layer of the mixed layer is in the range of 5/95 to 95/5. The thickness of the transition layer can be measured by shearing the film obliquely in the thickness direction and observing the composition distribution of the polyimide.

Regarding the thickness of the transition layer, when the multilayer polyimide film is a laminated structure of two layers, since there is only one interface between the layers, the thickness of the transition layer can be obtained from the thickness of the transition layer existing at the interface and the total thickness of the film. When the multilayer polyimide film is a laminated structure of three layers, since there are two interfaces between the layers, the thickness of each transition layer can be obtained from the total thickness of the film. When the multilayer polyimide film is a laminated structure of four or more layers, the thickness of all the transition layers can be obtained from the total thickness of the film.

The polyimide used in the (a) layer of the present invention is preferably a polyimide having a yellow index of less than 10 and a total transmittance of more than 85% when formed into a film having a thickness of 25±2μm. Furthermore, the polyimide used in the (a) layer has a CTE of less than 25ppm/K, more preferably less than 20ppm/K, a tensile strength of more than 100MPa, more preferably more than 120MPa, and a breaking elongation of more than 10%, more preferably more than 12%. As a preferred polyimide for this (a) layer, there can be exemplified a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having an amide bond in the molecule. In addition, as a polyimide used for the (a) layer, there can be exemplified a polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule. As the polyimide for any (a) layer, an alicyclic tetracarboxylic anhydride can also be used. The content of the alicyclic tetracarboxylic anhydride is preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 90% by mass or more, and further preferably 95% by mass or more of the total tetracarboxylic anhydride. By limiting the content of the alicyclic tetracarboxylic acid to a specified range, coloring is suppressed.

As the diamine having an amide bond in the molecule, 4-amino-N-(4-aminophenyl)benzamide is preferred. The diamine having an amide bond is preferably used in an amount of 70% by mass or more, 80% by mass or more, and more preferably 90% by mass or more of the whole diamine. In addition, as the diamine having a trifluoromethyl group, 2,2'-bistrifluoromethyl-4,4'-diaminobiphenyl, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, and 2,2'-trifluoromethyl-4,4'-diaminodiphenyl ether are preferred. These diamine compounds having fluorine atoms in the molecule, especially when using a diamine having a trifluoromethyl group in the molecule, are preferably used in an amount of 70% by mass or more, 80% by mass or more, and more preferably 90% by mass or more of the whole diamine.

The polyimide used in the (b) layer of the present invention is preferably a polyimide having a yellow index of 5 or less and a total light transmittance of 90% or more when a single film having a thickness of 25±2μm is formed. As the polyimide used in the (b) layer, there can be exemplified a polyimide having a chemical structure composed of a tetracarboxylic anhydride containing 70% by mass or more of an aromatic tetracarboxylic anhydride and a diamine containing at least 70% by mass or more of a diamine having a sulfur atom in the molecule. Examples of polyimide suitable for layer (b) include polyimide having a chemical structure obtained by polycondensation of a tetracarboxylic anhydride containing at least 30% by mass of a tetracarboxylic acid having a trifluoromethyl group in its molecule and a diamine containing at least 70% by mass of a diamine having a trifluoromethyl group in its molecule.

Aromatic tetracarboxylic anhydrides preferably used as the polyimide of the (b) layer are preferably 4,4'-oxydiphthalic acid, pyromelitic acid, and 3,3',4,4'-biphenyltetracarboxylic acid. Aromatic tetracarboxylic dianhydride used in the polyimide of the (b) layer is preferably 70% by mass or more of the total tetracarboxylic acids in the polyimide of the (b) layer, more preferably 80% by mass or more, still more preferably 90% by mass or more, and still more preferably 95% by mass or more. By limiting the content of the aromatic tetracarboxylic acid to a specified range, heat resistance is improved.

The tetracarboxylic acid containing a trifluoromethyl group in the molecule used as the polyimide of the (b) layer is preferably 4,4'-(2,2-hexafluoroisopropylidene)diphthalic dianhydride. The tetracarboxylic acid containing a trifluoromethyl group in the molecule used as the polyimide of the (b) layer is preferably 30% by mass or more of the total tetracarboxylic acid in the polyimide of the (b) layer, more preferably 45% by mass or more, further preferably 60% by mass or more, and further preferably 80% by mass or more. By limiting the content of the tetracarboxylic acid containing a trifluoromethyl group in the molecule to a specified range, the colorless transparency is improved.

In the polyimide preferably used as the (b) layer of the present invention, the diamine preferably used is a diamine having at least a sulfur atom in the molecule and/or a diamine having a trifluoromethyl group in the molecule. As the diamine having a sulfur atom in the molecule, 3,3'-diaminodiphenylsulfone, 3,4'-diaminodiphenylsulfone, and 4,4'-diaminodiphenylsulfone can be used. In the present invention, by using a diamine containing 70% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more of the diamine having a sulfur atom in the molecule, colorless transparency can be obtained even in the case of combining with an aromatic tetracarboxylic anhydride. As the diamine having a trifluoromethyl group, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, and 2,2'-trifluoromethyl-4,4'-diaminodiphenyl ether are preferred. When using such diamine compounds having fluorine atoms in the molecule, especially diamines having a trifluoromethyl group in the molecule, the usage amount is preferably 70% by mass or more, 80% by mass or more, and more preferably 90% by mass or more of the total diamine.

The characteristics of the polyimide layer (a) and the polyimide layer (b) of the present invention are determined by the yellowness index, total light transmittance, mechanical properties, etc. when the films are formed into individual films with a thickness of 25±2 μm. Here, the operation of a single film with a thickness of 25±2μm is set as the evaluation on a scale that can be performed in the laboratory. The polyimide solution or the polyimide precursor solution is applied to a glass plate with a size of 10cm square (preferably 20cm square) or more, preheated to a temperature of 120°C, and then preheated and dried until the residual solvent amount becomes less than 40% by mass of the coating, and then heated at 300°C for 20 minutes in an inert gas such as nitrogen, and the obtained film is evaluated to obtain the value. In the case of containing inorganic components such as lubricants and fillers for the purpose of adjusting physical properties, the physical property values of the film are used, and the film is obtained by using a solution containing inorganic components.

The polyimide of the (a) layer and the polyimide of the (b) layer in the present invention may contain a lubricant (filler), respectively. The lubricant may be an inorganic filler or an organic filler, but an inorganic filler is preferred. The lubricant is not particularly limited, and may include silica, carbon, ceramics, etc., among which silica is preferred. Such lubricants may be used alone or in combination of two or more types. The average particle size of the lubricant is preferably greater than 10 nm, more preferably greater than 30 nm, and even more preferably greater than 50 nm. Furthermore, it is preferably less than 1 μm, more preferably less than 500 nm, and even more preferably less than 100 nm. The content of lubricant in the polyimide of the (a) layer and the polyimide of the (b) layer is preferably 0.01% by mass or more. Since the smoothness of the polyimide film will be improved, it is more preferably 0.02% by mass or more, more preferably 0.05% by mass or more, and particularly preferably 0.1% by mass or more. In terms of transparency, it is preferably 30% by mass or less, more preferably 20% by mass or less, more preferably 10% by mass or less, and particularly preferably 5% by mass or less.

The following is a description of the manufacturing method for obtaining the multilayer polyimide film of the present invention. Among the multilayer polyimide films of the present invention, the polyimide film consisting of two layers can be manufactured by the following steps: preferably in an atmosphere or in an inert gas at a temperature of 10°C to 40°C and a humidity of 10% to 55%, 1: applying a polyimide solution or a polyimide precursor solution for forming the (a) layer on a temporary support to obtain a coating film. a1; 2: drying the coating a1 to obtain a coating a2 with a residual solvent content of 5-40% by mass; 3: applying the polyimide solution or polyimide precursor solution for forming the (b) layer to the coating a2 to obtain a coating ab1; 4: heating the entire layer to obtain a layered product with a residual solvent content of less than 0.5% by mass based on the entire layer.

The temporary support is preferably long and flexible. In addition, the heating time in step 4 is preferably more than 5 minutes and less than 60 minutes. In addition, the residual solvent amount based on the whole layer in step 4 is obtained only from the mass of the coating ab1, and does not include the mass of the temporary support.

Furthermore, step 4 can also be divided into the following two stages: 4': heating for more than 5 minutes and less than 45 minutes until the residual solvent content of the whole layer becomes more than 8 mass% and less than 40 mass%, and then peeling off from the temporary support to obtain a self-supporting film; 5: holding both ends of the self-supporting film and heating again until the residual solvent content of the whole layer becomes less than 0.5 mass%.

By peeling off the temporary support at the stage of a self-supporting film, byproducts generated by drying and chemical reactions can be quickly discharged from the film, further reducing the difference in physical properties and structure between the surface and the inside.

Furthermore, in the case of making a film with more than three layers, it is sufficient to apply (a) layer of polyimide solution or polyimide precursor solution again after the above 1 and 2. By repeatedly applying (a) layer and (b) layer, a multi-layer film can be obtained.

In the present invention, it is preferred to apply the polyimide solution or polyimide precursor solution on a long and flexible temporary support in an atmosphere or in an inert gas at a temperature of 10°C to 40°C (preferably 15°C to 35°C) and a humidity of 10%RH to 55%RH (preferably 20%RH to 50%RH). As a coating method, the first layer to be applied can be applied using a notch wheel coater, a rod coater, a slit coater, etc., and the second layer and thereafter can be applied using a die coater, a curtain coater, a spray coater, etc. Furthermore, by using a multi-layer mold, these multiple layers can actually be applied simultaneously.

The best environment for coating the solution is in the atmosphere or in an inert gas. The so-called inert gas can be interpreted as a gas with a substantially low oxygen concentration. From an economic point of view, nitrogen or carbon dioxide can be used.

The temperature in the coating environment affects the viscosity of the coating liquid. When two coating liquids are overlapped, it affects the thickness of the transition layer formed when the two coating liquids are mixed with each other at the interface. The viscosity of the polyimide solution or polyimide precursor solution of the present invention is preferably adjusted to an appropriate viscosity range in the non-contact coating method, especially after the second layer. This temperature range is also conducive to properly maintaining the fluidity of the viscosity range during the mixing of the two-layer interface.

Most solvents used in polyimide solutions or polyimide precursor solutions are hygroscopic. If the solvent absorbs moisture and the water content of the solvent increases, the solubility of the resin component decreases, the dissolved components will precipitate in the solution, and the viscosity of the solution will increase sharply. If this happens after coating, the internal structure of the film becomes inhomogeneous, which may hinder transparency. In the present invention, it is preferred to limit the humidity of the coating environment to a specified range and enter the heating and drying step within 100 seconds after coating.

As the temporary support used in the present invention, glass, metal plate, metal belt, metal drum, polymer film, metal foil, etc. can be used. In the present invention, it is preferred to use a long and flexible temporary support, and a film of polyethylene terephthalate, polyethylene naphthalate, polyimide, etc. can be used as the temporary support. One of the preferred aspects is to apply a release treatment to the surface of the temporary support.

In the present invention, it is preferred to apply a polyimide solution or a polyimide precursor solution on a temporary support, dry it until the residual solvent content of the coating reaches 5-40% by mass, and then apply the next layer. Drying it until the residual solvent content reaches 40% by mass means that the applied coating loses fluidity and reaches a semi-solid and fully dried state. In the case where the next solution is applied when the residual solvent content of the coating reaches less than 5% by mass, the re-swelling of the previous dried coating will be uneven, and the boundary between the two adjacent layers may be blurred. Therefore, when the residual solvent content is in the range of 5 to 40 mass%, the diffusion and migration of the solvent in the coating liquid at the interface is uniform, and a transition layer of appropriate thickness can be formed by microscopic flow mixing. The residual solvent content is preferably 6 mass% or more, and more preferably 8 mass% or more. Furthermore, it is preferably 35 mass% or less, and more preferably 30 mass% or less.

In the present invention, after all the layers are coated, drying and chemical reactions as required are promoted by heat treatment. In the case of using a polyimide solution, it is sufficient to simply dry the solution in the sense of removing the solvent, but in the case of using a polyimide precursor solution, both drying and chemical reactions are required. Here, the so-called polyimide precursor is preferably in the form of polyamide or polyisoimide. When converting polyamide into polyimide, a dehydration condensation reaction is required. The dehydration condensation reaction can be carried out by simply heating, but an imidization catalyst can also be allowed to act as required. In the case of polyisoimide, isoimide bonds can also be converted to imide bonds by heating. In addition, a suitable catalyst can also be used. The residual solvent content of the final film is 0.5% by mass or less, preferably 0.2% by mass or less, and more preferably 0.08% by mass or less, as an average value of the entire film layer. The heating time is 5 minutes or more and 60 minutes or less, preferably 6 minutes or more and 50 minutes or less, and more preferably 7 minutes or more and 30 minutes or less. By limiting the heating time to a specified range, the solvent can be removed and the necessary chemical reaction can be terminated. At the same time, the transition layer can be controlled to an appropriate thickness and high colorless transparency, mechanical properties, and especially elongation at break can be maintained. In the case of a short heating time, the transition layer is formed slowly. If the heating time is longer than required, the film coloring becomes stronger and the elongation at break of the film decreases.

In the present invention, as long as the applied solution can be dried or chemically reacted by heating and can be peeled off from the temporary support in a self-supporting manner, it can also be peeled off from the temporary support during the heating step.

More specifically, the following steps can be adopted: after heating for more than 5 minutes and less than 45 minutes (preferably more than 6 minutes and less than 30 minutes, and more preferably more than 7 minutes and less than 20 minutes) until the residual solvent amount of the entire film layer reaches a range of more than 5 mass% and less than 40 mass%, the self-supporting film is peeled off from the temporary support, and the two ends of the self-supporting film are clamped by a clip or pierced on a needle for holding, and transported to a heating environment, and then heated until the residual solvent amount of the entire layer becomes less than 0.5 mass% (preferably less than 0.2 mass%, and more preferably less than 0.08 mass%), thereby obtaining a multi-layer polyimide film.

During the heating step, the self-supporting film is peeled off from the temporary support, and the heating is continued to evaporate the solvent. When the polyamide undergoes dehydration and ring closure and is converted into polyimide, the water generated can be quickly discharged from both sides of the film, and a film with small difference in physical properties between the front and back can be obtained.

In the present invention, the aforementioned self-supporting film can be stretched. The stretching can be either in the longitudinal direction (MD direction) of the film or in the width direction (TD) of the film, or both. The stretching in the longitudinal direction of the film can be performed using the speed difference of the conveying rollers, or the speed difference between the conveying rollers and the two ends of the film. The stretching in the width direction of the film can be performed by expanding the interval of the gripping clamps or needles. Stretching and heating can also be performed simultaneously. The stretching ratio can be arbitrarily selected between 1.00 times and 2.5 times. In the present invention, by making the film into a multi-layer structure, a polyimide that is difficult to stretch in a single state is combined with a polyimide that can stretch, thereby making it possible to stretch the polyimide in a composition that is difficult to stretch, that is, easily breaks due to stretching, and improve the mechanical properties. In addition, the polyimide shrinks in volume during the film-forming process by drying or dehydration condensation, so even in a state where both ends are held at equal intervals (stretching ratio is 1.00 times), the stretching effect is also exhibited.

In the multilayer polyimide film of the present invention, it is preferred that a lubricant is added to the polyimide to impart fine irregularities to the surface of the layer (film) to improve the slipperiness of the film. The lubricant is preferably added only to the outer layer (a). As a lubricant, inorganic or organic particles with an average particle size of about 0.03 μm to 3 μm can be used. Specific examples include titanium oxide, aluminum oxide, silicon dioxide, calcium carbonate, calcium phosphate, calcium hydrogen phosphate, calcium pyrophosphate, magnesium oxide, calcium oxide, clay minerals, etc. [Example]

Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof. In addition, the physical property values in the manufacturing examples and examples were measured by the following methods.

<Measurement of the thickness of the polyimide film> Measurement was performed using a micrometer (Millitron 1245D, manufactured by FEINPRUF).

<Tensile elasticity, tensile strength (breaking strength), and elongation at break> Short strips of 100 mm × 10 mm were cut from the film in the flow direction (MD direction) and width direction (TD direction) during coating as test pieces. Using a tensile testing machine (manufactured by Shimadzu Corporation, Autoradiography (R) model AG-5000A), tensile elasticity, tensile strength, and elongation at break were measured in the MD direction and TD direction at a tensile speed of 50 mm/min and a distance between chucks of 40 mm, and the average of the measured values in the MD direction and TD direction was calculated.

<Coefficient of linear expansion (CTE)> The film was stretched in the flow direction (MD direction) and width direction (TD direction) under the following conditions. The stretch/temperature was measured at intervals of 15°C from 30°C to 45°C and from 45°C to 60°C. This measurement was continued until 300°C. The average of all measured values was calculated as CTE. The average of the measured values in the MD direction and TD direction was then calculated. Equipment name: TMA4000S manufactured by MAC Science Sample length: 20mm Sample width: 2mm Heating start temperature: 25°C Heating end temperature: 300°C Heating rate: 5°C/min Gas atmosphere: Argon

<Thickness of transition layer> The oblique section of the film was prepared by SAICAS DN-20S (DAIPLA WINTES), and then the oblique section was subjected to the microscopic ATR method using a germanium crystal (incident angle 30°) using an IRCary 620 FTIR (Agilent) microscope to obtain a spectrum. The composition change was calculated by mass ratio conversion based on the increase and decrease of the characteristic peaks of the (a) layer and the (b) layer and the calibration curve obtained in advance. The thickness in the range of 5/95 mass ratio to 95/5 mass ratio of the (a) layer composition/(b) layer composition was determined as the thickness of the transition layer.

<Haze> The haze of the film was measured using a HAZEMETER (NDH5000, manufactured by Nippon Denshoku Co., Ltd.). A D65 lamp was used as a light source. The same measurement was performed three times, and the arithmetic average value was adopted.

<Total transmittance> The total transmittance (TT) of the film was measured using HAZEMETER (NDH5000, manufactured by Nippon Denshoku Co., Ltd.). A D65 lamp was used as the light source. The same measurement was performed three times, and the arithmetic average was used. The results are shown in Tables 2 to 6.

<Yellowness Index> Using a colorimeter (ZE6000, manufactured by Nippon Denshoku Co., Ltd.) and C2 light source, the tristimulus values XYZ of the film were measured in accordance with ASTM D1925, and the yellowness index (YI) was calculated using the following formula. In addition, the same measurement was performed 3 times, and the arithmetic average value was adopted. YI=100×(1.28X-1.06Z)/Y

<Film warp> A film cut into a square of 100 mm x 100 mm was used as a test piece. The test piece was placed on a flat surface at room temperature in a concave shape. The distances from the flat surface at the four corners were measured (h1rt, h2rt, h3rt, h4rt: unit: mm). The average value was taken as the warp amount (mm).

[Production Example 1 Production of polyamide solution A] After nitrogen substitution in a reaction vessel equipped with a nitrogen inlet tube, a reflux tube, and a stirring rod, 22.73 parts by mass of 4,4'-diaminobenzamide (DABAN) was dissolved in 201.1 parts by mass of N,N-dimethylacetamide (DMAc), and then 19.32 parts by mass of solid 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) was added directly in a split manner and stirred at room temperature for 24 hours. 173.1 parts by mass of DMAc was added for dilution to obtain a polyamide solution A with an NV (solid content) of 10% by mass and a reduced viscosity of 3.10 dl/g.

[Production Example 2 (a) Production of polyamide solution As containing a lubricant for layer formation] A dispersion of silica gel dispersed in dimethylacetamide ("SNOWTEX (registered trademark) DMAC-ST-ZL" manufactured by Nissan Chemical Industries, Ltd.) was further added as a lubricant to the polyamide solution A obtained in Production Example 1 so that the amount of silicon dioxide (lubricant) was 1.4 mass % in the total amount of the polymer solid content in the polyimide solution, thereby obtaining a uniform polyamide solution As.

[Production Example 3 Production of polyamide solution B] After nitrogen replacement in a reaction vessel equipped with a nitrogen inlet tube, a reflux tube, and a stirring rod, 32.02 parts by mass of 2,2'-bistrifluoromethyl-4,4'-diaminobiphenyl (TFMB) was dissolved in 279.9 parts by mass of N,N-dimethylacetamide (DMAc), and then 9.81 parts by mass of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and 15.51 parts by mass of 4,4'-oxydiphthalic dianhydride (ODPA) were added directly as solids, and stirred at room temperature for 24 hours. Subsequently, polyamide solution B with a solid content of 17% by mass and a reduced viscosity of 3.60 dl/g was obtained.

[Production Example 4 (a) Production of polyamide solution Bs containing a lubricant for layer formation] A dispersion of silica gel dispersed in dimethylacetamide ("SNOWTEX (registered trademark) DMAC-ST-ZL" manufactured by Nissan Chemical Industries, Ltd.) was added as a lubricant to the polyamide solution B obtained in Production Example 3 so that the amount of silica (lubricant) was 0.45% by mass based on the total amount of polymer solid components in the polyimide solution, thereby obtaining a uniform polyamide solution Bs.

[Production Example 5 (b) Production of polyimide solution C for layer formation] In a reaction vessel equipped with a nitrogen inlet, a Dean-Stark apparatus, a reflux tube, a thermometer, and a stirring rod, 32.02 parts by mass of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) and 230 parts by mass of N,N-dimethylacetamide (DMAc) were added while nitrogen gas was introduced to completely dissolve them. Then, 44.42 parts by mass of 4,4'-(2,2-hexafluoroisopropylidene)diphthalic anhydride (6FDA) was added directly as a solid, and stirred at room temperature for 24 hours. Thereafter, a polyimide solution Caa having a solid content of 25% by mass and a reduced viscosity of 1.10 dl/g was obtained. Next, 204 parts by mass of DMAc was added to the obtained polyamide solution Caa, and after diluting the polyamide solution to a concentration of 15% by mass, 1.3 parts by mass of isoquinoline was added as an imidization accelerator. Next, while stirring the polyamide solution, 12.25 parts by mass of acetic anhydride as an imidization agent was slowly dripped. Thereafter, stirring was continued for 24 hours to carry out a chemical imidization reaction, and a polyimide solution Cpi was obtained. Next, 100 parts by mass of the obtained polyimide solution Cpi was transferred to a reaction vessel equipped with a stirring device and a stirrer, and 150 parts by mass of methanol was slowly dropped while stirring, and the precipitation of a powdery solid was confirmed. After that, the contents of the reaction vessel, i.e., the powder, were dehydrated and filtered, and then washed with methanol, vacuum dried at 50°C for 24 hours, and then heated at 260°C for 5 hours to obtain a polyimide powder Cpd. 20 parts by mass of the obtained polyimide powder was dissolved in 80 parts by mass of DMAc to obtain a polyimide solution C.

[Production Example 6 (b) Production of polyimide solution D for layer formation] In a reaction container equipped with a nitrogen inlet tube, a Dean-Stark apparatus, a reflux tube, a thermometer, and a stirring rod, 120.5 parts by mass of 4,4'-diaminodiphenyl sulfone (4,4'-DDS), 51.6 parts by mass of 3,3'-diaminodiphenyl sulfone (3,3'-DDS), and 500 parts by mass of γ-butyrolactone (GBL) were added while introducing nitrogen gas. Then, 217.1 parts by mass of 4,4'-oxydiphthalic dianhydride (ODPA), 223 parts by mass of GBL, and 260 parts by mass of toluene were added at room temperature, and the temperature was raised to an internal temperature of 160°C, and refluxed at 160°C for 1 hour to perform imidization. After the imidization was completed, the temperature was raised to 180°C and the reaction was continued while removing toluene. After 12 hours of reaction, the oil bath was removed and the temperature was returned to room temperature. GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide solution D.

[Production Example 7 Production of polyamide solution E] In a reaction vessel equipped with a nitrogen inlet tube, a reflux tube, and a stirring rod, 161 parts by mass of 2,2'-bistrifluoromethyl-4,4'-diaminobiphenyl (TFMB) and 1090 parts by mass of N-methyl-2-pyrrolidone were mixed, stirred, and dissolved in a nitrogen atmosphere, and then 112 parts by mass of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) was directly added in a solid state at room temperature and stirred at room temperature for 12 hours. Then, 400 parts by mass of xylene was added as an azeotropic solvent, the temperature was raised to 180°C, and the reaction was carried out for 3 hours to separate the azeotropically generated water. After confirming that water had stopped flowing out, xylene was removed while the temperature was raised to 190° C. over 1 hour, thereby obtaining a polyamide solution E.

[Production Example 8 (a) Production of polyamide solution Es containing a lubricant for layer formation] A dispersion of silica gel dispersed in dimethylacetamide ("SNOWTEX (registered trademark) DMAC-ST-ZL" manufactured by Nissan Chemical Industries) was added as a lubricant to the polyamide solution E obtained in Production Example 7 so that the amount of silicon dioxide (lubricant) was 1.0 mass % in the total amount of the polymer solid content in the polyimide solution to obtain a uniform polyamide solution Es.

[Production Example 9 (b) Production of a filler-added polyamide solution Ef for layer formation] A dispersion of silica gel dispersed in dimethylacetamide ("SNOWTEX (registered trademark) DMAC-ST-ZL" manufactured by Nissan Chemical Industries) was added as a lubricant to the polyamide solution E obtained in Production Example 7 so that the amount of silica (lubricant) was 25% by mass based on the total amount of the polymer solid component in the polyimide solution, thereby obtaining a filler-added polyamide solution Ef.

The polyimide solutions and polyamide acid solutions (polyimide precursor solutions) obtained in Preparation Examples 1 to 9 were made into thin films by the following method, and their optical and mechanical properties were measured. The results are shown in Table 1.

(Method for obtaining a thin film for single property measurement) Use a rod coater to apply a polyimide solution or polyamide solution to a 20 cm square area in the center of a 30 cm glass plate in a final thickness of 25 ± 2 μm, heat at 100°C for 30 minutes in an inert oven with a steady flow of dry nitrogen, and after confirming that the residual solvent content of the coating is less than 40% by mass, heat at 300°C for 20 minutes in a Monfort furnace that has been replaced with dry nitrogen. Then, take it out of the Monfort furnace, lift the edge of the dry coating (thin film) with a cutter, and carefully peel it off from the glass to obtain a thin film.

(Implementation Example 1)

In an air-conditioned atmosphere at 25°C and 45%RH, a roll-to-roll notch wheel coating machine and a continuous drying furnace were used to coat the polyamide solution As obtained in Preparation Example 2 on the lubricant-free surface of a polyethylene terephthalate film A4100 (manufactured by Toyobo Co., Ltd., hereinafter referred to as PET film) as a temporary support in a manner such that the final film thickness was 5μm. Then, the film was heated at 110°C for 5 minutes in a continuous drying machine as a primary heating to obtain a semi-dried film Agf with a residual solvent content of 28% by mass, and each temporary support was rolled into a roll.

The obtained roll was mounted again in the aforementioned device, the temporary support and the Agf were unwound together, and the polyimide solution C obtained in Preparation Example 5 was applied on the Agf using a notch wheel coating machine in such a way that the final film thickness became 20 μm. This was dried at 110°C for 10 minutes as a secondary heating.

After drying, the self-supporting film was peeled off from the PET film as a support, and passed through a pin-comb tenter with a pin plate equipped with pins. The film ends were inserted on the pins to hold the film, and the pin plate interval was adjusted to prevent the film from breaking and unnecessary slack. The film was transported, and finally heated at 200°C for 3 minutes, 250°C for 3 minutes, and 300°C for 6 minutes to allow the imidization reaction to proceed. After that, it was cooled to room temperature in 2 minutes, and the portions with poor flatness at both ends of the film were cut off with a cutter, and the film was rolled into a roll to obtain a roll of film (Example 1) with a width of 530 mm and a length of 80 m.

The evaluation results of the obtained film (Example 1) are shown in Table 2.

(Examples 2 to 4)

Below, by setting the conditions shown in Table 2, films (Example 2) to (Example 4) and a comparative film (Comparative 1) were obtained. The results of the same evaluation are shown in Table 2.

(Comparative example 1)

As Comparative Example 1, in the same apparatus and coating environment as in Example 1, only the polyamide solution As obtained in the Preparation Example was coated on a temporary support in such a manner that the final film thickness became 25 μm. Next, as a primary heating, the film was heated at 110°C for 10 minutes, and the self-supporting film was peeled off from the temporary support. The pin-bar tenter used in Example 1 was used to hold the film by inserting the film ends on the pins, and the pin plate interval was adjusted so that the film would not break and unnecessary slack would not occur, and the film was transported. As a final heating, the film was heated at 200°C for 3 minutes, 250°C for 3 minutes, and 300°C for 6 minutes to allow the imidization reaction to proceed. After that, the film was cooled to room temperature in 2 minutes, and the portions with poor flatness at both ends of the film were cut off with a cutter, and the film was wound into a roll to obtain a roll of film (ratio 1) with a width of 560 mm and a length of 50 m. The evaluation results of the obtained film (ratio 1) are shown in Table 3.

(Comparative Examples 2~4)

Similarly, the polyamide solution Bs, polyimide solution C, and polyimide solution D obtained in the manufacturing example were used separately, and the same conditions as in comparative example 1 were used to obtain films (comparison 2) to (comparison 4). The evaluation results of each are shown in Table 3.

The mechanical property values of the films (Ratio 1) to (Ratio 4) show higher fracture strength and higher fracture elongation than the values of the test piece obtained from the manufacturing example. This difference represents the difference in the case where the film of the manufacturing example is directly thinned after being coated on the glass, and the film is thinned while being peeled off from the PET film as a temporary support during the heat treatment process and the solvent and reaction products are discharged from the surface and inside of the film.

(Calculation examples 1~4)

Table 4 shows: the arithmetic mean value obtained by weighting the values of Comparative Examples 1 to 4 corresponding to the thickness of the (a) layer and (b) layer of Examples 1 to 4.

Example 1 corresponds to Calculation Example 1, and similarly, Examples 2 to 4 correspond to Calculation Examples 2 to 4, respectively. If the examples are compared with the calculation examples, the films obtained from the examples have lower haze and higher total light transmittance than the calculation examples. In addition, the yellow index also shows a lower value, indicating that the optical properties are improved. In addition, the tensile strength and elongation at break are both higher values in the examples, and the CTE is also suppressed, which shows that the mechanical properties are also improved.

In addition, the warp is naturally large because the film thickness direction has an asymmetric structure.

(Examples 5 to 10)

In an air-conditioned atmosphere at 25°C and 45% RH, the polyamide solution As obtained in Preparation Example 2 was applied to the lubricant-free surface of the PET film as a temporary support using a notch wheel coater so that the final film thickness was 3 μm, and then temporarily heated under the conditions shown in Table 5, and wound up with the temporary support to obtain a roll. The obtained roll was mounted again in the same device, and the polyimide solution C obtained in Preparation Example 5 was applied using a die coater so that the final film thickness was 31 μm, and then heated again under the conditions shown in Table 5, and then wound up with the temporary support to obtain a roll again. The obtained roll was mounted again in the same device, and the polyamide solution As was applied by a notch wheel coating machine so that the final film thickness became 3 μm. The film was heated three times according to the conditions in Table 5, and the self-supporting three-layer coating film was peeled off from the temporary support. The film was passed through a pin bar tenter, and the film ends were inserted into the upper pins to hold the film. The pin plate interval was adjusted so that the film would not break and unnecessary slack would not be generated, and the film was transported. The film was finally heated under the conditions shown in Table 5, and then cooled to room temperature for 3 minutes. The portions with poor flatness at both ends of the film were cut off by a cutter, and the film was wound into a roll to obtain a roll of a film (Example 5) with a width of 510 mm and a length of 100 m. The evaluation results of the obtained film (Example 5) are shown in Table 5.

Similarly, following the solutions and conditions shown in Tables 5 and 6, polyimide films (Example 6) to (Example 10) with a three-layer structure of (a)/(b)/(a) were obtained, and the results are shown in Tables 5 and 6.

As with Examples 1 to 4, the properties are shown to be improved compared to the thin film made of a single layer. Furthermore, compared with Examples 1 to 4, the warp is greatly reduced, but this is because the contrast in the thickness direction is improved.

(Comparative example 5)

Using the polyamide solution Ef to which the filler obtained in the manufacturing example was added, an attempt was made to produce a single-layer 50μm film. The set conditions are shown in Table 6. After primary drying, the self-supporting film was peeled off from the temporary support PET and introduced into a pin-bar tenter, but the film broke in the longitudinal direction at the beginning of heating. Although the test was continued by adjusting the needle width, the film became very fragile because the conversion reaction to polyimide would proceed if dried, and a film that could be evaluated for physical properties could not be obtained.

(Implementation Example 11)

The polyimide solution Es to which only a lubricant was added as a filler was used in the (a) layer, and the polyimide solution Ef containing a filler obtained in Preparation Example 9 was used in the (b) layer. Using the conditions set in Table 6, a film having a (a)/(b)/(a) structure was tested. Although the width adjustment was time-consuming, a polyimide film with a width of 510 mm and a length of 80 m was finally obtained (Example 11). The evaluation results are shown in Table 6.

[Production Example 10 (Production of polyamide solution Fs with lubricant added)]

After nitrogen substitution in a reaction vessel equipped with a nitrogen inlet tube, a reflux tube, and a stirring rod, 336.31 parts by weight of N-methyl-2-pyrrolidone (NMP) was added to 33.36 parts by weight of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB), and a dispersion (Japanese) prepared by dispersing silica gel in dimethylacetamide was added so that the amount of silica (lubricant) in the total amount of the polymer solid component in the polyimide solution was 0.3% by weight. "SNOWTEX (registered trademark) DMAC-ST-ZL" manufactured by Sankagaku Kogyo Co., Ltd. was used as a lubricant to completely dissolve it. Then, 9.81 parts by mass of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), 11.34 parts by mass of 3,3',4,4'-biphenyltetracarboxylic acid (BPDA), and 4.85 parts by mass of 4,4'-oxydiphthalic dianhydride (ODPA) were added separately and stirred at room temperature for 24 hours. Subsequently, a polyamide solution Fs with a solid content of 15% by mass and a reduced viscosity of 3.50 dl/g was obtained (the molar ratio of TFMB//CBDA/BPDA/ODPA = 1.00//0.48/0.37/0.15).

[Production Example 11 (Production of polyamide solution F without lubricant)]

After nitrogen replacement in a reaction vessel equipped with a nitrogen inlet tube, a reflux tube, and a stirring rod, 336.31 parts by mass of N-methyl-2-pyrrolidone (NMP) was added to 33.36 parts by mass of 2,2'-bistrifluoromethyl-4,4'-diaminobiphenyl (TFMB) to completely dissolve it. Then, 9.81 parts by mass of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), 11.34 parts by mass of 3,3',4,4'-biphenyltetracarboxylic acid (BPDA), and 4.85 parts by mass of 4,4'-oxydiphthalic dianhydride (ODPA) were added separately as solids, and stirred at room temperature for 24 hours. Afterwards, a polyamide solution F with a solid content of 15% by mass and a reduced viscosity of 3.50 dl/g was obtained (the molar ratio of TFMB//CBDA/BPDA/ODPA = 1.00//0.48/0.37/0.15).

(Example 12)

In an air-conditioned atmosphere of 25°C and 45%RH, a roll-to-roll notch wheel coater and a continuous drying furnace were used to coat the polyamide solution Fs obtained in Preparation Example 10 on the lubricant-free surface of the PET film as a temporary support in such a way that the final film thickness became 5μm. Then, the continuous drying machine was used as a primary heating at 110°C for 5 minutes to obtain a semi-dried film Fsgf with a residual solvent content of 28% by mass, and each temporary support was rolled into a roll.

The obtained roll was mounted again in the aforementioned device, the temporary support and Fsgf were unwound together, and the polyamide solution F obtained in Preparation Example 11 was applied to Fsgf using a notch wheel coating machine in such a way that the final film thickness became 20 μm. This was dried at 110°C for 10 minutes as a secondary heating to obtain a dry film Fgf2.

After drying, the self-supporting film was peeled off from the PET film as a support, passed through a pin bar tenter with a pin plate equipped with pins, and held by inserting the film ends on the pins. The pin plate interval was adjusted so that the film would not break and unnecessary slack would not occur, and the film was transported. As the final heating, it was heated at 200°C for 3 minutes, 250°C for 3 minutes, 300°C for 3 minutes, and 400°C for 3 minutes to allow the imidization reaction to proceed. After that, it was cooled to room temperature in 2 minutes, and the portions with poor flatness at both ends of the film were cut off with a cutter, and rolled into a roll to obtain a roll of film (12) with a width of 530 mm and a length of 80 m. The obtained film (Example 12) has a total film thickness of 25μm, a haze of 0.42%, a total light transmittance of 87.5%, a yellowness index of 4.1, a fracture strength of 212MPa, a fracture elongation of 10.5%, an elastic modulus of 4.3GPa, a CTE of 32ppm/K, a warp of less than 0.1mm, and a transition layer thickness of 0.1μm.

(Implementation Example 13)

The dry film Fgf2 obtained in the middle stage of Example 12 was further coated with the polyamide solution Fs obtained in Manufacturing Example 10 using a notch wheel coater in such a way that the final film thickness became 5μm, and then dried at 100°C for 10 minutes as a third heating to obtain a dry film Fgf3.

After drying, the self-supporting film was peeled off from the PET film as a support, and heated at 200°C for 3 minutes, 250°C for 3 minutes, 300°C for 3 minutes, and 400°C for 3 minutes using a pin-bar tenter as in Example 12 to allow the imidization reaction to proceed. The same operation was then performed to obtain a roll of a film (Example 13) having a width of 530 mm and a length of 80 m. The obtained film (Example 13) is a three-layer structure of Fs/F/Fs, with a total thickness of 30μm, a haze of 0.45%, a total transmittance of 87.1%, a yellowness index of 4.0, a fracture strength of 189MPa, a fracture elongation of 9.5%, a modulus of elasticity of 4.2GPa, a CTE of 31ppm/K, a warp of less than 0.1mm, and a transition layer thickness (air side/base side) of 0.1μm/0.1μm.

(Comparative example 6)

In an air-conditioned atmosphere of 25°C and 45%RH, a roll-to-roll notch wheel coater and a continuous drying furnace were used to coat the polyamide solution As obtained in Preparation Example 2 on the lubricant-free surface of the PET film as a temporary support in a manner such that the final film thickness became 20μm. Then, the continuous drying machine was used as a primary heating at 110°C for 5 minutes to obtain a semi-dried film Agfx with a residual solvent content of 28% by mass, and each temporary support was rolled into a roll.

The obtained self-supporting dry film Agfx was peeled off from the PET film as a support, and passed through a pin-bar tenter with a pin plate equipped with pins. The film ends were inserted into the pins to hold the film, and the pin plate interval was adjusted to prevent the film from breaking and unnecessary slack. The film was transported, and as the final heating, it was heated at 200°C for 3 minutes, 250°C for 3 minutes, and 300°C for 6 minutes to allow the imidization reaction to proceed. After that, it was cooled to room temperature in 2 minutes, and the portions with poor flatness at both ends of the film were cut off with a cutter, and rolled into a roll to obtain a roll of polyimide film (compared to 6a) with a width of 530 mm and a length of 50 m.

The obtained polyimide film (Compared to 6a) roll was mounted again in the aforementioned device, the polyimide film (Compared to 6a) was unrolled, and the polyimide solution C obtained in Preparation Example 5 was applied thereon using a notch wheel coating machine in such a way that the final film thickness became 5μm. This was dried at 110°C for 10 minutes as a secondary heating.

After drying, the film is held by inserting the ends of the film onto the pins through a pin-bar tenter with a pin plate equipped with pins. The pin plate interval is adjusted and the film is transported in a manner that the film does not break and does not produce unnecessary slack. As the final heating, the film is heated at 200°C for 3 minutes, 250°C for 3 minutes, and 300°C for 6 minutes to allow the imidization reaction to proceed. After that, the film is cooled to room temperature in 2 minutes, and the portions with poor flatness at both ends of the film are cut off with a cutter, and the film is wound into a roll to obtain a roll of polyimide film (compare to 6b) with a width of 450 mm and a length of 30 m.

The obtained polyimide film (Compared to 6b) has a two-layer structure of As (20μm)/C (5μm), a total film thickness of 25μm, a haze of 0.63%, a total transmittance of 86.9%, a yellowness index of 4.3, a fracture strength of 154MPa, a fracture elongation of 18%, an elasticity of 3.9GPa, a CTE of 19.6ppm/K, a warp of less than 2.8mm, and a transition layer thickness of 0.0μm. The warp of the film is greater than that of the embodiment.

[Table 1] Polyimide (PI) solution or polyamine (PAA) solution Manufacturing example 1 Manufacturing example 2 Manufacturing example 3 Manufacturing example 4 Manufacturing example 5 Manufacturing example 6 Manufacturing example 7 Manufacturing example 8 Manufacturing example 9 (a) Layer (a) Layer (a) Layer (a) Layer (b) Layering (b) Layering (b) Layering (a) Layer (b) Layering PAA solution A PAA solution As PAA Solution B PAA solution Bs PI Solution C PI solution D PAA Solution E PAA solution Es PAA solution Ef Acid component filling mass% 6FDA - - - - 100 - - - - ODPA - - 61 61 - 100 - - - CBDA 100 100 39 39 - - - - - CHDA - - - - - - 100 100 100 Diamine component filling mass% TFMB - - 100 100 100 - 100 100 100 4DDS - - - - - 70 - - - 3DDS - - - - - 30 - - - DABAN 100 100 - - - - - - - ODA - - - - - - - - - Inorganic filler (lubricant) (% of polymer mass) SiO2 - 1.4 - 0.45 - - - 1.0 25.0 Film thickness μm 25 25 25 25 25 25 25 25 25 Haze % 0.34 0.67 0.28 0.65 0.28 0.31 0.29 0.72 4.8 Full light transmittance % 86.3 85.2 85.2 84.3 90.3 92.4 90.5 90.3 86.3 Yellow Index 6.8 6.8 8.3 8.3 1.3 3.4 3.7 3.6 3.2 Breaking strength MPa 165 162 170 166 134 126 127 125 86 Elongation at break % 15.4 14.2 12.8 12.5 22.6 18.4 10.6 10.5 5.8 Elastic coefficient GPa 3.8 3.8 3.5 3.5 3.7 3.5 3.3 3.4 4.1 CTE ppm/K 17 17 18 18 48 51 38 37 27 Curve mm 1.8 1.5 1.2 1.1 0.6 0.3 1.0 0.8 1.2

[Table 2] Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Film (Real 1) Film (Real 2) Film (Real 3) Film (Real 4) Layer composition solution name (thickness μm) (a) Layer-Air Surface without without without without (b) Layer C(20) C(20) D(20) D(20) (a) Layer-base plane As(5) Bs(5) As(5) Bs(5) Temporary support PET PET PET PET Coating environment temperature and humidity ℃,%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH Primary heating temperature ℃ 110 110 110 110 One heating time minute 5 5 5 5 Residual solvent amount Quality% 28 30 26 27 Secondary heating temperature ℃ 110 110 110 110 Second heating time minute 10 10 10 10 Three heating temperatures ℃ - - - - Three heating times minute - - - - Final heating temperature ℃ 200-250-300 200-250-300 200-250-300 200-250-300 final heating time minute 3-3-6 3-3-6 3-3-6 3-3-6 Total film thickness μm 25 25 25 25 Haze % 0.22 0.26 0.31 0.36 Full light transmittance % 90.3 88.4 86.8 92.4 Yellow Index 1.3 0.9 3.2 1.2 Breaking strength MPa 178 177 156 151 Elongation at break % twenty two 25 twenty three twenty three Elastic coefficient GPa 4.1 3.7 3.4 3.7 CTE ppm/K 33.4 34.5 35.0 33.8 Curve mm 5.2 4.5 3.5 2.7 Transition layer thickness μm 0.34 0.27 0.31 0.23

[Table 3] Comparative example 1 Comparative example 2 Comparative example 3 Comparative example 4 Film (ratio 1) Film (than 2) Film (than 3) Film (than 4) Layer composition solution name (thickness μm) (a) Layer-Air Surface - - - - (b) Layer - - - - (a) Layer-base plane As(25) Bs(25) C(25) D(25) Temporary support PET PET PET PET Coating environment temperature and humidity ℃,%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH Primary heating temperature ℃ 110 110 110 110 One heating time minute 10 10 10 10 Residual solvent amount Quality% 28 27 27 31 Secondary heating temperature ℃ - - - - Second heating time minute - - - - Three heating temperatures ℃ - - - - Three heating times minute - - - - Final heating temperature ℃ 200-250-300 200-250-300 200-250-300 200-250-300 final heating time minute 3-3-6 3-3-6 3-3-6 3-3-6 Total film thickness μm 25 25 25 25 Haze % 0.64 0.65 0.26 0.41 Full light transmittance % 86.2 89.7 86.1 91.8 Yellow Index 4.5 6.8 0.9 2.3 Breaking strength MPa 176 160 133 123 Elongation at break % 20 16 twenty two 26 Elastic coefficient GPa 3.8 3.5 3.6 3.4 CTE ppm/K 16.3 16.8 48.0 51.3 Curve mm 0.9 0.3 0.5 0.2 Transition layer thickness μm - - - -

[Table 4] Calculation Example 1 Calculation Example 2 Calculation Example 3 Calculation Example 4 Layer composition solution name (thickness μm) (a) Layer-Air Surface The weighted average of the film (ratio 1) and (ratio 3) is 1/ratio 3 = 1/4 The weighted average of the film (ratio 2) and (ratio 3) is ratio 2/ratio 3 = 1/4 The weighted average of the film (ratio 1) and (ratio 4) is 1/ratio 4 = 1/4 The weighted average of the film (ratio 2) and (ratio 4) is ratio 2/ratio 4 = 1/4 (b) Layer (a) Layer-base plane Temporary support Coating environment temperature and humidity ℃,%RH Primary heating temperature ℃ One heating time minute Secondary heating temperature ℃ Second heating time minute Three heating temperatures ℃ Three heating times minute Final heating temperature ℃ final heating time minute Total film thickness μm 25 25 25 25 Haze % 0.3 0.3 0.5 0.5 Full light transmittance % 86.2 86.9 90.7 91.3 Yellow Index 1.6 2.1 2.7 3.2 Breaking strength MPa 142 139 134 131 Elongation at break % twenty one twenty one 25 twenty four Elastic coefficient GPa 3.7 3.6 3.5 3.4 CTE ppm/K 41.7 41.8 44.3 44.4 Curve mm 0.5 0.4 0.3 0.2 Transition layer thickness μm - - - -

[Table 5] Embodiment 5 Embodiment 6 Embodiment 7 Embodiment 8 Film (Real 5) Film (Real 6) Film (Real 7) Film (Real 8) Layer composition solution name (thickness μm) (a) Layer-Air Surface As(3) Bs(3) As(3) Bs(3) (b) Layer C(31) C(31) D(31) D(31) (a) Layer-base plane As(3) Bs(3) As(3) Bs(3) Temporary support PET PET PET PET Coating environment temperature and humidity ℃,%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH Primary heating temperature ℃ 110 110 110 110 One heating time minute 5 5 5 5 Residual solvent amount Quality% 25 19 twenty one twenty four Secondary heating temperature ℃ 110 110 110 110 Second heating time minute 15 15 15 15 Three heating temperatures ℃ 110 110 110 110 Three heating times minute 5 5 5 5 Final heating temperature ℃ 200-250-300 200-250-300 200-250-300 200-250-300 final heating time minute 4-4-8 4-4-8 4-4-8 4-4-8 Total film thickness μm 37 37 37 37 Haze % 0.23 0.27 0.31 0.38 Full light transmittance % 92.9 90.1 91.5 89.7 Yellow Index 1.4 0.9 1.1 1.3 Breaking strength MPa 164 163 141 159 Elongation at break % 26 27 twenty four twenty four Elastic coefficient GPa 3.8 3.7 3.6 3.7 CTE ppm/K 31.7 36.0 36.7 31.0 Curve mm 0.32 0.20 0.18 0.11 Transition layer thickness (air side/base side) μm 0.34 / 0.52 0.08 / 0.11 0.15 / 0.18 0.25 / 0.31

[Table 6] Embodiment 9 Embodiment 10 Comparative example 5 Embodiment 11 Film (Real 9) Film (Real 10) Film (than 5) Film (Real 11) Layer composition solution name (thickness μm) (a) Layer-Air Surface As(2) Bs(1) without Es(5) (b) Layer C(8) C(3) Ef(50) Ef(40) (a) Layer-base plane As(2) Bs(1) without Es(5) Temporary support PET PET PET PET Coating environment temperature and humidity ℃,%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH 25℃,45%RH Primary heating temperature ℃ 110 110 110 110 One heating time minute 2 1.5 18 5 Residual solvent amount Quality% 18 15 32 twenty three Secondary heating temperature ℃ 110 110 - 110 Second heating time minute 5 3 - 15 Three heating temperatures ℃ 110 110 - 110 Three heating times minute 2 3 - 5 Final heating temperature ℃ 200-250-300 200-250-300 200 200-250-300 final heating time minute 2-2-4 2-2-3 5 6-6-12 Total film thickness μm 12 5 Break in the process 50 Haze % 0.14 0.11 4.11 Full light transmittance % 94.3 95.2 87.3 Yellow Index 0.4 0.2 2.2 Breaking strength MPa 147 139 135 Elongation at break % twenty four twenty three twenty one Elastic coefficient GPa 3.8 3.7 3.7 CTE ppm/K 34.0 32.0 27.1 Curve mm 0.00 0.00 0.21 Transition layer thickness (air side/base side) μm 0.05 / 0.08 0.02 / 0.03 0.27 / 0.23 [Possibility of industrial application]

As described above, compared to the case where polyimide of different compositions is separately filmed, the multilayer polyimide film of the present invention exhibits good optical and mechanical properties. In addition, according to the manufacturing method of the present invention, a transition layer with a composition gradient of a specific thickness can be formed between the layers of different compositions that are divided into multiple layers for functional distribution, thereby forming a balanced film. The multilayer polyimide film of the present invention has excellent optical properties, colorless transparency, excellent mechanical properties, and exhibits a low CTE, so it can be used as a component of a flexible and lightweight display device, or a switch element such as a touch panel that requires transparency, a pointing device, etc.

without.

without.

without.

Claims (11)

A multilayer polyimide film, characterized in that it has a multilayer polyimide layer, which is formed by laminating at least two polyimide layers of different compositions in the thickness direction, and a transition layer, which exists between the (a) layer constituting the multilayer polyimide layer and the (b) layer adjacent to the (a) layer, and the chemical composition gradually changes; the overall thickness of the film is greater than 3μm and less than 120μm The yellow index of the film as a whole is less than 5, and the total light transmittance of the film as a whole is more than 86%. The transition layer is a layer that continuously changes from the polyimide of the (a) layer to the polyimide of the (b) layer. The polyimide of the (a) layer is: a tetracarboxylic acid anhydride containing 70% by mass or more of alicyclic tetracarboxylic acid anhydride and a polyimide containing 70% by mass or more of an amide in the molecule. The polyimide of the (b) layer is a polyimide having a chemical structure obtained by the polycondensation of a diamine containing a diamine having a trifluoromethyl group in the molecule; or a polyimide having a chemical structure obtained by the polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of an alicyclic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule. The polyimide of the (b) layer is a polyimide having a chemical structure obtained by the polycondensation of a tetracarboxylic anhydride containing 70% by mass or more of an aromatic tetracarboxylic anhydride and a diamine containing 70% by mass or more of a diamine having a sulfur atom in the molecule; or a polyimide having a chemical structure obtained by the polycondensation of a tetracarboxylic anhydride containing 30% by mass or more of a tetracarboxylic acid containing a trifluoromethyl group in the molecule and a diamine containing 70% by mass or more of a diamine having a trifluoromethyl group in the molecule. A multi-layer polyimide film, characterized in that it has a multi-layer polyimide layer, which is formed by laminating at least two polyimide layers of different compositions in the thickness direction, and a transition layer, which exists between the (a) layer constituting the multi-layer polyimide layer and the (b) layer adjacent to the (a) layer, and the chemical composition gradually changes; the thickness of the film as a whole is greater than 3μm and less than 120μm, the yellowness index of the film as a whole is less than 5, and the total light transmittance of the film as a whole is greater than 86% , the transition layer is a layer that continuously changes from the polyimide of the (a) layer to the polyimide of the (b) layer, the (a) layer contains more than 70% by mass of polyimide with a yellow index of less than 10 and a total transmittance of more than 85% when the film is formed into a single film with a thickness of 25±2μm, and the (b) layer contains more than 70% by mass of polyimide with a yellow index of less than 5 and a total transmittance of more than 90% when the film is formed into a single film with a thickness of 25±2μm. A multi-layer polyimide film as claimed in claim 2, wherein the yellowness index of the polyimide of the (a) layer is greater than 0.1, and the yellowness index of the polyimide of the (b) layer is greater than 0.1. A multi-layer polyimide film as claimed in claim 2, wherein the CTE of the polyimide of the (a) layer is less than 25 ppm/K. A multi-layer polyimide film as claimed in claim 1 or 2, wherein the thickness of the (a) layer is greater than 1 μm and less than 119 μm, and the thickness of the (b) layer is greater than 1 μm and less than 119 μm. For a multi-layer polyimide film as claimed in claim 1 or 2, the lower limit of the thickness of the transition layer is 0.01 μm, and the upper limit is either 3% of the total thickness of the film or 1 μm. A multilayer polyimide film as claimed in claim 1 or 2, wherein the (a) layer exists on both sides of the (b) layer, one side and the other side, and the transition layer exists between the (a) layer and the (b) layer on one side of the (b) layer, and between the (a) layer and the (b) layer on the other side of the (b) layer, and has a layer structure in which the (a) layer, the transition layer, the (b) layer, the transition layer, and the (a) layer are sequentially stacked. A method for producing a multilayer polyimide film as claimed in any one of claims 1 to 6, comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a residual solvent content of 5-60%. 40 mass% of coating a2; 3: applying the polyimide solution or polyimide precursor solution for forming the (b) layer on the coating a2 to obtain the coating ab1; 4: heating the entire layer to obtain a layered body with a residual solvent content of less than 0.5 mass% based on the entire layer. A method for producing a multilayer polyimide film as claimed in any one of claims 1 to 6, comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming a (a) layer on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming a (b) layer on a temporary support to obtain a coating a1; 4: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 5: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 6: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 7: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 8: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a2; 9: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 10: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 11: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a2; 12: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 13: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a2; 14: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 15: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating a1; 16: applying a polyimide solution or a polyimide precursor solution on a temporary support to obtain a coating The solution is applied to the coating a2 to obtain the coating ab1; 4: the whole layer is heated to obtain a layered body with a residual solvent content of 5% by mass or more and 40% by mass or less based on the whole layer, and then it is peeled off from the temporary support to obtain a self-supporting film; 5: the two ends of the self-supporting film are held to further obtain a film with a residual solvent content of 0.5% by mass or less based on the whole layer. A method for producing a multilayer polyimide film as claimed in any one of claims 1 to 7, comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming a (a) layer on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming a (b) layer on a temporary support to obtain a coating a1; 4: drying the coating ab1 to obtain the coating ab2 with a residual solvent content of 5-40% by mass based on the whole layer; 5: applying the polyimide solution or polyimide precursor solution for forming the (a) layer on the coating ab2 to obtain the coating ab1; 6: heating the whole layer to obtain a layered product with a residual solvent content of less than 0.5% by mass based on the whole layer. A method for producing a multilayer polyimide film as claimed in any one of claims 1 to 7, comprising at least: 1: applying a polyimide solution or a polyimide precursor solution for forming a layer (a) on a temporary support to obtain a coating a1; 2: drying the coating a1 to obtain a coating a2 having a residual solvent content of 5 to 40% by weight; 3: applying a polyimide solution or a polyimide precursor solution for forming a layer (b) on the coating a2 to obtain a coating ab1; 4: drying the coating ab1 to obtain a residual solvent content of the entire layer of 5: the step of applying the polyimide solution or polyimide precursor solution for forming the (a) layer on the coating ab2 to obtain the coating ab1; 6: the step of heating the whole layer to obtain a layered product with a residual solvent content of 8% to 40% based on the whole layer, and then peeling it from the temporary support to obtain a self-supporting film; 7: holding both ends of the self-supporting film to further obtain a film with a residual solvent content of 0.5% or less based on the whole layer.