Method for producing polyimide film

Abstract

This method for producing a polyimide film is provided with a step Sa for transporting a polyimide film precursor (10) while heating the polyimide film precursor (10) in a heating furnace in a state in which both end parts in the width direction of the polyimide film precursor (10) are gripped by a plurality of clips (12). In at least a part of the step Sa, the polyimide film precursor (10) is relaxed in the transport direction by shortening the intervals between the plurality of clips (12) in the transport direction while transporting the polyimide film precursor (10).

Description

Polyimide film manufacturing method

[0001] The present invention relates to a method for producing a polyimide film.

[0002] In recent years, there has been an increasing demand for lighter, smaller, and thinner electronic products such as smartphones and personal computers, and the wiring patterns of various flexible printed circuit boards (hereinafter sometimes referred to as "FPCs") have become finer. As a result, higher dimensional stability is also being demanded of the polyimide films used as substrate materials for FPCs.

[0003] Polyimide films are typically obtained by casting and coating a polyamic acid solution onto a support, drying it to obtain a self-supporting film (polyimide film precursor), and then baking the polyimide film precursor through a high-temperature heating furnace (tenter furnace method) to imidize the polyamic acid in the polyimide film precursor. The use of a tenter furnace baking method can result in a bowing phenomenon, which can degrade the overall physical properties of the polyimide film. Specifically, the bowing phenomenon can cause differences in molecular orientation between the film edges and the film center, resulting in non-uniform film properties across the film width and potentially causing parts of the film to fail quality control standards.

[0004] As a method for effectively controlling the occurrence of the bowing phenomenon and uniformly controlling the physical properties of a polyimide film across its entire width, Patent Document 1 discloses a method for producing a polyimide film in which a polyimide film precursor is relaxed along the conveying direction, and then baked (heated) while both widthwise ends are fixed. In Patent Document 1, a plurality of needle-shaped objects (such as pin sheets) are used as fixing means for fixing both widthwise ends of the polyimide film precursor. By piercing both widthwise ends of the relaxed polyimide film precursor with the needle-shaped objects, the polyimide film precursor is baked while maintaining its relaxed state.

[0005] Japanese Patent Application Laid-Open No. 2006-181986

[0006] In the method described in Patent Document 1, a polyimide film precursor is relaxed along the conveying direction, and then both ends of the polyimide film precursor are fixed in the width direction by piercing needle-shaped objects into both ends of the polyimide film precursor. The polyimide film precursor is then transported to a tenter furnace. Therefore, once the polyimide film precursor is fixed in a relaxed state along the conveying direction, the film is baked while maintaining this relaxed state. Therefore, in the method described in Patent Document 1, the relaxation rate along the conveying direction cannot be freely changed in the tenter furnace, making it difficult to produce a polyimide film with a uniform degree of molecular orientation across the entire width direction. In particular, when a monomer composition that is easily oriented, a monomer composition with a low storage modulus at high temperatures, or a monomer composition with low tear strength is selected for the polyimide film to be formed, it may be difficult to stably produce a polyimide film with uniform properties across the width direction.

[0007] In view of the above, an object of the present invention is to provide a method for producing a polyimide film having a uniform degree of molecular orientation across the entire width direction.

[0008] <Aspects of the Present Invention> The present invention includes the following aspects.

[0009] [1] A method for producing a polyimide film, in which a polyimide film precursor containing polyamic acid is heated while being transported to continuously form a polyimide film, the method comprising: a step Sa of transporting the polyimide film precursor while heating it in a heating furnace in a state where both ends of the polyimide film precursor in a width direction are held with a plurality of clips; and in at least a part of the step Sa, the polyimide film precursor is relaxed in the transport direction by shortening the intervals between the plurality of clips in the transport direction while being transported.

[0010] [2] The method for producing a polyimide film according to [1], wherein the clip is flat.

[0011] [3] The method for producing a polyimide film according to [1] or [2], wherein the distance between the clips in the conveying direction before the polyimide film precursor is relaxed is 70 mm or more and 100 mm or less.

[0012] [4] The method for producing a polyimide film according to any one of [1] to [3], wherein in the step Sa, two or more heating furnaces are used to heat the polyimide film precursor, and the relaxation rate of the polyimide film precursor in the transport direction is adjusted for each heating furnace.

[0013] [5] The method for producing a polyimide film according to [4], wherein in the step Sa, the heating temperature of the polyimide film precursor is adjusted for each heating furnace.

[0014] [6] The method for producing a polyimide film according to any one of [1] to [5], wherein the maximum relaxation rate of the polyimide film precursor in the conveying direction in the step Sa is 4.0% or more.

[0015] [7] The method for producing a polyimide film according to any one of [1] to [6], wherein the maximum heating temperature of the polyimide film precursor in the step Sa is 320°C or higher.

[0016] [8] The method for producing a polyimide film according to any one of [1] to [7], further comprising, before the step Sa, a step Sb of conveying the polyimide film precursor while heating it in a heating furnace, with both widthwise ends of the polyimide film precursor fixed with pin sheets.

[0017] [9] The method for producing a polyimide film according to any one of [1] to [8], wherein the width of the polyimide film to be formed is 1000 mm or more.

[0018]

[10] The method for producing a polyimide film according to any one of [1] to [9], wherein the polyimide film precursor has a laminated structure in which two or more layers containing different types of polyamic acids are laminated.

[0019] According to the present invention, a method for producing a polyimide film having a uniform degree of molecular orientation across the entire width direction can be provided.

[0020] 1 is a plan view of the inside of a heating furnace when a polyimide film precursor is heated while being conveyed, and FIG. 2 is a plan view of the inside of a heating furnace after a predetermined time has elapsed since the state of FIG. 1. FIG. 3 is a plan view for explaining measurement points of the degree of molecular orientation of a polyimide film.

[0021] Preferred embodiments of the present invention will be described in detail below, but the present invention is not limited thereto. In addition, all academic and patent documents described in this specification are incorporated herein by reference.

[0022] First, the terms used in this specification will be explained. "Polyamic acid" includes not only polyamic acid with an imidization rate of 0% but also polyamic acid in which some of the amide groups have been imidized. "Polyimide film precursor" refers to a film containing polyamic acid. "Conveying direction" refers to the conveying direction of the polyimide film precursor, unless otherwise specified. "Upstream" and "downstream" mean "upstream in the conveying direction" and "downstream in the conveying direction," respectively. "Clip spacing" refers to the spacing between the centers of the clips. "Spacing between multiple clips in the conveying direction" refers to the spacing between adjacent clips in the conveying direction. "Polyimide film width" refers to the width of the polyimide film before cutting (slitting) to narrow the film width, unless otherwise specified.

[0023] "Non-thermoplastic polyimide" refers to polyimide that retains a film shape (flat membrane shape) when fixed in a film state (thickness 17.0 μm) on a metal frame and heated at a heating temperature of 380° C. for 1 minute. "Thermoplastic polyimide" refers to polyimide that does not retain a film shape when fixed in a film state (thickness 17.0 μm) on a metal frame and heated at a heating temperature of 380° C. for 1 minute.

[0024] Hereinafter, the compound name may be followed by "based" to refer to the compound and its derivatives in a comprehensive manner. Furthermore, when the compound name is followed by "based" to represent the name of a polymer, unless otherwise specified, it means that the repeating unit of the polymer is derived from the compound or its derivative. Furthermore, tetracarboxylic acid dianhydrides may be referred to as "acid dianhydrides."

[0025] Unless otherwise specified, the components and functional groups exemplified in this specification may be used alone or in combination of two or more kinds.

[0026] The drawings referred to in the following description mainly show each component in a schematic manner for ease of understanding, and the size, number, shape, etc. of each component shown may differ from the actual size, number, shape, etc. of each component due to the convenience of creating the drawings. Furthermore, for convenience of explanation, in drawings described later, the same components as those in previously described drawings may be assigned the same reference numerals, and their explanation may be omitted.

[0027] <Method for Producing Polyimide Film> In the method for producing a polyimide film according to this embodiment, a polyimide film precursor containing polyamic acid is heated while being transported, thereby continuously forming a polyimide film. The method for producing a polyimide film according to this embodiment includes a step Sa in which the polyimide film precursor is transported while being heated in a heating furnace, with both widthwise ends of the polyimide film precursor held by a plurality of clips. In this embodiment, in at least a part of the step Sa, the polyimide film precursor is relaxed in the transport direction by shortening the spacing between the plurality of clips while being transported.

[0028] Hereinafter, the "process of relaxing the polyimide film precursor in the transport direction by shortening the intervals between multiple clips in the transport direction while transporting the polyimide film precursor" may be referred to as the "specific relaxation process."

[0029] According to the method for producing a polyimide film according to this embodiment, a polyimide film having a uniform degree of molecular orientation across the entire width direction can be produced. The reason for this is presumed to be as follows.

[0030] In the manufacturing method according to the present embodiment, the polyimide film precursor is subjected to a specific relaxation treatment while being held at both widthwise ends with multiple clips, thereby relaxing the polyimide film precursor in the conveying direction while heating, which tends to homogenize the degree of molecular orientation at the film edges and the center of the film. Therefore, the manufacturing method according to the present embodiment can produce a polyimide film having a homogenous degree of molecular orientation throughout the entire width direction.

[0031] The polyimide film manufacturing method according to this embodiment may include a step (other step) other than step Sa. For example, the other step may include step Sb, which is performed before step Sa, in which both widthwise ends of the polyimide film precursor are fixed with pin sheets and the polyimide film precursor is transported while being heated in a heating furnace. Hereinafter, step Sb may be referred to as the "first heating step." Furthermore, step Sa may be referred to as the "second heating step." Note that the polyimide film manufacturing method according to this embodiment may perform only the second heating step (step Sa) as the heating step for the polyimide film precursor, without performing the first heating step.

[0032] The method for producing a polyimide film according to the present embodiment may further include a "polyamic acid synthesis step" and a "polyimide film precursor formation step," which will be described later. Each step included in an example of the production method according to the present embodiment will be described in detail below.

[0033] [Polyamic Acid Synthesis Process] Polyamic acid synthesis (production) can be achieved by any known method or a combination thereof. A specific example of a polyamic acid synthesis (production) method is a method in which a diamine and a tetracarboxylic dianhydride are reacted in an organic solvent. The amounts of diamine and tetracarboxylic dianhydride used during the reaction are preferably substantially equal. When synthesizing polyamic acid using a diamine and a tetracarboxylic dianhydride, the desired polyamic acid (a polymer of a diamine and a tetracarboxylic dianhydride) can be obtained by adjusting the amounts of diamine (or, if multiple diamines are used, the amount of each diamine) and tetracarboxylic dianhydride (or, if multiple tetracarboxylic dianhydrides are used, the amount of each tetracarboxylic dianhydride). The temperature conditions for the reaction between the diamine and the tetracarboxylic dianhydride, i.e., the polyamic acid synthesis reaction, are not particularly limited, but are, for example, in the range of 10°C to 150°C. The reaction time for the polyamic acid synthesis reaction is, for example, in the range of 10 minutes to 30 hours.

[0034] Examples of acid dianhydrides (monomers) used in the synthesis of polyamic acids include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 3,4 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, p-phenylene bis(trimellitic acid monoester acid anhydride), ethylene bis(trimellitic acid monoester acid anhydride), bisphenol A bis(trimellitic acid monoester acid anhydride), and derivatives thereof.

[0035] By adjusting the amount of these acid dianhydrides added, it is possible to control the basic properties of the film within a range suitable for FPC applications. In order to control the elastic modulus, tensile strength, elongation, linear expansion coefficient, and other properties within a range suitable for FPC applications, it is preferable to use one or more selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 3,3',4,4'-benzophenonetetracarboxylic dianhydride. Furthermore, in order to control the tear strength and other properties within a range suitable for FPC applications, it is preferable to use one or more selected from the group consisting of 4,4'-oxydiphthalic anhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride. Furthermore, among the acid dianhydrides listed above, the use of one or more selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydiphthalic anhydride tends to facilitate the orientation of polymer chains, thereby more effectively demonstrating the effects of this embodiment.

[0036] Examples of diamines (monomers) used in the synthesis of polyamic acids include p-phenylenediamine, 4,4'-diamino-2,2'-dimethylbiphenyl, 1,3-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diamino Examples include diphenyl sulfone, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl N-methylamine, 4,4'-diaminodiphenyl N-phenylamine, 1,3-diaminobenzene, 1,2-diaminobenzene, and derivatives thereof.

[0037] By adjusting the amount of these diamines added, it is possible to control the basic properties of the film within a range suitable for FPC applications. In order to control the elastic modulus, tensile strength, elongation, linear expansion coefficient, etc. within a range suitable for FPC applications, it is preferable to use one or more selected from the group consisting of p-phenylenediamine, 4,4'-diaminodiphenyl ether, and 4,4'-diamino-2,2'-dimethylbiphenyl. Furthermore, among the diamines listed above, the use of one or more selected from the group consisting of p-phenylenediamine and 4,4'-diamino-2,2'-dimethylbiphenyl tends to make it easier for the polymer chains to be oriented, thereby more effectively demonstrating the effects of this embodiment.

[0038] Furthermore, when 3,3',4,4'-biphenyltetracarboxylic dianhydride is used as the acid dianhydride monomer, or when one or more diamine monomers selected from the group consisting of p-phenylenediamine, 4,4'-diamino-2,2'-dimethylbiphenyl, and 1,3-bis(4-aminophenoxy)benzene are used as the diamine monomer, the dielectric constant and dielectric dissipation factor of the film tend to be low. As a result, a polyimide film suitable for high-frequency applications can be obtained, but the storage modulus at high temperatures tends to be low. Films with a low storage modulus at high temperatures are generally susceptible to the effects of heat and tension during the film formation process, making it difficult to obtain a polyimide film with a uniform degree of molecular orientation across the entire width direction. However, the polyimide film manufacturing method of this embodiment makes it possible to stably produce a polyimide film with a uniform degree of molecular orientation across the entire width direction, even when producing a polyimide film with a storage modulus of 0.3 GPa or less at a temperature of 350°C.

[0039] Furthermore, when one or more dianhydride monomers selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride and p-phenylenebis(trimellitic acid monoester anhydride) are used as the dianhydride monomer, or when one or more diamine monomers selected from the group consisting of 4,4'-diamino-2,2'-dimethylbiphenyl and 1,3-bis(4-aminophenoxy)benzene are used as the diamine monomer, the tear strength of the film tends to be low. Polyimide films with low tear strength are also susceptible to the effects of heat and tension during the film formation process, making it difficult to obtain polyimide films with a uniform degree of molecular orientation across the entire width. However, the polyimide film manufacturing method of this embodiment makes it possible to stably produce polyimide films with a uniform degree of molecular orientation across the entire width, even when producing polyimide films with a tear strength of 300 gf / mm or less.

[0040] Any method of adding a monomer may be used for synthesizing a polyamic acid. Examples of methods for synthesizing (producing) a polyamic acid include a polymerization method (hereinafter sometimes referred to as "polymerization method A") that includes the following steps (A-a) and (A-b): (A-a): A step of reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent with an excess of aromatic diamine to obtain a prepolymer having amino groups at both ends; (A-b): A step of adding an additional aromatic diamine having a different structure from that used in step (A-a), and then adding an aromatic acid dianhydride having a different structure from that used in step (A-a) so that the aromatic diamine and aromatic acid dianhydride are substantially equimolar in all steps, thereby polymerizing the resulting mixture.

[0041] Another example of a method for producing polyamic acid is a polymerization method (hereinafter sometimes referred to as "polymerization method B") that involves the following steps (Ba) and (B-b): (Ba): a step of reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent with an excess of the aromatic acid dianhydride to obtain a prepolymer having acid anhydride groups at both ends; (B-b): a step of adding an aromatic acid dianhydride having a different structure from that used in step (Ba), and then adding an aromatic diamine having a different structure from that used in step (Ba) so that the aromatic diamine and aromatic acid dianhydride are substantially equimolar in all steps, and polymerizing the resulting mixture.

[0042] A polymerization method in which the addition order is set so that a specific diamine or specific acid dianhydride selectively reacts with any or specific diamine, or any or specific acid dianhydride (for example, the above-mentioned polymerization method A or polymerization method B) is referred to herein as sequence polymerization. Among polymers obtained by sequence polymerization, a polymer having two types of segments is called a diblock copolymer, and a polymer having three types of segments is called a triblock copolymer. In contrast, a polymerization method in which the addition order of diamine and acid dianhydride is not set (a polymerization method in which monomers react with each other randomly) is referred to herein as random polymerization. A polymer obtained by random polymerization is called a random copolymer.

[0043] When preparing a polyimide film precursor, a method of preparing the polyimide film precursor from a polyamic acid solution containing polyamic acid and an organic solvent may be employed. Examples of organic solvents that can be used in the polyamic acid solution include urea-based solvents such as N,N-dimethylethylurea; sulfoxide-based solvents such as dimethyl sulfoxide; sulfone-based solvents such as tetramethyl sulfone; amide-based solvents such as N,N-dimethylformamide and N-methyl-2-pyrrolidone; ester-based solvents such as γ-butyrolactone; alkyl halide solvents such as chloroform; aromatic hydrocarbon solvents such as toluene; phenol-based solvents such as phenol; ketone-based solvents such as cyclopentanone; and ether-based solvents such as tetrahydrofuran. These solvents are typically used alone, but two or more may be used in combination as needed. When polyamic acid is prepared by the polymerization method described above, the reaction solution (the solution after the reaction) itself may be used as the polyamic acid solution for preparing the polyimide film precursor. In this case, the organic solvent in the polyamic acid solution is the same organic solvent used in the reaction in the polymerization method described above. Alternatively, a polyamic acid solution may be prepared by removing the solvent from the reaction solution to obtain a solid polyamic acid, and dissolving the solid polyamic acid in an organic solvent.

[0044] The polyamic acid solution may contain additives such as dyes, surfactants, leveling agents, plasticizers, silicones, and sensitizers. Furthermore, a filler may be added to the polyamic acid solution to improve various film properties such as sliding properties, thermal conductivity, electrical conductivity, corona resistance, and loop stiffness. Any filler may be used, but preferred examples include fillers made of silica, titanium oxide, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, mica, and the like.

[0045] The concentration of polyamic acid in the polyamic acid solution is not particularly limited and is, for example, 5 to 40% by weight, preferably 10 to 30% by weight, based on the total amount of the polyamic acid solution. When the concentration of polyamic acid is 5 to 40% by weight, an appropriate molecular weight and solution viscosity can be obtained.

[0046] [Step of Forming Polyimide Film Precursor] The method of forming the polyimide film precursor is not particularly limited, and various known methods can be employed, including, for example, a method of forming the polyimide film precursor according to the following steps (i) and (ii): Step (i): A step of applying a dope liquid containing a polyamic acid solution onto a support to form a coating film; Step (ii): A step of heating the coating film on the support to form a self-supporting polyimide film precursor, and then peeling the polyimide film precursor from the support.

[0047] Hereinafter, the self-supporting polyimide film precursor may be referred to as a "precursor film."

[0048] In order to obtain a polyimide film having a uniform degree of molecular orientation across the entire width direction while increasing productivity, the width of the precursor film is preferably 500 mm or more and 5000 mm or less, and more preferably 1000 mm or more and 4000 mm or less.

[0049] The methods for producing polyimide films are broadly divided into thermal imidization and chemical imidization. The thermal imidization method is a method in which a polyamic acid solution is applied as a dope onto a support without using a dehydrating ring-closing agent or the like, and the imidization is promoted by heating. On the other hand, the chemical imidization method is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization accelerator is used as a dope to promote imidization. Either method can be used, but the chemical imidization method is superior in productivity.

[0050] As the dehydration ring-closing agent, an acid anhydride typified by acetic anhydride is preferably used. As the catalyst, a tertiary amine such as an aliphatic tertiary amine, an aromatic tertiary amine, or a heterocyclic tertiary amine is preferred, and isoquinoline is more preferred.

[0051] The amount of dehydration ring-closing agent added is preferably 0.5 to 10.0 molar equivalents, more preferably 0.7 to 5.0 molar equivalents, and even more preferably 0.8 to 3.0 molar equivalents, relative to the amide groups of the polyamic acid. The amount of catalyst added is preferably 0.5 to 5.0 molar equivalents, more preferably 0.7 to 2.5 molar equivalents, and even more preferably 0.8 to 2.0 molar equivalents, relative to the amide groups of the polyamic acid. In this specification, "amide groups of polyamic acid" refers to amide groups formed by the polymerization reaction of diamine and tetracarboxylic dianhydride.

[0052] In the step (i), the method for applying the dope solution onto the support is not particularly limited, and a method using a conventionally known coating device such as a die coater, a comma coater (registered trademark), a reverse coater, a knife coater, etc. Suitable supports to be applied with the dope solution in the step (i) include a glass plate, an aluminum foil, an endless stainless steel belt, a stainless steel drum, etc.

[0053] In step (ii), heating conditions are set according to the thickness of the final film to be obtained and the production rate, and after at least one of partial imidization and drying is performed, the precursor film is peeled off from the support to obtain a precursor film. The heating conditions in step (ii) are, for example, a heating temperature in the range of 50°C to 200°C and a heating time in the range of 1 minute to 100 minutes.

[0054] The precursor film immediately after peeling from the support in step (ii) contains components other than the polymer (residual components). Examples of residual components include organic solvents, catalysts, dehydration ring-closing agents, reaction products (post-reaction dehydration ring-closing agents, water, etc.), and additives. The proportion of the remaining components (unit: wt %) in the precursor film immediately after peeling from the support in step (ii) is calculated from the weight a (g) of the dried precursor film and the weight b (g) of the remaining components using the formula "residual component proportion = 100 × b / a." The weights a and b are calculated by first measuring the weight d of a 100 mm × 100 mm precursor film. The precursor film is then dried in an oven at 450°C for 20 minutes, cooled to room temperature, and weighed. The resulting weight is designated as weight a. Weight b is then calculated using the formula "b = d - a." For stable production of polyimide films, the proportion of the residual components is preferably 65% ​​by weight or less, more preferably 35% by weight or more and 60% by weight or less, and even more preferably 35% by weight or more and 55% by weight or less.

[0055] In this embodiment, the polyimide film precursor may be formed from a single-layer coating film containing one type of polyamic acid, or from a coating film in which two or more layers containing different types of polyamic acid are laminated. A polyimide film precursor obtained from a coating film in which two or more layers containing different types of polyamic acid are laminated has a laminate structure in which two or more layers containing different types of polyamic acid are laminated. A method for forming a coating film in which two or more layers containing different types of polyamic acid are laminated includes a co-extrusion-cast coating method. The "co-extrusion-cast coating method" is a method for forming a coating film using an extruder having an extrusion die with two or more layers. For example, the co-extrusion-cast coating method is a method in which a dope solution containing a first polyamic acid and a dope solution containing a second polyamic acid different from the first polyamic acid are extruded through the lip opening of the extrusion die in the form of two or more thin films, thereby forming a coating film having a layer structure of two or more layers on a support. Even when a polyimide film precursor is formed from a coating film in which two or more films containing different types of polyamic acid are laminated, the preferable conditions for forming the polyimide film precursor from the coating film are the same as those described above.

[0056] In this embodiment, for example, when forming a multilayer polyimide film having a three-layer structure of an adhesive layer containing a thermoplastic polyimide / a non-thermoplastic polyimide layer / an adhesive layer containing a thermoplastic polyimide, a polyamic acid (non-thermoplastic polyamic acid) for forming the non-thermoplastic polyimide layer and a polyamic acid (thermoplastic polyamic acid) for forming the adhesive layer (thermoplastic polyimide layer) can be used. When using these polyamic acids to form a three-layer coating film by a co-extrusion-casting coating method, a non-thermoplastic polyimide layer-forming solution (dope solution) containing a non-thermoplastic polyamic acid and an adhesive layer-forming solution (dope solution) containing a thermoplastic polyamic acid are prepared, the non-thermoplastic polyimide layer-forming solution is supplied to the central layer of an extrusion die, and the adhesive layer-forming solution is supplied to the two layers adjacent to (on both sides of) the central layer of the extrusion die, and these solutions are extruded onto a support.

[0057] The acid dianhydrides (monomers) for obtaining the non-thermoplastic polyamic acid and the thermoplastic polyamic acid include the acid dianhydrides (monomers) listed above. The acid dianhydrides used for synthesizing the non-thermoplastic polyamic acid and the acid dianhydrides used for synthesizing the thermoplastic polyamic acid may be the same or different.

[0058] Examples of diamines (monomers) used in the synthesis of non-thermoplastic polyamic acids include the diamines (monomers) listed above. Furthermore, in order to ensure thermoplasticity, diamines having a bent structure are preferred as diamines (monomers) used in the synthesis of thermoplastic polyamic acids. Examples of diamines (monomers) having a bent structure include 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane. As diamines (monomers) used in the synthesis of thermoplastic polyamic acids, diamines having a rigid structure and diamines having a bent structure may be used in combination.

[0059] [First Heating Step] In the first heating step, the polyimide film precursor (precursor film) is conveyed while being heated in a heating furnace with both widthwise ends of the polyimide film precursor fixed with pin sheets. Note that the first heating step does not involve a treatment to relax the polyimide film precursor in the conveying direction.

[0060] Because the precursor film may contain a large amount of organic solvent, if the precursor film immediately after peeling from the support in step (ii) is subjected to the second heating step described below, dirt may adhere to the clips, making the device more susceptible to contamination. Furthermore, if the precursor film immediately after peeling from the support in step (ii) is subjected to the second heating step described below, the precursor film may rapidly shrink due to the volatilization of remaining components, resulting in notches being formed in the portions gripped by the clips. Notches in the precursor film may cause breakage problems. In contrast, if the first heating step is performed before the second heating step to remove at least a portion of the organic solvent from the precursor film, the amount of dirt adhering to the clips in the second heating step can be reduced and the occurrence of rapid shrinkage of the precursor film can be suppressed. Note that pin sheets adhere to less dirt than clips.

[0061] In the first heating step, it is preferable to use two or more heating furnaces. Examples of the configuration of two or more heating furnaces include a configuration in which two or more hot air furnaces are connected together, a configuration in which two or more far-infrared ovens are connected together, and a configuration in which one or more hot air ovens and one or more far-infrared ovens are connected together.

[0062] In the first heating step, the temperature of the most upstream heating furnace is preferably 300°C or lower, more preferably 200°C or higher and 280°C or lower, and even more preferably 220°C or higher and 280°C or lower. In the first heating step, the maximum heating temperature is preferably 400°C or lower, more preferably 200°C or higher and 350°C or lower, and even more preferably 230°C or higher and 300°C or lower. In this specification, the term "maximum heating temperature" refers to the maximum temperature obtained by attaching a thermocouple to the surface of the film being transported through the heating furnace and measuring the temperature from the entrance to the exit of the heating furnace.

[0063] After the imidization of the polyamic acid in the precursor film and the removal of remaining volatile components have been completed in this manner, the precursor film is subjected to the second heating step described below. In this case, the precursor film subjected to the second heating step (hereinafter sometimes referred to as the "unrelaxed precursor film") preferably has a heat shrinkage of 1% to 10% in both the longitudinal direction (machine direction) and the width direction at a temperature of 350°C. A heat shrinkage of 1% to 10% can produce a polyimide film with a more uniform degree of molecular orientation throughout the entire width direction. The heat shrinkage at 350°C is measured by the same method as in the Examples described below.

[0064] The proportion of the remaining components in the unrelaxed precursor film is preferably 1.0 wt % or more, more preferably 2.0 wt % to 7.0 wt % or less, and even more preferably 3.0 wt % to 6.0 wt % or less. The proportion of the remaining components in the unrelaxed precursor film can be obtained by the same method as in the above-mentioned method for calculating the "proportion of the remaining components in the precursor film immediately after peeling from the support", except that the drying conditions are changed to "temperature of 350°C for 10 minutes".

[0065] [Second Heating Step] The second heating step is a step in which the polyimide film precursor (unrelaxed precursor film) is heated and transported in a heating furnace while both widthwise ends of the polyimide film precursor are held with multiple clips. In the second heating step, imidization is completed to obtain a polyimide film. In this embodiment, in at least a part of the second heating step, the polyimide film precursor is relaxed in the transport direction by shortening the spacing between the multiple clips in the transport direction while transporting the polyimide film precursor. The second heating step may include only a step of heating the polyimide film precursor in a relaxed state in the transport direction (relaxation step), or may include both a relaxation step and a step of heating the polyimide film precursor without relaxing it in the transport direction (non-relaxation step).

[0066] The second heating step may be performed using only one heating furnace, or may be performed using two or more heating furnaces. Examples of configurations of two or more heating furnaces include a configuration in which two or more hot air furnaces are connected together, a configuration in which two or more far-infrared ovens are connected together, and a configuration in which one or more hot air ovens and one or more far-infrared ovens are connected together.

[0067] An example of carrying out the second heating step using only one heating furnace will be described below with reference to the drawings. Fig. 1 is a plan view of the interior of a heating furnace during heating while conveying a polyimide film precursor, showing an arbitrary moment in the heating furnace. Fig. 2 is a plan view of the same heating furnace as Fig. 1 at a moment after a predetermined time has elapsed since the state shown in Fig. 1.

[0068] A polyimide film precursor 10 shown in FIG. 1 is heated in a heating furnace and transported along the conveying direction Y from a furnace entrance vicinity region 20a to a furnace exit vicinity region 20b. Clip chains are provided near both ends of the polyimide film precursor 10 in the width direction X. Each clip chain includes a connecting portion 11 and a plurality of clips 12. The polyimide film precursor 10 is transported along the conveying direction Y with both ends in the width direction X held by the clips 12. The clips 12 are connected to the connecting portion 11 via connecting portions 13 and are movable in a direction parallel to the conveying direction Y while holding both ends in the width direction X of the polyimide film precursor 10. A clip spacing adjustment mechanism (not shown) shortens the spacing between adjacent clips 12 in the conveying direction Y in the heating furnace, thereby relaxing the polyimide film precursor 10 along the conveying direction Y (specific relaxation treatment). Examples of the clip spacing adjustment mechanism include the clip spacing adjustment mechanisms described in International Publication No. 2020 / 116071 and Japanese Patent Application Laid-Open No. 2022-051372.

[0069] The clip spacing adjustment mechanisms described in International Publication No. 2020 / 116071 and Japanese Patent Application Laid-Open No. 2022-051372 are not intended for use in the high-temperature imidization process described in this embodiment, so it is preferable to change the material, such as by using a high-temperature solid lubricant bearing. Examples of such bearings that can be used include the "SJ Type High-Temperature Solid Lubricant Bearing" manufactured by NSK Ltd. and the "ULTAGE Deep Groove Ball Bearing for Ultra-High Temperature Environments" manufactured by NTN Corporation.

[0070] The conveying speed of the polyimide film precursor 10 is not particularly limited, but is preferably 3.0 m / min or more and 20.0 m / min or less in order to stably produce polyimide films while improving productivity.

[0071] The shape of the clip 12 may be round, flat, etc. In order to suppress plastic deformation or necking deformation that occurs when the stretch ratio is locally increased between adjacent clips 12, the clip 12 is preferably flat.

[0072] In order to suppress plastic deformation and necking deformation caused by a local increase in the stretch ratio between adjacent clips 12, it is preferable that the spacing between the multiple clips 12 in the conveying direction Y before relaxing the polyimide film precursor 10 be 70 mm or more and 100 mm or less.

[0073] In Fig. 1, clip 12a is the clip closest to the entrance of the heating furnace. Also, in Fig. 1, clip 12b is the clip adjacent to clip 12a in the conveying direction Y. The distance D between clips 12a and 12b shown in Fig. 1 is １ is the spacing before the specific relaxation process is started in the heating furnace (the spacing between the multiple clips 12 in the conveying direction Y before the polyimide film precursor 10 is relaxed). The spacing between clips 12a and 12b is gradually shortened by the clip spacing adjustment mechanism as the polyimide film precursor 10 is conveyed. Because FIG. 1 is a plan view showing an instantaneous state inside the heating furnace, in FIG. 1, the spacing between the multiple clips 12 in the conveying direction Y gradually shortens from the heating furnace entrance vicinity region 20a to the heating furnace exit vicinity region 20b.

[0074] In the example shown in Figure 1, the total number of clips 12 housed in one heating furnace is 20, but in this embodiment, the total number of clips housed in one heating furnace is not limited to 20 and can be changed as appropriate depending on the size of the heating furnace and the size of the clips, for example, in the range of 6 to 60.

[0075] 2 is a plan view of the clip 12b shown in FIG. 1 at the moment when the clip 12b reaches the furnace outlet vicinity region 20b. In other words, the clip 12b shown in FIG. 2 is the clip closest to the furnace outlet. In FIG. 2, the distance D between the clips 12a and 12b is ２ is the interval after the specific relaxation treatment in the furnace is completed.

[0076] The relaxation rate in the conveying direction Y of the polyimide film precursor 10 during the specific relaxation treatment is the rate of change in the distance between adjacent clips 12. Hereinafter, the "relaxation rate in the conveying direction of the polyimide film precursor" may be simply referred to as the "relaxation rate." The relaxation rate (unit: %) during the specific relaxation treatment is, for example, the rate of change in the distance D １ (unit: mm) and spacing D ２ (unit: mm) and the formula "relaxation rate in specific relaxation treatment = 100 × (D １ -D ２ ) / D １ In the following, unless otherwise specified, the term "relaxation rate" refers to the relaxation rate in a specific relaxation treatment.

[0077] Conventional conveying devices used in tenter furnaces for producing polyimide films can stretch the film in the width direction or induce a relaxed state in the width direction by widening or narrowing the rail widths on the left and right sides. However, they are not designed for stretching or relaxing the film in the transport direction. Therefore, the precursor film is stretched in the transport direction using dancer rolls. Furthermore, the only way to relax the precursor film along the transport direction is by using the method described in JP 2006-181986 A. In this embodiment, the spacing between the clips in the transport direction is shortened in the heating furnace, allowing the film to be heated while relaxing in the transport direction, thereby enabling the relaxation rate to be freely adjusted in the heating furnace. This embodiment allows the shrinkage rate of the film at a specific heating temperature to be estimated in advance and the relaxation rate to be set according to that shrinkage rate. This allows the production of polyimide films with a more uniform degree of molecular orientation across the entire width direction compared to conventional methods.

[0078] When the relaxation process is carried out using two or more heating furnaces, the relaxation rate in each heating furnace is calculated as follows. For convenience, the case where the relaxation process is carried out in two heating furnaces, a first heating furnace (upstream heating furnace) and a second heating furnace (downstream heating furnace), is explained as an example. Also, the distance between adjacent clips 12 before the specific relaxation process is started in the first heating furnace is defined as D. １ (see FIG. 1), and the distance between adjacent clips 12 after the specific relaxation treatment in the first heating furnace is D ２ (See FIG. 2), and the distance between adjacent clips 12 after the specific relaxation treatment in the second heating furnace is D ３ In this case, the relaxation rate (R １ ) is "R １ = 100 × (D １ -D ２ ) / D １ The relaxation rate in the second heating furnace (R ２ ) is "R ２ = 100 × (D １ -D ３ ) / D １In this way, when the relaxation step is performed using two or more heating furnaces, the clip spacing that serves as the basis for calculating the relaxation rate in each heating furnace is the clip spacing before the polyimide film precursor 10 is relaxed (in the above case, D １ )

[0079] When the second heating step is performed using two or more heating furnaces, it is preferable to adjust the relaxation rate for each heating furnace. When the relaxation rate is adjusted for each heating furnace, it is possible to change the relaxation rate depending on the heating temperature as the polyimide film precursor 10 is transported, making it possible to produce a polyimide film having a more uniform degree of molecular orientation throughout the width direction. When the second heating step is performed using two or more heating furnaces, it is preferable to adjust the relaxation rate for each heating furnace and the heating temperature for each heating furnace in order to produce a polyimide film having a more uniform degree of molecular orientation throughout the width direction.

[0080] To produce a polyimide film having a more uniform degree of molecular orientation across the entire width direction, the maximum relaxation rate of the polyimide film precursor 10 in the machine direction Y is preferably 4.0% or more, more preferably 5.0% or more, and even more preferably 6.0% or more. When the second heating step is performed using only one heating furnace, the "maximum relaxation rate" refers to the relaxation rate in that heating furnace. When the second heating step is performed using two or more heating furnaces, the "maximum relaxation rate" refers to the maximum relaxation rate among the relaxation rates in each heating furnace. When producing a polyimide film having a storage modulus of 0.3 GPa or less at 350°C, the maximum relaxation rate of the polyimide film precursor 10 is preferably 5.0% or more, more preferably 6.0% or more, and even more preferably 8.0% or more. To stably produce a polyimide film, the maximum relaxation rate of the polyimide film precursor 10 in the machine direction Y is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 9.0% or less. The relaxation rate of each heating furnace in which the relaxation step is carried out is preferably set in the range of 1.0% to 15.0%.

[0081] The second heating step is preferably carried out using a staged heating furnace consisting of two or more connected heating furnaces, more preferably a staged heating furnace consisting of five or more heating furnaces, and even more preferably a staged heating furnace consisting of seven or more heating furnaces. In order to increase productivity, the second heating step is preferably carried out using a staged heating furnace consisting of nine or less heating furnaces. Furthermore, when the second heating step is carried out using two or more heating furnaces, it is preferable to increase the relaxation rate stepwise in the multiple heating furnaces.

[0082] Furthermore, when the second heating step is performed using two or more heating furnaces, it is preferable to perform the relaxation step in the second or subsequent heating furnace from the upstream side, rather than in the most upstream heating furnace. When the relaxation step is performed in the second or subsequent heating furnace from the upstream side, the relaxation step can be performed after at least a portion of the organic solvent has evaporated from the polyimide film precursor 10 in the most upstream heating furnace, thereby achieving heating conditions that match the thermal shrinkage behavior of the film. This allows the production of a polyimide film with a more uniform degree of molecular orientation across the entire width direction. Furthermore, performing the relaxation step in the second or subsequent heating furnace from the upstream side is also preferable because it avoids breakage problems caused by rapid shrinkage of the film due to the evaporation of remaining components.

[0083] When the second heating step is carried out without carrying out the first heating step, the heating temperature in the heating furnace located most upstream of the second heating step is preferably 300° C. or lower, more preferably 200° C. or higher and 270° C. or lower, and even more preferably 210° C. or higher and 250° C. or lower. When the second heating step is carried out after carrying out the first heating step, the heating temperature in the heating furnace located most upstream of the second heating step is preferably 360° C. or lower, and more preferably 300° C. or higher and 350° C. or lower.

[0084] Furthermore, when the second heating step is performed without performing the first heating step, the temperature at which the relaxation step is started is preferably 300° C. or higher. Furthermore, when the second heating step is performed after performing the first heating step, the temperature at which the relaxation step is started is preferably 350° C. or higher. Furthermore, the temperature of each heating furnace when performing the second heating step is preferably set in the range of 300° C. or higher and 450° C. or lower.

[0085] The maximum heating temperature in the second heating step is preferably 320° C. or higher, more preferably 340° C. or higher and 450° C. or lower, even more preferably 350° C. or higher and 450° C. or lower, and particularly preferably 360° C. or higher and 420° C. In addition, when producing a polyimide film having a storage modulus of 0.3 GPa or lower at a temperature of 350° C., the maximum heating temperature in the second heating step is preferably 380° C. or lower, more preferably 360° C. or lower.

[0086] In this embodiment, at least part of the second heating step may include a step of relaxing or stretching the polyimide film precursor 10 along the width direction X by adjusting the spacing between the pair of clip chains in the width direction X while transporting the polyimide film precursor 10. This step may be performed simultaneously with the relaxation step described above.

[0087] [Preferred Aspects of the Present Embodiment] In order to stably produce a polyimide film having a more uniform degree of molecular orientation throughout the width direction, the production method of the present embodiment preferably satisfies the following condition 1, more preferably satisfies the following condition 2, and even more preferably satisfies the following condition 3. Condition 1: The maximum relaxation rate of the polyimide film precursor in the conveying direction is 4.0% or more and 15.0% or less. Condition 2: The above condition 1 is satisfied, and the spacing between the multiple clips in the conveying direction before relaxing the polyimide film precursor is 70 mm or more and 100 mm or less. Condition 3: The above condition 2 is satisfied, and the maximum heating temperature in the second heating step is 320°C or more and 450°C or less.

[0088] [Polyimide Film Obtained by the Production Method According to the Present Embodiment] In order to more effectively exhibit the effects of this embodiment, the width of the polyimide film obtained by the production method according to this embodiment (the polyimide film formed) is preferably 500 mm or more, and more preferably 1000 mm or more. Furthermore, in order to stably produce a polyimide film, the width of the polyimide film obtained by the production method according to this embodiment is preferably 3000 mm or less. The length of the polyimide film obtained by the production method according to this embodiment is, for example, 50 m or more and 20,000 m or less. Hereinafter, the polyimide film obtained by the production method according to this embodiment may be referred to as "polyimide film A."

[0089] The polyimide film A may be a single-layer polyimide film or a multi-layer polyimide film in which two or more layers containing different types of polyimides are laminated. Examples of multi-layer polyimide films include a multi-layer polyimide film in which a thermoplastic polyimide layer (adhesive layer) and a non-thermoplastic polyimide layer are laminated. In the case of a multi-layer polyimide film containing an adhesive layer, a metal foil can be attached to the adhesive layer to form a metal-clad laminate.

[0090] The thickness of the polyimide film A (in the case of a multilayer polyimide film, the total thickness of each layer) is, for example, 6 μm or more and 60 μm or less. The thinner the polyimide film A, the easier it is to reduce the weight of the resulting flexible printed circuit board. In order to make it easier to reduce the weight of the resulting flexible printed circuit board, the thickness of the polyimide film A is preferably 50 μm or less, and more preferably 25 μm or less. In order to ensure the mechanical strength of the resulting flexible printed circuit board, the thickness of the polyimide film A is preferably 7 μm or more, and more preferably 10 μm or more. The thickness of the polyimide film A can be measured using a laser hologram.

[0091] When the polyimide film A is a multilayer polyimide film including an adhesive layer, in order to easily achieve a thin flexible printed circuit board while ensuring adhesion to the metal foil, the thickness of the adhesive layer (if two adhesive layers are provided, the thickness of each adhesive layer) is preferably 1 μm or more and 15 μm or less. Furthermore, when the polyimide film A is a multilayer polyimide film including a non-thermoplastic polyimide layer and an adhesive layer, in order to easily adjust the linear expansion coefficient of the polyimide film A, the thickness ratio of the non-thermoplastic polyimide layer to the adhesive layer (thickness of the non-thermoplastic polyimide layer / thickness of the adhesive layer) is preferably 55 / 45 or more and 95 / 5 or less. When two adhesive layers are provided, the thickness of the adhesive layer when calculating the thickness ratio is the total thickness of the two adhesive layers.

[0092] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0093] <Method of measuring physical properties> First, the method of measuring the physical properties of the film will be described. When measuring the physical properties shown below, the measurement specimens (samples) used were taken from any position in the longitudinal direction of the film. For the heat shrinkage rate, storage modulus, and tear strength, the measurement specimens were taken from the center of the film in the width direction.

[0094] [Heat Shrinkage] First, two holes with a diameter of 0.1 mm were formed along the longitudinal direction (conveyance direction) of the film, spaced apart by a distance e (75 mm), and a film piece (measurement sample) with the two holes was then taken. The film piece thus taken was then heated for 30 minutes in an oven set at 350°C. The distance f between the holes in the heated film piece was then measured, and the heat shrinkage in the longitudinal direction (unit: %) was calculated according to the formula "heat shrinkage = 100 x (e - f) / e". Furthermore, two holes with a diameter of 0.1 mm were formed along the width direction of the film, spaced apart by a distance e (75 mm), and the heat shrinkage in the width direction (unit: %) was calculated using the same method as for the longitudinal direction.

[0095] [Storage modulus at 350°C] The dynamic viscoelasticity of the film was measured in an air atmosphere using a dynamic viscoelasticity measuring device (Hitachi High-Tech Science Corporation, "DMS6100"), and the correlation between the storage modulus and the measurement temperature was plotted to read the storage modulus at a measurement temperature of 350°C. The measurement conditions are as follows: Sample (film) width: 10 mm, Sample holder (gripper) spacing: 20 mm, Measurement temperature range: 0°C to 440°C, Heating rate: 3°C / min, Strain amplitude: 10 μm, Measurement frequency: 5 Hz, Minimum tension / compression force: 100 mN, Tension / compression gain: 1.5, Initial force amplitude: 100 mN

[0096] [Molecular Orientation Degree] The molecular orientation degree of the polyimide film was measured at seven measurement points (measurement points 102a, 103a, 104a, 105, 104b, 103b, and 102b) shown in Figure 3 using a molecular orientation measurement device (Oji Scientific Instruments, "MOA-6015"). Regarding the details of the measurement points, as shown in Figure 3, measurement points 102a and 102b were located 25 mm inward from end portions 101a and 101b in the width direction X of the polyimide film 100. Measurement points 103a, 104a, 105, 104b, and 103b were provided between measurement point 102a and measurement point 102b at the intervals shown in Figure 3. The straight line connecting the measurement points 102 a , 103 a , 104 a , 105 , 104 b , 103 b and 102 b was parallel to the width direction X of the polyimide film 100 .

[0097] Note that measurement points where the molecular orientation degree obtained by the molecular orientation degree measurement device is 1 have isotropic molecular orientation. Measurement points where the molecular orientation degree obtained by the molecular orientation degree measurement device is greater than 1 have in-plane anisotropy of molecular orientation, and the greater the molecular orientation degree, the greater the in-plane anisotropy of molecular orientation.

[0098] [Tear Strength] The tear strength of the film was measured in an atmosphere of 25°C (±2°C) using a tensile tester (Shimadzu Corporation's "AGS-J") according to the trouser tear test method based on JIS K7128-1. The measurement conditions are as follows: Measurement sample size: 50 mm (width direction) x 150 mm (length direction); Chuck spacing: 10 mm; Pulling speed: 200 mm / min

[0099] <Preparation of Polyamic Acid Solutions> Below, we will explain how to prepare polyamic acid solutions P1 to P3, which are non-thermoplastic polyamic acid solutions, and how to prepare polyamic acid solutions P4 and P5, which are thermoplastic polyamic acid solutions. All of the polyamic acid solutions P1 to P5 were prepared in a nitrogen atmosphere at a temperature of 23°C. Below, compounds and reagents are referred to by the following abbreviations. DMF: N,N-dimethylformamide ODA: 4,4'-diaminodiphenyl ether PDA: p-phenylenediamine BAPP: 2,2-bis[4-(4-aminophenoxy)phenyl]propane m-TB: 4,4'-diamino-2,2'-dimethylbiphenyl TPE-R: 1,3-bis(4-aminophenoxy)benzene PMDA: pyromellitic dianhydride BTDA: 3,3',4,4'-benzophenonetetracarboxylic dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride ODPA: 4,4'-oxydiphthalic anhydride

[0100] [Preparation of Polyamic Acid Solution P1] 796.7 kg of DMF, 57.0 kg of ODA, and 25.5 kg of PMDA were added to a reaction vessel and the contents of the reaction vessel were stirred. After visually confirming that the PMDA had dissolved, an additional 31.0 kg of PMDA was added to the reaction vessel and the contents of the reaction vessel were stirred until the PMDA was dissolved. Next, 26.3 kg of PMDA was added to the reaction vessel, and while stirring the contents of the reaction vessel, 23.6 kg of a pre-prepared PDA solution (solvent: DMF, amount of PDA dissolved: 10.0 kg, PDA concentration: 20.0 wt%) was added to the reaction vessel and the contents of the reaction vessel were stirred for 1 hour. Next, 23.6 kg of a pre-prepared PDA solution (solvent: DMF, amount of PDA dissolved: 10.0 kg, PDA concentration: 20.0 wt%) was added to the reaction vessel and the contents of the reaction vessel were stirred for 1 hour. Next, a previously prepared PDA solution (solvent: DMF, dissolved amount of PDA: 1.0 kg, PDA concentration: 5.0 wt %) was added to the reaction vessel for a predetermined time at an addition rate that did not cause a sudden increase in the viscosity of the reaction vessel content. When the viscosity of the reaction vessel content at a temperature of 23° C. reached 200 Pa s, the addition of the PDA solution and stirring of the reaction vessel content were stopped, thereby obtaining polyamic acid solution P1.

[0101] [Preparation of Polyamic Acid Solution P2] 822.3 kg of DMF, 13.2 kg of ODA, and 40.5 kg of BAPP were added to a reaction vessel, followed by 21.2 kg of BTDA and 17.9 kg of PMDA, and the contents of the reaction vessel were stirred. After visually confirming that the BTDA and PMDA had dissolved, 17.8 kg of PDA was added to the reaction vessel while stirring the contents of the reaction vessel, and the reaction vessel contents were stirred for 5 minutes. Next, while stirring the contents of the reaction vessel, 37.3 kg of PMDA was added to the reaction vessel and the reaction vessel contents were stirred for 30 minutes. Next, while stirring the contents of the reaction vessel, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 5.0 kg, PMDA concentration: 7.2 wt%) was continuously added to the reaction vessel for a predetermined period of time at an addition rate that did not cause a sudden increase in the viscosity of the reaction vessel contents. When the viscosity of the content of the reaction vessel reached 200 Pa·s at a temperature of 23° C., the addition of the PMDA solution and the stirring of the content of the reaction vessel were stopped, to obtain a polyamic acid solution P2.

[0102] [Preparation of Polyamic Acid Solution P3] 820.9 kg of DMF, 7.3 kg of m-TB, 10.7 kg of TPE-R, and 29.9 kg of PDA were added to the reaction vessel, followed by stirring of 60.9 kg of BPDA. After visually confirming that the BPDA had dissolved, 26.76 kg of ODPA was added to the reaction vessel while stirring the reaction vessel contents, and the reaction vessel contents were stirred for 30 minutes. Next, while stirring the reaction vessel contents, a pre-prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 5.0 kg, PMDA concentration: 7.2 wt%) was added to the reaction vessel for a predetermined period of time at an addition rate that did not cause a sudden increase in the viscosity of the reaction vessel contents. When the viscosity of the reaction vessel contents at 23°C reached 200 Pa·s, the addition of the PMDA solution and stirring of the reaction vessel contents were stopped, yielding polyamic acid solution P3.

[0103] [Preparation of Polyamic Acid Solution P4] 841.6 kg of DMF, 89.8 kg of BAPP, and 9.7 kg of BPDA were added to a reaction vessel, and the contents of the reaction vessel were stirred for 30 minutes. Next, 39.1 kg of PMDA was added to the reaction vessel while stirring the contents of the reaction vessel, and the contents of the reaction vessel were stirred for 30 minutes. Next, while stirring the contents of the reaction vessel, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 5.0 kg, PMDA concentration: 7.2 wt%) was added to the reaction vessel for a predetermined time at an addition rate that did not cause a sudden increase in the viscosity of the reaction vessel contents. Then, when the viscosity of the reaction vessel contents at a temperature of 23°C reached 50 Pa s, the addition of the PMDA solution and the stirring of the reaction vessel contents were stopped, yielding polyamic acid solution P4.

[0104] [Preparation of Polyamic Acid Solution P5] 837.5 kg of DMF, 54.6 kg of TPE-R, and 39.3 kg of BPDA were added to a reactor, and the contents of the reactor were stirred for 30 minutes. Next, 8.7 kg of PMDA was added to the reactor while stirring the contents of the reactor, and the contents of the reactor were stirred for 30 minutes. Next, 17.0 kg of m-TB and 18.6 kg of PMDA were added to the reactor while stirring the contents of the reactor, and the contents of the reactor were stirred for 30 minutes. Next, while stirring the contents of the reactor, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 5.0 kg, PMDA concentration: 7.2 wt%) was added to the reactor for a predetermined period of time at an addition rate that did not cause a sudden increase in the viscosity of the reactor contents. When the viscosity of the content of the reaction vessel reached 50 Pa·s at a temperature of 23° C., the addition of the PMDA solution and the stirring of the content of the reaction vessel were stopped, to obtain a polyamic acid solution P5.

[0105] <Preparation of Polyimide Film> Next, a method for preparing the polyimide films of Examples and Comparative Examples will be described. Each polyimide film was prepared using an apparatus that continuously forms a film while transporting a polyimide film precursor.

[0106] Example 1: A mixture of 30.4 kg of DMF, 41.0 kg of isoquinoline, and 28.6 kg of acetic anhydride was prepared as an imidization accelerator. This imidization accelerator was added to polyamic acid solution P3, and the mixture was stirred using a mixer to obtain a solution for forming a non-thermoplastic polyimide layer. The amount of imidization accelerator added was 35 parts by weight per 100 parts by weight of polyamic acid solution P3. Polyamic acid solution P5 was also prepared as a solution for forming an adhesive layer.

[0107] Next, using a three-layer coextrusion die, the non-thermoplastic polyimide layer-forming solution and the adhesive layer-forming solution were applied onto a stainless steel endless belt support by a coextrusion-casting coating method. Specifically, the non-thermoplastic polyimide layer-forming solution was first supplied to the central layer of the extrusion die, and the adhesive layer-forming solution was supplied to the two layers adjacent (on both sides) to the central layer of the extrusion die. Next, the non-thermoplastic polyimide layer-forming solution and the adhesive layer-forming solution were extruded from the lip opening of the extrusion die by a coextrusion-casting coating method to form a 2,300 mm-wide coating film (a coating film consisting of three layers: adhesive layer-forming solution / non-thermoplastic polyimide layer-forming solution / adhesive layer-forming solution) on the endless belt.

[0108] The resulting coating film was dried on an endless belt at 90°C for 60 seconds, then at 110°C for 60 seconds, and then at 160°C for 120 seconds to obtain a polyimide film precursor (precursor film). The resulting precursor film had a residual component ratio of 50% by weight. After peeling the precursor film from the endless belt, both widthwise ends were fixed to pin sheets, and the first heating step was carried out. Specifically, the precursor film with both widthwise ends fixed to the pin sheets was heated while being transported through a first hot air oven (set temperature: 280°C), a second hot air oven (set temperature: 250°C), a third hot air oven (set temperature: 250°C), a first far-infrared oven (set temperature: 200°C), and a second far-infrared oven (set temperature: 200°C) in this order, to obtain an unrelaxed precursor film. The residual component ratio of the resulting unrelaxed precursor film was 5.0% by weight. The heat shrinkage of the unrelaxed precursor film obtained was 5.5% in the machine direction (transport direction) and 4.2% in the width direction.

[0109] Next, both widthwise ends of the unrelaxed precursor film were held with multiple clips, and a second heating step was performed. Specifically, the unrelaxed precursor film held with multiple clips at both widthwise ends was heated while being transported through the first hot air oven (set temperature: 320°C), the second hot air oven (set temperature: 360°C), the third hot air oven (set temperature: 360°C), the fourth hot air oven (set temperature: 360°C), the fifth hot air oven (set temperature: 360°C), the sixth hot air oven (set temperature: 360°C), and the seventh hot air oven (set temperature: 320°C) in that order, thereby obtaining a roll-shaped multilayer polyimide film having a length of 500 m and a width of 2160 mm. In the second, third, fourth, and fifth hot air ovens, the polyimide film precursor was transported while shortening the spacing between the multiple clips in the transport direction, thereby relaxing the polyimide film precursor in the transport direction. The relaxation rates of the polyimide film precursors in each hot air oven were as shown in Table 2 below.

[0110] The obtained multilayer polyimide film (polyimide film of Example 1) was a polyimide film having a three-layer structure of a thermoplastic polyimide layer (thickness: 4.0 μm) / a non-thermoplastic polyimide layer (thickness: 17.0 μm) / a thermoplastic polyimide layer (thickness: 4.0 μm).

[0111] In the second heating step of Example 1, flat clips were used, and the spacing between the clips in the conveying direction before relaxation was adjusted to 86 mm. In addition, in the relaxation step of the second heating step of Example 1, the clip spacing adjustment mechanism described in WO 2020 / 116071 was used as the clip spacing adjustment mechanism.

[0112] [Example 2] A roll-shaped multilayer polyimide film having a length of 500 m and a width of 2160 mm was obtained in the same manner as in Example 1, except that the polyamic acid solution used in the non-thermoplastic polyimide layer-forming solution, the polyamic acid solution used in the adhesive layer-forming solution, and the conditions for the second heating step were as shown in Tables 1 and 2. The obtained multilayer polyimide film (polyimide film of Example 2) was a polyimide film having a three-layer structure of thermoplastic polyimide layer (thickness: 4.0 μm) / non-thermoplastic polyimide layer (thickness: 17.0 μm) / thermoplastic polyimide layer (thickness: 4.0 μm).

[0113] Example 3: A mixture of 32.1 kg of DMF, 7.6 kg of isoquinoline, and 60.1 kg of acetic anhydride was prepared as an imidization accelerator. This imidization accelerator was added to polyamic acid solution P1, and the mixture was stirred using a mixer to obtain a solution for forming a non-thermoplastic polyimide layer. The amount of imidization accelerator added was 50 parts by weight per 100 parts by weight of polyamic acid solution P1.

[0114] Next, using a single-layer extrusion die, the non-thermoplastic polyimide layer-forming solution was applied to a stainless steel endless belt support to form a 2300 mm-wide coating film. The resulting coating film was dried on the endless belt at 120°C for 60 seconds, then at 130°C for 60 seconds, and then at 140°C for 120 seconds to obtain a polyimide film precursor (precursor film). The resulting precursor film had a residual component ratio of 35 wt%. After peeling the precursor film from the endless belt, the first and second heating steps were performed in this order to obtain a roll-shaped polyimide film (thickness: 25.0 μm) with a length of 500 m and a width of 2160 mm. The first and second heating steps of Example 3 were performed in the same manner as Example 1, except that the conditions were as shown in Tables 1 and 2 below. The residual component ratio of the unrelaxed precursor film obtained in the first heating step of Example 3 was 3.0 wt%.

[0115] Comparative Example 1 A roll-shaped multilayer polyimide film having a length of 500 m and a width of 2160 mm was obtained in the same manner as in Example 1, except that the conditions for the first heating step were as shown in Table 1 below and that the second heating step was not performed. The obtained multilayer polyimide film (polyimide film of Comparative Example 1) was a polyimide film having a three-layer structure of a thermoplastic polyimide layer (thickness: 4.0 μm) / a non-thermoplastic polyimide layer (thickness: 17.0 μm) / a thermoplastic polyimide layer (thickness: 4.0 μm).

[0116] Comparative Example 2 A roll-shaped multilayer polyimide film having a length of 500 m and a width of 2160 mm was obtained in the same manner as in Example 2, except that the conditions for the first heating step were as shown in Table 1 below and that the second heating step was not performed. The obtained multilayer polyimide film (polyimide film of Comparative Example 2) was a polyimide film having a three-layer structure of a thermoplastic polyimide layer (thickness: 4.0 μm) / a non-thermoplastic polyimide layer (thickness: 17.0 μm) / a thermoplastic polyimide layer (thickness: 4.0 μm).

[0117] Comparative Example 3 A roll-shaped polyimide film (thickness: 25.0 μm) having a length of 500 m and a width of 2160 mm was obtained in the same manner as in Example 3, except that the conditions of the first heating step were as shown in Table 1 described below and the second heating step was not performed.

[0118] Comparative Example 4 A roll-shaped multilayer polyimide film measuring 100 m in length and 2160 mm in width was obtained by the same method as in Comparative Example 1, except that in the first heating step, the precursor film was heated in a relaxed state along the conveying direction by the method described in JP 2006-181986 A. The resulting multilayer polyimide film (polyimide film of Comparative Example 4) was a polyimide film with a three-layer structure consisting of a thermoplastic polyimide layer (thickness: 4.0 μm), a non-thermoplastic polyimide layer (thickness: 17.0 μm), and a thermoplastic polyimide layer (thickness: 4.0 μm). In the first heating step of Comparative Example 4, the relaxation rate of the precursor film in the conveying direction (more specifically, the relaxation rate calculated as the ratio of the conveying speed of the precursor film before and after fixation by the pin sheet) was adjusted to 0.89 using a film relaxation and conveyance device. In the first heating step of Comparative Example 4, the spacing between the pin sheets (pin width) was adjusted to 2200 mm, and the precursor film was fixed in a state with no slack in the width direction.

[0119] In Comparative Example 4, when the precursor membrane was heated in a relaxed state along the conveyance direction in the first heating step, breakage occurred due to notches formed at the fixing points on both ends of the precursor membrane. In contrast, in Examples 1 to 3 and Comparative Examples 1 to 3 described above, as well as Comparative Example 5 described later, no breakage problems occurred, and polyimide films 500 m in length were successfully produced.

[0120] Comparative Example 5 A roll-shaped multilayer polyimide film measuring 500 m in length and 2160 mm in width was obtained by the same method as in Comparative Example 2, except that in the first heating step, the precursor film was heated in a relaxed state along the conveying direction by the method described in JP 2006-181986 A. The resulting multilayer polyimide film (polyimide film of Comparative Example 5) was a polyimide film with a three-layer structure consisting of a thermoplastic polyimide layer (thickness: 4.0 μm), a non-thermoplastic polyimide layer (thickness: 17.0 μm), and a thermoplastic polyimide layer (thickness: 4.0 μm). In the first heating step of Comparative Example 5, the relaxation rate of the precursor film in the conveying direction (more specifically, the relaxation rate calculated as the ratio of the conveying speed of the precursor film before and after fixation by the pin sheet) was adjusted to 0.89 using a film relaxation and conveyance device. In the first heating step of Comparative Example 5, the spacing between the pin sheets (pin width) was adjusted to 2200 mm, and the precursor film was fixed in a state without slack in the width direction.

[0121] <Results> Table 1 shows the polyamic acid solutions used, whether or not a relaxation treatment was performed in the first heating step, and the heating conditions for the first heating step for Examples 1 to 3 and Comparative Examples 1 to 5. Table 2 shows the conditions for the second heating step for Examples 1 to 3. Table 3 shows the heat shrinkage of the precursor films subjected to the second heating step and the physical properties (storage modulus, degree of molecular orientation, and tear strength at a temperature of 350°C) of the resulting polyimide films for Examples 1 to 3 and Comparative Examples 1 to 5. In Table 1, a "-" in the thermoplastic polyamic acid solution column indicates that no thermoplastic polyamic acid solution was used. In Table 2, a "-" indicates that a relaxation treatment was not performed in the hot air oven. In Table 3, a "-" indicates that no measurement was performed. In Table 3, "storage modulus" refers to the storage modulus at a temperature of 350°C.

[0122]

[0123]

[0124]

[0125] As shown in Table 3, the variation in the degree of molecular orientation in Example 1, which was subjected to the specific relaxation treatment, was smaller than the variation in the degree of molecular orientation in Comparative Examples 1 and 4, which were not subjected to the specific relaxation treatment. Furthermore, the variation in the degree of molecular orientation in Example 2, which was subjected to the specific relaxation treatment, was smaller than the variation in the degree of molecular orientation in Comparative Examples 2 and 5, which were not subjected to the specific relaxation treatment. Furthermore, the variation in the degree of molecular orientation in Example 3, which was subjected to the specific relaxation treatment, was smaller than the variation in the degree of molecular orientation in Comparative Example 3, which was not subjected to the specific relaxation treatment. These results demonstrate that the present invention can produce a polyimide film having a uniform degree of molecular orientation throughout the width direction.

[0126] Furthermore, the tear strength of Example 2 was greater than that of Comparative Example 5. This result demonstrates that the present invention can suppress the decrease in tear strength of the polyimide film more effectively than the relaxation treatment using the method described in JP 2006-181986 A.

[0127] 10: Polyimide film precursor 12: Clip 100: Polyimide film

Claims

1. A method for producing a polyimide film, in which a polyimide film precursor containing polyamic acid is heated while being transported to continuously form a polyimide film, the method comprising: a step Sa in which the polyimide film precursor is held at both widthwise ends with a plurality of clips and is transported while being heated in a heating furnace; and in at least a part of the step Sa, the polyimide film precursor is relaxed in the transport direction by shortening the spacing between the plurality of clips in the transport direction while being transported.

2. The method for producing a polyimide film according to claim 1, wherein the clip is flat.

3. The method for producing a polyimide film according to claim 1, wherein the distance between the clips in the conveying direction before the polyimide film precursor is relaxed is 70 mm or more and 100 mm or less.

4. The method for producing a polyimide film according to claim 1, wherein in step Sa, two or more heating furnaces are used to heat the polyimide film precursor, and the relaxation rate of the polyimide film precursor in the transport direction is adjusted for each heating furnace.

5. The method for producing a polyimide film according to claim 4, wherein in step Sa, the heating temperature of the polyimide film precursor is adjusted for each heating furnace.

6. The method for producing a polyimide film according to claim 1, wherein the maximum relaxation rate of the polyimide film precursor in the conveying direction in step Sa is 4.0% or more.

7. The method for producing a polyimide film according to claim 1, wherein the maximum heating temperature of the polyimide film precursor in step Sa is 320°C or higher.

8. The method for producing a polyimide film according to claim 1, further comprising, before step Sa, step Sb, in which the polyimide film precursor is transported while being heated in a heating furnace, with both widthwise ends of the polyimide film precursor fixed with pin sheets.

9. The method for producing a polyimide film according to claim 1, wherein the width of the polyimide film formed is 1000 mm or more.

10. The method for producing a polyimide film according to claim 1, wherein the polyimide film precursor has a laminated structure in which two or more layers containing different types of polyamic acid are laminated.

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