Polyimide film

Abstract

To provide a novel polyimide film.SOLUTION: The present invention provides a polyimide film that has a water absorption of 2.2 mass% or less and is used for graphite sheets.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to polyimide films and the like. [Background technology]

[0002] In recent years, with the increasing performance of electronic devices such as personal computers and mobile phones, the amount of heat generated within these devices has risen dramatically, making efficient heat dissipation a crucial issue.

[0003] Methods of heat dissipation include methods that utilize thermal radiation (e.g., using ceramic plates), methods that utilize thermal convection (e.g., using air cooling fins), and methods that utilize thermal conduction (e.g., using thermal conductive sheets, thermal conductive resins). In methods that utilize heat conduction, for example, a solid that acts as a heat source and a solid that acts as a heat dissipation material are stacked. However, simply stacking solids (for example, metals stacked together, or metal and ceramic) does not allow for efficient heat transfer from the heat source to the heat dissipation material because air exists between the layers due to surface irregularities. Therefore, it is believed that efficient heat conduction from the heat source to the heat dissipation material can be achieved by interposing a heat conductive sheet or a heat conductive resin layer between the solids.

[0004] As such a heat-conducting sheet, graphite sheets derived from polyimide film are used. Such graphite sheets can be obtained, for example, by heat-treating (calcining) a polyimide film to convert it into graphite (Patent Documents 1 and 2, etc.). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-75211 [Patent Document 2] Japanese Patent Application Publication No. 4-21508 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The object of the present invention is to provide novel polyimide films and the like. [Means for solving the problem]

[0007] As mentioned above, polyimide film is used as a raw material for graphite sheets. However, the inventors have found that water (water absorption) contained in the polyimide film can affect the quality or performance of the graphite sheet, and that this tendency is particularly pronounced in roll-type polyimide films (polyimide film wound into a roll).

[0008] In this context, the inventors discovered that by setting the amount of water (water absorption rate) contained in the polyimide film within a predetermined range, it is possible to efficiently manufacture graphite sheets [for example, graphite sheets with little thickness variation, graphite sheets with little (or no) blistering or wrinkling], and furthermore, that such efficient manufacturing can be achieved even when using roll-shaped polyimide film. Further research led to the completion of the present invention.

[0009] In other words, the present invention relates to the following inventions, etc. [1] A polyimide film with a water absorption capacity of 2.2% by mass or less, and a polyimide film for use as a graphite sheet. [2] The polyimide film according to [1], wherein the water absorption is 1.8% by mass or less. [3] A polyimide film according to [1] or [2], wherein the polymerization components constituting the polyimide include at least an aromatic diamine component containing 4,4'-diaminodiphenyl ether and an acid anhydride component containing pyromellitic dianhydride. [4] A polyimide film according to any one of [1] to [3], which contains inorganic particles and has a maximum dispersion diameter of 15 μm or less (for example, 12 μm or less, 10 μm or less). [5] A polyimide film according to any of [1] to [4], having a thickness of 25 to 100 μm. [6] A polyimide film in roll form (film), as described in any of [1] to [5]. [7] A polyimide film in the form of a roll (film) with a length of 5m or more, as described in any of [1] to [6]. [8] A polyimide film according to any one of [1] to [7], wherein the water absorption capacity is 0.5% by mass or more. [9] A method for producing a polyimide film for graphite sheets (for example, the polyimide film described in any of [1] to [8]), comprising the step of leaving the polyimide film in a space with a humidity of 70 RH or less for 24 hours or more (humidity control, storage).

[10] A method for producing a graphite sheet, comprising the step of heat-treating a polyimide film with a water absorption rate of 2.2% by mass or less (for example, a polyimide film as described in any of [1] to [8]).

[11] The manufacturing method according to [9] or

[10] , wherein the thickness of the graphite sheet is 1.5 times or less the thickness (average thickness) of the polyimide film (over the entire area of ​​the sheet).

[12] A manufacturing method according to any one of the following [9] to

[11] , wherein the thickness of the graphite sheet is within the range of 0.95 to 1.05 times the average thickness.

[13] The method according to any one of [9] to

[12] , wherein the graphite sheet is in roll form, the ratio of the maximum thickness G1 to the minimum thickness G2 (G1 / G2) is 1.1 or less, and / or the difference between the maximum thickness G2 and the minimum thickness G1 (G1-G2) is 10 μm or less.

[13] A graphite sheet in roll form that satisfies at least one of the following conditions (1) to (3). (1) The thickness of the graphite sheet is within the range of 0.95 to 1.05 times the average thickness. (2) The ratio (G1 / G2) of the maximum thickness G1 to the minimum thickness G2 is 1.1 or less. (3) The difference (G1 - G2) between the maximum thickness G2 and the minimum thickness G1 is 10 μm or less.

Advantages of the Invention

[0010] According to the present invention, a novel polyimide film can be provided. Such a film is particularly useful as a film for a graphite sheet (for manufacturing a graphite sheet).

[0011] For example, according to the film of the present invention, a graphite sheet [for example, a graphite sheet with little thickness unevenness, a graphite sheet with few bulges, wrinkles, undulations, etc. (a flat graphite sheet), etc.] can be efficiently manufactured. Moreover, as such a film, a roll-shaped one can also be used. Therefore, according to the present invention, it is also possible to efficiently manufacture a graphite sheet having a long length and a large area.

[0012] When a roll-shaped polyimide film is heat-treated at a high temperature (for example, 2000 °C or higher), problems such as film fusion between films and film shrinkage due to thermal decomposition may affect, and problems such as large thickness unevenness, bulges, and wrinkles (furthermore, undulations) are likely to occur in the graphite sheet. However, according to the present invention, even when a roll-shaped polyimide film is used as a raw material, a graphite sheet with few (especially no) such problems can be efficiently manufactured, which is extremely useful.

Embodiments for Carrying Out the Invention

[0013] <Polyimide Film> [Polyimide] The polyimide film is composed of polyimide. Such polyimide has a diamine component and an acid anhydride component as polymerization components.

[0014] In particular, the polymerization components are usually mainly aromatic diamine components and aromatic acid anhydride components (the main components of the diamine component and the acid anhydride component (aromatic acid anhydride component), respectively).

[0015] Aromatic diamine components include, for example, paraphenylenediamine, metaphenylenediamine, paraxylylenediamine, benzidine, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, diaminodiphenylpropane (e.g., 4,4'-diaminodiphenylpropane, 3,4'-diaminodiphenylpropane, 3,3'-diaminodiphenylpropane, etc.), and diaminodiphenylmethane (e.g., 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenyl (e.g., ylmethane, 3,3'-diaminodiphenylmethane), diaminodiphenyl sulfides (e.g., 4,4'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, etc.), diaminodiphenyl sulfones (e.g., 4,4'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, etc.), 2,6-diaminopyridine, bis-(4-aminophenyl)diethylsulfate Lan, 3,3'-dichlorobenzidine, bis-(4-aminophenyl)ethylphosphinoxide, bis-(4-aminophenyl)phenylphosphinoxide, bis-(4-aminophenyl)-N-phenylamine, bis-(4-aminophenyl)-N-methylamine, 1,5-diaminonaphthalene, 3,3'-dimethyl-4,4'-diaminobiphenyl, 3,4'-dimethyl-3',4-diaminobiphenyl, 3,3'-dimethoxybenzidine, 2,4-bis(p-β-amino-t-butyl) Examples include phenyl ether, bis(p-β-amino-t-butylphenyl) ether, p-bis(2-methyl-4-aminopentyl)benzene, p-bis-(1,1-dimethyl-5-aminopentyl)benzene, m-xylylenediamine, p-xylylenediamine, 2,5-diamino-1,3,4-oxadiazole, 2,2-bis(4-aminophenyl)hexafluoropropane, N-(3-aminophenyl)-4-aminobenzamide, 4-aminophenyl-3-aminobenzoate, etc.

[0016] Aromatic diamine components may be used individually or in combination of two or more types.

[0017] The aromatic diamine component may typically include at least paraphenylenediamine and / or 4,4'-diaminodiphenyl ether [in particular, it may be included as a main component (main component of polymerization components, main component of aromatic diamine components) (for example, it may be included in a proportion of 50 mol% or more of the aromatic diamine component)].

[0018] If the aromatic diamine component includes paraphenylenediamine and / or 4,4'-diaminodiphenyl ether, the proportion of paraphenylenediamine and / or 4,4'-diaminodiphenyl ether in the total aromatic diamine component (and even the diamine component) may be selected from a range of, for example, 10 mol% or more (e.g., 20 mol% or more, 25 mol% or more, 30 mol% or more, 40 mol% or more, etc.), and in particular, 50 mol% or more (e.g., 55 mol% or more, 60 mol% or more, 65 mol% or more, 70 mol% or more, 75 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 100 mol%, etc.).

[0019] Furthermore, the diamine component may consist solely of aromatic diamine components, and may also contain other diamine components (other diamine components) as long as aromatic diamine components constitute the main component. The proportion of aromatic diamine components in the total diamine component may be 50 mol% or more (for example, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 100 mol%, etc.).

[0020] Aromatic acid anhydride components (acid anhydride components) include, for example, pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,3,6,7-naphthalenedicarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl) ether, pyridine-2,3,5,6-tetracarboxylic acid dianhydride, naphthalenetetracarboxylic acid dianhydride (for example, 1,2,4,5-naphthalenetetracarboxylic acid dianhydride, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride, 1,4,5,8-decahydronaphthalenetetracarboxylic acid dianhydride, 4,8-dimethyl-1,2,5,6-hexahydronaphthalenetetracarboxylic acid dianhydride, 2,6-dichloro-1,4, Examples include 5,8-naphthalenetetracarboxylic dianhydride, 2,7-dichloro-1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-tetrachloro-1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,8,9,10-phenanthrenetetracarboxylic dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, benzene-1,2,3,4-tetracarboxylic dianhydride, and 3,4,3',4'-benzophenonetetracarboxylic dianhydride.

[0021] Aromatic acid anhydride components may be used individually or in combination of two or more types.

[0022] The aromatic acid anhydride component may typically include at least pyromellitic dianhydride and / or 3,3',4,4'-biphenyltetracarboxylic acid dianhydride [in particular, it may be included as a main component (main component of polymerization components, main component of aromatic acid anhydride components) (for example, it may be included in a proportion of 50 mol% or more of the aromatic acid anhydride components)].

[0023] If the aromatic acid anhydride component includes pyromellitic dianhydride and / or 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, the proportion of pyromellitic dianhydride and / or 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in the total aromatic acid anhydride component (and even the acid anhydride component) may be selected from a range of, for example, 10 mol% or more (e.g., 20 mol% or more, 25 mol% or more, 30 mol% or more, 40 mol% or more, etc.), and in particular, 50 mol% or more (e.g., 55 mol% or more, 60 mol% or more, 65 mol% or more, 70 mol% or more, 75 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 100 mol%, etc.).

[0024] Furthermore, the acid anhydride component may consist solely of aromatic acid anhydride components, and may also contain other acid anhydride components (other acid anhydride components) as long as aromatic acid anhydride components constitute the main component. The proportion of aromatic acid anhydride components in the total acid anhydride component may be 50 mol% or more (for example, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 100 mol%, etc.).

[0025] [Inorganic particles] Polyimide films may typically contain inorganic particles. In this case, the inorganic particles are usually dispersed in the polyimide film (contained in a dispersed form).

[0026] Examples of inorganic particles include oxides {e.g., SiO2 (silica), TiO2 (titanium(IV) oxide), etc.} and inorganic salts {e.g., phosphate (hydrogen) salts such as CaHPO4 (calcium hydrogen phosphate), CaPO4 (calcium phosphate), Ca2P2O7 (calcium diphosphate), etc., and carbonates such as CaCO3 (calcium carbonate)}. In particular, when CaHPO4 containing phosphoric acid is used, good foaming (swelling) occurs due to the gas generated when it sublimes from inside the polyimide film, resulting in a good graphite sheet with excellent thermal conductivity, etc. Therefore, it is especially preferable to use CaHPO4 as the main component (for example, 50 to 100% by mass, preferably 70 to 100% by mass of the total inorganic particles).

[0027] The average particle size of the inorganic particles (inorganic particles used for dispersion) may be, for example, 0.3 to 3.5 μm (e.g., 0.5 to 3 μm), preferably 0.5 to 2.5 μm (e.g., 0.5 to 2.2 μm), and may also be 0.5 to 2.0 μm (e.g., 0.5 to 1.9 μm), 1 to 3.5 μm (e.g., 1.5 to 3.0 μm, 2.0 to 2.5 μm), etc.

[0028] With such particle size, it becomes easier to efficiently obtain a better quality graphite sheet when combined with the water absorption capacity, etc., as described later.

[0029] Furthermore, such a particle size is preferable because it reduces the aggregation of inorganic particles in the polyimide film and ensures uniform foaming of inorganic particles by sublimation gas during the firing of the polyimide film.

[0030] More specifically, a moderately large particle size is preferable from the viewpoint of improving properties such as the slipperiness of the polyimide film and the flexibility and tensile strength of the graphite sheet. In addition, a moderate particle size that is not too large is preferable from the viewpoint of suppressing the aggregation of inorganic particles in the polyimide film, which in turn suppresses partial excessive foaming caused by the sublimation gas of inorganic particles during firing, and also reduces surface protrusion defects on the graphite sheet.

[0031] Regarding the particle size distribution of inorganic particles (inorganic particles used for dispersion), a narrow distribution is preferable, meaning that inorganic particles of similar size make up a high proportion of the total inorganic particles. Specifically, it is preferable that inorganic particles of a specific particle size (e.g., 0.5-2.5 μm, 1-3.5 μm, etc.) account for 80% or more (e.g., 80-100% by volume) of the total inorganic particles.

[0032] With this particle size distribution, when combined with the water absorption rate and other factors described later, it becomes easier to efficiently obtain a higher quality graphite sheet.

[0033] Furthermore, such a particle size distribution is preferable from the viewpoint of achieving good properties such as the slipperiness of the film, the flexibility of the graphite sheet, and the tensile strength.

[0034] Furthermore, the particle size (average particle size) of the inorganic particles (inorganic particles used for dispersion) may be the volume diameter (volume average diameter). Also, the method for measuring the particle size (particle size distribution) of the inorganic particles is not particularly limited, and for example, the method described in the examples below may be used.

[0035] In a polyimide film, the proportion (amount added) of inorganic particles may be, for example, 0.03 parts by mass or more (for example, 0.04 parts by mass or more), preferably 0.05 parts by mass or more, per 100 parts by mass of polyimide (solid content of polyimide resin).

[0036] The upper limit of the proportion (amount added) of inorganic particles may be, for example, 1 part by mass or less (for example, 0.9 parts by mass or less), preferably 0.8 parts by mass or less (for example, 0.7 parts by mass or less), and more preferably 0.6 parts by mass or less (for example, 0.58 parts by mass or less, 0.55 parts by mass or less, 0.52 parts by mass or less, 0.4 parts by mass or less, 0.3 parts by mass or less, 0.2 parts by mass or less), per 100 parts by mass of polyimide (solid content of polyimide resin).

[0037] Furthermore, these upper and lower limits may be combined as appropriate to set a suitable range (for example, 0.05 to 1 part by mass) (the same applies to the others).

[0038] Typically, in a polyimide film, the proportion (amount added) of inorganic particles may be 0.03 to 0.8 parts by mass, 0.04 to 0.8 parts by mass, 0.05 to 0.8 parts by mass, 0.05 to 0.52 parts by mass, 0.05 to 0.3 parts by mass, 0.1 to 0.2 parts by mass, etc., per 100 parts by mass of polyimide (solid content of polyimide resin).

[0039] With this ratio, when combined with the water absorption amount and other factors described later, it becomes easier to efficiently obtain a higher quality graphite sheet. Furthermore, including inorganic particles in a sufficient proportion is preferable because it increases the amount of gas generated when the inorganic particles in the polyimide film sublimate from within the film, and the resulting graphite sheet obtained by rolling exhibits excellent properties such as flexibility and tensile strength.

[0040] Furthermore, by including an appropriate amount of inorganic particles, not too much, aggregation of inorganic particles in the polyimide film is easily suppressed. This is preferable because it can suppress localized excessive foaming caused by sublimation gas of inorganic particles during the firing of the polyimide film, and can also reduce surface protrusion defects on the graphite sheet.

[0041] In a polyimide film (a polyimide film containing inorganic particles), the maximum dispersion diameter (maximum dispersed particle diameter) of the inorganic particles may be relatively small. For example, the maximum dispersion diameter (maximum dispersed particle diameter) of inorganic particles in a polyimide film may be, for example, 15 μm or less (e.g., 1 to 15 μm, 3 to 15 μm), preferably 13 μm or less (e.g., 1 to 13 μm, 3 to 12 μm), and more preferably 11 μm or less (e.g., 1 to 11 μm, 3 to 11 μm), or 10 μm or less (e.g., 2 to 10 μm, 5 to 10 μm), etc.

[0042] With such a maximum dispersion diameter, it becomes easier to efficiently obtain a higher quality graphite sheet when combined with factors such as water absorption.

[0043] Furthermore, by using a dispersion diameter that is not too large, it is preferable from the viewpoint that the swelling of the polyimide film during graphitization is good without causing partial excessive foaming (swelling) due to the sublimation gas of inorganic particles, resulting in fewer surface protrusion defects in the graphitized sheet and a superior appearance.

[0044] Furthermore, the dispersion diameter of inorganic particles refers to the particle size of dispersed inorganic particles in a polyimide film, and includes not only the primary particle size but also the secondary particle size (particle size of aggregated particles). In other words, the maximum dispersion diameter of inorganic particles refers to the maximum particle size of inorganic particles in the polyimide film.

[0045] The method for measuring the dispersion diameter of inorganic particles is not particularly limited, and the method described in the examples below may be used.

[0046] The thickness of the polyimide film (e.g., average thickness) can be appropriately selected depending on the application, but is, for example, 10 μm or more (e.g., 20 μm or more), and preferably 25 μm or more (e.g., 30 μm or more, 40 μm or more, or 50 μm or more).

[0047] The upper limit of the thickness of the polyimide film is not particularly limited, but may be, for example, 200 μm or less, 180 μm or less, 150 μm or less, 100 μm or less, 90 μm or less, 80 μm or less, etc.

[0048] The thickness of the polyimide film may typically be 10-150 μm, 25-100 μm, 30-95 μm, 40-90 μm, 50-85 μm, 60-80 μm, etc.

[0049] With this thickness, it becomes easier to efficiently obtain a higher quality graphite sheet when combined with factors such as water absorption.

[0050] Using a polyimide film of sufficient thickness is preferable from the viewpoint that a flexible graphite sheet can be obtained after firing the polyimide film. Furthermore, using a polyimide film that is not too thick is preferable from the viewpoint that, due to its high gas permeability, good quality foaming occurs in the graphite sheet, and there are fewer surface protrusion defects on the graphitized sheet, resulting in a superior appearance.

[0051] The polyimide film may have a relatively large size. The length of such a polyimide film may be, for example, 1 m or more (e.g., 5 m or more), 10 m or more (e.g., 20 m or more), preferably 30 m or more (e.g., 40 m or more), more preferably 50 m or more (e.g., 100 m or more), and may also be 200 m or more, 300 m or more, 500 m or more, 1000 m or more, 2000 m or more, 3000 m or more, 5000 m or more, and so on.

[0052] The width of the polyimide film is not particularly limited, but may be, for example, 30 mm or more (e.g., 45 mm or more, 50 mm or more, 75 mm or more, 100 mm or more), preferably 150 mm or more (e.g., 155 mm or more), more preferably 200 mm or more (e.g., 250 mm or more), and may also be 500 mm or more, 1000 mm or more, 1500 mm or more, 2000 mm or more, etc.

[0053] Specific widths for polyimide films include, for example, 50-1000mm, 75-500mm, and 100-300mm.

[0054] The polyimide film may be in a laminated state, or in particular in a wound state [roll form (roll), for example, a roll wound in a relatively large size (length) as described above].

[0055] "Rolled" refers to a state in which polyimide film is wound around an inner core or similar object. While there are no restrictions on the shape of the inner core, it is generally cylindrical.

[0056] The material for the inner core is not particularly limited depending on the application of the polyimide film, but if it is subsequently heat-treated to obtain a graphite sheet, it is necessary to withstand continuous use in high-temperature environments (e.g., 2000°C or higher), and carbon material is one example.

[0057] The core diameter may be selected from a range of, for example, 20 mm or more, preferably 30 mm or more, preferably 50 mm or more, and even more preferably 70 mm or more. The upper limit of the core diameter may be, for example, 500 mm or less, 400 mm or less, 300 mm or less, 250 mm or less, 200 mm or less, 150 mm or less, 120 mm or less, etc.

[0058] Specific examples of core diameters include 30-400mm, 50-200mm, and 70-120mm.

[0059] With this core diameter, even in a rolled form, it is easier to efficiently obtain a good quality graphite sheet when combined with factors such as water absorption capacity.

[0060] By making the inner core diameter moderately large, it is preferable to prevent the resulting graphite sheet from developing a curling tendency and to reduce the likelihood of warping. Furthermore, by keeping the inner core diameter not too large, it is preferable to be able to increase the amount of polyimide film per unit volume (and thus increase the amount of graphitization treatment).

[0061] In a roll of film, the number of turns may be, for example, 10 or more, preferably 50 or more, and more preferably 100 or more. The upper limit of the number of turns is not particularly limited and may be, for example, 3000 or less, 2000 or less, 1000 or less, etc.

[0062] The polyimide film of the present invention typically has a specific water absorption rate. The water absorption rate of such a polyimide film may be selected from a range of, for example, 2.5% by mass or less (e.g., 2.4% by mass or less, 2.3% by mass or less), preferably 2.2% by mass or less (e.g., 2.1% by mass or less), preferably 2% by mass or less (e.g., 1.9% by mass or less), and more preferably 1.8% by mass or less, and can also be 1.7% by mass or less, 1.5% by mass or less, 1.3% by mass or less, etc.

[0063] The lower limit of the water absorption of the polyimide film may be 0 mass% (or the detection limit), or it may be a finite value (for example, 0.01 mass%, 0.05 mass%, 0.1 mass%, 0.2 mass%, 0.3 mass%, 0.4 mass%, 0.5 mass%, 0.6 mass%, 0.7 mass%, 0.8 mass%, 0.9 mass%, 1 mass%, 1.1 mass%, 1.2 mass%, 1.3 mass%, etc.).

[0064] With such a water absorption rate, it is easy to efficiently obtain high-quality graphite sheets (for example, graphite sheets with minimal thickness variations, and graphite sheets with minimal blistering, wrinkles, or waviness). In particular, with roll-shaped films, water absorption, along with graphitization, tends to cause blistering and variations in thickness. However, by using the water absorption amount described above, such blistering and variations in thickness can be effectively suppressed.

[0065] Furthermore, as described above, in order to obtain such a graphite sheet, it is not necessarily required to reduce the water absorption rate to the absolute minimum. Rather, a slight water absorption rate is preferable, as it eliminates the need for excessive water absorption adjustment treatments (e.g., drying treatments) and allows for more efficient production of the graphite sheet (for example, the film shrinks somewhat when moisture is removed during heating, creating a space between the films, which facilitates degassing during subsequent graphitization).

[0066] The water absorption rate may be the average value of the entire film, a pseudo-average value (for example, the arithmetic mean of the maximum water absorption rate and the minimum water absorption rate), or the maximum water absorption rate, or the entire film may satisfy the above water absorption rate. For example, if there is variation in the water absorption rate of the film, the maximum value may be used as the water absorption rate, or the arithmetic mean of the maximum value (maximum water absorption rate A1) and the minimum value (minimum water absorption rate A2) [(A1+A2)÷2] may be used as the water absorption rate [for example, in a roll of film, the arithmetic mean of the maximum water absorption rate (for example, the water absorption rate of the innermost part (closest to the core)) or the water absorption rate of the maximum water absorption rate and the minimum water absorption rate (for example, the water absorption rate of the outermost layer (the part furthest from the core)) may satisfy the above water absorption rate].

[0067] The polyimide film may be subjected to various treatments (e.g., stretching, annealing, and other treatments described later). In a stretched polyimide film, the stretching ratio may be, for example, 1.01 times or more (e.g., 1.01 to 1.90 times, 1.05 to 1.60 times, 1.10 to 1.50 times). Stretching may be performed in the length direction (running direction, longitudinal direction of the film) and / or in the width direction, or in both directions.

[0068] [Method for manufacturing polyimide film] Next, the method for producing the polyimide film of the present invention will be described. To obtain a polyimide film, a polyamic acid solution (hereinafter also referred to as a polyamic acid solution) is first obtained by polymerizing the polymerization components (components including aromatic diamine components and aromatic acid anhydride components) in an organic solvent.

[0069] Specific examples of organic solvents used in the formation of polyamic acid solutions include sulfoxide solvents such as dimethyl sulfoxide and diethyl sulfoxide; formamide solvents such as N,N-dimethylformamide and N,N-diethylformamide; acetamide solvents such as N,N-dimethylacetamide and N,N-diethylacetamide; pyrrolidone solvents such as N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone; phenolic solvents such as phenol, o-,m-, or p-cresol, xylenol, halogenated phenol, and catechol; and aprotic polar solvents such as hexamethylphosphoramide and γ-butyrolactone. These can be used individually or in combination of two or more. Furthermore, they may be used in combination with aromatic hydrocarbons such as xylene and toluene.

[0070] The polymerization method for the polyamic acid solution may be any known method, for example, (1) A method of polymerization in which the entire amount of the diamine component (aromatic diamine component) is first placed in the solvent, and then the acid anhydride component (aromatic acid anhydride component) is added in an amount equivalent to the total amount of the diamine component (aromatic diamine component), (2) A method in which the entire amount of the acid anhydride component (aromatic acid anhydride component) is first placed in the solvent, and then the diamine component (aromatic diamine component) is added in an equivalent amount to the acid anhydride component (aromatic acid anhydride component) to polymerize. (3) A method of polymerization in which one diamine component (aromatic diamine component) is placed in a solvent, and then mixed for the time required for the reaction in a ratio in which one acid anhydride component (aromatic acid anhydride component) is 95 to 105 mol% relative to the reaction components, the other aromatic diamine component is added, and then the other acid anhydride component (aromatic acid anhydride component) is added so that the total diamine component (aromatic diamine component) and the total acid anhydride component (aromatic acid anhydride component) are approximately equivalent in amount. (4) A method of polymerization in which one acid anhydride component (aromatic acid anhydride component) is placed in a solvent, one diamine component (aromatic diamine component) is mixed for the time required for the reaction in a ratio of 95 to 105 mol% relative to the reaction components, the other acid anhydride component (aromatic acid anhydride component) is added, and then the other diamine component (aromatic diamine component) is added so that the total diamine component (aromatic diamine component) and the total acid anhydride component (aromatic acid anhydride component) are approximately equivalent in amount. (5) Prepare polyamic acid solution (A) by reacting one diamine component (aromatic diamine component) and an acid anhydride component (aromatic acid anhydride component) in a solvent such that one is in excess, and prepare polyamic acid solution (B) by reacting the other diamine component (aromatic diamine component) and an acid anhydride component (aromatic acid anhydride component) in a separate solvent such that one is in excess. Mix the resulting polyamic acid solutions (A) and (B) to complete the polymerization. When preparing polyamic acid solution (A), if there is an excess of diamine component (aromatic diamine component), polyamic acid solution (B) may contain an excess of acid anhydride component (aromatic acid anhydride component). Conversely, if there is an excess of acid anhydride component (aromatic acid anhydride component) in polyamic acid solution (A), polyamic acid solution (B) may contain an excess of diamine component (aromatic diamine component). By mixing polyamic acid solutions (A) and (B), the total amount of diamine component (aromatic diamine component) and total amount of acid anhydride component (aromatic acid anhydride component) used in these reactions may be adjusted to be approximately equivalent.

[0071] The polymerization method is not limited to these methods, and other known methods may also be used.

[0072] The acid anhydride component (aromatic acid anhydride component) and the diamine component (aromatic diamine component) that constitute the polyamic acid are polymerized in a ratio in which the number of moles of each is approximately equal, but one may be added in excess of the other within the range of 10 mol%, preferably 5 mol%.

[0073] The polymerization reaction is preferably carried out in an organic solvent with stirring. The polymerization temperature is not particularly limited, but is usually carried out at an internal temperature of 0 to 80°C in the reaction solution. The polymerization time is not particularly limited, but is preferably carried out continuously for 10 minutes to 30 hours. The polymerization reaction may be divided into stages or the temperature may be raised or lowered as needed. There are no particular restrictions on the order in which the two reactants are added, but it is preferable to add the aromatic acid anhydride to the solution of the aromatic diamine component. Vacuum degassing during the polymerization reaction is an effective method for producing a high-quality organic solvent solution of polyamic acid. Furthermore, the polymerization reaction may be controlled by adding a small amount of end-capping agent to the aromatic diamines before the polymerization reaction. The end-capping agent is not particularly limited, and known ones can be used.

[0074] The resulting polyamic acid solution typically contains 5-40% by weight, preferably 10-30% by weight, of solids. Its viscosity is not particularly limited, but the measurement using a Brookfield viscometer is typically 10-2000 Pa·s, and preferably 100-1000 Pa·s for stable liquid delivery. The polyamic acid in the organic solvent solution may be partially imidized.

[0075] Furthermore, polyamic acid solutions typically contain inorganic particles. To obtain a polyamic acid solution containing inorganic particles, the inorganic particles may be added to a pre-polymerized polyamic acid solution, or the polyamic acid solution may be polymerized in the presence of inorganic particles.

[0076] Inorganic particles are preferably used as a slurry (inorganic particle slurry) dispersed in a solvent (for example, polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone) because this prevents aggregation. Because the particle size of this slurry is very small, it has a slow and stable sedimentation rate. Furthermore, even if sedimentation occurs, it can be easily redispersed by re-stirring.

[0077] The method for producing the inorganic particle slurry is not particularly limited and may follow conventionally known methods. Examples of methods for producing the inorganic particle slurry include mixing inorganic particles and a solvent using a mixer. It is preferable to use a mixer with high shear force, such as a high-speed disperser, homomixer, ball mill, coreless mixer, or agitated disperser. Alternatively, wet grinding may be performed to reduce the average particle size. For wet grinding, for example, a bead mill or sand mill can be used.

[0078] As the inorganic particle slurry, a commercially available product in which inorganic particles are pre-dispersed in a solvent may be used. Furthermore, the inorganic particle slurry may contain other organic solvents or compounding agents as needed.

[0079] The concentration of inorganic particles in the inorganic particle slurry is not particularly limited, but is, for example, 1 to 80% by weight, preferably 1 to 60% by weight, and more preferably 1 to 40% by weight.

[0080] Furthermore, filtering the inorganic particle slurry through a filter having a predetermined pore size (for example, a cut filter with a pore size of 15 μm or less, preferably 13 μm or less, more preferably 11 μm or less, even more preferably 5 μm or less, and particularly preferably 3 μm or less) is preferable from the viewpoint that it can suppress aggregation of inorganic particles and remove inorganic particles of a predetermined particle size (for example, inorganic particles of 15 μm or more) from the polyimide film.

[0081] The material of the filter is not particularly limited and includes, for example, polymer materials (e.g., polyethylene, polypropylene, polytetrafluoroethylene, etc.) and metals (e.g., stainless steel, etc.).

[0082] The amount of inorganic particles added should be appropriately selected to correspond to the desired content in the polyimide film (for example, so that when the polyimide is formed, the amount is 0.03 to 0.8 parts by mass per 100 parts by mass of polyimide).

[0083] Methods for producing polyimide films include, for example, a method in which a polyamic acid solution is cast into a film and thermally decyclized and desolvated to obtain a polyimide film, and a method in which a cyclization catalyst and a dehydrating agent are mixed with a polyamic acid solution, chemically decyclized to produce a gel film, and then heated and desolvated to obtain a polyimide film. The latter method is preferable because it can keep the coefficient of thermal expansion of the resulting polyimide film low, improve the height distribution in the direction of the film surface, and result in a graphite sheet with excellent thermal conductivity and the ability to increase the thickness.

[0084] In the method of chemical decyclization, the above polyamic acid solution is first prepared. The above polyamic acid solution may contain a cyclization catalyst (imidization catalyst), a dehydrating agent, a gelation retarder, etc.

[0085] Specific examples of cyclization catalysts include aliphatic tertiary amines such as trimethylamine and triethylenediamine, aromatic tertiary amines such as dimethylaniline, and heterocyclic tertiary amines such as isoquinoline, pyridine, and β-picoline. These can be used individually or in combination of two or more. Among these, the use of at least one heterocyclic tertiary amine is preferred.

[0086] Specific examples of dehydrating agents include aliphatic carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, and butyric anhydride, and aromatic carboxylic acid anhydrides such as benzoic anhydride, but acetic anhydride and / or benzoic anhydride are preferred among these.

[0087] One method for producing a polyimide film from a polyamic acid solution is to cast a polyamic acid solution containing a cyclization catalyst and a dehydrating agent onto a support using a slit-type die or the like to form a film, partially allow imidization to proceed on the support to form a self-supporting gel film, and then peel it off the support and heat treat it to obtain a polyimide film.

[0088] The above polyamic acid solution is cast onto a heated support, undergoes a thermal ring-closing reaction on the support, and is peeled off from the support as a self-supporting gel film.

[0089] The above-mentioned support is a metal rotating drum or an endless belt, and its temperature is controlled by a liquid or gaseous heat transfer medium and / or radiant heat such as an electric heater.

[0090] The gel film described above is heated to a temperature of typically 30 to 200°C, preferably 40 to 150°C, by heat received from the support and / or from a heat source such as hot air or an electric heater, causing a ring-closing reaction. The released volatile components such as organic solvents are dried, allowing the gel film to become self-supporting and be peeled off the support.

[0091] The gel film peeled from the support is typically stretched in the direction of travel while its travel speed is controlled by a rotating roll. The stretching is usually carried out at a temperature of 140°C or lower at a magnification of 1.01 to 1.90 times, preferably 1.05 to 1.60 times, and more preferably 1.10 to 1.50 times. The gel film stretched in the direction of travel is then introduced into a tenter device, for example, where both ends in the width direction are gripped by tenter clips, and it is stretched in the width direction while traveling together with the tenter clips.

[0092] The stretched gel film described above is typically heated for 15 seconds to 30 minutes using air, an infrared heater, etc. Then, it is heat-treated with hot air and / or an electric heater, etc., at a temperature of typically 250 to 550°C for 15 seconds to 30 minutes.

[0093] Furthermore, the thickness of the polyimide film can be adjusted by controlling the travel speed.

[0094] The polyimide film obtained in this manner may be further annealed. The method of annealing is not particularly limited and may follow conventional methods. The temperature of the annealing treatment is not particularly limited, but is preferably 200 to 500°C, more preferably 200 to 370°C, and particularly preferably 210 to 350°C. Specifically, it is preferable to run the film under low tension in a furnace heated to the above temperature range to perform the annealing treatment. The time the film remains in the furnace becomes the treatment time, which is controlled by changing the running speed, and is preferably 5 seconds to 5 minutes. The film tension during running is preferably 10 to 50 N / m, and more preferably 20 to 30 N / m.

[0095] The polyimide film may be wound up (or rolled) as described above. The winding method is not particularly limited, and conventional methods [for example, a method of continuously winding a gel film (after stretching or annealing)] can be used. The roll configuration (length of the film wound, core material, core diameter, etc.) is as described above.

[0096] As described above, the polyimide film of the present invention has a specific water absorption capacity. Such a film can be used as is for any application (especially in the manufacture of graphite sheets) without any special treatment, provided that it satisfies the specific water absorption capacity. However, from the viewpoint of being able to stably adjust the water absorption capacity, some kind of treatment is usually performed to obtain a polyimide film with a specific water absorption capacity.

[0097] The method (technique) for achieving such a specific amount of water absorption is not particularly limited, but it may also be a method (process) of leaving the polyimide film in a space with predetermined humidity conditions (storage, humidity control), as it is easy to adjust to the predetermined amount of water absorption efficiently (and even easily and stably).

[0098] In this method, the humidity may be 80 RH% or less (e.g., 75 RH% or less), preferably 70 RH% or less (e.g., 65 RH% or less), or 60 RH% or less (55 RH% or less, 50 RH% or less, 45 RH% or less, 40 RH% or less), depending on the amount of water absorbed by the polyimide film before standing, the desired amount of water absorbed, etc.

[0099] The storage (humidity control) time depends on the amount of water absorbed by the polyimide film before storage, the storage time, the desired amount of water absorbed, the form of the film [whether it is in roll form or not, the size (length) of the film, etc.], but may be, for example, 3 hours or more (e.g., 6 hours or more), preferably 8 hours or more (e.g., 12 hours or more), and even more preferably 18 hours or more (e.g., 20 hours or more), or 24 hours or more (e.g., 30 hours or more, 36 hours or more, 48 hours or more, 60 hours or more, 72 hours or more), etc.

[0100] <Applications of polyimide film and graphite sheets> The polyimide film of the present invention is particularly suitable for use in graphite sheets and the like.

[0101] Graphite sheets can be obtained by firing (and graphitizing) a polyimide film (through a firing process).

[0102] Furthermore, as mentioned above, if the polyimide film is in roll form, it may be fired while still in roll form. In this case, the graphite sheet will also be in roll form.

[0103] The firing of the polyimide film preferably includes a pre-firing step (I) and a main firing step (II). The main firing step (II) is usually performed after the pre-firing step (I).

[0104] The final firing temperature in the pre-firing process (I) does not need to be higher than the firing temperature in the main firing process (II), for example, it may be around 900 to 1500°C.

[0105] The heating rate in the pre-firing process (I) is preferably 1 to 15°C / min. Within this range, the density is 0.3 to 1.5 g / cm³. 3 This is preferable from the standpoint of easily obtaining graphite sheets.

[0106] Furthermore, in the pre-firing process (I), after raising the temperature to the final firing temperature, it may be held at the final firing temperature. The holding time may be, for example, 30 minutes to 3 hours.

[0107] The firing temperature in the main firing process (II) is, for example, 2000°C or higher (e.g., 2000 to 3500°C), preferably 2400°C or higher (e.g., 2400 to 3500°C), and more preferably 2600°C or higher (e.g., 2600 to 3500°C).

[0108] Furthermore, the final firing temperature in the main firing process (II) is preferably 2700°C or higher (for example, 2700 to 3500°C), more preferably 2800°C or higher (for example, 2800 to 3500°C), and even more preferably around 2900°C (for example, 2900 to 3200°C). If the firing temperature is 3500°C or lower, the heat resistance degradation of the firing furnace is small, allowing for long-term production. If the maximum firing temperature is 2000°C or higher, the resulting graphite sheets tend to be both flexible and strong.

[0109] The heating rate in the main firing process (II) is preferably 1 to 15°C / min. Within this range, a sufficient density (for example, 0.3 to 1.5 g / cm³) is achieved. 3 This is preferable from the viewpoint that it is easier to obtain graphite sheets.

[0110] Furthermore, in the main firing process (II), after raising the temperature to the final firing temperature, it may be held at the final firing temperature. The holding time may be, for example, 30 minutes to 3 hours.

[0111] The pre-sintering process (I) and the main sintering process (II) are usually carried out in an inert gas. The inert gas is not particularly limited and may include helium, argon, nitrogen, etc., but argon is preferred.

[0112] Furthermore, the graphite sheet after firing may be rolled by sandwiching it between rolling rollers or the like, if necessary. Rolling can improve the thermal conductivity of the graphite sheet after firing. The rolling method is not particularly limited and may be carried out according to conventionally known methods.

[0113] A graphite sheet can be obtained as described above. Therefore, the present invention also includes such a graphite sheet. Such a graphite sheet is usually a sheet manufactured by the above method (a sheet manufactured using the polyimide film), but it may not be otherwise.

[0114] The graphite sheets may be in a laminated state, or in particular, in a wound state [roll shape (roll), for example, a roll wound in a relatively large size (length) as described above].

[0115] "Rolled" refers to a state in which graphite sheets are wound around an inner core or similar structure. While there are no restrictions on the shape of the inner core, it is generally cylindrical.

[0116] Considering the need for the inner core material to withstand continuous use in high-temperature environments (e.g., over 2000°C), carbon fiber material is a suitable choice.

[0117] The core diameter may be selected from a range of, for example, 20 mm or more, preferably 30 mm or more, preferably 50 mm or more, and even more preferably 70 mm or more. The upper limit of the core diameter may be, for example, 500 mm or less, 400 mm or less, 300 mm or less, 250 mm or less, 200 mm or less, 150 mm or less, 120 mm or less, etc.

[0118] Specific examples of core diameters include 30-400mm, 50-200mm, and 70-120mm.

[0119] In a roll of graphite sheet, the number of turns (layers) may be, for example, 10 or more, preferably 50 or more, and more preferably 100 or more. The upper limit of the number of turns is not particularly limited and may be, for example, 3000 or less, 2000 or less, 1000 or less, etc.

[0120] The thickness of the graphite sheet depends on the thickness of the polyimide film used as the raw material, but is, for example, 10 μm or more (e.g., 20 μm or more), and preferably 25 μm or more (e.g., 30 μm or more, 40 μm or more, or 50 μm or more).

[0121] The upper limit of the thickness of the graphite sheet is not particularly limited, but may be, for example, 500 μm or less, 300 μm or less, 200 μm or less, 180 μm or less, 150 μm or less, 100 μm or less, 90 μm or less, 80 μm or less, etc.

[0122] The thickness of the graphite sheet can typically be 10-150 μm, 25-100 μm, 30-95 μm, 40-90 μm, 50-85 μm, 60-80 μm, etc.

[0123] The thickness of graphite sheets may vary depending on their length, whether they are in roll form, etc.

[0124] Therefore, the thickness of the graphite sheet may be the average value of the entire sheet, a pseudo-average value (for example, the arithmetic mean of the maximum and minimum thicknesses), the maximum thickness, or the thickness may be satisfied throughout the entire sheet (it may be the thickness of the entire sheet or the entire area).

[0125] For example, in the case of a sheet, if there is variation in thickness, the maximum value (maximum thickness G1) may be used as the thickness, or the arithmetic mean of the maximum value (maximum thickness G1) and the minimum value (minimum thickness G2) [(G1 + G2) ÷ 2] may be used as the thickness. [For example, in the case of a roll of sheet, the maximum thickness G1 (for example, the thickness of the innermost part (closest to the core)) or the arithmetic mean of the maximum thickness G1 and the minimum thickness G2 (for example, the thickness of the outermost layer (the part furthest from the core)) may satisfy the above thickness requirement.]

[0126] As mentioned above, graphite sheets may have variations in thickness, but the present invention makes it possible to efficiently provide (manufacture) graphite sheets with small variations (unevenness) in thickness.

[0127] For example, the thickness of the graphite sheet may be within the range of 0.95 to 1.05 times (e.g., 0.96 to 1.04, 0.97 to 1.03, 0.98 to 1.02) the average thickness of the graphite sheet (or the arithmetic mean of the maximum thickness G1 and the minimum thickness G2) across the entire sheet.

[0128] In addition, the ratio of the maximum thickness G1 to the minimum thickness G2 of the graphite sheet (G1 / G2) may be, for example, 1.1 or less (for example, 1.09 or less, 1.08 or less, 1.07 or less, 1.06 or less, 1.05 or less, 1.04 or less).

[0129] Furthermore, the difference between the maximum thickness G1 and the minimum thickness G2 of the graphite sheet (G1-G2) may be, for example, 10 μm or less (for example, 9 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less).

[0130] Furthermore, since blistering and the like can be efficiently suppressed in the present invention, it is easier to efficiently achieve the target thickness. For example, the thickness of the graphite sheet in the entire sheet [or the maximum thickness G1 and / or minimum thickness G2 of the graphite sheet] may be 1.6 times or less (for example, 1.55 times or less, 1.5 times or less, 1.45 times or less, 1.4 times or less, 1.38 times or less, 1.35 times or less, 1.32 times or less, 1.3 times or less, 1.28 times or less, 1.25 times or less, 1.22 times or less, 1.2 times or less, etc.) the thickness of the polyimide film (the polyimide film used to manufacture the graphite sheet).

[0131] The length of the graphite sheet also depends on the polyimide film used as the raw material. For example, the length of the graphite sheet may be relatively large, for example, 1 m or more (e.g., 5 m or more), 10 m or more (e.g., 20 m or more), preferably 30 m or more (e.g., 40 m or more), more preferably 50 m or more (e.g., 100 m or more), and may also be 200 m or more, 300 m or more, 500 m or more, 1000 m or more, 2000 m or more, 3000 m or more, 5000 m or more, etc.

[0132] The width of the graphite sheet also depends on the polyimide film used as the raw material, but may be, for example, 30 mm or more (e.g., 45 mm or more, 50 mm or more, 75 mm or more, 100 mm or more), preferably 150 mm or more (e.g., 155 mm or more), and even more preferably 200 mm or more (e.g., 250 mm or more), and may also be 500 mm or more, 1000 mm or more, 1500 mm or more, 2000 mm or more, etc.

[0133] Specific graphite sheet widths include, for example, 50-1000mm, 75-500mm, and 100-300mm.

[0134] The density of graphite sheets is, for example, 0.2 to 1.6 g / cm³. 3 Preferably 0.3 to 1.5 g / cm³ 3 This is also acceptable. 0.2 g / cm³ 3The above is preferable from the viewpoint of preventing delamination of the graphite sheet. Also, 1.6 g / cm³ 3 The following conditions are preferable from the viewpoint of having excellent flexibility in the graphite sheet, which makes it less likely for voids to form between the graphite sheet and solids when laminated with solids such as heat sources and heat dissipation components, thereby reducing the thermal resistance of the laminated surface.

[0135] The thermal diffusivity of a graphite sheet is, for example, 7 cm 2 / s or more (for example, 7-15cm) 2 / s) is also acceptable.

[0136] The breaking strength of the graphite sheet is, for example, 10 MPa or more (e.g., 10 to 200 MPa), preferably 15 MPa or more (e.g., 15 to 200 MPa), and more preferably 20 MPa or more (e.g., 30 to 200 MPa).

[0137] The applications of graphite sheets are not particularly limited, and they may be used, for example, as heat dissipation components [for example, heat dissipation components for small portable electronic devices (e.g., smartphones)]. [Examples]

[0138] The present invention will now be described in more detail based on examples, but the present invention is not limited to such examples.

[0139] [Evaluation of polyimide film thickness and graphite sheet thickness] Measurements were taken using a Mitutoyo Lightmatic (Series 318). The thickness of the graphite sheet was measured by selecting five evenly spaced points in the width direction, including 1 cm from both ends, on both the surface and inner core sheets, cutting out 1 cm squares, and evaluating the average value.

[0140] [Water absorption capacity of polyimide film] The mass (W1) of the polyimide film was measured, and then the mass (W0) of the polyimide film after heat treatment at 150 °C for 1 hour to dehydrate it was measured, and it was determined by the following formula. Water absorption (%) = (W1 - W0) / W0 × 100 The water absorption of the polyimide film was measured by cutting out a 6 cm square from the central part in the width direction of the surface layer and inner core side films, respectively.

[0141] [Evaluation of Inorganic Particles Added to Polyimide Film] Using a laser diffraction / scattering particle size distribution measuring device LA-910 manufactured by Horiba, Ltd., a sample dispersed in a polar solvent was measured, and from the results of analyzing the laser diffraction and scattered light intensity patterns, as the volume average diameter, the particle size range, the average particle diameter, and the occupancy rate of particles with a particle diameter of 1.0 to 3.5 μm in all particles were read.

[0142] [Dispersion Diameter of Inorganic Particles in Polyimide Film] Using a microscope VHX-2000 manufactured by Keyence Corporation, the dispersed particle diameter of inorganic particles present in a 50 cm 2 polyimide film was observed in transmission mode, and the number of particles with a dispersed particle diameter of 10 μm or more, the maximum dispersed particle diameter, and the number thereof were read.

[0143] [Evaluation of the Amount of Inorganic Particles in Polyimide Film] Using a fluorescence X-ray S2 Ranger manufactured by Bruker AXS GmbH, the addition amount of inorganic particles (calcium hydrogen phosphate) was evaluated from the energy amount of the Kα line of the component (phosphorus) contained in the inorganic particles (calcium hydrogen phosphate).

[0144] [Protrusion Defects on the Surface of Graphite Sheet] Using a microscope VHX-2000 manufactured by Keyence Corporation, the surface of a 1 m 2 graphite sheet was observed, and the number of protrusion defects with a size of Φ (diameter) 0.1 mm or more was evaluated.

[0145] (Example 1) A polyimide film (stretched film) with a thickness of 60 μm, a width of 250 mm, and a length of 200 m was manufactured by conventional methods. This film contained polymer components (100 mol% 4,4'-diaminodiphenyl ether, 100 mol% pyromellitic dianhydride), inorganic particles for dispersion (calcium hydrogen phosphate, average particle diameter 2.2 μm, 81.5 volume of inorganic particles between 1.0 and 3.5 μm), and inorganic particles (calcium hydrogen phosphate, maximum dispersion diameter 9.2 μm, number of particles with a dispersion particle diameter of 10 μm or more = 0) in a ratio of 0.1 parts by mass per 100 parts by mass of polyimide. This film was then wound onto a carbon fiber core with an outer diameter of 75 mm and a width of 300 mm (665 turns).

[0146] Two sets of these were prepared and each was subjected to humidity control for 48 hours in a room adjusted to 50 RH. From one of the sets, samples were taken from the surface polyimide film and the polyimide film in contact with the inner core, and the amount of water absorbed was measured. The surface side (minimum water absorption) was 1.5 mass%, and the inner core side (maximum water absorption) was 1.6 mass%.

[0147] The other batch was placed in a firing furnace, heated to 1000°C at a rate of 5°C / min in argon gas and held for 1 hour (pre-firing process), and then heated to 2800°C at a rate of 8°C / min and held for 1 hour (main firing process) to produce graphite, and a graphite sheet was obtained.

[0148] The obtained graphite sheets had thicknesses of 68 μm on the surface side (minimum thickness G2) (1.13 times the thickness of the polyimide film, 0.98 times the average thickness (arithmetic mean of the surface side (minimum) thickness G2 and the inner core side (maximum) thickness G1)) and 71 μm on the inner core side (maximum thickness G1) (1.18 times the thickness of the polyimide film, 1.02 times the average thickness). Both sides were flat graphite sheets (G1 / G2=1.04, G1-G2=3 μm) without blistering, wrinkles, or waviness. Furthermore, the number of protrusion defects with a diameter of 0.1 mm or larger in the graphite sheets was 0 per square meter. 2 That was the case.

[0149] (Example 2) The procedure was the same as in Example 1, except that the room was humidified for 48 hours in a room adjusted to 60 RH. The water absorption of the obtained polyimide film was 1.6% by mass on the surface side (minimum water absorption) and 1.7% by mass on the inner core side (maximum water absorption). The thickness of the obtained graphite sheet was 70 μm on the surface side (minimum thickness G2) (1.17 times the thickness of the polyimide film, 0.97 times the average thickness) and 74 μm on the inner core side (maximum thickness G1) (1.23 times the thickness of the polyimide film, 1.03 times the average thickness). Both sides were flat graphite sheets (G1 / G2=1.04, G1-G2=4 μm) without blistering, wrinkles, or waviness. In addition, the number of protrusion defects with a diameter of 0.1 mm or larger in the graphite sheet was 0 per m 2 That was the case.

[0150] (Example 3) A polyimide film (stretched film) with a thickness of 68 μm, a width of 250 mm, and a length of 200 m was manufactured by conventional methods. This film contained polymer components (100 mol% 4,4'-diaminodiphenyl ether, 100 mol% pyromellitic dianhydride), inorganic particles for dispersion (calcium hydrogen phosphate, average particle diameter 1.0 μm, 90% by volume of inorganic particles between 1.0 and 3.5 μm), and inorganic particles (calcium hydrogen phosphate, maximum dispersion diameter 9.9 μm, number of particles with a dispersion particle diameter of 10 μm or more = 0) in a ratio of 0.5 parts by mass per 100 parts by mass of polyimide. This film was then wound onto a carbon fiber core with an outer diameter of 75 mm and a width of 300 mm (660 turns).

[0151] The obtained polyimide film had a water absorption of 1.6% by mass on the surface side (minimum water absorption) and 1.7% by mass on the inner core side (maximum water absorption). The obtained graphite sheet had a thickness of 88 μm on the surface side (minimum thickness G2) (1.29 times the thickness of the polyimide film, 0.97 times the average thickness) and 93 μm on the inner core side (maximum thickness G1) (1.37 times the thickness of the polyimide film, 1.02 times the average thickness). Both sides were flat graphite sheets (G1 / G2=1.06, G1-G2=5 μm) without blistering, wrinkles, or waviness. Furthermore, the number of protrusion defects with a diameter of 0.1 mm or larger in the graphite sheet was 0 per m 2 That was the case.

[0152] (Comparative Example 1) The procedure was the same as in Example 1, except that the room was humidified for 48 hours in a room adjusted to 90 RH. The water absorption of the obtained polyimide film was 2.6% by mass on the surface side (minimum water absorption) and 2.7% by mass on the inner core side (maximum water absorption). The thickness of the obtained graphite sheet was 92 μm on the surface side (minimum thickness G2) (1.53 times the thickness of the polyimide film, 0.94 times the average thickness) and 103 μm on the inner core side (maximum thickness G1) (1.72 times the thickness of the polyimide film, 1.06 times the average thickness). Blisters and wrinkles were observed on both sides, and were particularly pronounced on the inner core side (G1 / G2=1.12, G1-G2=11 μm). [Industrial applicability]

[0153] The present invention provides a polyimide film useful for applications such as graphite sheets.

Claims

1. A polyimide film for use as a graphite sheet, having a water absorption of 2.2% by mass or less, containing inorganic particles, and having a maximum dispersion diameter of inorganic particles of 15 μm or less.

2. A polyimide film having a water absorption capacity of 0.1 to 1.8% by mass, wherein the polymerization components constituting the polyimide include at least an aromatic diamine component containing 4,4'-diaminodiphenyl ether and an acid anhydride component in which pyromellitic dianhydride accounts for 60 mol% or more of the total acid anhydride components, and is a polyimide film for graphite sheets.

3. The water absorption capacity is 0.1 to 1.8% by mass. The polymerization components constituting the polyimide include at least an aromatic diamine component containing 4,4'-diaminodiphenyl ether and an acid anhydride component in which the proportion of pyromellitic dianhydride in the total acid anhydride components is 60 mol% or more. A polyimide film containing inorganic particles, wherein the maximum dispersion diameter of the inorganic particles is 15 μm or less, and which is a polyimide film for use as a graphite sheet.

4. A polyimide film according to any one of claims 1 to 3, wherein the water absorption amount is 0.5 to 1.8% by mass.

5. The polyimide film according to claim 1, wherein the polymerization components constituting the polyimide include at least an aromatic diamine component containing 4,4'-diaminodiphenyl ether and an acid anhydride component containing pyromellitic dianhydride.

6. A polyimide film according to any one of claims 1 to 5, comprising inorganic particles in a ratio of 0.03 to 1 part by mass per 100 parts by mass of polyimide, wherein the maximum dispersion diameter of the inorganic particles is 1 to 15 μm.

7. A polyimide film according to any one of claims 1 to 6, having a thickness of 25 to 100 μm.

8. A polyimide film according to any one of claims 1 to 7, which is in the form of a roll.

9. A polyimide film according to any one of claims 1 to 8, which is in the form of a roll with a length of 50 m or more.

10. A polyimide film according to any one of claims 1 to 9, wherein the water absorption amount is 1 to 1.8% by mass.

11. A method for producing a polyimide film according to any one of claims 1 to 10, comprising the step of leaving the polyimide film in a space with a humidity of 70 RH or less for 24 hours or more.

12. A method for producing a graphite sheet, comprising the step of heat-treating a polyimide film according to any one of claims 1 to 10.

13. The manufacturing method according to claim 12, wherein the thickness of the graphite sheet is 1.5 times or less the thickness of the polyimide film.

14. The manufacturing method according to claim 12 or 13, wherein the thickness of the graphite sheet is within the range of 0.95 to 1.05 times the average thickness.

15. The method according to any one of claims 12 to 14, wherein the graphite sheet is in roll form, the ratio of the maximum thickness G1 to the minimum thickness G2 (G1 / G2) is 1.1 or less, and / or the difference between the maximum thickness G1 and the minimum thickness G2 (G1-G2) is 10 μm or less.