Method for producing polyimide film for graphite sheet and method for producing graphite sheet

Abstract

The present invention provides a method for producing a polyimide film for a graphite sheet and a method for producing a graphite sheet using the same. The method for producing a polyimide film for a graphite sheet comprises the following steps: preparing a polyamic acid solution, adding a sublimable inorganic filler solution having a zeta potential of +30 mV to +40 mV or -40 mV to -30 mV to the polyamic acid solution to produce a polyimide film precursor composition, and then obtaining a polyimide film from the precursor composition.

Description

Technical Field

[0001] The present invention relates to a method for producing a polyimide film for a graphite sheet and a method for producing a graphite sheet, and more particularly, to a method for producing a polyimide film for a graphite sheet having excellent thermal conductivity and a method for producing a graphite sheet. Background Art

[0002] In recent years, electronic devices have become increasingly lightweight, compact, thin, and highly integrated, generating significant amounts of heat. This heat can shorten product lifespans or cause malfunctions and operational errors. Therefore, thermal management of electronic devices has become a critical issue.

[0003] Graphite sheets have higher thermal conductivity than metal sheets such as copper and aluminum, and therefore have attracted attention as heat dissipation components for electronic devices. Such graphite sheets can be produced using a variety of methods, for example, by carbonizing and graphitizing polymer films. Polyimide films, in particular, have attracted attention as polymer films for graphite sheet production due to their excellent mechanical, thermal, dimensional, and chemical stability.

[0004] It is known that the physical properties of graphite sheets produced from polyimide films are affected by the physical properties of the polyimide films themselves. Therefore, despite the development of various polyimide films for graphite sheets, there is still a demand for polyimide films that can produce graphite sheets with higher thermal conductivity. Summary of the Invention

[0005] Technical issues

[0006] The present invention provides a method for producing a polyimide film for a graphite sheet having excellent thermal conductivity and a method for producing a graphite sheet.

[0007] Solutions to Problems

[0008] 1. According to one aspect, a method for producing a polyimide film for a graphite sheet is provided. The method may include preparing a polyamic acid solution, adding a sublimable inorganic filler solution having a zeta potential of +30 mV to +40 mV or -40 mV to -30 mV to the polyamic acid solution to produce a polyimide film precursor composition, and then obtaining a polyimide film from the precursor composition.

[0009] 2. In the above 1, the polyamic acid solution is produced by reacting a diamine monomer and a dianhydride monomer in a solvent, wherein the diamine monomer may include 4,4'-diaminodiphenyl ether (4,4'-oxydianiline), 3,4'-diaminodiphenyl ether, p-phenylene diamine, m-phenylenediamine, 4,4'-methylenedianiline, 3,3'-methylenedianiline, or a combination thereof, and the dianhydride monomer may include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, oxydiphthalic anhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, or a combination thereof.

[0010] 3. In the above 1 or 2, the average particle size (D 50 ) can be 2μm to 10μm.

[0011] 4. In any one of 1 to 3 above, the sublimable inorganic filler may include calcium hydrogen phosphate, barium sulfate, calcium carbonate, or a combination thereof.

[0012] 5. In any one of the above 1 to 4, the amount of the sublimable inorganic filler added may be 0.1 to 0.3 parts by weight based on 100 parts by weight of the polyamic acid.

[0013] 6. In any of items 1 to 5 above, the precursor composition may further comprise a dehydrating agent and an imidizing agent, and the step of obtaining the polyimide film from the precursor composition may comprise forming the precursor composition on a support and drying it to produce a gel film, and then heat-treating the gel film.

[0014] 7. In any one of 1 to 6 above, the polyimide film may have a roughness (Ra) of 10 nm to 15 nm as measured according to ISO 1997.

[0015] 8. According to another aspect, a method for manufacturing a graphite sheet is provided, which may include: manufacturing a polyimide film according to any one of 1 to 7 above, and then carbonizing and graphitizing the polyimide film to obtain the graphite sheet.

[0016] 9. In the above 8, the graphite sheet may have a thickness of 20 μm to 40 μm, and a thermal conductivity of 1,400 W / m·K or higher.

[0017] Effects of the Invention

[0018] The present invention has the effect of providing a method for producing a polyimide film for a graphite sheet having excellent thermal conductivity and a method for producing a graphite sheet. DETAILED DESCRIPTION

[0019] Best Practice

[0020] In this specification, an expression in the singular includes an expression in the plural unless the context clearly indicates otherwise.

[0021] When the positional relationship of two parts is described by “on”, “upper”, “lower”, “beside”, etc., one or more other parts may exist between the two parts unless “directly” is used.

[0022] In this specification, the terms "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility of adding one or more other features or components.

[0023] When interpreting constituent elements, they should be interpreted as including a margin of error even if there is no separate explanatory note.

[0024] In this specification, “to” in “a to b” indicating a numerical range is defined as ≥a and ≤b.

[0025] According to one aspect of the present invention, a method for producing a polyimide film for a graphite sheet (hereinafter referred to as the "polyimide film production method") is provided. The method comprises the steps of preparing a polyamic acid solution, adding a sublimable inorganic filler solution having a zeta potential of +30 mV to +40 mV or -40 mV to -30 mV to the polyamic acid solution to produce a polyimide film precursor composition, and then obtaining a polyimide film from the precursor composition.

[0026] Below, each step is described in more detail.

[0027] First, a polyamic acid solution is prepared.

[0028] The polyamic acid solution can be prepared using conventional methods known in the art. For example, the polyamic acid solution can be produced by reacting a diamine monomer with a dianhydride monomer in a solvent. In this case, the types and quantities of the solvent, diamine monomer, and dianhydride monomer used are not particularly limited.

[0029] As long as the solvent can make polyamic acid dissolve, it is not particularly limited.For example, the solvent can include aprotic polar solvent (aprotic polar solvent).As the example of aprotic polar solvent, the amide solvents such as N, N'-dimethylformamide (DMF), N, N'-dimethylacetamide (DMAc), phenolic solvents such as p-chlorophenol, o-chlorophenol, N-methyl-pyrrolidone (NMP), gamma-butyrolactone (GBL), diglyme (Diglyme) etc. can be enumerated, and they can be used alone or in combination of two or more.In some cases, the auxiliary solvents such as toluene, tetrahydrofuran (THF), acetone, methyl ethyl ketone (MEK), methanol, ethanol, water can also be used to regulate the solubility of polyamic acid.

[0030] As the diamine monomer, various diamine monomers known in the art can be used without limitation, as long as they do not hinder the objectives of the present invention. For example, the diamine monomer may include 4,4'-oxydianiline (ODA), 3,4'-oxydianiline, p-phenylene diamine (PPD), m-phenylenediamine, 4,4'-methylenedianiline, 3,3'-methylenedianiline, or a combination thereof. In such cases, a polyimide film with favorable molecular orientation can be formed, thereby forming a graphite sheet with excellent thermal conductivity during carbonization and graphitization.

[0031] As the dianhydride monomer, various dianhydride monomers known in the art can be used without limitation, as long as they do not hinder the purpose of the present invention. For example, the dianhydride monomer may include pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, oxydiphthalic anhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, or a combination thereof. In such a case, a polyimide film having favorable molecular orientation can be formed, thereby forming a graphite sheet having excellent thermal conductivity during carbonization and graphitization.

[0032] The diamine monomer and the dianhydride monomer are added to the solvent in a manner such that they are substantially equimolar. Here, the term "substantially equimolar" may mean that the dianhydride monomer is contained in an amount of 99.8 mol% to 100.2 mol% based on the total molar number of the diamine monomer. As a method for reacting the diamine monomer and the dianhydride monomer in substantially equimolar amounts, for example, the following method can be mentioned:

[0033] (a) a method of adding all the diamine monomers (or dianhydride monomers) and substantially equimolar amounts of the dianhydride monomers (or diamine monomers) to a solvent to carry out the reaction;

[0034] (b) a method of adding a portion of a diamine monomer (or a dianhydride monomer) to a solvent and adding a dianhydride monomer (or a diamine monomer) at a ratio of 95 mol% to 105 mol% relative to the diamine monomer (or a dianhydride monomer), and then adding the diamine monomer and / or the dianhydride monomer in substantially equimolar amounts to carry out the reaction;

[0035] (c) A method in which a portion of a diamine monomer (or dianhydride monomer) and a portion of a dianhydride monomer (or diamine monomer) are added to a solvent so that one of the two is in excess to form a first composition, a portion of a diamine monomer (or dianhydride monomer) and a portion of a dianhydride monomer (or diamine monomer) are added to another solvent so that one of the two is in excess to form a second composition, and the first and second compositions are mixed and reacted, wherein when the diamine monomer (or dianhydride monomer) is in excess in the first composition, the dianhydride monomer (or diamine monomer) is in excess in the second composition. In (a) to (c) above, the diamine monomer and the dianhydride monomer may refer to one or more (e.g., one or two) types of diamine monomers and dianhydride monomers.

[0036] According to one embodiment, based on 100 parts by weight of polyamic acid solution, the content of polyamic acid can be 5 parts by weight to 35 parts by weight. In the above range, the polyamic acid solution can have a molecular weight and viscosity suitable for forming a film. For example, based on 100 parts by weight of polyamic acid solution, the content of polyamic acid can be 5 parts by weight to 30 parts by weight, as another example, can be 15 parts by weight to 20 parts by weight, but is not limited thereto.

[0037] According to one embodiment, the polyamic acid solution is heated at 23°C and a shear rate of 1s -1 The viscosity of the polyamic acid solution can be 100,000 cP to 500,000 cP. Within the above range, the polyamic acid can have a predetermined molecular weight and can be excellent in processability when the polyimide film is formed. Here, the "viscosity" can be measured using a rotational rheometer (HAAKE Mars Rheometer). For example, the viscosity of the polyamic acid solution at 23 ° C and a shear rate of 1s -1 The lower limit may be 150,000 cP to 450,000 cP, as another example, it may be 200,000 cP to 400,000 cP, as yet another example, it may be 250,000 cP to 350,000 cP, but is not limited thereto.

[0038] According to one embodiment, the weight average molecular weight of polyamic acid can be 100,000 g / mol to 500,000 g / mol. In the above range, it can be beneficial to manufacture a graphite sheet with more excellent thermal conductivity. Here, "weight average molecular weight" can be measured using gel permeation chromatography (GPC) and polystyrene as a standard sample. For example, the weight average molecular weight of polyamic acid can be 150,000 g / mol to 500,000 g / mol, as another example, it can be 100,000 g / mol to 400,000 g / mol, but is not limited thereto.

[0039] Next, a sublimable inorganic filler solution having a zeta potential of +30 mV to +40 mV or -40 mV to -30 mV is added to the polyamic acid solution to produce a polyimide film precursor composition.

[0040] A "sublimable inorganic filler" refers to an inorganic filler that sublimates due to heat during the carbonization and / or graphitization process during graphite sheet production. When a polyimide film contains a sublimable inorganic filler, gases generated by the sublimation of the sublimable inorganic filler can form pores in the graphite sheet during graphite sheet production. This not only facilitates the release of sublimated gases during graphite sheet production, resulting in high-quality graphite sheets, but also improves the flexibility of the graphite sheet, ultimately enhancing its handleability and formability. Examples of sublimable inorganic fillers include, but are not limited to, calcium hydrogen phosphate, barium sulfate, and calcium carbonate.

[0041] The inventors of the present invention have found that when the Zeta potential of the sublimable inorganic filler solution is controlled to be +30mV to +40mV or -40mV to -30mV and added to the polyamic acid solution to manufacture a polyimide film, the sublimable inorganic filler can have a suitable size, uniform particle size distribution and be uniformly dispersed in the polyimide film, as a result of which a graphite sheet with excellent thermal conductivity can be manufactured, thereby completing the present invention. Here, "Zeta potential" is measured using a Zeta potential measuring instrument according to ISO 13099-2 (colloidal systems-methods for zeta-potential determination-part 2:optical methods). According to one embodiment, the Zeta potential of the sublimable inorganic filler solution can be +32mV to +40mV or -40mV to -32mV. According to another embodiment, the zeta potential of the sublimable inorganic filler solution may be +35 mV to +40 mV or -35 mV to -40 mV, but is not limited thereto.

[0042] The method for controlling the zeta potential is not particularly limited, and various methods known to those skilled in the art can be used. For example, the zeta potential of a sublimable inorganic filler solution can be controlled by adding a surfactant, adding a charged polymer, or adjusting the pH of the solution containing the sublimable inorganic filler.

[0043] The sublimable inorganic filler solution may contain a solvent and a sublimable inorganic filler. The description of the solvent contained in the sublimable inorganic filler solution refers to the description of the solvent contained in the polyamic acid solution.

[0044] According to one embodiment, the average particle size (D 50 ) can be 2 μm to 10 μm. When it is within the above range, the sublimable inorganic filler can have a suitable size, uniform particle size distribution and be uniformly dispersed in the polyimide film, resulting in the production of graphite sheets with excellent thermal conductivity. Here, the "average particle size (D 50 )" can be measured by ultrasonically dispersing the sublimable inorganic filler solution at 25°C for 5 minutes using a particle size analyzer (laser diffraction particle size analyzer) (SALD-2201, Shimadzu). For example, the average particle size (D) of the sublimable inorganic filler in the sublimable inorganic filler solution is 50 ) can be 3 μm to 8 μm, and as another example, can be 4 μm to 7 μm, but is not limited thereto.

[0045] According to one embodiment, the amount of the sublimable inorganic filler added can be 0.05 to 0.3 parts by weight based on 100 parts by weight of polyamic acid. Within the above range, the sublimable inorganic filler can have a suitable size, uniform particle size distribution and be uniformly dispersed in the polyimide film, resulting in the manufacture of a graphite sheet with excellent thermal conductivity. For example, based on 100 parts by weight of polyamic acid, the amount of the sublimable inorganic filler added can be 0.10 to 0.28 parts by weight, and as another example, can be 0.12 to 0.26 parts by weight, but is not limited thereto.

[0046] Thereafter, a polyimide film is obtained from the precursor composition.

[0047] The method for obtaining the polyimide film from the precursor composition is not particularly limited, and various methods known to those skilled in the art can be used. For example, the polyimide film can be obtained by a thermal imidization method, a chemical imidization method, or a composite imidization method that combines thermal imidization and chemical imidization methods.

[0048] The thermal imidization method is a method of performing an imidization reaction only by heating without using a dehydrating agent, an imidizing agent, etc. For example, it is a method of obtaining a polyimide film by coating a precursor composition on a support, slowly heating it in a temperature range of 40°C to 400°C (for example, 40°C to 300°C) and performing a heat treatment for 1 hour to 8 hours.

[0049] The chemical imidization method is a method in which a dehydrating agent and / or an imidizing agent is applied to a precursor composition to promote the imidization of the polyamic acid.

[0050] The composite imidization method is a method of obtaining a polyimide film by adding a dehydrating agent and an imidizing agent to a precursor composition and coating the precursor composition on a support, heating the precursor composition at 80°C to 200°C (e.g., 100°C to 180°C) to activate the dehydrating agent and the imidizing agent and partially curing the precursor composition, and then heating the precursor composition at 200°C to 400°C for 5 seconds to 400 seconds.

[0051] According to one embodiment, the precursor composition may further include a dehydrating agent and an imidizing agent, and the step of obtaining the polyimide film from the above-mentioned precursor composition may include applying the above-mentioned precursor composition on a support (for example, casting) and drying to produce a gel film, and then heat-treating the above-mentioned gel film. The order of adding the sublimable inorganic filler solution, the dehydrating agent, and the imidizing agent is not particularly limited. The sublimable inorganic filler solution, the dehydrating agent, and the imidizing agent can be added to the polyamic acid solution at the same time, or the dehydrating agent and the imidizing agent can be added after adding the sublimable inorganic filler solution to the polyamic acid solution.

[0052] A "dehydrating agent" is a substance that promotes the ring-closure reaction by dehydrating the polyamic acid. Examples of dehydrating agents include aliphatic anhydrides, aromatic anhydrides, N,N'-dialkylcarbodiimides, lower aliphatic halides, halogenated lower aliphatic anhydrides, arylphosphonic acid dihalides, and thionyl halides. These can be used alone or in combination. Among these, aliphatic anhydrides such as acetic anhydride, propionic anhydride, and lactic anhydride are suitable for ease of availability and cost.

[0053] The so-called "imidizing agent" is a substance that promotes the ring closure reaction of polyamic acid. Examples of imidizing agents include aliphatic tertiary amines, aromatic tertiary amines, and heterocyclic tertiary amines. Among them, heterocyclic tertiary amines can be used from the perspective of reactivity as a catalyst. Examples of heterocyclic tertiary amines include quinoline, isoquinoline, β-picoline, pyridine, etc. These can be used alone or in combination of two or more.

[0054] The addition amount of dehydrating agent and imidizing agent is not particularly limited, and relative to 1 mole of amic acid groups in polyamic acid, the use ratio of dehydrating agent can be 0.5 mole to 7 moles (another example is 1 mole to 6 moles), and relative to 1 mole of amic acid groups in polyamic acid, the use ratio of imidizing agent can be 0.05 mole to 3 moles (another example is 0.2 mole to 2 moles). When within the above range, imidization is sufficient and can be easily cast into a film.

[0055] Examples of supports used in the gel film production process include glass plates, aluminum foil, endless stainless steel belts, and stainless steel drums. The drying temperature can be 40°C to 300°C (e.g., 80°C to 200°C), and the drying time can be 1 minute to 10 minutes (e.g., 3 minutes to 7 minutes), but are not limited thereto. The gel film is an intermediate step in the curing of polyamic acid to polyimide and can be self-supporting.

[0056] In some cases, in order to adjust the thickness and size of the finally obtained polyimide film and improve the orientation, the gel film may be further stretched. The stretching may be performed in at least one of the machine direction (MD) and the transverse direction (TD).

[0057] The heat treatment temperature of the gel film can be, for example, 50°C to 700°C, as another example, 150°C to 600°C, and as yet another example, 200°C to 600°C. The heat treatment time can be, for example, 1 minute to 10 minutes (e.g., 3 minutes to 7 minutes), but is not limited thereto. Heat treatment of the gel film can remove the solvent and the like remaining in the gel film, and most of the remaining amic acid groups can be imidized to obtain a polyimide film.

[0058] In some cases, the polyimide film thus obtained may be further cured by heating the film at a temperature of 400°C to 650°C for 5 seconds to 400 seconds. Optionally, heating may be performed under a predetermined tension to relieve any internal stress that may remain in the obtained polyimide film.

[0059] According to one embodiment, the roughness (Ra) of the polyimide film measured according to ISO 1997 may be 10 nm to 15 nm. Within this range, the thermal conductivity of a graphite sheet produced from the polyimide film may be improved, but the present invention is not limited thereto.

[0060] In the polyimide film produced by the above-described method for producing a polyimide film, the sublimable inorganic filler can have an appropriate size and uniform particle size distribution and be uniformly dispersed in the polyimide film, resulting in the production of a graphite sheet having excellent thermal conductivity.

[0061] According to another aspect, a method for producing a graphite sheet from the polyimide film is provided. The method may include the steps of producing the polyimide film according to the method, and then carbonizing and graphitizing the polyimide film to obtain the graphite sheet.

[0062] Carbonization is a process of thermally decomposing the polymer chains of the polyimide film to form primary graphite sheets containing amorphous carbon and / or amorphous carbon. For example, it may include heating the polyimide film from room temperature to a maximum temperature of 1,000°C to 1,500°C over 10 to 20 hours under reduced pressure or in an inert gas atmosphere and maintaining the temperature, but is not limited thereto. Optionally, in order to achieve high carbon orientation, pressure may be applied to the polyimide film during carbonization by means of a hot press, for example, 5 kg / cm 2 As another example, the above can be 15 kg / cm 2 As another example, the above can be 25kg / cm 2 The above, but not limited to.

[0063] Graphitization is a process of reorienting carbon in amorphous carbon and / or amorphous carbon to form graphite sheets. For example, it may include heating the primary graphite sheet from room temperature to a maximum temperature of 2,500°C to 3,000°C over 20 to 30 hours in an inert gas atmosphere and maintaining the temperature, but is not limited thereto. Optionally, to achieve high carbon orientation, pressure may be applied to the primary graphite sheet during graphitization, such as by hot pressing. The pressure may be, for example, 100 kg / cm 2 As another example, the above can be 200 kg / cm 2 As another example, the above can be 300 kg / cm 2 The above, but not limited to.

[0064] According to one embodiment, the graphite sheet can have a thickness of 20 to 40 μm (e.g., 22 to 32 μm) and a thermal conductivity of 1,400 W / m·K or greater. The graphite sheet according to one embodiment of the present invention can exhibit excellent thermal conductivity because it is manufactured using a polyimide film having a uniform particle size distribution and a sublimable inorganic filler having a suitable size. Therefore, the graphite sheet can have a thermal conductivity of 1,500 W / m·K or greater, 1,600 W / m·K or greater, 1,700 W / m·K or greater, and 1,800 W / m·K or greater, but is not limited to these.

[0065] Implementation Method

[0066] The present invention will be described in more detail below with reference to Examples, but these are provided merely as preferred examples of the present invention and are not to be construed as limiting the present invention in any way.

[0067] Example

[0068] Example 1

[0069] 15 g of pyromellitic dianhydride as a dianhydride monomer, 15 g of 4,4′-diaminodiphenyl ether as a diamine monomer, and 100 g of dimethylformamide as a solvent were mixed and reacted to prepare a polyamic acid solution having a viscosity of 300,000 cP.

[0070] The polyamic acid solution was added with calcium hydrogen phosphate (average particle size (D)) as a sublimable inorganic filler and controlled to have a zeta potential of +40 mV. 50 ): 5μm)) 10g, a sublimable inorganic filler solution of 200g of dimethylformamide as a solvent, then relative to 1 mol of the amic acid group of polyamic acid, acetic anhydride as a dehydrating agent, β-picoline as an imidizing agent are added in a 5 mol ratio and a 1 mol ratio, respectively, to manufacture a precursor composition. At this time, for every 100 parts by weight of polyamic acid in the precursor composition, the sublimable inorganic filler is 0.14 parts by weight. Here, Zeta potential is measured using a Zeta potential measuring instrument (Zeta potential and particle size analyzer (Zeta-potential & Particle size Analyzer) ELSZ-2000ZS, Otsuka Electronics (Photo OTSUKA ELECTRONICS)) according to ISO 13099-2 (Colloidal system-zeta potential determination method-Part 2: Optical method).

[0071] The precursor composition was cast onto a SUS plate (100SA, Sandvik) using a doctor blade to a thickness of 80 μm and dried at 100°C for 5 minutes to produce a gel film. The gel film was separated from the SUS plate and then heat-treated at 300°C for 5 minutes to produce a polyimide film having a thickness of 60 μm.

[0072] Examples 2 to 8 and Comparative Examples 1 to 4

[0073] A polyimide film was produced by the same method as in Example 1 except that a sublimable inorganic filler solution having the zeta potential shown in Table 1 below was used.

[0074] Evaluation Example 1

[0075] The polyimide films produced in Examples and Comparative Examples were measured for roughness (Ra) (unit: nm) using a surface roughness measuring instrument (SE600, Kosaka Laboratory Ltd.) in accordance with ISO 1997. The results are shown in Table 1 below.

[0076] [Table 1]

[0077]

[0078] As can be seen from Table 1 above, the roughness of the polyimide films of Examples 1 to 8 produced using the sublimable inorganic filler solution having the zeta potential of the present invention is lower than that of Comparative Examples 1 to 4 in which no sublimable inorganic filler solution was used. Therefore, it can be expected that the sublimable inorganic filler is more uniformly dispersed in the polyimide films of Examples 1 to 8.

[0079] Evaluation Example 2

[0080] The surfaces of the polyimide films produced in Examples 1 and 2, and Comparative Example 2, were etched using a plasma surface etching method to produce test pieces. Etching was performed using a K1050X RF Plasma Etcher (EMITECH) at 100 W for 30 minutes, with air as the etching gas. Subsequently, scanning electron microscopy (SEM) was used to photograph 10 distinct locations on the surface of the test piece at 1,000x magnification. The particle size of the sublimable inorganic filler particles was measured at each of these 10 locations.

[0081] The measurement results showed that in Example 1, among all the sublimable inorganic fillers measured, the inorganic fillers having a particle size of 2 μm or less accounted for 54%, and the inorganic fillers having a particle size of 5 μm or more accounted for 8%.

[0082] In the case of Example 2, among all the sublimable inorganic fillers measured, the inorganic fillers having a particle size of 2 μm or less accounted for 45%, and the inorganic fillers having a particle size of 5 μm or more accounted for 16%.

[0083] In the case of Comparative Example 2, among all the sublimable inorganic fillers measured, the inorganic fillers having a particle size of 2 μm or less accounted for 33%, and the inorganic fillers having a particle size of 5 μm or more accounted for 52%.

[0084] This shows that the polyimide films of Examples produced using the sublimable inorganic filler solution having the zeta potential of the present invention have a more uniform distribution of the sublimable inorganic filler than the polyimide films of Comparative Examples in which no sublimable inorganic filler solution was used.

[0085] Example 9

[0086] The polyimide film produced in Example 1 was heated to 1,500°C at a rate of 2.0°C / min under argon in an electric furnace and then held at this temperature for 1 hour to carbonize it. The carbonized polyimide film was then heated to 2,900°C at a rate of 2.5°C / min under argon and then held at this temperature for 1 hour to graphitize it, thereby producing a graphite sheet having a thickness of 30 μm.

[0087] Examples 10 to 16 and Comparative Examples 5 to 8

[0088] A graphite sheet was produced by the same method as in Example 9 except that the polyimide films described in the following Table 2 were used.

[0089] Evaluation Example 3

[0090] The graphite sheets produced in the Examples and Comparative Examples were measured for thermal diffusivity in the planar direction using a thermal diffusivity measuring apparatus (LFA 467, Netsch) by the laser flash method. The thermal conductivity was calculated by multiplying the measured thermal diffusivity values ​​by the density (weight / volume) and specific heat (specific heat measured using DSC). The results are shown in Table 2 below.

[0091] [Table 2]

[0092]

[0093] As can be seen from Table 2, the graphite sheets of Examples 9 to 16 produced from the polyimide films produced using the production method of the present invention have more excellent thermal conductivity than those of Comparative Examples 5 to 8 in which no polyimide films were used.

[0094] So far, the present invention has been described with the embodiment as the center. A person skilled in the art of the present invention should understand that the present invention can be implemented in a modified manner without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered from an illustrative point of view rather than a restrictive point of view. The scope of the present invention is represented by the scope of the claims rather than the above description, and all differences included in the equivalent scope should be interpreted as also included in the present invention.

[0095] Industrial applicability

[0096] The present invention has the effect of providing a method for producing a polyimide film for a graphite sheet having excellent thermal conductivity and a method for producing a graphite sheet.

Claims

1. A method for producing a polyimide film for a graphite sheet, comprising the following steps: Prepare polyamic acid solution, A sublimable inorganic filler solution having a zeta potential of +30 mV to +40 mV or -40 mV to -30 mV is added to the polyamic acid solution to produce a polyimide film precursor composition, and then obtaining a polyimide film from the precursor composition, in, The average particle size D of the sublimable inorganic filler in the sublimable inorganic filler solution 50 2μm to 10μm, and Wherein, based on 100 parts by weight of the polyamic acid, the added amount of the sublimable inorganic filler is 0.1 parts by weight to 0.3 parts by weight.

2. The method for producing a polyimide film for a graphite sheet according to claim 1, wherein the polyamic acid solution is produced by reacting a diamine monomer and a dianhydride monomer in a solvent. The diamine monomer comprises 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 4,4'-methylenedianiline, 3,3'-methylenedianiline or a combination thereof. The dianhydride monomer comprises pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, oxydiphthalic anhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride or a combination thereof. 3 . The method for producing a polyimide film for a graphite sheet according to claim 1 , wherein the sublimable inorganic filler comprises calcium hydrogen phosphate, barium sulfate, calcium carbonate, or a combination thereof.

4. The method for producing a polyimide film for a graphite sheet according to claim 1, wherein the precursor composition further comprises a dehydrating agent and an imidizing agent. The step of obtaining a polyimide film from the precursor composition comprises: The precursor composition is cast on a support and dried to produce a gel film, and then the gel film is subjected to a heat treatment step. 5 . The method for producing a polyimide film for a graphite sheet according to claim 1 , wherein the polyimide film has a roughness Ra of 10 nm to 15 nm as measured in accordance with ISO 1997.

6. A method for manufacturing a graphite sheet, comprising: A step of manufacturing a polyimide film according to any one of claims 1 to 5, and then carbonizing and graphitizing the polyimide film to obtain a graphite sheet. 7 . The method for producing a graphite sheet according to claim 6 , wherein the graphite sheet has a thickness of 20 μm to 40 μm and a thermal conductivity of 1,400 W / m·K or higher.

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