JPWO2019187621A1 - Graphite sheet manufacturing method and polyimide film for graphite sheet - Google Patents

Abstract

Provided are a method for producing a graphite sheet and a polyimide film for the graphite sheet for producing a graphite sheet having good thermal diffusivity and flexibility. A method for producing a graphite sheet, which comprises a step of heat-treating a polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less to 2400 ° C. or higher.

Description

The present invention relates to a method for producing a graphite sheet and a polyimide film for a graphite sheet.

Since the graphite sheet has excellent heat dissipation characteristics, it is used as a heat dissipation component in semiconductor elements and other heat generating components mounted on various electronic devices such as computers or electric devices.

Such a graphite sheet can be obtained by firing a polyimide film. For example, Patent Document 1 describes a technique for producing a graphite sheet by firing a polyimide film containing inorganic particles.

Japanese Unexamined Patent Publication No. 2014-136721

Conventionally, various graphite sheets have been known, but there is still room for improvement in order to obtain a graphite sheet having both thermal diffusivity and flexibility.

One aspect of the present invention is to provide a method for producing a graphite sheet and a polyimide film for the graphite sheet for producing a graphite sheet having good thermal diffusivity and flexibility.

As a result of diligent studies to solve the above problems, the present inventors have made a graphite sheet having both thermal diffusivity and flexibility by using a polyimide film having a phosphorus content within a predetermined range as a raw material. The present invention has been completed by finding that it can be produced. The present invention includes the following.
[1] A method for producing a graphite sheet, which comprises a step of heat-treating a polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less to 2400% by weight or more.
[2] A polyimide film for a graphite sheet having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less.

According to one aspect of the present invention, a graphite sheet having good thermal diffusivity and flexibility can be obtained.

An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the configurations described below, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments and examples can be used. Embodiments and examples obtained in appropriate combinations are also included in the technical scope of the present invention. In addition, all the academic documents and patent documents described in the present specification are incorporated as references in the present specification. Further, unless otherwise specified in the present specification, "A to B" representing a numerical range is intended to be "A or more and B or less".

<1. Graphite sheet manufacturing method>
The method for producing a graphite sheet according to one aspect of the present invention may include a step of heat-treating a polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less at 2400 ° C. or higher. ..

This production method is a so-called polymer pyrolysis method in which a polyimide film is heat-treated under an inert gas atmosphere or under reduced pressure. Specifically, the polyimide film is preheated to a temperature of about 1000 ° C. to obtain a carbonized film, and the carbonized film produced in the carbonization step is heated to a temperature of 2400 ° C. or higher to graphitize graphite. A graphite sheet is obtained through a carbonization step and a compression step of compressing the graphite sheet. The carbonization step and the graphitization step may be continuously performed, or the carbonization step may be completed and then only the graphitization step may be performed independently. Further, in the method for producing a graphite sheet according to an embodiment of the present invention, the compression step may or may not be performed.

(Carbonization process)
The carbonization step is a step of heat-treating the polyimide film to a temperature of about 1000 ° C. to carbonize the polyimide film. For example, the maximum temperature is preferably 700 ° C. to 1800 ° C., more preferably 800 ° C. to 1500 ° C., further preferably 900 ° C. to 1200 ° C., and particularly preferably 1000 ° C.

The rate of temperature rise in the carbonization step is, for example, 0.01 ° C / min or more and less than 20 ° C / min, 0.1 ° C / min to 10 ° C / min, 0.2 ° C / min to less than 5.0 ° C / min. 0.5 ° C./min to 2.0 ° C./min can be preferably exemplified. When the heating rate is within the above range, a graphite sheet having good thermal diffusivity and flexibility can be obtained.

The holding time in the carbonization step (specifically, the holding time at the maximum carbonization temperature) is preferably 1 minute to 1 hour, more preferably 5 minutes to 30 minutes, and 8 minutes to 15 minutes. Is more preferable. If the holding time is within the above range, a graphite sheet having good thermal diffusivity and flexibility can be obtained.

In the carbonization step, the laminated polyimide film in which the rectangular polyimide film is laminated may be carbonized, the roll-shaped polyimide film may be carbonized as it is in the roll shape, or the film is unwound from the roll-shaped polyimide film and carbonized. May be good.

(Graphitization process)
The graphitization step is a step of heat-treating the carbonaceous film obtained in the carbonization step to a temperature of 2400 ° C. or higher to graphitize the carbonaceous film. For example, the maximum temperature is preferably 2400 ° C. or higher, 2500 ° C. or higher, 2600 ° C. or higher, 2700 ° C. or higher, 2800 ° C. or higher, 2900 ° C. or higher, or 3000 ° C. or higher. The upper limit is not particularly limited, but is preferably 3300 ° C. or lower, and more preferably 3200 ° C. or lower. The graphitization step is carried out under reduced pressure or in an inert gas, and argon or helium is suitable as the inert gas.

The rate of temperature rise in the graphitization step is preferably 0.01 ° C./min or more and less than 20 ° C./min, more preferably 0.1 ° C./min to 10 ° C./min, and 0.5 ° C./min. It is more preferably min to 5.0 ° C./min. When the heating rate is within the above range, a graphite sheet having good thermal diffusivity and flexibility can be obtained.

The holding time in the graphitization step (specifically, the holding time at the maximum graphitization temperature) is preferably 1 minute to 1 hour, more preferably 5 minutes to 30 minutes, and 8 minutes to 30 minutes. It is more preferably 15 minutes. When the holding time is within the above range, a graphite sheet having good thermal diffusivity and flexibility can be obtained.

In the graphitization step, the laminated carbonized film in which the rectangular carbonized films are laminated may be graphitized, the roll-shaped carbonized film may be graphitized in the roll shape, or the film is unwound from the roll-shaped carbonized film to be graphitized. It may be carbonized.

(Compression process)
The carbonaceous film after graphitization may be subjected to a compression step. By applying the compression step, flexibility can be imparted to the obtained graphite sheet. As the compression step, a method of compressing in a planar shape, a method of rolling using a metal roll or the like can be used. The compression step may be carried out at room temperature or during the graphitization step.

<2. Graphite sheet ＞
Thermal diffusivity of the graphite sheet obtained by the production method, is preferably 8.0 cm ² / s or more, more preferably 8.3 cm ² / s or more, is 8.5 cm ² / s or more Is even more preferable.

Further, the flexibility of the graphite sheet is preferably "C" or higher, more preferably "B" or higher, and even more preferably "A" or higher in the flexibility evaluation of Examples described later. ..

The thickness of the graphite sheet is preferably 5 μm to 60 μm, more preferably 10 μm to 50 μm, and even more preferably 15 μm to 40 μm.

The density of the graphite sheet is ^preferably from ^{1.0g / cm 3 ~2.26g / cm 3} , more preferably from ^{1.3g / cm 3 ~2.2g / cm 3} , 1.6g / cm 3 ~2 It is more preferably .18 g / cm ³ .

<3. Polyimide film for graphite sheet>
Hereinafter, the polyimide film that can be used in one embodiment of the present invention will be described in detail. The polyimide film for a graphite sheet used in the above-mentioned production method is a polyimide film using an acid dianhydride component and a diamine component as raw materials, and contains a predetermined amount of phosphorus.

(Rin)
The polyimide film preferably has a phosphorus content of 0.025% by weight to 0.032% by weight, more preferably 0.027% by weight to 0.030% by weight. Within such a range, both the thermal diffusivity and the flexibility of the finally obtained graphite sheet are excellent.

Phosphorus can be added to the polyimide film as inorganic particles (ie, fillers). Examples of the filler that can be used in one embodiment of the present invention include CaHPO ₄ , (NH ₄ ) ₂ HPO ₄ , and Ca ₂ P ₂ O ₇ . Among them, CaHPO ₄ and (NH ₄ ) ₂ HPO ₄ , which contain phosphoric acid, have good swelling due to the gas generated when sublimating from the inside of the polyimide film, and good graphite having excellent thermal conductivity can be obtained. It can be preferably used.

(Acid dianhydride component)
The acid dianhydride components that can be used are pyromellitic acid dianhydride, 2,3,6,7, -naphthalenetetracarboxylic hydride, 3,3', 4,4'-biphenyltetracarboxylic hydride. , 1,2,5,6-naphthalenetetracarboxylic hydride, 2,2', 3,3'-biphenyltetracarboxylic hydride, 3,3', 4,4'-benzophenone tetracarboxylic acid Dianhydride, 2,2-bis (3,4-dicarboxyphenyl) propane dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, 1,1- (3,4-dicarboxyphenyl) Phenyl) ethane dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, bis (2,3) -Dicarboxyphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) methane dianhydride, oxydiphthalic acid dianhydride, bis (3,4-dicarboxyphenyl) sulfonate dianhydride, p-phenylene bis (Trimeric acid monoesteric anhydride), ethylene bis (trimellitic acid monoesteric anhydride), bisphenol A bis (trimellitic acid monoesteric anhydride) and their analogs can be mentioned. These can be mixed in any proportion. Of these, it is preferable to use pyromellitic dianhydride or 3,3', 4,4'-biphenyltetracarboxylic dianhydride. By using such an acid dianhydride component, the physical properties of the finally obtained graphite sheet become good.

(Diamine component)
The diamine components that can be used are 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenylmethane, benzene, 3,3'-dichlorobenzidine, 4,4'-diaminodiphenylsulfide, 3,3. ′ -Diaminodiphenylsulfone, 4,4′-diaminodiphenylsulfone, 3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 1,5-diaminonaphthalene, 4,4′-diaminodiphenyldiphenylsilane, 4, 4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenylN-methylamine, 4,4'-diaminodiphenylN-phenylamine, 1,3-diaminobenzene, 1 , 2-Diaminobenzene and their analogs can be mentioned. These can be mixed in any proportion. Of these, 4,4'-diaminodiphenyl ether and p-phenylenediamine are preferably used. By using such a diamine component, the physical properties of the finally obtained graphite sheet become good.

(Thickness of polyimide film)
The thickness of the polyimide film is 12.5 μm to 125 μm, preferably 25 μm to 100 μm, and more preferably 35 μm to 75 μm. If it is within the above range, the heat treatment is uniformly performed in the thickness direction, so that the heat diffusibility is improved.

(Immidization method)
The imidization method of polyimide is a thermal cure method in which the precursor polyamic acid is heated to imidize, a dehydrating agent typified by an acid anhydride such as acetic anhydride, picolin, quinoline, isoquinolin, pyridine and the like. Any of the chemical cure method of imide-converting the polyimide polyamic acid as a precursor by using an imidization accelerator typified by the tertiary amines of the above may be used. As the imidization accelerator when the chemical cure method is used, the tertiary amines mentioned above are preferable.

In particular, from the viewpoint that the obtained polyimide film has a small linear expansion coefficient, a high elastic modulus, birefringence tends to be large, and rapid graphitization is possible at a relatively low temperature, so that a high-quality graphite sheet can be obtained. Therefore, the chemical cure method is preferable. In particular, it is preferable to use a dehydrating agent and an imidization accelerator in combination because the linear expansion coefficient of the obtained polyimide film is small, the elastic modulus is large, and the birefringence can be large. Further, the chemical cure method is an industrially advantageous method having excellent productivity because the imidization reaction proceeds faster, so that the imidization reaction can be completed in a short time in the heat treatment.

(Manufacturing method of polyamic acid)
The method for producing the polyamic acid is not particularly limited, but for example, aromatic dianhydride and diamine are dissolved in an organic solvent in substantially equal molar amounts, and this organic solution is dissolved in the organic solvent to aromatic dianhydride and diamine. Polyamic acid can be produced by stirring under controlled temperature conditions until the polymerization with and is completed. The polymerization method is not particularly limited, but for example, any of the following polymerization methods (1)-(5) is preferable. In addition, in (1)-(5), the case where the aromatic tetracarboxylic dianhydride is used as the aromatic acid dianhydride and the aromatic diamine compound is used as the diamine is illustrated.

(1) A method in which an aromatic diamine compound is dissolved in an organic polar solvent, and the aromatic diamine compound is reacted with a substantially equimolar aromatic tetracarboxylic dianhydride to polymerize.

(2) Aromatic tetracarboxylic dianhydride is reacted with an aromatic diamine compound in a small molar amount in an organic polar solvent to obtain a prepolyma having an acid anhydride group at both ends. Subsequently, a method of polymerizing this prepolyma with an aromatic diamine compound which is substantially equimolar to the aromatic tetracarboxylic dianhydride.

In a specific example of the method (2) above, a prepolyamide having the acid dianhydride at both ends is synthesized using a diamine and an acid dianhydride, and the prepolyamide is the same as the diamine used for the synthesis of the prepolyma. It is similar to the method for synthesizing a polyamic acid by reacting the diamines of the above or different types of diamines. Also in the method (2), the aromatic diamine compound to be reacted with the prepolyma may be the same type of aromatic diamine compound as the aromatic diamine compound used in the synthesis of the prepolyma, or a different type of aromatic diamine compound.

(3) Aromatic tetracarboxylic dianhydride and an excess molar amount of an aromatic diamine compound are reacted with each other in an organic polar solvent to obtain a prepolyma having amino groups at both ends. Subsequently, after the aromatic diamine compound is additionally added to this prepolyma, the prepolyma and the aromatic tetracarboxylic dianhydride are prepared so that the aromatic tetracarboxylic dianhydride and the aromatic diamine compound are substantially equimolar. A method of polymerizing with.

(4) After dissolving and / or dispersing the aromatic tetracarboxylic dianhydride in an organic polar solvent, an aromatic diamine compound is added so as to be substantially equimolar to the acid dianhydride. , A method of polymerizing an aromatic tetracarboxylic dianhydride and an aromatic diamine compound.

(5) A method in which a mixture of a substantially equimolar aromatic tetracarboxylic dianhydride and an aromatic diamine compound is reacted in an organic polar solvent for polymerization.

The present invention can also have the following configuration.
[1] A method for producing a graphite sheet, which comprises a step of heat-treating a polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less to 2400% by weight or more.
[2] The method for producing a graphite sheet according to [1], wherein the polyimide film has a phosphorus content of 0.027% by weight or more and 0.030% by weight or less.
[3] A polyimide film for a graphite sheet having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less.
[4] The polyimide film for a graphite sheet according to [3], wherein the phosphorus content is 0.027% by weight or more and 0.030% by weight or less.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

<Phosphorus content of polyimide film>
The phosphorus content of the polyimide film was calculated from the ratio of the molecular weight of the phosphate used and the atomic weight of phosphorus.

<Measurement of the number of bends in the MIT bending resistance test (flexibility evaluation)>
The number of times of bending of the graphite sheet obtained by the method described later in the MIT bending resistance test was measured and used as a method for evaluating flexibility. The test method is shown below. In the MIT bending resistance test, a MIT kneading fatigue resistance tester model D manufactured by Toyo Seiki Co., Ltd. was used. The test conditions were R = 2 mm, left and right bending angle: 135 °, and spring: φ14 mm.

The number of times of bending (MIT) in the bending resistance test means that the greater the number of times, the more flexible the graphite sheet is and the more excellent the bending resistance is. Therefore, even if a graphite sheet having a large number of MITs is used for the bent portion, it is less likely to be broken.

The evaluation criteria are as follows.
A: Number of bends is 50,000 or more and B: Number of bends is 40,000 or more and less than 50,000 C: Number of bends is 30,000 or more and less than 40,000 D: Number of bends is 20,000 or more and less than 30,000 E: Number of bends is less than 20,000 < Thermal diffusivity>
The thermal diffusivity of the graphite sheet obtained by the method described later was measured by the following method. Specifically, using a thermal diffusivity measuring device (“LaserPit” manufactured by ULVAC Riko Co., Ltd.) based on the optical AC method, a sample of a graphite sheet cut into a shape of 4 mm × 40 mm was subjected to an atmosphere of 20 ° C. Was measured under AC conditions of 10 Hz.

<Method of producing polyimide film>
(Manufacturing Example 1)
In a dimethylformamide solution in which 4,4'-diaminodiphenyl ether (ODA) was dissolved, pyromellitic dianhydride (PMDA) was dissolved so that the amount of ODA and PMDA was equivalent to that of 18. A polyamic acid solution containing 5% by weight was obtained. Calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.11% by weight based on the solid content of the polyamic acid. While cooling this solution, an imidization catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and dimethylformamide was added to the carboxylic acid group contained in the polyamic acid to defoam. Next, this mixed solution was applied onto an aluminum foil so as to have a thickness of 75 μm after drying to obtain a mixed solution layer. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

The drying conditions are as follows. First, the mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to obtain a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Further, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 42 seconds, 450 ° C. for 50 seconds, and a far-infrared heater at 460 ° C. for 22 seconds. And dried. As described above, a polyimide film (A-1) having a phosphorus content of 0.025% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 2)
The same as in Production Example 1 was carried out except that calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.12% by weight based on the solid content of the polyamic acid. A polyimide film (A-2) having a phosphorus content of 0.027% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 3)
The same as in Production Example 1 except that calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.13% by weight based on the solid content of the polyamic acid. A polyimide film (A-3) having a phosphorus content of 0.030% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 4)
The same as in Production Example 1 was carried out except that calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.14% by weight based on the solid content of the polyamic acid. A polyimide film (A-4) having a phosphorus content of 0.032% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 5)
The same as in Production Example 1 was carried out except that diammonium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of diammonium hydrogen phosphate was 0.12% by weight based on the solid content of the polyamic acid. A polyimide film (A-5) having a phosphorus content of 0.028% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 6)
The same as in Production Example 1 except that calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.10% by weight based on the solid content of the polyamic acid. A polyimide film (A-6) having a phosphorus content of 0.023% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 7)
The same as in Production Example 1 except that calcium hydrogen phosphate was added to the obtained polyamic acid solution so that the concentration of calcium hydrogen phosphate was 0.15% by weight based on the solid content of the polyamic acid. A polyimide film (A-7) having a phosphorus content of 0.034% by weight and a thickness of 75 μm was prepared.

(Manufacturing Example 8)
Similar to Production Example 1 except that calcium carbonate was added to the obtained polyamic acid solution instead of calcium hydrogen phosphate so that the concentration of calcium carbonate was 0.15% by weight based on the solid content of the polyamic acid. A polyimide film (A-8) having a phosphorus content of 0% by weight and a thickness of 75 μm was prepared.

<Manufacturing method of graphite sheet>
(Example 1)
A polyimide film (A-1) having a size of 200 mm × 200 mm and a thickness of 75 μm is sandwiched between graphite sheets having a size of 220 mm × 220 mm (one polyimide film and a graphite sheet are alternately laminated), and 0.5 in a nitrogen atmosphere. After raising the temperature to 1000 ° C. at a heating rate of ° C./min, the film was carbonized by heat treatment at 1000 ° C. for 10 minutes.

Then, under reduced pressure in the temperature range of room temperature to 2200 ° C., and under an argon atmosphere in the temperature range higher than 2200 ° C., the temperature was raised to 2800 ° C. (maximum graphitization temperature) at a heating rate of 1 ° C./min, and then 2800 ° C. A graphite sheet was prepared by holding for 10 minutes. One of the obtained graphite sheets was sandwiched between PET films having a size of 200 mm × 200 mm × thickness of 400 μm, and compression treatment was carried out using a compression molding machine. The applied pressure was 10 MPa. The thickness of the graphite sheet after compression was 36 μm, and the density was 1.87 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Example 2)
The graphite sheet of Example 2 was produced in the same manner as in Example 1 except that the polyimide film (A-2) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 36 μm, and the density was 1.87 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Example 3)
The graphite sheet of Example 3 was produced in the same manner as in Example 1 except that the polyimide film (A-3) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 37 μm, and the density was 1.92 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Example 4)
The graphite sheet of Example 4 was produced in the same manner as in Example 1 except that the polyimide film (A-4) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 37 μm, and the density was 1.92 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Example 5)
The graphite sheet of Example 5 was produced in the same manner as in Example 1 except that the polyimide film (A-5) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 36 μm, and the density was 1.87 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Comparative Example 1)
A graphite sheet of Comparative Example 1 was produced in the same manner as in Example 1 except that the polyimide film (A-6) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 35 μm, and the density was 1.97 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Comparative Example 2)
A graphite sheet of Comparative Example 2 was produced in the same manner as in Example 1 except that the polyimide film (A-7) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 38 μm, and the density was 1.82 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

(Comparative Example 3)
A graphite sheet of Comparative Example 3 was produced in the same manner as in Example 1 except that the polyimide film (A-8) was used instead of the polyimide film (A-1). The thickness of the graphite sheet after compression was 34 μm, and the density was 2.03 g / cm ³ . The characteristics of the compressed graphite sheet were examined by the above-mentioned test.

Table 1 shows the production conditions and physical properties of the graphite sheets of Examples 1 to 5 and Comparative Examples 1 to 3.

According to Examples 1 to 5, it can be seen that the graphite sheet obtained from the polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less is excellent in both thermal diffusivity and flexibility. On the other hand, according to Comparative Examples 1 and 3, it can be seen that the graphite sheet obtained from the polyimide film having a phosphorus content of less than 0.025% by weight is excellent in thermal diffusivity but inferior in flexibility. Further, according to Comparative Example 2, it can be seen that the graphite sheet obtained from the polyimide film having a phosphorus content of more than 0.032% by weight is excellent in flexibility but inferior in thermal diffusivity.

The graphite sheet obtained in the present invention has good heat diffusivity and flexibility, and can be suitably used as a heat radiating member for electronic devices.

Claims (4)

A method for producing a graphite sheet, which comprises a step of heat-treating a polyimide film having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less to 2400 ° C. or higher. The method for producing a graphite sheet according to claim 1, wherein the phosphorus content of the polyimide film is 0.027% by weight or more and 0.030% by weight or less. A polyimide film for a graphite sheet having a phosphorus content of 0.025% by weight or more and 0.032% by weight or less. The polyimide film for a graphite sheet according to claim 3, wherein the phosphorus content is 0.027% by weight or more and 0.030% by weight or less.