METHOD FOR PRODUCING GRAPHITE FILM

Abstract

A method of producing a graphite film has an excellent appearance and excellent thermal diffusivity. A graphite film production method includes the steps of: preparing a polyimide film having a heating loss rate X of 0.13% to 10%, which heating loss rate X is represented by: heating loss rate X=(ba)/a . . . Formula (1); and graphitizing the polyimide film by subjecting the polyimide film to a heat treatment. Where (i) a represents a mass of the polyimide film after the polyimide film is heated at 400 C. for 15 minutes and (ii) b represents a mass of the polyimide film after the polyimide film is heated at 150 C. for 15 minutes.

Claims

1. A method of producing a graphite film, comprising the steps of: preparing a polyimide film having a heating loss rate X of 0.13% to 10%, which heating loss rate X is represented by Formula (1) below; and graphitizing the polyimide film by subjecting the polyimide film to a heat treatment,
Heating loss rate X=(ba)/a Formula (1) where (i) a represents a mass of the polyimide film after the polyimide film is heated at 400 C. for 15 minutes and (ii) b represents a mass of the polyimide film after the polyimide film is heated at 150 C. for 15 minutes.

2. A method of producing a graphite film, comprising the steps of: preparing a polyimide film having a thermal shrinkage rate of not less than 0.30% after being heated at 400 C. for 15 minutes; and graphitizing the polyimide film by subjecting the polyimide film to a heat treatment.

3. The method as set forth in claim 1, wherein the polyimide film contains: a dianhydride containing a pyromellitic dianhydride; and diamine containing at least one of 4,4-oxydianiline and paraphenylenediamine.

4. The method as set forth in claim 3, wherein: the polyimide film contains 4,4-oxydianiline and paraphenylenediamine in an amount of not less than 90% relative to an entirety of the diamine; and a ratio between an amount of the 4,4-oxydianiline and an amount of the paraphenylenediamine are contained is 100:0 to 70:30.

5. The method as set forth in claim 3, wherein: the polyimide film contains 4,4-oxydianiline and paraphenylenediamine in an amount of not less than 90% relative to an entirety of the diamine; and a ratio between an amount of the 4,4-oxydianiline and an amount of the paraphenylenediamine are contained is 100:0 to 80:20.

6. The method as set forth in claim 1, wherein: the step of graphitizing the polyimide film includes the steps of carbonizing the polyimide film so as to obtain a carbonized film and further heating the carbonized film at a high temperature; and a heating rate during the carbonizing is not more than 5 C./min.

7. The method as set forth in claim 2, wherein the polyimide film contains: a dianhydride containing a pyromellitic dianhydride; and diamine containing at least one of 4,4-oxydianiline and paraphenylenediamine.

8. The method as set forth in claim 7, wherein: the polyimide film contains 4,4-oxydianiline and paraphenylenediamine in an amount of not less than 90% relative to an entirety of the diamine; and a ratio between an amount of the 4,4-oxydianiline and an amount of the paraphenylenediamine are contained is 100:0 to 70:30.

9. The method as set forth in claim 7, wherein: the polyimide film contains 4,4-oxydianiline and paraphenylenediamine in an amount of not less than 90% relative to an entirety of the diamine; and a ratio between an amount of the 4,4-oxydianiline and an amount of the paraphenylenediamine are contained is 100:0 to 80:20.

10. The method as set forth in claim 2, wherein: the step of graphitizing the polyimide film includes the steps of carbonizing the polyimide film so as to obtain a carbonized film and further heating the carbonized film at a high temperature; and a heating rate during the carbonizing is not more than 5 C./min.

Description

EXAMPLES

[0101] The following description will discuss the present invention in further detail through Examples. Note, however that the present invention is not limited to these Examples.

[0102] (Method of Measuring Heating Loss Rate of Polyimide Film)

[0103] The heating loss rate was measured as follows. The following are prepared: (i) a polyimide film which was cut into a 5 cm5 cm piece and (ii) an aluminum container having an opening which was large enough for the polyimide film to pass through. The polyimide film was placed in the aluminum container, and then the aluminum container was placed in an oven at 150 C. 15 minutes later, the aluminum container was taken out. Then, the polyimide film was cooled to room temperature, and then the mass of the polyimide film was measured with use of an electronic balance. This mass was regarded as the mass of the polyimide film after the polyimide film was heated at 150 C. for 15 minutes. Next, the oven was heated to 400 C., and then the aluminum container, in which the polyimide film was placed, was placed in the oven, 15 minutes later, the aluminum container was taken out. Then, the polyimide film was cooled to room temperature, and then the mass of the polyimide film was measured with use of an electronic balance. This mass was regarded as the mass of the polyimide film after the polyimide film was heated at 400 C. for 15 minutes.

[0104] The heating loss rate X is represented by the following Formula (1):

Heating loss rate X=(ba)/a Formula (1)

[0105] where (i) a represents a mass of the polyimide film after the polyimide film is heated at 400 C. for 15 minutes and (ii) b represents a mass of the polyimide film after the polyimide film is heated at 150 C. for 15 minutes.

[0106] (Method of Measuring Thermal Shrinkage Rate of Polyimide Film)

[0107] The thermal shrinkage rate was measured as follows. A polyimide film which was cut into a 200 mm200 mm piece was prepared, and then the dimensions of the polyimide film in a machine direction (MD) and in a transverse direction (TD) were measured. Next, the polyimide film was heated at 400 C. for 15 minutes, and then cooled to room temperature. Then, the dimensions in the machine direction (MD) and in the transverse direction (TD) were measured again. Respective change rates in the machine direction (MD) and in the transverse direction (TD) were calculated, and an average of the change rates was regarded as a thermal shrinkage rate of the film.

[0108] (Appearance of Graphite Film)

[0109] An appearance of a graphite film was evaluated as follows. The number of seeds and surface-peeled parts, which were visually observable within a 5 cm5 cm area, were counted, and the appearance was considered as Excellent if the number was 0, Good if the number was 1 to 5, Unsatisfactory if the number was 6 to 20, and Poor if the number was 21 or more.

[0110] (Thermal Diffusivity of Graphite Film)

[0111] The thermal diffusivity of the graphite film was measured as follows. That is, a sample having a size of 4 mm40 mm was cut out from a center part of the graphite film. A thermal diffusivity of the sample was then measured in an atmosphere of 23 C. and at 10 Hz with use of a thermal diffusivity measurement device (Laser Pit manufactured by ULVAC-RIKO, which employs the light alternating-current method.

Example 1

[0112] <Production of Polyimide Film>

[0113] A polyamic acid solution was synthesized by (i) adding 4,4-oxydianiline (ODA) at a proportion of 75 mol %, paraphenylenediamine (PDA) at a proportion of 25 mol %, and pyromellitic dianhydride (PMDA) at a proportion of 100 mol %, to N,N-dimethylformamide (DMF) which is an organic solvent for polymerization and (ii) stirring a resulting product so as to polymerize the resulting product. In so doing, the polyamic acid solution was synthesized so that a solid content concentration in the polyamic acid solution to be obtained would be 18.5% by mass.

[0114] Then, acetic anhydride and isoquinoline were added to the polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0115] This gel film was fired in stages (in a first heating furnace (hot air) at 250 C., in a second heating furnace (hot air) at 300 C., in a third heating furnace (hot air) at 340 C., and in a fourth heating furnace (far-infrared) at 400 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 0.24%, and the thermal shrinkage rate was 0.58%.

[0116] <Production of Graphite Film>

[0117] The polyimide film thus prepared was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 25 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 0 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 2

[0118] A polyimide film and a graphite film were produced as in Example 1 except that the conditions under which the polyimide film was dried were changed as follows.

[0119] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0120] This gel film was fired in stages (in a first heating furnace at 250 C., in a second heating furnace at 300 C., and in a third heating furnace at 450 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 1.48%, and the thermal shrinkage rate was 0.75%. Then, the graphite film (having a thickness of 25 m) was obtained as in Example 1.

Example 3

[0121] A polyimide film and a graphite film were produced as in Example 1 except that the conditions under which the polyimide film was dried were changed as follows.

[0122] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0123] This gel film was fired in stages (in a first heating furnace at 250 C., in a second heating furnace at 300 C., in a third heating furnace at 340 C., and in a fourth heating furnace at 350 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 3.09%, and the thermal shrinkage rate was 0.90%. Then, the graphite film (having a thickness of 25 m) was obtained as in Example 1.

Example 4

[0124] A polyimide film and a graphite film were produced as in Example 1 except that the conditions under which the polyimide film was dried were changed as follows.

[0125] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0126] This gel film was fired in stages (in a first heating furnace at 270 C., in a second heating furnace at 340 C., in a third heating furnace at 370 C., and in a fourth heating furnace at 400 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 0.15%, and the thermal shrinkage rate was 0.50%. Then, the graphite film (having a thickness of 25 m) was obtained as in Example 1.

Comparative Example 1

[0127] A polyimide film and a graphite film were produced as in Example 1 except that the conditions under which the polyimide film was dried were changed as follows.

[0128] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0129] This gel film was fired in stages (in a first heating furnace at 250 C., in a second heating furnace at 300 C., in a third heating furnace at 340 C., and in a fourth heating furnace at 480 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 0.05%, and the thermal shrinkage rate was 0,10%, Then, the graphite film (having a thickness of 25 m) was obtained as in Example 1.

Comparative Example 2

[0130] A polyimide film and a graphite film were produced as in Example 1 except that the conditions under which the polyimide film was dried were changed as follows.

[0131] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the auric acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0132] This gel film was fired in stages (in a first heating furnace at 250 C., in a second heating furnace at 300 C., in a third heating furnace at 340 C., and in a fourth heating furnace at 450 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 0.12%, and the thermal shrinkage rate was 0.28%. Then, the graphite film (having a thickness of 25 m) was obtained as in Example 1.

Example 5

[0133] A polyimide film having a thickness of 38 m was obtained as in Example 3. In so doing, the heating loss rate was 2.42%, and the thermal shrinkage rate was 0.83%. Then, the graphite film (having a thickness of 18 m) was obtained as in Example 1.

Example 6

[0134] A polyimide film having a thickness of 52 m was obtained as in Example 3. In so doing, the heating loss rate was 3.76%, and the thermal shrinkage rate was 0.95%. Then, the graphite film (having a thickness of 32 m) was obtained as in Example 1.

Example 7

[0135] A polyimide film as produced as in Example 1 except that (i) 4,4-oxydianiline (ODA) at a proportion of 100 mol % and pyromellitic dianhydride (PMDA) at a proportion of 100 mol % were used as monomers and (ii) the conditions under which the polyimide film was dried were changed as follows.

[0136] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0137] This gel film was fired in stages a first heating furnace at 250 C., in a second heating furnace at 300 C., in a third heating furnace at 340 C., and in a fourth heating furnace at 350 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 2.66%, and the thermal shrinkage rate was 1.05%.

[0138] In addition, a graphite film as produced as follows. The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Comparative Example 3

[0139] A polyimide film was produced as in Example 7 except that the conditions under which the polyimide film was dried were changed as follows.

[0140] Then, acetic anhydride and isoquinoline were added to a polyamic acid solution so that 2.0 equivalents of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an endless belt. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the endless belt, and then both of widthwise end parts of the gel film were fixed to a pin sheet that continuously conveys the gel film.

[0141] This gel film was fired in stages a first heating furnace at 250 C., in a second heating furnace at 300 C., in a third heating furnace at 340 C., and in a fourth heating furnace at 480 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 m. In so doing, the heating loss rate was 0.05%, and the thermal shrinkage rate was 0.20%.

[0142] In addition, a graphite film was produced as follows. The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 8

[0143] A polyimide film and a graphite film were produced as in Example 1 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0144] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 9

[0145] A polyimide film and a graphite film were produced as in Example 2 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0146] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2300 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 10

[0147] A polyimide film and a graphite film were produced as in Example 3 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0148] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 11

[0149] A polyimide film and a graphite film were produced as in Example 4 except that the carbonization conditions in the production of the graphite film were changed as follows

[0150] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Comparative Example 4

[0151] A polyimide film and a graphite film were produced as in Comparative Example 1 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0152] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Comparative Example 5

[0153] A polyimide film and a graphite film were produced as in Comparative Example 2 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0154] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Example 12

[0155] A polyimide film and a graphite film were produced as in Example 5 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0156] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 18 m).

Example 13

[0157] A polyimide film and a graphite film were produced as in Example 6 except that the carbonization conditions in the production of the graphite film were changed as follows.

[0158] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 32 m).

Reference Example 1

[0159] A polyamic acid solution was synthesized by (i) adding 4,4-oxydianiline (ODA) at a proportion of 75 mol %, paraphenylenediamine (PDA) at a proportion of 25 mol %, and pyromellitic dianhydride (PMDA) at a proportion of 100 mol %, to N,N-dimethylformamide (DMF) which is an organic solvent for polymerization and (ii) stirring a resulting product so as to polymerize the resulting product in so doing, the polyamic acid solution was synthesized so that a solid content concentration in the polyamic acid solution to be obtained would be 18.5% by mass.

[0160] Then, acetic anhydride and isoquinoline were added to the polyamic acid solution so that 1.0 equivalent of the acetic anhydride and 1.0 equivalent of the isoquinoline were added with respect to the equivalent of the amic acid. Then, a resulting product was cast onto an aluminum foil. Then, the resulting product was subjected to hot air drying in a range of 12010 C. for 4 minutes. This produced a gel film (polyimide precursor film) which was self-supporting. This gel film was removed from the aluminum foil, and the four sides of the gel film were fixed to a frame.

[0161] This gel film was fired in stages (in a first heating furnace at 275 C., in a second heating furnace at 400 C., in a third heating furnace at 450 C., and in a far-infrared heating furnace at 460 C.) so that imidization of the gel film advanced. This produced a polyimide film having a thickness of 50 rim. In so doing, the heating loss rate was 0.07, and the thermal shrinkage rate was 0.06%.

[0162] The polyimide film thus prepared was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature eras raised to 1000 C. at a rate of 16.7 C./ min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min, in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

Reference Example 2

[0163] A polyimide film and a graphite film were produced as in Reference Example 1 except that the carbonization. conditions in the production of the graphite film were changed as follows.

[0164] The polyimide film was cut into a 5 cm5 cm piece. Then the piece was sandwiched between graphite plates, and then subjected to carbonization with use of an electric furnace in which a temperature was raised to 1400 C. at a rate of 5 C./ min. in nitrogen. A carbonized film obtained by the carbonization was sandwiched between graphite plates, and then subjected to graphitization with use of a graphitization furnace in which a temperature was raised to 2800 C. at a heating rate of 1 C./min. in argon. Then, a resulting product was compressed at a pressure of 20 MPa with use of a single-plate press. This produced a graphite film (having a thickness of 25 m).

[0165] Table 1 shows the results of Examples 1 through 9, Comparative Examples 1 through 3, and Reference Examples 1 and 2.

TABLE-US-00001 TABLE 1 Physical properties Physical properties Heating of graphite film of polyimide film rate in Thermal Temperature ( C.) of each furnace Heating Thermal carboniza- diffu- Monomer ratio Belt 1st 2nd 3rd 4th Thickness loss rate shrinkage tion sivity PMDA ODA PDA oven furnace furnace furnace furnace (m) (%) rate (%) ( C./min.) Appearance (cm.sup.2/s) Ex 1 100 75 25 120 250 300 340 400 (IR) 50 0.24 0.58 25 E 8.5 Ex 2 100 75 25 120 250 300 450 50 1.48 0.75 25 E 8.5 Ex 3 100 75 25 120 250 300 340 350 (IR) 50 3.09 0.9 25 E 8.6 Ex 4 100 75 25 120 270 340 370 400 (IR) 50 0.15 0.5 25 G 8.3 CE 1 100 75 25 120 250 300 340 480 (IR) 50 0.05 0.1 25 U 7.7 CE 2 100 75 25 120 250 300 340 450 (IR) 50 0.12 0.28 25 U 7.9 Ex 5 100 75 25 120 250 300 340 350 (IR) 38 2.42 0.83 25 E 8.8 Ex 6 100 75 25 120 250 300 340 350 (IR) 62 3.76 0.95 25 E 8.4 Ex 7 100 100 0 120 250 300 340 350 (IR) 50 2.66 1.05 5 E 8.5 CE 3 100 100 0 120 250 300 340 480 (IR) 50 0.05 0.2 5 G 7.9 Ex 8 100 75 25 120 250 300 340 400 (IR) 50 0.24 0.58 5 G 6.8 Ex 9 100 75 25 120 250 300 450 50 1.48 0.75 5 G 7.2 Ex 10 100 75 25 120 250 300 340 350 (IR) 50 3.09 0.9 5 G 7.3 Ex 11 100 75 25 120 270 340 370 400 (IR) 50 0.15 0.5 5 U 6.6 CE 4 100 75 25 120 250 300 340 480 (IR) 50 0.05 0.1 5 P 5.8 CE 5 100 75 25 120 250 300 340 450 (IR) 50 0.12 0.28 5 P 5.9 Ex 12 100 75 25 120 250 300 340 350 (IR) 38 2.42 0.83 5 E 8 Ex 13 100 75 25 120 250 300 340 350 (IR) 62 3.76 0.95 5 G 6.9 RE 1 100 75 25 120 275 400 450 460 (IR) 50 0.07 0.05 16.7 G 8.5 RE 2 100 75 25 120 275 400 450 460 (IR) 50 0.07 0.05 5 P 6.3