Graphite sheet polyimide film comprising spherical PI-based filler, manufacturing method therefor, and graphite sheet manufactured using same

Abstract

The present invention provides: a graphite sheet polyimide film, which is derived from a first precursor composition comprising a first polyamic acid and comprises a sublimable inorganic filler and a spherical polyimide-based filler; a manufacturing method therefor; and a graphite sheet manufactured using the same.

Claims

1. A method of manufacturing polyimide film for graphite sheets, comprising: (a) mixing an organic solvent, a diamine monomer and a dianhydride monomer to prepare a first polyamic acid solution; (b) mixing inorganic fillers and polyimide fillers along with a first catalyst having a linear structure and a second catalyst having a ring structure with the first polyamic acid solution to prepare a first precursor composition; (c) forming a gel film by casting the first precursor composition onto a support, followed by drying the first precursor composition; and (d) imidizing the first precursor composition through heat treatment of the gel film to form a polyimide film, the polyimide film including sublimable inorganic fillers and spherical polyimide fillers, wherein the inorganic fillers are present in an amount of 0.2 parts by weight to 0.5 arts by weight relative to 100 parts by weight of the first polyamic acid and the polyimide fillers are present in an amount of 0.1 parts by weight to 5 parts by weight relative to 100 parts by weight of the first polyamic acid, wherein the inorganic fillers have an average particle diameter of 1.5 μm to 4.5 μm.

2. The method according to claim 1, wherein the first catalyst comprises at least one selected from the group consisting of dimethylacetamide (DMAc), N,N-diethylacetamide, dimethylformamide (DMF), and diethylformamide (DEF).

3. The method according to claim 1, wherein the first catalyst is dimethylformamide.

4. The method according to claim 1, wherein the second catalyst is N-methyl-2-pyrrolidone (NMP).

5. The method according to claim 1, wherein the first catalyst and the second catalyst are added in a total amount of 1.5 moles to 4.5 moles per 1 mole of an amic acid group in a polyamic acid.

6. The method according to claim 1, wherein the second catalyst is present in an amount of 10 mol % to 30 mol % based on the total amount of the first catalyst and the second catalyst.

7. The method according to claim 1, wherein, in Step (b), a dehydration agent and an imidization agent are further added to the first polyamic acid solution.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a picture of a surface of a polyimide film manufactured in Comparative Example 4.

(2) FIG. 2 is a picture of a surface of a polyimide film manufactured in Example 1.

MODE FOR INVENTION

(3) Next, the present invention will be described in more detail with reference to some examples. It should be understood that these examples are provided for illustration only and are not to be in any way construed as limiting the invention.

(4) <Example 1>

(5) Preparative Example 1-1: Preparation of Polyimide Fillers

(6) With 200 g of N,N′-dimethylacetamide (DMAc) placed in a 1 L reactor, the temperature of the reactor was reduced to 0° C. and 17.23 g (84.4 mmol) of 4,4′-oxyphenylenediamine (ODA) was added to the reactor.

(7) Then, 18.4 g (84.4 mmol) of 1,2,4,5-benzenetetracarboxylicdianhydride (PMDA) was added dropwise to the resulting solution.

(8) The resulting mixture was stirred for 30 minutes while adjusting the reaction temperature of the mixture so as not to exceed 40° C. and the temperature of the reactor was slowly raised to 80° C., followed by stirring and aging the mixture at the raised temperature for 4 hours, thereby preparing a polyamic acid solution.

(9) The prepared polyamic acid solution had a viscosity of 75 poise and an inherent viscosity of 1.31 dl/g.

(10) Then, the prepared polyamic acid solution was added in 800 g of methanol to obtain a thread-like polyamic acid, and left for 10 hours.

(11) Methanol floating on the thread-like polyamic acid was removed once every 3 hours, followed by adding 600 g of methanol to remove the solvent.

(12) After 10 hours, all methanol was removed and the remaining solid product was pulverized using a pulverizing machine. Then, the resulting powder was washed with water and methanol, filtered, and dried in a vacuum oven at 40° C. for 10 hours, thereby preparing powder-shaped polyimide fillers having an average particle diameter of 3 μm.

(13) Preparative Example 1-2: Preparation of First Precursor Composition

(14) As an organic solvent, 404.8 g of dimethylformamide (DMF) was placed in a 0.5 L reactor under a nitrogen atmosphere.

(15) With the temperature of the reactor set to 25° C., 45.59 g of ODA was added as a diamine monomer to the reactor and stirred for 30 minutes until the diamine monomer was dissolved, 49.66 g of PMDA was added as a dianhydride monomer, and a small amount of PMDA was further added until the resulting solution had a viscosity of 200,000 cP to 250,000 cP, thereby preparing a first polyamic acid.

(16) Thereafter, as inorganic fillers, 0.26 g of dicalcium phosphate particles having an average particle diameter of 3 μm were added to the polyamic acid together with 0.86 g of the polyimide fillers prepared in Preparative Example 1-1, and stirred for 1 hour while maintaining the temperature of the reactor, thereby preparing a first precursor composition.

(17) In conversion for comparison, the inorganic fillers were present in an amount of 0.3 parts by weight and the polyimide fillers were present in an amount of 1 part by weight relative to 100 parts by weight of the first polyamic acid in the first precursor composition in terms of solid content.

(18) Preparative Example 1-3: Preparation of Polyimide Film

(19) 2.25 g of beta-picoline (BP) as an imidization agent, 16.73 g of acetic anhydride (AA) as a dehydration agent, 9.5 g of DMF as a first catalyst, and 3.2 g of NMP as a second catalyst were added to 70 g of the first precursor composition prepared in Preparative Example 1-2, and were evenly stirred therewith. Then, the resulting composition was cast to a thickness of 350 μm on an SUS plate (100SA, Sandvik) using a doctor blade and dried in a temperature range of 100° C. to 200° C.

(20) Thereafter, the dried film was peeled off of the SUS plate and secured to a pin frame for transfer to a hot tenter.

(21) In the hot tenter, the film was heated from 200° C. to 600° C., cooled to 25° C., and separated from the pin frame, thereby providing a polyimide film having a size of 20 cm×20 cm×50 μm (length×width×thickness).

(22) <Example 2>

(23) A polyimide film was manufactured in the same manner as in Example 1 except that polyimide fillers having an average particle diameter of 1 μm were prepared in Preparative Example 1-1.

(24) <Example 3>

(25) A polyimide film was manufactured in the same manner as in Example 1 except that polyimide fillers having an average particle diameter of 10 μm were prepared in Preparative Example 1-1.

(26) <Example 4>

(27) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the polyimide fillers was changed to 0.1 parts by weight.

(28) <Example 5>

(29) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the polyimide fillers was changed to 5 parts by weight.

(30) <Example 6>

(31) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the inorganic fillers was changed to 0.5 parts by weight.

(32) <Example 7>

(33) A polyimide film was manufactured in the same manner as in Example 1 except that barium sulfate particles having an average particle diameter of 3 μm were added in an amount of 0.3 parts by weight as the inorganic fillers.

(34) <Comparative Example 1>

(35) A polyimide film was manufactured in the same manner as in Example 1 except that the polyimide fillers were not used.

(36) <Comparative Example 2>

(37) A polyimide film was manufactured in the same manner as in Example 1 except that the inorganic fillers were not used.

(38) <Comparative Example 3>

(39) A polyimide film was manufactured in the same manner as in Example 1 except that the polyimide fillers and the inorganic fillers were not used.

(40) <Comparative Example 4>

(41) A polyimide film was manufactured in the same manner as in Example 1 except that polyimide fillers having an average particle diameter of 15 μm was prepared in Preparative Example 1-1.

(42) <Comparative Example 5>

(43) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the polyimide fillers was changed to 10 parts by weight.

(44) <Comparative Example 6>

(45) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the polyimide fillers was changed to 0.05 parts by weight.

(46) <Comparative Example 7>

(47) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the inorganic fillers was changed to 0.1 parts by weight.

(48) <Comparative Example 8>

(49) A polyimide film was manufactured in the same manner as in Example 1 except that the amount of the inorganic fillers was changed to 0.6 parts by weight.

(50) <Comparative Example 9>

(51) A polyimide film was manufactured in the same manner as in Example 1 except that dicalcium phosphate particles having an average particle diameter of 5 μm were used.

(52) <Comparative Example 10>

(53) A polyimide film was manufactured in the same manner as in Example 1 except that dicalcium phosphate particles having an average particle diameter of 1 μm were used.

(54) <Comparative Example 11>

(55) A polyimide film was manufactured in the same manner as in Example 1 except that the second catalyst was not used and 11.84 g of DMF was used as the first catalyst in Preparative Example 1-3.

(56) <Experimental Example 1>

(57) For each of the polyimide films of Examples and Comparative Examples, external appearance was observed depending upon the average particle diameter and the input amount of the polyimide fillers and/or the inorganic fillers.

(58) In the experiment, the polyimide films manufactured in Examples 1 to 7, which satisfied conditions for the average particle diameters and the input amounts according to the present invention, were compared with the polyimide films manufactured in Comparative Examples 4, 5, 8 and 9, which failed to satisfy the conditions for the average particle diameters and the input amounts according to the present invention. The number of surface defects such as protrusions or pin holes of each of the polyimide films was counted with the naked eye and evaluation results are shown Table 1, FIG. 1 (Comparative Example 4) and FIG. 2 (Example 1).

(59) TABLE-US-00001 TABLE 1 Inorganic fillers PI fillers Dicalcium Barium Content phosphate sulfate parts (parts (parts Surface Size (by Size by by defect Kind (μm) weight) (um) weight) weight) (Number) Example 1 3 1 3 0.3 — 0 Example 2 1 1 3 0.3 — 0 Example 3 10 1 3 0.3 — 0 Example 4 3 0.1 3 0.3 — 0 Example 5 3 5 3 0.3 — 0 Example 6 3 1 3 0.5 — 0 Example 7 3 1 3 — 0.3 0 Comparative 15 1 3 0.3 — 35 Example 4 Comparative 3 10 3 0.3 — 21 Example 5 Comparative 3 1 3 0.6 — 6 Example 8 Comparative 3 1 5 0.3 — 8 Example 9

(60) It could be seen that numerous surface defects were generated on each of the polyimide films of Comparative Examples 4, 5, 8 and 9 prepared using the polyimide fillers and/or the inorganic fillers, the average particle diameters and the input amounts of which did not fall within the numerical ranges according to the present invention. Further, referring to FIG. 1 showing a picture of a surface of the polyimide film of Comparative Example 4, it could be seen that the polyimide film of Comparative Example 4 had poor surface quality, as observed with the naked eye. Accordingly, it could be seen that use of an excess of the polyimide fillers and/or the inorganic fillers or the use of the polyimide fillers and/or the inorganic fillers having too large particle size provided polyimide films having unsmooth surfaces.

(61) Conversely, it could be seen that the polyimide films of Examples manufactured using the polyimide fillers and/or the inorganic fillers, the average particle diameters and the input amounts of which were within the numerical ranges according to the present invention, did not suffer from surface defects. Further, referring to FIG. 2 showing a picture of a surface of the polyimide film of Example 1, it could be seen that the polyimide film of Example 1 had good surface quality, as observed with the naked eye.

(62) <Experimental Example 2>

(63) Each of the polyimide films of Examples and Comparative Examples was heated to 1,200° C. at a heating rate of 3° C./min in a high temperature furnace under a nitrogen atmosphere and was left at the same temperature for about 2 hours (carbonization). Then, the polyimide film was heated to 2,800° C. at a heating rate of 5° C./min in an ultra-high temperature furnace under an argon atmosphere and was left for 1 hour (graphitization), followed by cooling, thereby preparing a graphite sheet having a thickness of 30 μm.

(64) For each of the manufactured graphite sheets, thermal conductivity in the plane direction and in the thickness direction and the number of bright spots were measured and results are shown in Table 2.

(65) For each of the graphite sheets, heat diffusion rates in the thickness direction and in the plane direction thereof were measured by a laser flash method using a diffusion rate measurement instrument (Model: LFA 467, Netsch), and thermal conductivity was calculated by multiplying the measured heat diffusion rate by density (weight/volume) and specific heat (specific heat value measured by DSC).

(66) The number of bright spots is a factor that causes surface defects of the graphite sheets and the number of protrusions having a size of 0.05 mm or more in a square of 50 mm×50 mm of the graphite sheet was counted.

(67) TABLE-US-00002 TABLE 2 Thermal conductivity Thermal conductivity in in plane direction thickness direction Number of bright spots Kind (W/m .Math. K) (W/m .Math. K) (EA) Example 1 1127.3 33.2 2 Example 2 1188.4 30.1 1 Example 3 1058.9 34.0 5 Example 4 1174.6 31.5 0 Example 5 1043.5 34.9 3 Example 6 1135.0 30.2 2 Example 7 1048.1 30.5 4 Comparative Example 1 1142.2 5.4 3 Comparative Example 2 — — Graphitization did not proceeded Comparative Example 3 — — Graphitization did not proceeded Comparative Example 4 894.5 43.2 27 Comparative Example 5 832.2 51.3 12 Comparative Example 6 1139.4 4.9 3 Comparative Example 7 986.0 23.6 2 Comparative Example 8 1104.5 27.1 21 Comparative Example 9 951.5 23.5 13 Comparative Example 10 880.6 19.8 1 Comparative Example 11 997.1 17.5 2

(68) From Table 2, the following results could be obtained.

(69) First, all of the polyimide films of Examples were manufactured using a suitable amount of the polyimide fillers having an average particle diameter according to the present invention. Each of the graphite sheets manufactured using such polyimide films had very good thermal conductivity, that is, a thermal conductivity of 1,000 W/m.Math.K or more in the plane direction and a thermal conductivity of 30 W/m.Math.K or more in the thickness direction, as shown in Table 2.

(70) It is estimated that this result was obtained due to graphitization of at least some polyimide particles between the layers of the multilayer graphite structure to form a linking portion acting as a heat transfer path between the layers thereof.

(71) Conversely, it could be seen that the graphite sheet of Comparative Example 1 prepared using the polyimide film free from the polyimide fillers had much lower thermal conductivity in the thickness direction than the graphite sheets of Examples.

(72) It is understood that, since the graphite sheet of Comparative Example 1 has a gap between the layers of the multilayer graphite structure instead of the linking portion as described above, the graphite sheet suffers from inefficient heat transfer between the layers.

(73) That is, there can be a significant difference in thermal conductivity of the graphite sheet in the thickness direction thereof depending upon the presence of the polyimide particles.

(74) Further, the graphite sheets of Examples had 5 or fewer bright spots, indicating better surface quality than the graphite sheets of Comparative Examples 4, 8 and 9.

(75) Secondly, although an excess of the polyimide fillers can be taken into account in order to form more linking portions, Comparative Example 5 shows that the graphite sheet including an excess of the polyimide fillers has much lower thermal conductivity in the plane direction than the graphite sheets of Examples.

(76) Upon carbonization and graphitization, most components of the fillers in the polyimide film are sublimated and a higher amount of sublimation gas provides a higher possibility of fracture of the graphite structure. Consequently, in Comparative Example 5, it is estimated that a large amount of gas derived from an excess of the polyimide fillers partially damaged the multilayer graphite structure while obstructing rearrangement of carbon atoms in the course of carbonization and graphitization, thereby causing deterioration in thermal conductivity of the graphite sheet in the plane direction thereof.

(77) On the other hand, use of the polyimide fillers having a larger particle size can be taken into account in order to form more linking portions and this will be described with reference to Comparative Example 4.

(78) In Comparative Example 4, it could be seen that, although the polyimide film was manufactured using a suitable amount of polyimide fillers having an average particle diameter of 15 μm, the polyimide film did not exhibit desired thermal conductivity in the plane direction thereof, and it is estimated that rearrangement of carbon atoms in the course of carbonization and graphitization was obstructed, thereby failing to provide a desired result.

(79) Further, it could be seen that the polyimide film of Comparative Example 6 including a relatively small amount of the polyimide fillers had significantly low thermal conductivity in the thickness direction thereof. It is understood that this result was caused by insufficient formation of the linking portion.

(80) From the above results, it can be seen that the content and the particle size of the polyimide fillers are critical factors in realization of a graphite sheet having good thermal conductivity.

(81) Thirdly, it could be seen that a graphite sheet could not be obtained using the polyimide films of Comparative Examples 2 and 3, which were prepared without using the inorganic fillers and did not allow graphitization, and that the graphite sheets of Comparative Example 7 and 10 suffered from deterioration in thermal conductivity both in the plane direction and in the thickness direction, in which the polyimide film of Comparative Example 7 was prepared using a small amount of the inorganic fillers and the polyimide film of Comparative Example 10 was prepared using large inorganic fillers.

(82) From these results, it could be seen that the inorganic fillers acted as a critical factor in conversion from polyimide into graphite and allowed significant improvement in thermal conductivity particularly when the amount and the particle size of the inorganic fillers fall within certain ranges.

(83) Fourthly, in Comparative Example 11, the polyimide film was manufactured using the first catalyst alone and it could be seen that the graphite sheet manufactured using the polyimide film of Comparative Example 11 had relatively low thermal conductivity in the plane direction and in the thickness direction, as shown in Table 2.

(84) It is estimated that this result was caused by relatively low packing efficiency of polyimide chains upon imidization of the polyamic acid.

(85) Contrary to Comparative Example 11, the graphite sheets manufactured using the polyimide films of Examples using the second catalyst together with the first catalyst exhibited much better thermal conductivity in the plane direction than the graphite sheet manufactured using the polyimide film of Comparative Example 11. From this result, it can be anticipated that use of a suitable amount of the second catalyst can induce improvement in packing efficiency of the polyimide chains and such improvement in packing efficiency advantageously will allow regular arrangement of carbon atoms upon carbonization and graphitization.

(86) It should be understood that various modifications, changes, alterations, and equivalent embodiments can be made by those skilled in the art without departing from the spirit and scope of the present invention.

INDUSTRIAL APPLICABILITY

(87) Various advantages of the present invention obtained by including the sublimable inorganic fillers and the polyimide fillers have been described above in detail.

(88) In summary, in the polyimide film according to the present invention, the polyimide fillers may form at least one linking portion between the layers of the multilayer graphite structure to provide a heat transfer path upon carbonization and graphitization. With this structure, the polyimide film can realize a graphite sheet achieving significant improvement in thermal conductivity not only in the plane direction thereof but also in the thickness direction thereof.

(89) The polyimide film according to the present invention includes a suitable amount of inorganic fillers to induce foaming phenomenon of the polyimide film, thereby realizing a graphite sheet having good flexibility.

(90) Advantages obtained by use of two or more types of catalysts are described in detail in the above description. In summary, a combination of two or more types of catalysts having different properties can improve packing efficiency of polymer chains in a polyamic acid and a polyimide film derived from such a polyamic acid has a regular arrangement of the polymer chains. Such a polyimide film can realize a graphite sheet having improved thermal conductivity.