Polyimide film for graphite sheet, graphite sheet prepared by using the same and method for preparing graphite sheet

Abstract

The present disclosure provides a polyimide film prepared from a precursor composition containing a polyamic acid and an organic solvent and having a value of (first FWHMsecond FWHM)/(first FWHM+second FWHM) which is less than 0.4, a graphite sheet prepared from the polyimide film, and a method for preparing a graphite sheet.

Claims

1. A polyimide film prepared from a precursor composition containing a polyamic acid and an organic solvent and having a value of the following Formula 1 less than 0.4:
(first FWHMsecond FWHM)/(first FWHM+second FWHM)Formula 1 wherein, when a diffraction peak of (010) plane according to XRD analysis of the polyimide film is in the form of a single peak, the first FWHM (full width at half maximum) indicates a FWHM of the single peak; and when the diffraction peak of (010) plane is in the form of a plurality of single peaks or in the form of a plurality of single peaks overlapped with each other, the first FWHM indicates an average of individual FWHMs of the single peaks, and when a diffraction peak of (102) plane according to XRD analysis of the polyimide film is in the form of a single peak, the second FWHM indicates a FWHM of the single peak; and when the diffraction peak of (102) plane is in the form of a plurality of single peaks or in the form of a plurality of single peaks overlapped with each other, the second FWHM indicates an average of individual FWHMs of the single peaks, wherein the polyamic acid is contained in an amount 15 to 20% by weight based on the total weight of the precursor composition, and a film intermediate derived from the precursor composition is stretched at least once, wherein the film intermediate is stretched at a temperature of 20 degrees C. to 40 degrees C.

2. The polyimide film of claim 1, wherein the film intermediate is stretched so that the polyimide film has a thickness of 20 to 125 micrometers.

3. The polyimide film of claim 1, wherein a stretching ratio in the at least one of the machine direction and the transverse direction is +3% or more and +25% or less.

4. The polyimide film of claim 1, wherein the polyamic acid is prepared by polymerization of dianhydride monomers and diamine monomers.

5. The polyimide film of claim 1, wherein the first FWHM is 35 degrees or more and 80 degrees or less, and the second FWHM is 43% or more and 92% or less of the first FWHM.

6. The polyimide film of claim 1, wherein the value of Formula 1 is 0.01 or more and 0.37 or less.

7. A method for preparing a graphite sheet, comprising: preparing a polyimide film having a value of the following Formula 1 less than 0.4 from a precursor composition containing a polyamic acid and an organic solvent; carbonizing the polyimide film; and obtaining a graphite sheet by graphitizing the carbonized polyimide film, wherein the graphite sheet has a thermal conductivity of 1400 W/mK or more:
(first FWHMsecond FWHM)/(first FWHM+second FWHM)Formula 1 wherein, when a diffraction peak of (010) plane according to XRD analysis of the polyimide film is in the form of a single peak, the first FWHM (full width at half maximum) indicates a FWHM of the single peak; and when the diffraction peak of (010) plane is in the form of a plurality of single peaks or in the form of a plurality of single peaks overlapped with each other, the first FWHM indicates an average of individual FWHMs of the single peaks, and when a diffraction peak of (102) plane according to XRD analysis of the polyimide film is in the form of a single peak, the second FWHM indicates a FWHM of the single peak; and when the diffraction peak of (102) plane is in the form of a plurality of single peaks or in the form of a plurality of single peaks overlapped with each other, the second FWHM indicates an average of individual FWHMs of the single peaks, wherein the polyamic acid is contained in an amount 15 to 20% by weight based on the total weight of the precursor composition, and a film intermediate derived from the precursor composition is stretched at least once, wherein the film intermediate is stretched at a temperature of 20 degrees C. to 40 degrees C.

8. The method for preparing a graphite sheet of claim 7, wherein the film intermediate is stretched so that the polyimide film has a thickness of 20 to 125 micrometers.

9. The method for preparing a graphite sheet of claim 7, wherein the film intermediate is stretched at a stretching ratio of +3% or more and +25% or less in at least one of the machine direction and the transverse direction.

10. The method for preparing a graphite sheet of claim 7, wherein the precursor composition is prepared by mixing the polyamic acid in an amount of 15 to 20% by weight and the organic solvent in an amount of 80 to 85% by weight based on the total weight of the precursor composition.

11. The method for preparing a graphite sheet of claim 7, wherein the polyamic acid is prepared by polymerization of dianhydride monomers and diamine monomers.

12. The method for preparing a graphite sheet of claim 7, wherein the first FWHM is 35 degrees or more and 80 degrees or less, and the second FWHM is 43% or more and 92% or less of the first FWHM.

13. The method for preparing a graphite sheet of claim 7, wherein the value of Formula 1 is 0.01 or more and 0.37 or less.

14. The method for preparing a graphite sheet of claim 7, wherein the polyimide film is carbonized by heat-treating the polyimide film at a temperature of 1,000 degrees C. to 1,500 degrees C.

15. The method for preparing a graphite sheet of claim 7, wherein the carbonized polyimide film is graphitized by heat-treating the carbonized polyimide film at a temperature of 2,500 degrees C. to 3,000 degrees C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the XRD result for a (010) plane of a polyimide film according to example 1.

(2) FIG. 2 is a graph showing the XRD result for (102) plane of a polyimide film according to example 1.

DETAILED DESCRIPTION

(3) Hereinafter, the action and effect of the present disclosure will be described in more detail through specific examples of the present disclosure. It is to be understood, however, that such examples are merely illustrative of the present disclosure and are not intended to limit the scope of the present disclosure.

Preparation Example of Precursor Composition

Preparation Example 1

(4) 400 g of dimethylformamide was put into a 0.5 L reactor and the temperature was set to 20 degrees C. Then, 33.79 g of 4,4-diaminodiphenylether was added and dissolved. Thereafter, 35.33 g of pyromellitic dianhydride was added and dissolved. When dissolution was completed, pyromellitic dianhydride was gradually added to the solution, and the viscosity was measured to obtain a varnish of about 100,000 cP. At this time, the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 2

(5) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 250,000 cP was obtained. At this time, the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 3

(6) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 400,000 cP was obtained. At this time, the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 4

(7) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 500,000 cP was obtained. At this time, the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 5

(8) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 500,000 cP was obtained and polymerization was performed so that the polyamic acid solid content is 20% based on the total amount of the precursor composition.

Preparation Example 6

(9) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 900,000 cP was obtained and polymerization was performed so that the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 7

(10) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 30,000 cP was obtained and polymerization was performed so that the polyamic acid solid content is 15% based on the total amount of the precursor composition.

Preparation Example 8

(11) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 30,000 cP was obtained and polymerization was performed so that the polyamic acid solid content is 10% based on the total amount of the precursor composition.

Preparation Example 9

(12) A precursor composition was obtained in the same manner as in preparation example 1 except that a varnish of about 30,000 cP was obtained and polymerization was performed so that the polyamic acid solid content is 25% based on the total amount of the precursor composition.

EXAMPLE

Example 1

(13) 4.5 g of beta-picoline (boiling point: 144 degrees C.) as an imide curing catalyst, 17.0 g of acetic anhydride as a dehydrating agent and 23.5 g of dimethylformamide as a polar organic solvent were mixed and stirred in 100 g of the precursor composition obtained in preparation example 1. After mixing 45 g of imide conversion liquid with the solution, thus obtained, the solution was cast on a stainless steel plate and was dried in a 150 degrees C. oven for 4 minutes with a hot air. Thereafter, the film thus obtained was stretched by +3% in the machine direction and the transverse direction, respectively, at a temperature of 20 degrees C. to obtain a polyimide film.

Example 2

(14) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 2 was used.

Example 3

(15) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 3 was used.

Example 4

(16) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 4 was used.

Example 5

(17) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 5 was used.

Example 6

(18) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 2 was used and the film was stretched by +25% in the machine direction and the transverse direction, respectively.

Example 7

(19) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 6 was used.

Comparative Example 1

(20) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 7 was used.

Comparative Example 2

(21) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 8 was used.

Comparative Example 3

(22) A polyimide film was obtained in the same manner as in example 1 except that the precursor composition obtained in preparation example 9 was used.

Comparative Example 4

(23) A polyimide film was obtained in the same manner as in example 1 except that the film was stretched by +50% in the machine direction and the transverse direction, respectively.

Experimental Example 1: Evaluation of Morphological Stability of Polyimide Film

(24) In the polyimide films obtained in example 1 and comparative example 4, the degree of shrinkage in the transverse direction which occurs when stretching in the machine direction was evaluated.

(25) In the evaluation, the film width before stretching was measured and the film width after stretching was measured to calculate a shrinkage percentage. The shrinkage percentage in the transverse direction (TD) as a result is shown in Table 1 below. The breakage of the film was visually confirmed, and the result is also shown in Table 1.

(26) TABLE-US-00001 TABLE 1 TD shrinkage percentage (%) Breakage Example 1 1 X Comparative Example 4 18 (film broken)

(27) Referring to Table 1, it can be seen that the polyimide film according to example 1 has a small shrinkage percentage in the transverse direction caused by the stretching in the machine direction. As a result, it can be noted that there is no problem such as breakage in the transverse direction stretching performed after the machine direction stretching.

(28) Accordingly, the polyimide film according to example 1 has an advantage of improving the orientation by proper stretching and does not show any apparent breakage. Thus, the polyimide film can be used for the preparation of a graphite sheet. As a result, it is expected that a graphite sheet having excellent physical properties can be prepared.

(29) On the other hand, in the case of comparative example 4 in which the stretching in the machine direction was excessively performed, it can be noted that the transverse direction shrinkage percentage increases at the time of machine direction stretching, and the film is broken at the time of performing the transverse direction stretching again.

(30) This indicates that if the polyimide film is excessively stretched beyond the stretching range of the present disclosure, severe damage to the outer appearance may occur, making it impossible to prepare a graphite sheet.

Experimental Example 2: Evaluation of Physical Properties of Polyimide Film

(31) X-ray diffraction (XRD) analysis was performed on the polyimide films obtained in examples 1 to 6 and comparative examples 1 to 3 to measure the diffraction intensities with respect to the azimuthal angles on the crystal planes (010) and (102). Detailed conditions related to the XRD analysis are as follows. Light source: flexural magnet synchrotron/6D UNIST-PAL beam line (Pohang radiation accelerator) Energy used: 18.986 keV (wavelength: 0.653 ) Light source size: 100 (H)40 (V) um.sup.2 X-ray exposure time: 60 to 240 seconds Detector: Rayonix MX225-HS (28802880 pixels, pixel size: 78 um)

(32) Based on this, a first FWHM of (010) plane and a second FWHM of (102) plane were measured, and a value of the following Formula 1 was calculated using the first FWHM and the second FWHM. The results are shown in Table 2.
(first FWHMsecond FWHM)/(first FWHM+second FWHM)Formula 1

(33) TABLE-US-00002 TABLE 2 First FWHM Second FWHM Calculated (degrees) (degrees) value Example 1 78.9 36.6 0.37 Example 2 55.8 37.2 0.20 Example 3 59.0 43.9 0.15 Example 4 53.1 41.9 0.12 Example 5 57.9 41.1 0.17 Example 6 44.3 38.3 0.07 Example 7 65.1 59.6 0.04 Comparative 85.4 35.1 0.42 example 1 Comparative 84.5 33.1 0.44 example 2 Comparative 88.5 34.0 0.44 example 3

(34) As a result of the XRD analysis, it was confirmed that the polyimide films according to examples 1 to 6 have the value of Formula 1 which falls within the specific numerical value range according to the present disclosure, i.e., less than 0.4, and the polyimide films according to the comparative examples has the value of Formula 1 which falls outside the specific numerical value range according to the present disclosure.

(35) Meanwhile, FIG. 1 shows an XRD analysis graph of (010) plane of the polyimide film according to example 1. Referring to FIG. 1, the difference between the maximum azimuth angle and the minimum azimuth angle at (010) plane diffraction intensity of 50% can be calculated as a FWHM. The FWHM of the diffraction peak may be the first FWHM for (010) plane, the value of which is 78.9 degrees.

(36) FIG. 2 shows an XRD analysis graph of (102) plane of the polyimide film according to example 1.

(37) It can be noted that the diffraction peak of (102) plane is also has a form in which a plurality of peaks are superimposed. The plurality of peaks may be separated into individual peaks by calculation.

(38) In each of the separated diffraction peaks, the difference between the maximum azimuth angle and the minimum azimuth angle at the diffraction intensity of 50% can be calculated as a FWHM. The average of the FWHMs of the diffraction peaks may be the second FWHM for (102) plane, the value of which is 36.6 degrees.

Experimental Example 3: Evaluation of Physical Properties of Graphite Sheet

(39) The polyimide films obtained in examples 1 to 6 and comparative examples 1 to 3 were heated to 1,200 degrees C. at a rate of 1 degree C./min under the presence of a nitrogen gas using an electric furnace capable of carbonization and were maintained for about 2 hours (carbonization). Then, the polyimide films were heated to 2,800 degrees C. at a rate of 20 degree C./min under the presence of an argon gas using an electric furnace and were maintained for about 8 hours. Then, the polyimide films were cooled to obtain graphite sheets.

(40) The thermal diffusivity of each of the graphite sheets was measured by a laser flash method using a thermal diffusivity measuring instrument (Model LFA 447, Netzsch Korea co., ltd.). The thermal diffusivity thus measured was multiplied by the density (weight/volume) and the specific heat (theoretical value: 0.85 kJ/kg.Math.K) to calculate the thermal conductivity. The results are shown in Table 3 below.

(41) TABLE-US-00003 TABLE 3 Thermal diffusivity Thermal conductivity (m.sup.2/s) (W/m .Math. K) Example 1 782.5 1410 Example 2 781.7 1422 Example 3 813.6 1473 Example 4 825.2 1487 Example 5 811.2 1441 Example 6 841.4 1531 Example 7 873.9 1560 Comparative example 1 706.4 1315 Comparative example 2 767.3 1350 Comparative example 3 702.6 1284

(42) From the results shown in Table 3, it can be seen that the graphite sheets prepared from the polyimide films of the examples has significantly high thermal conductivity and thermal diffusivity as compared with those of the comparative examples.

(43) This clearly indicates that the polyimide films having the value of Formula 1 which falls within the specific numerical value range according to the present disclosure are capable of realizing excellent thermal conductivity, and further that the polyimide films having the value of Formula 1 which falls outside the specific numerical value range according to the present disclosure cannot realize graphite sheets having a desired thermal conductivity.

(44) In another aspect, the relationship between the first FWHM and the second FWHM can be quantitatively calculated using the value of Formula 1. The results shown in Tables 2 and 3 prove and indicate that the thermal conductivity of the graphite sheet can be predicted using the quantitative values for the relationship between the FWHMs.

(45) While the present disclosure has been described with reference to the embodiments, it is to be understood that various changes and modifications may be made without departing from the spirit and scope of the present disclosure.