POLYIMIDE FILM COMPRISING OMNIDIRECTIONAL POLYMER CHAIN, METHOD FOR MANUFACTURING SAME, AND GRAPHITE SHEET MANUFACTURED USING SAME

Abstract

The present invention provides a method of manufacturing a polyimide film including unit polymers that are omnidirectionally distributed, and a polyimide film. The present invention also provides a graphite sheet having good quality manufactured using the polyimide film.

Claims

1. A method of manufacturing a polyimide film, comprising: preparing a polyamic acid solution comprising first unit polymers by mixing a first organic solvent, a diamine monomer and a dianhydride monomer; preparing a precursor composition by mixing the polyamic acid solution, at least one imidization accelerator, and at least one dehydration agent; and forming a polyimide film comprising second unit polymers into which the first unit polymers are converted by imidizing the precursor composition at a variable temperature, wherein a processing condition (a) below is satisfied:
540 m<C.sup.2*T<610 m(a) wherein C is a sum of a molar amount of the imidization accelerator that is added and a molar amount of the dehydration agent that is added relative to 1 mol of an amic acid group of the polyamic acid, and T is a thickness of the polyimide film.

2. The method of claim 1, wherein the precursor composition comprises a first state, in which the first unit polymers of the polyamic acid are omnidirectional, and when the processing condition (a) is satisfied, at least a portion of the first unit polymers is imidized to the first state, thus forming the second unit polymers, which are omnidirectionally distributed.

3. The method of claim 1, wherein the molar amount of the imidization accelerator that is added is 0.43 to 0.6 relative to 1 mol of the amic acid group of the polyamic acid, and the molar amount of the dehydration agent that is added is 2.50 to 3 relative to 1 mol of the amic acid group of the polyamic acid.

4. The method of claim 3, wherein the imidization accelerator is a component having an effect of promoting a ring-closing dehydration reaction of the amic acid group of each of the first unit polymers.

5. The method of claim 3, wherein the first catalyst is at least one selected from the group consisting of quinoline, isoquinoline, -picoline, and pyridine.

6. The method of claim 3, wherein the dehydration agent is a component that enables dehydration and ring closure of the amic acid group of each of the first unit polymers.

7. The method of claim 3, wherein the dehydration agent is at least one selected from the group consisting of acetic anhydride, propionic anhydride, and lactic anhydride.

8. The method of claim 3, wherein, in the preparing the precursor composition, a second organic solvent is added along with the imidization accelerator and the dehydration agent.

9. The method of claim 1, wherein, when the polyimide film is subjected to corona treatment and tested using an adhesive, adhesion of the polyimide film is 1400 gf/mm or more.

10. The method of claim 1, wherein the imidizing comprises first imidization through heat treatment at a relatively low temperature and second imidization through heat treatment at a relatively high temperature, the second imidization comprises first heat treatment at a variable temperature in a range from 200 C. to 450 C. and second heat treatment at a variable temperature in a range of 550 C. or less but exceeding 450 C., and a processing condition (b) below is further satisfied in addition to the processing condition (a):
1.0 m/ C. (C.sup.2*T)/1.25 m/ C.(b) wherein C is a sum of a molar amount of the imidization accelerator that is added and a molar amount of the dehydration agent that is added relative to 1 mol of an amic acid group of the polyamic acid, T is a thickness of the polyimide film, and K is an average value of a highest temperature and two temperatures closest thereto among temperatures formed in the second heat treatment.

11. The method of claim 10, wherein the first imidization and the second imidization are sequentially performed, whereby conversion from the first unit polymers into the second unit polymers is induced stepwise, and the second unit polymers are formed in an omnidirectionally distributed state in each of the first imidization and the second imidization.

12. The method of claim 10, wherein at least one of the first heat treatment and the second heat treatment is performed using an infrared-ray heater.

13. The method of claim 10, wherein the first imidization is performed at a variable temperature in a range from 60 C. to less than 200 C.

14. A polyimide film comprising second unit polymers in which an amic acid group of each of first unit polymers is converted into an imide group by catalyzing polyamic acid comprising the first unit polymers that are omnidirectionally distributed in a state of satisfying a processing condition (a) below, at least a portion of the second unit polymers being omnidirectionally distributed:
540 m<C.sup.2*T<610 m(a) wherein C is a sum of a molar amount of the imidization accelerator that is added and a molar amount of the dehydration agent that is added relative to 1 mol of an amic acid group of the polyamic acid, and T is a thickness of the polyimide film.

15. The polyimide film of claim 14, wherein imidization is accelerated in the processing condition (a), and thus omnidirectionality of the first unit polymers is maintained in at least a portion of the second unit polymers, and T is 30 m to 75 m.

16. The polyimide film of claim 14, wherein adhesion of the polyimide film that is subjected to corona treatment is 1400 gf/mm or more.

17. A graphite sheet manufactured using the polyimide film of claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0120] FIG. 1 schematically shows the distribution of second unit polymers constituting a polyimide film;

[0121] FIG. 2 shows an image of the surface of the graphite sheet of Example 4;

[0122] FIG. 3 shows an image of the surface of the graphite sheet of Example 5;

[0123] FIG. 4 shows an image of the surface of the graphite sheet of Example 6;

[0124] FIG. 5 shows an image of the surface of the graphite sheet of Comparative Example 4;

[0125] FIG. 6 shows an image of the surface of the graphite sheet of Comparative Example 5; and

[0126] FIG. 7 shows an image of the surface of the graphite sheet of Comparative Example 6.

DETAILED DESCRIPTION

[0127] A better understanding of the actions and effects of the present invention may be obtained through the following examples, which are set forth to illustrate and are not to be construed as limiting the scope of the present invention.

EXAMPLE 1

Preparation of Precursor Composition

[0128] 354.8 g of dimethylformamide was placed in a 0.7 L reactor, and the temperature was set to 30 C., after which 38.8 g of 4,4-diaminodiphenylether was added thereto and dissolved therein, after which 41.8 g of pyromellitic dianhydride was added thereto and dissolved therein. After completion of dissolution, the viscosity of the resulting solution was measured while pyromellitic dianhydride was added little by little thereto, thereby obtaining a varnish having a viscosity of about 250,000 cP to 300,000 cP.

[0129] Thereafter, 263.7 g of dimethylformamide, 54.3 g of -picoline as an imidization accelerator, and 381.1 g of acetic anhydride as a dehydration agent were placed in the reactor, thus preparing a precursor composition.

[0130] Here, the molar amount of the imidization accelerator that was added relative to 1 mol of the amic acid group of polyamic acid was 0.46, and the molar amount of the dehydration agent that was added relative thereto was 3.00. These are summarized in Table 1 below for clear comparison between Examples and Comparative Examples.

Preparation of Gel Film

[0131] A gel film was manufactured in a manner in which the precursor composition prepared as described above was applied onto an endless stainless steel belt and the temperature was variably adjusted using hot air in the range from 60 C. to less than 200 C.

Preparation of Polyimide Film

[0132] The gel film prepared as described above was peeled from the endless belt and then fixed to a tenter. Thereafter, first heat treatment was performed by variably adjusting the temperature using hot air in the range from 200 C. to 400 C., and then second heat treatment was performed continuously by variably adjusting the temperature using an infrared-ray heater in the range of 550 C. or less but exceeding 450 C. Here, the average value of the highest temperature and the two temperatures closest thereto among the temperatures formed in the second heat treatment was 537 C. Thereafter, a polyimide film having a thickness of 50 m was obtained.

[0133] Moreover, the values in the following processing conditions (a) and (b) were determined, and whether or not the processing conditions were satisfied was confirmed, and the results thereof are shown in Table 1 below.

540 m<C.sup.2*T<610 m(a)

1.0 m/ C.(C.sup.2* T)/K1.25 m/ C.(b)

[0134] Here, C is the sum of the molar amount of the imidization accelerator that is added and the molar amount of the dehydration agent that is added relative to 1 mol of the amic acid group of polyamic acid, T is the thickness of the polyimide film, and K is the average value of the highest temperature and the two temperatures closest thereto among the temperatures formed in the second heat treatment, and is 537 C.

EXAMPLE 2

[0135] A polyimide film was manufactured in the same manner as in Example 1, with the exception that the molar amounts of the imidization accelerator and the dehydration agent that were added were changed as shown in Table 1 below, and the K value was adjusted to 529 C.

[0136] Moreover, the values determined in the processing conditions (a) and (b) are shown in Table 1 below, and whether these conditions were satisfied was confirmed.

EXAMPLE 3

[0137] A polyimide film was manufactured in the same manner as in Example 1, with the exception that the molar amounts of the imidization accelerator and the dehydration agent that were added were changed as shown in Table 1 below, the K value was adjusted to 520 C., and the amount of the precursor composition that was applied was changed to form a polyimide film having a thickness of 62.5 m.

[0138] Moreover, the values determined in the processing conditions (a) and (b) are shown in Table 1 below, and whether these conditions were satisfied was confirmed.

Comparative Example 1

[0139] A polyimide film was manufactured in the same manner as in Example 1, with the exception that the molar amounts of the imidization accelerator and the dehydration agent that were added were changed as shown in Table 1 below, and the K value was adjusted to 547 C.

[0140] Moreover, the values determined in the processing conditions (a) and (b) are shown in Table 1 below, and whether these conditions were satisfied was confirmed.

Comparative Example 2

[0141] A polyimide film was manufactured in the same manner as in Example 1, with the exception that the molar amounts of the imidization accelerator and the dehydration agent that were added were changed as shown in Table 1 below, and the K value was adjusted to 564 C.

[0142] Moreover, the values determined in the processing conditions (a) and (b) are shown in Table 1 below, and whether these conditions were satisfied was confirmed.

Comparative Example 3

[0143] A polyimide film was manufactured in the same manner as in Example 1, with the exception that the molar amounts of the imidization accelerator and the dehydration agent that were added were changed as shown in Table 1 below, the K value was adjusted to 534 C., and the amount of the precursor composition that was applied was changed to form a polyimide film having a thickness of 62.5 m.

[0144] Moreover, the values determined in the processing conditions (a) and (b) are shown in Table 1 below, and whether these conditions were satisfied was confirmed.

TABLE-US-00001 TABLE 1 Whether Whether Film Molar amount Molar amount Processing processing K Processing processing thick. of imidization of dehydration condition (a) condition (a) is value condition (b) condition (b) is (m) accelerator agent (m) satisfied ( C.) (m/ C.) satisfied Example 1 50 0.46 3.00 598 537 1.11 Example 2 50 0.47 3.00 602 529 1.14 Example 3 62.5 0.43 2.51 540 520 1.04 Comparative 50 0.43 2.75 505 X 547 0.92 X Example 1 Comparative 50 0.51 3.30 726 X 564 1.29 X Example 2 Comparative 62.5 0.4 2.5 525 X 534 0.98 X Example 3

[0145] As is apparent from Table 1, the polyimide films manufactured in Example 1 to Example 3 satisfied not only processing condition (a) but also processing condition (b), but the polyimide films manufactured in Comparative Example 1 to Comparative Example 3 did not satisfy processing condition (a), which is the highest priority, and also did not satisfy processing condition (b).

[0146] The importance of satisfying the processing condition (a) and the processing condition (b) will be clearly stated later, but these conditions can act decisively on the formation of the omnidirectional second unit polymers, and in connection therewith, it is possible to realize a graphite sheet having a good appearance and superior thermal properties.

Test Example 1: Test of Adhesion of Polyimide Film

[0147] The polyimide film manufactured in each of Example 1 to Example 3 and Comparative Example 1 and Comparative Example 3 was subjected to corona treatment.

[0148] Thereafter, an adhesive was applied onto one surface of the polyimide film, and a copper foil, an interlayer paper and a hot plate were sequentially laminated on the adhesive and then compressed and bonded at a temperature of 180 C. under a pressure of 0.3 Kpa, thereby completing a layup structure.

[0149] The finished layup structure was cut to a width of 10 mm*a length 150 mm, and while the copper foil directly adhered to the polyimide film was peeled off, the adhesion z of the polyimide film was measured. The results thereof are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Adhesion (gf/mm) Example 1 1400 or more Example 2 1400 or more Example 3 1400 or more Comparative 880 Example 1 Comparative 1400 or more Example 2 Comparative 820 Example 3

[0150] In general, the higher the proportion of the omnidirectional polymer chain, the higher the adhesion after corona treatment.

[0151] As shown in Table 2, the polyimide films of Example 1 to Example 3 and Comparative Example 2 exhibited a remarkable difference in adhesion compared to Comparative Example 1 and Comparative Example 3.

[0152] As discussed above, adhesion may be regarded as an indicator of the proportion of polymers in an omnidirectional state, which suggests that the proportion of the omnidirectional second unit polymers in the polyimide films of Example 1 to Example 3 and Comparative Example 2 showing high adhesion was high.

[0153] On the other hand, the polyimide films of Comparative Example 1 and Comparative Example 3 were measured to have relatively low adhesion, indicating that the proportion of the second unit polymers that were omnidirectionally distributed was relatively small.

[0154] Also, in Comparative Example 2, in which the imidization accelerator and the dehydration agent were added in excess in order to increase the proportion of the omnidirectional second unit polymers, the proportion of the omnidirectional second unit polymers was also regarded as high based on the observed high adhesion. However, Comparative Example 2 did not satisfy the processing conditions (a) and (b), and, as is apparent from the following test results, not only did appearance defects occur on the surface, but the thermal conductivity of the graphite sheet obtained therefrom was also reduced, which is undesirable.

[0155] This suggests that the second unit polymers in an omnidirectional state are formed and distributed in the polyimide film that satisfies the processing conditions (a) and (b) without causing surface appearance defects.

Test Example 2: Test of Color L of Polyimide Film

[0156] The color L of the polyimide films manufactured in Example 1 to Example 3 and Comparative Example 1 to Comparative Example 3 was tested. The results thereof are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Color L Example 1 73 Example 2 73.5 Example 3 65.5 Comparative 71 Example 1 Comparative 70.5 Example 2 Comparative 65 Example 3

[0157] In general, when comparing films having the same thickness, the higher the proportion of the omnidirectional polymer chain, the higher the color L of the film. As is apparent from Table 3, there was a slight difference in color L between Examples 1 and 2 and Comparative Examples 1 and 2, having the same thickness.

[0158] This can be understood to be the main evidence suggesting that the second unit polymers are mainly omnidirectionally distributed in the polyimide films of Examples 1 and 2, along with the results of Table 2.

[0159] In contrast, the polyimide films of Comparative Examples 1 and 2 have relatively low color L, and unlike the results of Examples, it can be determined that there are relatively few second unit polymers that are omnidirectionally distributed in the polyimide film.

[0160] Likewise, in Example 3 and Comparative Example 3, having the same thickness, the same results as the above results were obtained, and the significance thereof will be considered the same.

EXAMPLE 4 TO EXAMPLE 6

[0161] The polyimide film obtained in each of Example 1 to Example 3 was subjected to a carbonization process by raising the temperature to 1200 C. at a rate of 1.5 C./min under nitrogen gas using an electric furnace capable of carbonization, after which formation of graphite was completed by raising the temperature to 2800 C. using an electric furnace at a heating rate of 5 C./min under argon gas, followed by cooling to afford the graphite sheet of each of Example 4 to Example 6. The thicknesses of the graphite sheets thus obtained are summarized in Table 4 below.

Comparative Example 4 to Comparative Example 6

[0162] The polyimide film obtained in each of Comparative Example 1 to Comparative Example 3 was subjected to a carbonization process by raising the temperature thereof to 1200 C. at a rate of 1.5 C./min under nitrogen gas using an electric furnace capable of carbonization, after which the formation of graphite was completed by raising the temperature to 2800 C. using an electric furnace at a heating rate of 5 C./min under argon gas, followed by cooling to afford the graphite sheet of each of Comparative Example 4 to Comparative Example 6. The thicknesses of the graphite sheets thus obtained are summarized in Table 4 below.

Test Example 3: Evaluation of Properties of Graphite Sheet

[0163] The thermal diffusivity of the graphite sheets obtained in Example 4 to Example 6 and Comparative Example 4 to Comparative Example 6 was measured through a laser flash method using a thermal diffusivity meter (model name LFA 467, Netsch), and the thermal conductivity was calculated by multiplying the measured thermal diffusivity value by density (weight/volume) and specific heat (theoretical value: 0.85 kJ/(kg.Math.K)).

TABLE-US-00004 TABLE 4 Graphite PI film sheet Thermal Thermal thickness thickness diffusivity conductivity (m) (m) (mm.sup.2/s) (W/m .Math. K) Example 4 50 25 945 1681 Example 5 50 25 960 1673 Example 6 62.5 35 867 1254 Comparative 50 25 848 1574 Example 4 Comparative 50 25 792 1456 Example 5 Comparative 62.5 35 818 1147 Example 6

[0164] Table 4 shows that, when comparing graphite sheets having the same thickness, Examples exhibited significantly better thermal conductivity and thermal diffusivity than Comparative Examples.

[0165] When specifically analyzing Table 4, the following results are evident.

[0166] Example 4 exhibited thermal conductivity that was improved by about 7% and 15% compared to Comparative Example 4 and Comparative Example 5, respectively, and similar results were obtained for thermal diffusivity.

[0167] Example 5 exhibited thermal conductivity that was improved by about 6% and 15% compared to Comparative Example 4 and Comparative Example 5, respectively, and similar results were obtained for thermal diffusivity.

[0168] Example 6 exhibited thermal conductivity that was improved by 9% compared to Comparative Example 6, and similar results were obtained for thermal diffusivity.

[0169] Specifically, when the graphite sheet is manufactured from the polyimide film satisfying both processing conditions (a) and (b), it has properties superior to a graphite sheet manufactured from a polyimide film not satisfying the same. Briefly, the processing condition (a) and the processing condition (b) can be regarded as essential factors in achieving the above results.

Test Example 4: Evaluation of Appearance of Graphite Sheet

[0170] The number of defects per unit area (220 mm*254 mm) and the size of defects were observed for surface defects such as protrusions and the like in the graphite sheets manufactured in Examples and Comparative Examples, and the results of appearance evaluation grading according to the following criteria are shown in Table 5 below. For reference, a grade of B or higher is considered good, and a grade of C or lower can be judged to be defective. [0171] grade A: average protrusion size less than 0.3 mm; fewer than 10 protrusions [0172] grade B: average protrusion size of 0.3 mm to 0.5 mm; fewer than 10 protrusions [0173] grade C: average protrusion size of 0.3 mm to 0.5 mm; 10 protrusions or more [0174] grade D: average protrusion size greater than 0.5 mm or generation over the entire area

[0175] Respective surface images of the graphite sheets of Example 4 to Example 6 are shown in FIG. 2 to FIG. 4, and respective surface images of the graphite sheets of Comparative Example 4 to Comparative Example 6 are shown in FIG. 5 to FIG. 7.

TABLE-US-00005 TABLE 5 Appearance evaluation (grade) Example 4 A Example 5 A Example 6 A Comparative C Example 4 Comparative D Example 5 Comparative C Example 6

[0176] In the graphite sheets of Examples, protrusions were fine enough to make it difficult to check the size thereof, or defects such as protrusions were not observed, and thus the appearance evaluation grade thereof was excellent, as shown in Table 5.

[0177] With reference to the images of FIG. 2 to FIG. 4, in the graphite sheets of Examples, defects such as protrusions and the like were not observed on the surface, and the surface was smooth. This is deemed to be because the processing conditions (a) and (b) were satisfied, whereby the polyimide film including omnidirectional unit polymers could be manufactured without surface defects, and also because, in the graphite sheets of Examples 4 to 6 manufactured using the same, damage to the graphite structure on the surface was suppressed while the sublimation gas was efficiently exhausted during the carbonization and graphitization processes.

[0178] In Comparative Examples 4 to 6, the graphite sheet was manufactured using the polyimide film obtained through a method not satisfying the processing conditions (a) and (b). As shown in Table 5 and in FIGS. 4 to 7, results that contrasted strongly with the Examples were obtained.

[0179] In particular, in the graphite sheet of Comparative Example 5, manufactured using Comparative Example 2, in which the imidization accelerator and the dehydration agent were added in excess in order to increase the proportion of omnidirectional second unit polymers, even when the polyimide film included the omnidirectional second unit polymers, severe appearance defects were generated on the surface, so the thermal conductivity of the graphite sheet was reduced. This is deemed to be because imidization was partially promoted in excess in the casting step during the process of manufacturing the polyimide film of Comparative Example 2, resulting in appearance defects such as pinholes and the like. It is expected that the appearance defects generated in the polyimide film step suppress the formation of the graphite structure even in the step of manufacturing the graphite sheet, leading to a decrease in overall thermal conductivity.

[0180] In Comparative Examples 4 and 6, the omnidirectional polymer was present in excess, or the advantage of the omnidirectional state was not exhibited in Comparative Examples, which is presumed to be caused by the destruction of the graphite structure due to the sublimation gas.

[0181] Although the present invention has been described above with reference to embodiments thereof, those skilled in the art to which the present invention belongs will understand that it is possible to implement various applications and modifications without exceeding the scope of the present invention based on the above description.

INDUSTRIAL APPLICABILITY

[0182] The present invention discloses specific processing conditions related to the imidization accelerator, the dehydration agent and the heat-treatment temperature, and provides a method of manufacturing a polyimide film that satisfies the specific processing conditions.

[0183] The manufacturing method according to the present invention is advantageously capable of manufacturing a polyimide film including second unit polymers that are omnidirectionally distributed. This polyimide film is suitable for realizing a graphite sheet having good quality because multiple and complex gas exhaust pathways can be set between the omnidirectionally distributed second unit polymers, so even when the polyimide film is processed through carbonization and graphitization, gas is efficiently exhausted.

[0184] The present invention also provides a polyimide film in which the second unit polymers maintain the omnidirectionality of the first unit polymers, so spaces having a variety of complex shapes are formed between the second unit polymers.

[0185] When manufacturing the graphite sheet using such a polyimide film, gas flows efficiently from the inside of the film to the outside, resulting in a graphite sheet having good quality. This can be due to the fact that relatively low thermal energy is required for carbon rearrangement, which is economical and enables realization of desired crystallinity. In addition, assuming that graphitization proceeds almost simultaneously on the surface layer of the polyimide film and inside, relatively low pressure is created in the gas generated from the inside, and the gas exhausted through the surface layer does not destroy the carbonization structure that is forming or formed on the surface layer, or the extent of destruction thereof is low, whereby graphite in an intact surface state can be manufactured. Therefore, the polyimide film according to the present invention is suitable for formation of a graphite sheet having good quality.

[0186] The present invention can also provide a graphite sheet having a good appearance by being manufactured using the polyimide film as described above.