Plastic substrate, method for producing the same and touch screen panel containing the same

Abstract

The present invention provides a plastic substrate, including: a polyimide film; a hard coating layer formed on one side of the polyimide film; and a transparent electrode layer formed on the other side of the polyimide film. The plastic substrate has excellent light transmittance high hardness characteristics, superior ITO processability and flexibility. Further, the plastic substrate can function as both a window film and an electrode film when it is applied to a touch screen panel. Thus, the present invention provides a touch screen panel which can be slimmed by reducing the number of laminated films including the plastic substrate.

Claims

1. A plastic substrate, comprising: a polyimide film including a polyimide resin which is obtained by copolymerizing monomers comprising an aromatic dianhydride, an aromatic dicarbonyl compounds and an aromatic diamine, wherein the polyimide resin comprises a unit structure derived from the aromatic dianhydride, a unit structure derived from the aromatic dicarbonyl compound, and a unit structure derived from the aromatic diamine; and wherein the polyimide film has a yellowness of 5 or less and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, measured by a UV spectrometer; a first hard coating layer formed on one side of the polyimide film; and a transparent electrode layer formed on the other side of the polyimide film.

2. The plastic substrate of claim 1, wherein the first hard coating layer includes a polysilazane or silica, and an acrylic resin or polyurethane resin.

3. The plastic substrate of claim 1, wherein the transparent electrode layer includes at least one selected from the group consisting of ITO, IZO, Ag and Ag nanowire.

4. The plastic substrate of claim 1, wherein the polyimide film is surface-modified.

5. The plastic substrate of claim 4, wherein the surface modification of the polyimide film is conducted by plasma treatment or corona treatment.

6. The plastic substrate of claim 1, further comprising: an inorganic layer formed between the polyimide film and the transparent electrode layer, wherein the inorganic layer contains at least one selected from the group consisting of silicon oxide and silicon nitride.

7. The plastic substrate of claim 1, further comprising: a first organic layer formed on at least one side of the transparent electrode layer, wherein the organic layer is a cured layer of an active energy ray-curable composition including: polyurethane (meth)acrylate obtained by reacting a (meth)acrylate compound containing (meth)acrylate having one hydroxyl group in one molecule thereof; and a compound having one (meth)acryloyl group.

8. The plastic substrate of claim 1, further comprising: a second organic layer formed on the hard coating layer, wherein the organic layer is a cured layer of an active energy ray-curable composition including: polyurethane (meth)acrylate obtained by reacting a (meth)acrylate compound containing (meth)acrylate having one hydroxyl group in one molecule thereof; and a compound having one (meth)acryloyl group.

9. The plastic substrate of claim 1, further comprising: a second hard coating layer formed between the polyimide film and the transparent electrode layer.

10. The plastic substrate of claim 1, wherein the first hard coating layer has a thickness of 1-50 m.

11. The plastic substrate of claim 1, wherein the transparent electrode layer has a thickness of 1-100 nm.

12. The plastic substrate of claim 7, wherein the first organic layer has a thickness of 1-10 m.

13. The plastic substrate of claim 6, wherein the inorganic layer has a thickness of 1-100 nm.

14. The plastic substrate of claim 1, wherein the polyimide resin comprises: a unit structure derived from at least one aromatic dianhydride selected from the group consisting of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-I,2-dicarboxylic anhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxyl dianhydride and 4,4-(4,4-isoproylidenediphenoxy)bis(phthalic anhydride), a unit structure derived from at least one aromatic dianhydride selected from the group consisting of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride and oxydiphthalic dianhydride; a unit structure derived from at least one aromatic diamine selected from the group consisting of 2,2-bis[4-(4-aminophenoxy)-phenyl]propane, 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl, 3,3-bis(trifluoromethyl)-4,4-diaminobiphenyl, 4,4-bis(3-aminophenoxy)diphenylsulfone, bis(3-aminophenyl)sulfone, bis(4-aminophenyl)sulfone, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[3(3-aminophenoxy)phenyl]hexafluoropropane, 2,2-bis[4(4-aminophenoxy)phenyl]hexafluoropropane, 2,2-bis(3-aminophenyl)hexafluoropropane (3,3-6F), 2,2-bis(4-aminophenyl)hexafluoropropane (4,4-6F) and oxydianiline; and a unit structure derived from at least one aromatic dicarbonyl compound selected from the group consisting of p-terephthaloyl chloride, terephthalic acid, iso-phthaloyl dichloride and 4,4-benzoyl chloride.

15. The plastic substrate of claim 1, wherein the polyimide film has an average linear expansion coefficient of 30.0 ppm/ C. or less, which was measured at a temperature range of 50-250 C. by thermo-mechanical analysis based on a thickness of 50-200 m.

16. The plastic substrate of claim 1, wherein the plastic substrate has a surface resistance of 50 /sq. or less, when a thickness of the transparent electrode layer is 100 nm.

17. A method of manufacturing a plastic substrate, comprising the steps of: forming a hard coating layer on one side of a polyimide film including imides of polyimide resin precursors prepared by a polymerization of aromatic dianhydrides, aromatic dicarbonyl compounds and aromatic dimaines, having a yellowness of 5 or less, and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, as measured by a UV spectrometer; and forming a transparent electrode layer on the other side of the polyimide film, wherein the step of forming the transparent electrode layer includes a step of depositing the transparent electrode layer on the other side of the polyimide film and heat-treating the deposited transparent electrode layer, wherein the deposition or heat treatment is performed at 100 to 250 C.

18. A touch screen panel, comprising the plastic substrate of claim 1.

19. The plastic substrate of claim 9, wherein the hard coating layer has a thickness of 1-50 m.

20. The plastic substrate of claim 8, wherein the organic layer has a thickness of 1-10 m.

21. The plastic substrate of claim 9, wherein the second hard coating layer has a thickness of 1-50 m.

22. The plastic substrate of claim 8, wherein the organic layer has a thickness of 1-10 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Exemplary embodiments are illustrated in the referenced figures of the drawings. It is intended that the embodiments and the figures disclosed herein are to be considered illustrative rather than limiting.

(2) The FIGURE is a schematic cross sectional view of an exemplary embodiment of the inventive plastic substrate.

DISCLOSURE

Technical Problem

(3) The present invention intends to provide a plastic substrate having excellent light transmittance and satisfying high hardness characteristics, ITO processability, and flexibility.

(4) The present invention intends to provide a plastic substrate which can function as both a window film and an electrode film.

(5) The present invention intends to provide a touch screen panel which can be slimmed by reducing the number of laminated films including the plastic substrate.

Technical Solution

(6) An aspect of the present invention provides a plastic substrate, including: a polyimide film including imides of polyimide resin precursors prepared by polymerization of dianhydrides and diamines or polymerization of dianhydrides, aromatic dicarbonyl compounds and diamines, having a yellowness of 5 or less and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, measured by a UV spectrometer; a hard coating layer formed on one side of the polyimide film; and a transparent electrode layer formed on the other side of the polyimide film.

(7) Here, the hard coating layer may be formed of an organic-inorganic hybrid hard coat composition including polysilazane or silica and acrylic resin or polyurethane resin.

(8) Further, the transparent electrode layer may include at least one selected from among ITO, IZO, Ag and Ag nanowire.

(9) Further, the polyimide film may be surface-modified, and the surface modification of the polyimide film may be conducted by plasma treatment or corona treatment.

(10) The plastic substrate may further include: an inorganic layer formed between the polyimide film and the transparent electrode layer and containing at least one selected from among silicon oxide and silicon nitride.

(11) The plastic substrate may further include: an organic layer formed on at least one side of the transparent electrode layer, wherein the organic layer is a cured layer of an active energy ray-curable composition including: polyurethane (meth)acrylate obtained by reacting a (meth)acrylate compound containing (meth)acrylate having one hydroxyl group in one molecule thereof; and a compound having one (meth)acryloyl group.

(12) The organic layer may be formed on the hard coating layer.

(13) The hard coating layer may be formed between the polyimide film and the transparent electrode layer.

(14) The hard coating layer may have a thickness of 150 m. The transparent electrode layer may have a thickness of 1100 nm.

(15) The inorganic layer may have a thickness of 1100 nm.

(16) The polyimide film may include a polyimide resin including: a unit structure derived from at least one aromatic dianhydride selected from among 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (FDA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride (TDA), 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxyl dianhydride (6FCDA) and 4,4-(4,4-isoproylidenediphenoxy)bis(phthalic anhydride) (HBDA), and at least one aromatic dianhydride selected from among pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA) and oxydiphthalic dianhydride (ODPA); and a unit structure derived from at least one aromatic diamine selected from among 2,2-bis[4-(4-aminophenoxy)-phenyl]propane (6HMDA), 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl (2,2-TFDB), 3,3-bis(trifluoromethyl)-4,4-diaminobiphenyl (3,3-TFDB), 4,4-bis(3-aminophenoxy)diphenylsulfone (DBSDA), bis(3-aminophenyl)sulfone (3DDS), bis(4-aminophenyl)sulfone (ODDS), 1,3-bis(3-aminophenoxy)benzene (APB-133), 1,4-bis(4-aminophenoxy)benzene (APB-134), 2,2-bis[3(3-aminophenoxy)phenyl]hexafluoropropane (3-BDAF), 2,2-bis[4(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF), 2,2-bis(3-aminophenyl)hexafluoropropane (3,3-6F), 2,2-bis(4-aminophenyl)hexafluoropropane (4,4-6F) and oxydianiline (ODA), or the polyimide film may include a polyimide resin including: a unit structure derived from the aromatic dianhydride; a unit structure derived from at least one aromatic dicarbonyl compound selected from among p-terephthaloyl chloride (TPC), terephthalic acid, iso-phthaloyl dichloride and 4,4-benzoyl chloride; and a unit structure derived from the aromatic diamine.

(17) The polyimide film may have an average linear expansion coefficient of 30.0 ppm/ C. or less, which was measured at a temperature range of 50250 C. by thermo-mechanical analysis based on a thickness of 50200 m.

(18) The plastic substrate may have a surface resistance of 50 /sq. or less, when the thickness of the transparent electrode layer is 100 nm, thus realizing low resistance.

(19) Another aspect of the present invention provides a method of manufacturing a plastic substrate, including the steps of: forming a hard coating layer on one side of a polyimide film including imides of polyimide resin precursors prepared by polymerization of dianhydrides and diamines or polymerization of dianhydrides, aromatic dicarbonyl compounds and diamines, having a yellowness of 5 or less and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, measured by a UV spectrometer; and forming a transparent electrode layer on the other side of the polyimide film, wherein the step of forming the transparent electrode layer includes the step of depositing the transparent electrode layer on the other side of the polyimide film and heat-treating the deposited transparent electrode layer, wherein the deposition or heat treatment is performed at 100 to 250 C.

Advantageous Effects

(20) According to the plastic substrate of the present invention, it does not cause deformation such as yellowing even during an ITO process, can exhibit high hardness characteristics even though it is thin, and it can realize low surface resistance. Therefore, for example, when it is applied to a touch screen panel, the structure of the touch screen panel can be simplified.

BEST MODE

(21) Referring to the FIGURE, the plastic substrate according to an embodiment of the present invention includes: a polyimide film including imides of polyimide resin precursors prepared by polymerization of dianhydrides and diamines or polymerization of dianhydrides, aromatic dicarbonyl compounds and dimaines, having a yellowness of 5 or less and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, measured by a UV spectrometer; a hard coating layer formed on one side of the polyimide film; and a transparent electrode layer formed on the other side of the polyimide film. Here, the hard coating layer is a layer for imparting high hardness to the plastic substrate. Thanks to the hard coating layer, a window film is not required at the time of forming a touch screen panel. Further, the hard coating layer can improve the chemical resistance of the plastic substrate together with the following organic layer. For example, the hard coating layer can improve the chemical resistance of the plastic substrate to such a degree that the plastic substrate is not changed when it is observed by the naked eye after it is dipped into an organic solvent such as tetramethylammonium hydroxide, magnesium sulfate, potassium hydroxide, N-methyl-2-pyrrolidone or methyl ethyl ketone.

(22) According to the plastic substrate of the present invention, the light transmittance of the polyimide film may be 88% or more at a wavelength of 550 nm and 70% or more at a wavelength of 440 nm, as measured by a UV spectrometer based on its thickness of 50200 m.

(23) The polyimide film satisfying the above yellowness and light transmittance can be used in the fields of optical windows and transmissive displays requiring transparency, the usage of which has been limited due to the yellow color of a conventional polyimide film, and can be used for flexible display substrates.

(24) According to an embodiment of the present invention, the hard coating layer may be formed of an organic-inorganic hybrid hard coat composition including polysilazane or silica and acrylic resin or polyurethane resin.

(25) According to an embodiment of the present invention, the transparent electrode layer may include at least one selected from among ITO, IZO, Ag and Ag nanowire.

(26) According to an embodiment of the present invention, the polyimide film may be surface-modified, and the surface modification of the polyimide film may be conducted by plasma treatment or corona treatment. Thanks to the surface modification of the polyimide film, the surface adhesivity of the polyimide film to other various layers exemplified in the present invention can be improved, thus further improving the light transmittance, surface hardness and moisture permeability of the plastic substrate.

(27) According to an embodiment of the present invention, the plastic substrate may further include: an inorganic layer formed between the polyimide film and the transparent electrode layer and containing at least one selected from among silicon oxide and silicon nitride.

(28) Further, according to an embodiment of the present invention, the plastic substrate may further include: an organic layer formed on at least one side of the transparent electrode layer, wherein the organic layer is a cured layer of an active energy ray-curable composition including: polyurethane (meth)acrylate obtained by reacting a (meth)acrylate compound containing (meth)acrylate having one hydroxyl group in one molecule thereof; and a compound having one (meth)acryloyl group.

(29) This organic layer may be formed on the hard coating layer.

(30) The hard coating layer may be formed between the polyimide film and the transparent electrode layer.

(31) According to an embodiment of the present invention, the hard coating layer may have a thickness of 150 m.

(32) According to an embodiment of the present invention, the transparent electrode layer may have a thickness of 1100 nm.

(33) According to an embodiment of the present invention, the inorganic layer may have a thickness of 10100 nm.

(34) According to an embodiment of the present invention, the polyimide film may include a polyimide resin including: a unit structure derived from at least one aromatic dianhydride selected from among 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (FDA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride (TDA), 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxyl dianhydride (6FCDA) and 4,4-(4,4-isoproylidenediphenoxy)bis(phthalic anhydride) (HBDA), and at least one aromatic dianhydride selected from among pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA) and oxydiphthalic dianhydride (ODPA); and a unit structure derived from at least one aromatic diamine selected from among 2,2-bis[4-(4-aminophenoxy)-phenyl]propane (6HMDA), 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl (2,2-TFDB), 3,3-bis(trifluoromethyl)-4,4-diaminobiphenyl (3,3-TFDB), 4,4-bis(3-aminophenoxy)diphenylsulfone (DBSDA), bis(3-aminophenyl)sulfone (3DDS), bis(4-aminophenyl)sulfone (ODDS), 1,3-bis(3-aminophenoxy)benzene (APB-133), 1,4-bis(4-aminophenoxy)benzene (APB-134), 2,2-bis[3(3-aminophenoxy)phenyl]hexafluoropropane (3-BDAF), 2,2-bis[4(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF), 2,2-bis(3-aminophenyl)hexafluoropropane (3,3-6F), 2,2-bis(4-aminophenyl)hexafluoropropane (4,4-6F) and oxydianiline (ODA), or the polyimide film may include a polyimide resin including: a unit structure derived from the aromatic dianhydride; a unit structure derived from at least one aromatic dicarbonyl compound selected from among p-terephthaloyl chloride (TPC), terephthalic acid, iso-phthaloyl dichloride and 4,4-benzoyl chloride; and a unit structure derived from the aromatic diamine.

(35) According to an embodiment of the present invention, the polyimide film may have an average linear expansion coefficient (CTE) of 30.0 ppm/ C. or less, which was measured at a temperature range of 50250 C. by thermo-mechanical analysis based on a thickness of 50200 m.

(36) The plastic substrate of the present invention may be used in a substrate for forming a color filter, an OLED TFT substrate, a substrate for PV, an upper electrode substrate and the like. In this case, a process of forming a transparent electrode layer on a polyimide film, referred to as an ITO process, must be conducted. Therefore, when the linear expansion coefficient of the polyimide film is high, the polyimide film is extended by the linear expansion coefficient thereof during a high-temperature ITO process, and is then contracted again during a cooling process at room temperature. In this case, when the difference in expansion or contraction between the polyimide film and electrode material increases, the polyimide film is damaged, and the performance of a device is deteriorated. Therefore, it is preferred that the linear expansion coefficient of the polyimide film be low. Considering these points, in the plastic substrate of the present invention, it is preferred that the average linear expansion coefficient of the polyimide film be 30.0 ppm/ C. or less, which was measured at a temperature range of 50250 C. by thermo-mechanical analysis based on a thickness of 50200 m.

(37) The plastic substrate of the present invention may have a surface resistance of 50 /sq. or less, when the thickness of the transparent electrode layer is 100 nm, thus realizing low resistance.

(38) The method of manufacturing a plastic substrate according to another embodiment of the present invention includes the steps of: forming a hard coating layer on one side of a polyimide film including imides of polyimide resin precursors prepared by polymerization of dianhydrides and diamines or polymerization of dianhydrides, aromatic dicarbonyl compounds and diamines, having a yellowness of 5 or less and satisfying a light transmittance of 80% or more at a wavelength of 550 nm, measured by a UV spectrometer; and forming a transparent electrode layer on the other side of the polyimide film, wherein the step of forming the transparent electrode layer includes the step of depositing the transparent electrode layer on the other side of the polyimide film and heat-treating the deposited transparent electrode layer, wherein the deposition or heat treatment is performed at 100 to 250 C.

(39) According to the plastic substrate satisfying the above-mentioned structural characteristics, a process of forming a transparent electrode layer can be performed under high temperature, thus providing a plastic substrate having a lower resistance.

MODE FOR INVENTION

(40) Hereinafter, the present invention will be described in more detail with reference to the following Examples.

Examples 1 to 3

(41) As a reactor, a 100 mL 3-neck round-bottom flask provided with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller and a cooler was filled with 28.78 g of N,N-dimethylacetamide (DMAc) while supplying nitrogen, and then cooled to 0 C., and then 3.2023 g (0.01 mol) of 2,2-TFDB was dissolved in DMAc to prepare a solution, and then this solution was maintained at 0 C. Then, 0.88266 g (0.003 mol) of BPDA was added to the solution and stirred for 1 hour to completely dissolve BPDA, and then 3.10975 g (0.007 mol) of 6FDA was added and completely dissolved. In this case, the concentration of solid matter in the resultant solution was 20 wt %. Thereafter, this solution was stirred at room temperature for 8 hours to obtain a polyamic acid solution having a solution viscosity of 2100 poise at 23 C.

(42) Subsequently, 24 equivalents of acetic anhydride (acetic oxide, manufactured by Samjeon Co., Ltd.), as a curing agent, and 24 equivalents of pyridine (manufactured by Samjeon Co., Ltd.) were added to the polyamic acid solution, and then this polyamic acid solution was heated in a temperature range of 20180 C. at a heating rate of 10 C./min for 110 hours to imidize the polyamic acid solution. Then, 30 g of this imidized solution was mixed with 300 g of a nonpolar solvent such as water or alcohol (methanol, ethanol or the like) and then precipitated to obtain a solid precipitate. Then, the solid precipitate was filtered and pulverized to be finely powdered, and then the finely-powdered precipitate was dried in a vacuum oven at 80100 C. for 6 hours to obtain about 8 g of solid polyimide resin powder. The obtained solid polyimide resin powder was dissolved in 32 g of a polymerization solvent such as DMAc or DMF to obtain a polyimide solution. This polyimide solution was heated in a temperature range of 40400 C. at a heating rate of 10 C./min for 18 hours to obtain a polyimide film having a thickness of 100 m.

(43) Then, an organic-inorganic hybrid hard coat composition including polysilazane or silica and acrylic resin or urethane resin was applied to one side of the obtained polyimide film (serving as a substrate) to form a hard coating layer.

(44) Next, an ITO electrode layer was formed by depositing the other side of the polyimide film with ITO using sputtering and then heat-treating this polyimide film using rapid thermal annealing (RTA).

(45) The thicknesses of the hard coating layers and ITO electrode layers obtained in Examples 1 to 3 depending on the change of process temperature are summarized in Table 1 below.

Example 4

(46) As a reactor, a 100 mL 3-neck round-bottom flask provided with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller and a cooler was filled with 28.78 g of N,N-dimethylacetamide (DMAc) while supplying nitrogen, and then cooled to 0 C., and then 3.2023 g (0.01 mol) of 2,2-TFDB was dissolved in DMAc to prepare a solution, and then this solution was maintained at 0 C. Then, 0.88266 g (0.003 mol) of BPDA was added to the solution and stirred for 1 hour to completely dissolve BPDA, and then 1.33275 g (0.003 mol) of 6FDA was added and completely dissolved. Then, 0.8121 g (0.004 mol) of TPC was added to the resultant solution to obtain a polyamic acid solution having a solid content of 15 wt %.

(47) A plastic substrate film was formed in the same manner as in Example 1 using the obtained polyamic acid solution.

(48) The thicknesses of the polyimide film, hard coating layer and ITO electrode layer obtained in Example 4 are summarized in Table 1 below.

Example 5

(49) A polyamic acid solution was prepared in the same manner as in Example 1, except that 6FCDA (9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxyl dianhydride) was used as a composition for preparing the same instead of 6FDA. Then, a plastic substrate film was formed using the prepared polyamic acid solution in the same manner as in Example 1.

(50) The thicknesses of the polyimide film, hard coating layer and ITO electrode layer obtained in Example 5 are summarized in Table 1 below.

Comparative Example 1

(51) As a reactor, a 100 mL 3-neck round-bottom flask provided with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller and a cooler was filled with 28.78 g of N,N-dimethylacetamide (DMAc) while supplying nitrogen, and then cooled to 0 C., and then 3.2023 g (0.01 mol) of 2,2-TFDB was dissolved in DMAc to prepare a solution, and then this solution was maintained at 0 C. Then, 0.88266 g (0.003 mol) of BPDA was added to the solution and stirred for 1 hour to completely dissolve BPDA, and then 3.10975 g (0.007 mol) of 6FDA was added and completely dissolved. In this case, the concentration of solid matter in the resultant solution was 20 wt %. Thereafter, this solution was stirred at room temperature for 8 hours to obtain a polyamic acid solution having a solution viscosity of 2100 poise at 23 C.

(52) Subsequently, 24 equivalents of acetic anhydride (acetic oxide, manufactured by Samjeon Co., Ltd.), as a curing agent, and 24 equivalents of pyridine (manufactured by Samjeon Co., Ltd.) were added to the polyamic acid solution, and then this polyamic acid solution was heated in a temperature range of 20180 C. at a heating rate of 10 C./min for 110 hours to imidize the polyamic acid solution. Then, 30 g of this imidized solution was mixed with 300 g of a nonpolar solvent such as water or alcohol (methanol, ethanol or the like) and then precipitated to obtain a solid precipitate. Then, the solid precipitate was filtered and pulverized to be finely powdered, and then the finely-powdered precipitate was dried in a vacuum oven at 80100 C. for 6 hours to obtain about 8 g of solid polyimide resin powder. The obtained solid polyimide resin powder was dissolved in 32 g of a polymerization solvent such as DMAc or DMF to obtain a polyimide solution. This polyimide solution was heated in a temperature range of 40400 C. at a heating rate of 10 C./min for 18 hours to obtain a polyimide film having a thickness of 100 m.

Comparative Example 2

(53) A plastic substrate was manufactured in the same manner as in Example 1, except that a hard coating layer was not formed.

Comparative Example 3

(54) A PET film, which is used as a substrate film for touch screen panels, was formed.

(55) The physical properties of the plastic substrate films prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were measured as follows, and the results thereof are summarized in Table 1 below.

(56) (1) Coefficient of Linear Expansion (CTE)

(57) The linear expansion coefficient of each of the prepared polyimide films was measured at 50250 C. using a thermo-mechanical analyzer (TMA) (Q400, manufactured by TA Instrument Corporation).

(58) Specimen size: 20 mm4 mm

(59) Temperature: room temperature (30 C.)250 C., heating rate: 10/min

(60) Load: 10 g (weight of a pendulum hanging on specimen)

(61) (2) Yellowness

(62) The yellowness of each of the prepared polyimide films was measured using a UV spectrometer (Cary 100, manufactured by Varian Corporation) based on ASTM E313 standard.

(63) (3) Transmittance

(64) The visible light transmittance of each of the prepared polyimide films was measured using a UV spectrometer (Cary 100, manufactured by Varian Corporation).

(65) (4) Surface Hardness

(66) The surface hardness of each of the prepared films was measured using a pencil hardness meter based on ASTM D3363 standard under the condition that a pencil was pressed and moved on each of the polyimide films at a load of 1 Kg and an inclination of 45.

(67) (5) Surface Resistance

(68) The surface resistance of each of the prepared films was measured ten times using a resistance meter (CMT-SR 2000N (Advanced Instrument Technology (AIT) Corporation), 4-Point Probe System, measuring range: 1010.sup.31010.sup.5), and the average resistance value thereof was calculated.

(69) TABLE-US-00001 TABLE 1 Thickess Thickness Transmittance of Thickness of of ITO CTE of Yellowness polyimide of H/C electrode polyimide process polyimide of Surface Molar film layer layer film temperature film polyimide Surface resistance Class. Composition ratio (m) (m) (nm) (550 nm) ( C.) (ppm/ C.) film hardness (/sq.) Ex. 1 6FDA + BPDA/ 7:3:10 100 5 100 87.51 100 17.22 2.78 5 30 2,2-TFDB Ex. 2 6FDA + BPDA/ 7:3:10 100 30 100 86.81 150 17.22 2.82 9 21 2,2-TFDB Ex. 3 6FDA + BPDA/ 7:3:10 100 30 100 85.43 250 17.22 3.01 9 13 2,2-TFDB Ex. 4 (6FDA + BPD 3:3:4:10 100 25 100 87.1 100 14.75 4.56 9 28 A + TPC/2,2- TFDB) Ex. 5 (6FDA + 3:3:4:10 102 25 100 86.4 100 15.68 4.35 8 30 BPDA + TPC/2,2-TFDB) Comp. 6FDA + BPDA/ 7:3:10 100 88.71 17.22 2.66 3 Ex. 1 2,2-TFDB Comp. 6FDA + BPDA/ 7:3:10 100 ITO 83.60 25 17.22 3.26 3 55 Ex. 2 2,2-TFDB 100 nm Comp. PET 188 50 20 88.55 25 1.98 H 260 Ex. 2

(70) From the results of Table 1 above, it can be ascertained that each of the plastic substrates of Examples 1 to 3, each including a polyimide film, a hard coating layer and a transparent electrode layer, maintains transparency and exhibits excellent surface hardness. Further, it can be ascertained that each of the plastic substrates of Examples 1 to 3 is stable at an ITO process temperature of 250 C. or lower. That is, it can be ascertained that each of the plastic substrates of Examples 1 to 3 exhibits good results in yellowness even at such high temperature. In contrast, it can be ascertained that a PET film, which is used as a conventional plastic substrate for transparent electrodes, causes yellowing at a process temperature of about 100 C., and has an excessively high surface resistance value when it is applied as an electrode substrate.

(71) The surface resistance of the plastic substrate is influenced by the thickness of an electrode layer or by the process temperature at the time of forming the electrode layer. According to Examples 1 to 5, it can be ascertained that, when electrode layers having the same thickness are formed, surface resistance becomes low with the increase in process temperature. From the results, it can be ascertained that, according to the plastic substrate of the present invention, a process of forming a transparent electrode layer can be performed under high temperature, thus lowering surface resistance.