POLYIMIDE PRESERVATIVE SOLUTION AND POLYIMIDE FILM PRODUCED WITH IT

Description

BACKGROUND OF THE INVENTION1. Field of the Invention

[0001] The present invention relates to a polyimide precursor solution having a high solid content and a low viscosity and a method for forming a polyimide film prepared using the same.2. Description of the Related Art

[0002] Polyimide (PI) is a polymer having a relatively low crystallinity or mostly noncrystalline structure, which has an advantage that it is easy to synthesize, can form a thin film and does not require a crosslinking agent for curing. Also, polyimide is a polymeric material that has excellent heat resistance and chemical resistance, excellent mechanical properties, electrical properties and dimensional stability due to its rigid chain structure in addition to its transparency. Therefore, it is widely used as electrical and electronic materials for automobiles, aerospace, flexible circuit boards, liquid crystal alignment films for LCDs, and adhesives and coatings.

[0003] Generally, a flexible device is manufactured by applying a polyimide precursor solution onto a carrier substrate and then curing to form a film, completing the device through a subsequent process, and then removing the completed device from the carrier substrate.

[0004] Therefore, storage stability at room temperature of the polyimide precursor solution is particularly important. If the storage stability of the polyimide precursor solution is poor and the process viscosity is changed, application and curing processes of the polyimide precursor solution becomes unstable. Polyamic acid, which is a polyimide precursor, is known to have poor storage stability because it has a carboxylic acid that promotes hydrolysis adjacent to an amide bond.

[0005] However, a flexible device involving a high-temperature process is required to have high heat resistance. In particular, in the case of an organic light emitting diode (OLED) device using a low temperature polysilicon (LTPS) process, the process temperature may approach 500 °C. However, at such a temperature, the polyimide even having excellent heat resistance tends to undergo thermal decomposition.

[0006] Further, in the case where a light emitted from a display device is emitted through a film substrate (for example, a bottom emission type organic EL or the like), transparency of the film substrate is required. In particular, it is required to have a high light transmittance in a wavelength region of 400 nm or less which is a visible light region. In addition, when light passes through a retardation film or a polarizing plate, liquid crystal displays, touch panels, and the like, for example, are required to have high transparency.

[0007] However, since the imidized polyimide resin has a low solubility, it is difficult to prepare it in a solution state, and its use for coating may be restricted. However, if solubility is to be improved, heat resistance can be reduced, and then if heat resistance is to be improved, light transmittance is lowered.

[0008] Therefore, there is a need for a polyimide precursor solution which can exhibit excellent chemical resistance and storage stability due to reduced hydrolysis of the polyimide precursor solution in order to manufacture flexible devices, and simultaneously improve heat resistance and transparency while maintaining mechanical properties.

[0009] US 2017 / 137 571 A1 relates to a a method for producing a polyimide film including (i) applying a polyamic acid solution composition, which includes at least one of the following solvents: N-methylformamide, N,N-dimethylpropionamide, N,N-dimethylisobutylamide and tetramethylurea, and a polyamic acid, to a substrate; and then (ii) imidizing the polyamic acid by subjecting the composition to heat treatment, to obtain the polyimide film.

[0010] US 2017 / 096 530 A1 relates to a composition for the production of a polyimide film for a flexible board of a photoelectronic device having good dimensional stability without causing a rise in the stress of a substrate despite high-temperature heat treatment.

[0011] WO 2017 / 068 936 A1 relates to a resin composition for a display substrate, a heat-resistant resin film using the resin composition, an organic EL display substrate, and a method of manufacturing an organic EL display, wherein a heat-resistant resin or a precursor thereof may be a polyimide or a polyimide precursor.SUMMARY OF THE INVENTION

[0012] A problem to be solved by the present invention is to provide a polyimide precursor solution having a high solid content and a low viscosity.

[0013] Other problem to be solved by the present invention is to provide a method for forming a polyimide film produced using the polyimide precursor solution.

[0014] In order to solve the problems, the present invention provides a polyimide precursor solution consisting of a polyimide precursor and an amide-based organic solvent having a positive distribution coefficient (Log P) at 25 °C and a density of 1 g / cm 3< or less, the amide-based organic solvent is diethylpropionamide (DEPA), the polyimide precursor solution has a solid content of 12% by weight, and a viscosity of 7000 mPa*s (cP) or less, the polyimide precursor is a polyamic acid consisting of a repeating structure of the following formula 1: wherein, X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 4,4'-oxydiphthalic anhydride, N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide] or 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, and Y consists of a divalent organic derived from 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine, wherein, when X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, the viscosity is 2900 mPa*s (cP) and the weight average molecular weight is 72,000 g / mol; when X is a tetravalent organic group derived from 4,4'-oxydiphthalic anhydride, the viscosity is 3250 mPa*s (cP) and the weight average molecular weight is 95,000 g / mol; when X is a tetravalent organic group derived from N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide], the viscosity is 3020 mPa*s (cP) and the weight average molecular weight is 91,200 g / mol; and when X is a tetravalent organic group derived from 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, the viscosity is 3050 mPa*s (cP) and the weight average molecular weight is 81,000 g / mol, the distribution coefficient (Log P) is calculable using an ACD / Log P module of ACD / Percepta platform from ACD / Labs, wherein the ACD / Log P module uses an algorithm based on QSPR (Quantitative Structure-Property Relationship) methodology using 2D molecular structures, the density is as measured according to Standard ASTM D1475, the viscosity is as measured at 25 °C with a Brookfield rotational viscometer, and the weight average molecular weight is as measured by using Gel Permeation Chromatography (GPC) and eluent THF: DMF 50:50 by volume.

[0015] In order to solve other problem of the present invention, there is provided a method for forming a polyimide film produced using the polyimide precursor solution.EFFECT OF THE INVENTION

[0016] According to the present invention, by using an amide-based organic solvent having a positive distribution coefficient and a density of 1 g / cm 3< or less in the production of a polyimide precursor, an interaction between a polyamic acid which is a polyimide precursor and the organic solvent is alleviated to reduce a viscosity of the polyimide precursor solution. Therefore, the polyimide precursor solution having a high solid content and a low viscosity can be obtained. Further, according to the present invention, the difference in polarity is reduced due to the amphiphilic characteristic of the amide-based solvent having a positive Log P, so that the phase separation phenomenon between the polyamic acid and the organic solvent can be suppressed.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figs. 1 and 2 show the results of SEM observation of cross-sections of the films of Comparative Example 1 and Reference Example 1 after treatment with a focused ion beam (FIB).DETAILED DESCRIPTION OF THE INVENTION

[0018] In the following description of the present invention, detailed description of known functions will be omitted if it is determined that it may obscure the gist of the present invention.

[0019] Herein, the term "substituted" means that at least one hydrogen atom contained in the compound or the organic group is substituted with a substituent selected from the group consisting of a halogen atom, an alkyl group having 1 to 10 carbon atoms, a halogenated alkyl group, a cycloalkyl group having 3 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, a hydroxyl group, an alkoxy group having 1 to 10 carbon atoms, a carboxylic group, an aldehyde group, an epoxy group, a cyano group, a nitro group, an amino group, a sulfonic group or a derivative thereof.

[0020] The polyimide film is one of the materials attracting attention for use as flexible display substrates. The manufacturing process of the polyimide film substrate is as follows. First, a polyamic acid solution that is a polyimide precursor solution is coated on a carrier substrate and thermally cured to form a polyimide film on the carrier substrate. Thereafter, a process necessary for manufacturing the device is performed on the polyimide film, and then the polyimide film is delaminated from the carrier substrate to obtain a flexible display. The most important step of this process is forming a polyimide film on a carrier substrate. When the polyamic acid solution is coated on the carrier substrate, it should be coated with low surface roughness without Mura and bubble generation. However, the polyamic acid has a high polarity, and also a commonly used solvent (for example, N-methylpyrrolidone) has a relatively high polarity. Accordingly, due to the interaction between the solvent and the polyamic acid, the polyamic acid solution has a high viscosity relative to its solid content. This makes it very difficult to control the bubbles generated during coating. In addition, as the solution has a high water-absorption rate, it absorbs water when being left after coating and white turbidity of solution is occurred. In case of thermally curing the coating layer with white turbidity, it may have a high surface roughness and thus defects in the display process are occurred. Therefore, in order to lower the viscosity of the solution to improve processability, it is inevitably required to lower the solid content. However, when the solid content is lowered, the ratio of the solvent is relatively increased, so that a large amount of solvent evaporates during thermal curing, resulting in coating defects.

[0021] In order to solve the above conventional problems, the present invention provides a polyimide precursor solution consisting of polyimide precursor and an amide-based organic solvent having a positive distribution coefficient (Log P) and a density of 1 g / cm 3< or less, the amide-based organic solvent is diethylpropionamide (DEPA), the polyimide precursor solution has a solid content of 12% by weight, and a viscosity of 7000 mPa*s (cP) or less, the polyimide precursor is a polyamic acid consisting of a repeating structure of the following formula 1: wherein, X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 4,4'-oxydiphthalic anhydride, N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide] or 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, and Y consists of a divalent organic derived from 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine, wherein, when X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, the viscosity is 2900 mPa*s (cP) and the weight average molecular weight is 72,000 g / mol; when X is a tetravalent organic group derived from 4,4'-oxydiphthalic anhydride, the viscosity is 3250 mPa*s (cP) and the weight average molecular weight is 95,000 g / mol; when X is a tetravalent organic group derived from N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide], the viscosity is 3020 mPa*s (cP) and the weight average molecular weight is 91,200 g / mol; and when X is a tetravalent organic group derived from 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, the viscosity is 3050 mPa*s (cP) and the weight average molecular weight is 81,000 g / mol, the distribution coefficient (Log P) is calculable using an ACD / Log P module of ACD / Percepta platform from ACD / Labs, wherein the ACD / Log P module uses an algorithm based on QSPR (Quantitative Structure-Property Relationship) methodology using 2D molecular structures, the density is as measured according to Standard ASTM D1475, the viscosity is as measured at 25 °C with a Brookfield rotational viscometer, and the weight average molecular weight is as measured by using Gel Permeation Chromatography (GPC) and eluent THF: DMF 50:50 by volume.

[0022] The distribution coefficient (Log P) can be calculated using an ACD / Log P module of ACD / Percepta platform from ACD / Labs. The ACD / Log P module uses an algorithm based on QSPR (Quantitative Structure-Property Relationship) methodology using 2D molecular structures.

[0023] A density of the organic solvent is 1 g / cm 3< or less as measured according to Standard ASTM D1475. If the density is 1 g / cm 3< or more, the relative viscosity may be increased and the process efficiency may be reduced.

[0024] In the present invention, due to the amphiphilic characteristic of the amide-based solvent having a positive distribution coefficient (Log P), the polarity difference between the polyamic acid and the organic solvent can be reduced and the phase separation between the polyamic acid and the organic solvent can be suppressed.

[0025] In the present invention, by using an amide-based organic solvent having a positive Log P and a density of 1 g / cm 3< or less as a polymerization solvent for preparing a polyimide precursor and incorporating the amide-based organic solvent in a polyimide precursor solution, the viscosity can be lowered even at a high solid content of 12 by weight. The viscosity of the polyimide precursor solution according to the present invention is 7,000 mPa*s (cP) or less, preferably 6,000 mPa*s (cP) or less, and more preferably 5,000 mPa*s (cP) or less at the above solid content.

[0026] If the viscosity of the polyimide precursor solution is high, efficiency of degassing during processing of the polyimide film is lowered, so that not only process efficiency is lowered but also electrical, optical, and mechanical properties are deteriorated. Accordingly, in the present invention, by using an amide-based solvent having a positive Log P value and a density of 1 g / cm 3< or less, there is provided a polyimide precursor solution having a low viscosity even at a high solid content. As a result, the coating defects due to the formation of bubbles can be reduced.

[0027] The amide-based solvent having a positive Log P and a density of 1 g / cm 3< or less is diethylpropionamide (DEPA). This is because due to its relatively low polarity and low boiling point, coating property is excellent, volatility of the solvent is good even at a low temperature and the amount of the solvent remaining in the film after film formation is low.

[0028] On the other hand, if polyamic acid meets with water, a salt is formed and white turbidity occurs. As the white turbidity becomes severe, aggregation may occur. When cured in this state, a film with rough surface may be formed. That is, when the haze of the coating of the polyimide-based solution is too high, the coating property is poor and surface roughness may occur after curing. Such improvement of the white turbidity may contribute to improvement of the surface properties after curing.

[0029] When the distribution coefficient value is positive, it means that the polarity of the solvent is hydrophobic. According to the studies of the inventors of the present invention, if a specific solvent having a positive distribution coefficient is used to prepare a polyimide precursor solution, the formation of salt by the polyamic acid and water or a polar organic solvent can be suppressed due to low affinity to water, and as a result, the haze can be improved. Therefore, the polyimide-based film according to the present invention can have high heat resistance and mechanical properties together with excellent transparency.

[0030] In the process of coating a polyimide precursor solution on a glass substrate, dewetting of the solution may be occurred due to shrinkage of the coating layer during curing or leaving the coating solution. This dewetting phenomenon of the coating solution leads to a variation in the film thickness, resulting in insufficient bending resistance of the film. Therefore, film breakage may occur or edge cracking may appear when cutting. That is, there may be problems of poor processability and lowered yield. If the amide-based solvent having a positive distribution coefficient value according to the present invention is used, the dewetting phenomenon of the polyimide precursor solution can be improved.

[0031] If polar fine foreign substances are introduced to the substrate on which the polyimide precursor solution containing a polar solvent having a negative Log P is coated, the polarity of the foreign substances may cause sporadic coating cracks or thickness change around the portions where the foreign substances exist. On the contrary, when a hydrophobic solvent having a positive log P is used, coating cracks or thickness changes can be reduced or suppressed even when polar fine foreign substances are introduced.

[0032] According to the present invention, the polyimide precursor is a polyamic acid consisting of a repeating structure represented by the following formula 1.

[0033] In the formula 1, X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 4,4'-oxydiphthalic anhydride, N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide] or 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, and Y consists of a divalent organic derived from 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine, wherein, when X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, the viscosity is 2900 mPa*s (cP) and the weight average molecular weight is 72,000 g / mol; when X is a tetravalent organic group derived from 4,4'-oxydiphthalic anhydride, the viscosity is 3250 mPa*s (cP) and the weight average molecular weight is 95,000 g / mol; when X is a tetravalent organic group derived from N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide], the viscosity is 3020 mPa*s (cP) and the weight average molecular weight is 91,200 g / mol; and when X is a tetravalent organic group derived from 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, the viscosity is 3050 mPa*s (cP) and the weight average molecular weight is 81,000 g / mol.

[0034] The reaction of tetracarboxylic dianhydride and diamine may be carried out by a conventional polymerization method of polyimide precursor such as solution polymerization. Specifically, diamine is dissolved in an organic solvent and then is subjected to a polymerization reaction by adding tetracarboxylic dianhydride. The reaction may be carried out in an inert gas or a nitrogen stream, and may be carried out under anhydrous conditions.

[0035] The polymerization reaction may be carried out at a temperature of -20 °C to 60 °C, preferably 0 °C to 45 °C. If the reaction temperature is too high, the reactivity may become high and the molecular weight may become large, and the viscosity of the precursor composition may increase, which may be unfavorable in the process.

[0036] Then, the obtained polyimide precursor as a result of the polymerization reaction may be imidized to prepare a transparent polyimide film. At this time, the imidization process may be a chemical imidization or thermal imidization process. Preferably, the polyimide precursor solution is coated on a substrate, followed by heat treatment to imidize the polyamic acid.

[0037] The polyimide precursor composition may be in the form of a solution in which the polyimide precursor is dissolved in an organic solvent. For example, when the polyimide precursor is synthesized in an organic solvent, the solution may be the reaction solution as obtained, or may be obtained by diluting this reaction solution with another solvent. When the polyimide precursor is obtained as a solid powder, it may be dissolved in an organic solvent to prepare a solution.

[0038] A method for producing a film with a polyimide precursor solution according to the present invention comprises the steps of: applying the polyimide precursor solution onto a substrate; and heat treating the applied polyimide precursor solution.

[0039] As the substrate, a glass substrate, a metal substrate or a plastic substrate can be used without any particular limitation. Among them, a glass substrate may be preferable which is excellent in thermal and chemical stability during the imidization and curing process for the polyimide precursor and can be easily separated even without any treatment with additional release agent without damaging the formed polyimide film after curing.

[0040] The applying step may be carried out according to a conventional application method. Specifically, a spin coating method, a bar coating method, a roll coating method, an air knife method, a gravure method, a reverse roll method, a kiss roll method, a doctor blade method, a spray method, a dipping method or a brushing method may be used. Of these, it is more preferable to carry out by a casting method which allows a continuous process and enables to increase the imidization rate of the polyimide.

[0041] In addition, the polyimide precursor solution may be applied on the substrate in a thickness such that the polyimide film to be finally produced has a thickness suitable for a display substrate.

[0042] Specifically, it may be applied in an amount such that the thickness is 10 to 30 µm. After the application of the polyimide precursor solution, a drying process for removing the solvent remained in the polyimide precursor solution may be further optionally performed prior to the curing process.

[0043] The drying process may be carried out according to a conventional method. Specifically, the drying process may be carried out at a temperature of 140 °C or lower, or from 80 °C to 140 °C. If the drying temperature is lower than 80 °C, the drying process becomes longer. If the drying temperature exceeds 140 °C, the imidization proceeds rapidly, making it difficult to form a polyimide film having a uniform thickness.

[0044] The polyimide precursor solution is applied on a substrate and heat-treated in an IR oven, in a hot air oven, or on a hot plate. The heat treatment temperature may range from 300 to 500 °C, preferably from 320 to 480 °C, and may be performed in a multi-step heating process within the above temperature range. The heat treatment process may be performed for 20 to 70 minutes, and preferably for 20 to 60 minutes.

[0045] The organic solvent contained in the polyimide precursor solution of the present invention is the same as the organic solvent used in the synthesis reaction. The organic solvent is an amide-based organic solvent having a positive Log P and a density of 1 g / cm 3< or less, and is diethyl propionamide (DEPA).

[0046] The polyimide-based film may have a haze of 1 or less, preferably 0.9 or less, or 0.7 or less, more preferably 0.5 or less, and thus the transparency is improved. At this time, the thickness of the polyimide film may be 8 to 15 µm, preferably 10 to 12 µm.

[0047] In addition, the transmittance to light at a wavelength of 380 to 760 nm in a film thickness range of 5 to 30 µm may be 70% or more.

[0048] Further, the polyimide film according to the present invention may have excellent heat resistance characteristics depending on the temperature change. For example, the polyimide film according to the present invention may have a thermal expansion coefficient of -20 to 90 ppm / °C and a thermal decomposition temperature Td_1% of 400 °C or more or 500 °C or more at which the weight decreases by 1%, after the n+1 times heating and cooling processes in a temperature range of 100 to 450 °C.

[0049] The polyimide-based film has excellent mechanical properties such as a tensile modulus of at least about 1 GPa or at least 3 GPa, a tensile strength of at least about 30 MPa or at least about 100 MPa or at least about 150 MPa, and a maximum elongation of about 15%, or about 10% or more.

[0050] Accordingly, the polyimide-based film can be applied to display substrates and devices. Specifically, the device may be a flexible display device such as any solar cell having a flexible substrate (e.g., a flexible solar cell), organic light emitting diode (OLED) lighting (e.g., flexible OLED lighting), any semiconductor device having a flexible substrate, or an organic electroluminescent device, an electrophoretic device, or an LCD device having a flexible substrate.

[0051] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention.

[0052] The solvents used in Examples and Comparative Examples of the present invention are shown in Table 1. [Table 1]DMPADEPADMAcNMPBoiling point (bp) (°C)175194166202Flash point (°C)65.8±9.6573.6±9.6563.8±9.686.1±0.0Pka-0.41±0.70-0.41±0.70-0.41±0.70-0.41±0.20Density (g / cm 3< )0.876±0.060.870±0.060.880±0.061.029±0.06Log P0.256±0.2491.275±0.249-0.253±0.249-0.637±0.349Vapor pressure (Pa (Torr))149 (1.12)59.2 (0.444)241 (1.81)39.9 (0.299)

[0053] In Table 1, the abbreviations have the following meanings: DMPA: Dimethylpropionamide DEPA: Diethylpropionamide DMAc: Dimethylacetamide NMP: 1-Methyl-2-pyrrolidone

[0054] A viscosity of the polyimide precursor solution was measured at 25 °C with a Brookfield rotational viscometer.

[0055] A weight average molecular weight of the polyamic acid was determined by using Gel Permeation Chromatography (GPC) and eluent (THF: DMF 50:50 by volume).<Reference Example 1>

[0056] 279 g of DMPA was charged in a reactor through which nitrogen gas flows, and then 29 g (0.091 mol) of TFMB (2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine) was added and dissolved while the temperature of the reactor was maintained at 25 °C. At the same temperature, 20 g (0.091 mol) of PMDA (Pyromellitic Dianhydride) was added to the TFMB solution and stirred for a predetermined time. DMPA was added in an amount such that the solid content of the polyimide precursor solution obtained from the reaction was 12% by weight. The resulting polyimide precursor solution had a viscosity of 5,010 mPa*s (cP), and the weight average molecular weight of the polyimide precursor, i.e., the polymerized polyamic acid was 101,000 g / mol.<Reference Examples 2 to 10>

[0057] A polyimide precursor solution was prepared in the same manner as in Reference Example 1 except that the tetracarboxylic dianhydride and the diamine shown in Table 2 were used. The viscosity of the polyimide precursor solution and the molecular weight of the polyamic acid are shown in Table 2. [Table 2]Example No.DianhydrideDiamineSolventSolid content (wt%)Viscosity (mPa*s (cP))Mw1PMDATFMBDMPA125010101,0002BPFATFMBDMPA12320070,5003ODPATFMBDMPA12460097,1004TBIS-BANTFMBDMPA12390093,00056FDATFMBDMPA12510078,0006PMDA-HSTFMBDMPA12450079,0007TAHQTFMBDMPA124800101,0008DSDATFMBDMPA12360089,0009BPDAPDADMPA126800100,00010BPDATFMBDMPA126700107000

[0058] In Table 2, the abbreviations have the following meanings: PMDA: pyromellitic dianhydride BPFA: 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride ODPA: 4,4'-oxydiphthalic anhydride TBIS-BAN: N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide] 6FDA: 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride PMDA-HS: 1R,2S,4S,5R-cyclohexanetetracarboxylic dianhydride TAHQ: hydroquinone bis(trimellitate anhydride) DSDA: 3,3',4,4'-Diphenylsulfonetetracarboxylic dianhydride TFMB: 2,2'-bis(trifluoromethyl)benzidine PDA: para-phenylenediamine BPDA: 3,3',4,4'-Biphenyltetracarboxylic dianhydride <Reference Example 11>

[0059] 279 g of DEPA was charged in a reactor through which nitrogen gas flows, and 29 g (0.091 mol) of TFMB (2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine) was added and dissolved while the temperature of the reactor was maintained at 25 °C. At the same temperature, 20 g (0.091 mol) of PMDA (Pyromellitic Dianhydride) was added to the TFMB solution and stirred for a predetermined time. DEPA was added in an amount such that the solid content of the polyimide precursor solution obtained from the reaction was 12% by weight. The resulting polyimide precursor solution had a viscosity of 4300 mPa*s (cP), and the weight average molecular weight of the polymerized polyamic acid was 99,000 g / mol.<(Reference) Examples 12 to 20>

[0060] A polyimide precursor solution was prepared in the same manner as in Reference Example 11 except that the tetracarboxylic dianhydride and the diamine shown in Table 3 were used. The viscosity of the polyimide precursor solution and the molecular weight of the polyamic acid are shown in Table 3. [Table 3]Exampl e No.DianhydrideDiamineSolventSolid content (wt%)Viscosity (mPa*s (cP))Mw11 (Refere nce)PMDATFMBDEPA12430099,00012BPFATFMBDEPA12290072,00013ODPATFMBDEPA12325095,00014TBIS-BANTFMBDEPA12302091,20015 (Refere nce)6FDATFMBDEPA12460077,00016 (Refere nce)PMDA-HSTFMBDEPA12381073,00017 (Refere nce)TAHQTFMBDEPA12420098,90018DSDATFMBDEPA12305081,00019 (Refere nce)BPDAPDADEPA124800107,00020 (Refere nce)BPDATFMBDEPA124600108,000 <Comparative Example 1>

[0061] 279 g of DMAc was charged in a reactor through which nitrogen gas flows, and 29 g (0.091 mol) of TFMB (2,2'-bis (trifluoromethyl)-4,4'-biphenyl diamine) was added and dissolved while the temperature of the reactor was maintained at 25 °C. At the same temperature, 20 g (0.091mol) of PMDA (Pyromellitic Dianhydride) was added to the TFMB solution and stirred for a predetermined time. DMAc was added in an amount such that the solid content of the polyimide precursor solution obtained from the reaction was 12% by weight. The resulting polyimide precursor solution had a viscosity of 12,000 mPa*s (cP), and the weight average molecular weight of the polymerized polyamic acid was 100,400 g / mol.< Comparative Examples 2 to 10>

[0062] A polyimide precursor solution was prepared in the same manner as in Comparative Example 1 except that the tetracarboxylic dianhydride and the diamine shown in Table 4 were used. The viscosity of the polyimide precursor solution and the molecular weight of the polyamic acid are shown in Table 4. [Table 4]Comp. Example No.DianhydrideDiamin eSolventSolid content (wt%)Viscosity (mPa*s (cP))Mw1PMDATFMBDMAc1212,000100,4002BPFATFMBDMAc1210,60090,5003ODPATFMBDMAc128,10096,5004TBIS-BANTFMBDMAc128,20094,50056FDATFMBDMAc128,90098,0006PMDA-HSTFMBDMAc127,10076,5007TAHQTFMBDMAc128,600100,0008DSDATFMBDMAc127,30091,0009BPDAPDADMAc1210,090102,00010BPDATFMBDMAc1216,000996,000 <Comparative Example 11>

[0063] 279 g of NMP was charged in a reactor through which nitrogen gas flows, and 29 g (0.091 mol) of TFMB (2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine) was added and dissolved while the temperature of the reactor was maintained at 25 °C. At the same temperature, 20 g (0.091mol) of PMDA (Pyromellitic Dianhydride) was added to the TFMB solution and stirred for a predetermined time. NMP was added in an amount such that the solid content of the polyimide precursor solution obtained from the reaction was 12% by weight. The resulting polyimide precursor solution had a viscosity of 16,000 mPa*s (cP), and the molecular weight of the polymerized polyamic acid was 109,000 g / mol.<Comparative Examples 12 to 20>

[0064] A polyimide precursor solution was prepared in the same manner as in Comparative Example 11 except that the tetracarboxylic dianhydride and the diamine shown in Table 5 were used. The viscosity of the polyimide precursor solution and the molecular weight of the polyamic acid are shown in Table 5. [Table 5]Comp. Example No.Dianhydrid eDiamineSolventSolid content (wt%)Viscosity (mPa*s (cP))Mw11PMDATFMBNMP1216,000109,00012BPFATFMBNMP127,30087,00013ODPATFMBNMP129,800102,00014TBIS-BANTFMBNMP128,90099,000156FDATFMBNMP128,30082,00016PMDA-HSTFMBNMP128,20080,50017TAHQTFMBNMP129,700103,00018DSDATFMBNMP128,90093,00019PMDAPDANMP1215,000108,00020BPDATFMBNMP1219,500110,000

[0065] As can be seen from the results of (Reference) Examples 1 to 20 and Comparative Examples 1 to 20, it can be seen that the polyimide precursor solution containing the organic solvent according to the present invention has a low viscosity even at high solid content. On the other hand, it can be seen that since the polyimide precursor solution of the comparative example contains a polar solvent having a negative distribution coefficient, the viscosity thereof is higher than that of Examples at same solid content. It means that the defoaming effect of the bubbles in the solution during production of the polyimide film may be reduced and pores may be generated due to defective defoaming in the film after coating.<Experimental Example 1>

[0066] Figs. 1 and 2 show the results of SEM observation of cross-sections of the films of Comparative Example 1 and Reference Example 1 after treatment with FIB. It can be seen that micro-sized pores exist in the cross-section of the film of Comparative Example 1 in which the defoaming effect is reduced due to the relatively high viscosity of the polyimide precursor solution, while the pores are not present in the cross-section of the film of (Reference) Example 1.<Experimental Example 2>

[0067] Each of the polyimide precursor solutions prepared in Reference Examples 1, 2, 5, 9 and 10 was spin-coated onto a glass substrate. The glass substrate coated with the polyimide precursor solution was placed in an oven and heated at a rate of 2 °C / min. The curing process was carried out by maintaining at 80 °C for 15 minutes, 150 °C for 30 minutes, 220 °C for 30 minutes, and 380 °C for 1 hour. After completion of the curing process, the glass substrate was immersed in water to remove the film formed on the glass substrate, and then dried in an oven at 100 °C to prepare a polyimide film.

[0068] The CTE, the thermal decomposition temperature, the elongation, the tensile strength, the tensile modulus and the transmittance of the film were measured by the following methods, and the results are shown in Table 6.<Measurement of CTE>

[0069] The film was cut into 5 x 20 mm to prepare a sample, and then the sample was loaded using an accessory. A length of the film to be actually measured was equal to 16 mm. A pulling force was set at 0.02 N. The first temperature-rising step was carried out at a heating rate of 5 °C / min from 100 to 400 °C, then cooling was carried out at a cooling rate of 4 °C / min from 400 to 100 °C. The change in the thermal expansion was measured by TMA (Q400, TA Company).<Measurement of thermal decomposition temperature Td_1%>

[0070] The thermal decomposition temperature was measured by the method of ISO 11359.<Measurement of elongation, tensile strength and tensile modulus>

[0071] Universal Testing Machine (UTM) of Zwick was used to measure the mechanical properties (tensile modulus, tensile strength, elongation) of the film. The film was cut to a width of 5 mm and a length of 60 mm or more. A distance between the grips was set to 40 mm and the sample was pulled at a speed of 20 mm / min.<Measurement of transmittance>

[0072] The transmittance was measured with a transmittance meter (model name HR-100, manufactured by Murakami Color Research Laboratory) according to JIS K 7105. [Table 6]Unit(Reference ) Example 1(Reference ) Example 2(Reference ) Example 5(Referenc e) Example 9(Reference ) Example 10PMDA_TF MBBPFA_TF MB6FDA_TFM BBPDA_P DABPDA_TF MBSolid contentwt.%1212121212ViscositymPa* s (cP)50103200510068006700Molecular weightMw101,00070,50078,000100,000107,000Thicknessµm1010101010CTE (100~450°C1 st< cooling)ppm / °C-1775 @~350°C70 @ ~300°C3.320 @ ~300°CTd_1%°C535530501565546Elongation%23152015.420Tensile strengthMpa290150190343.3230Tensile modulusGpa7.53.23.59.34.6Transmittanc e (Tavar. 380-780nm)%8088887989

[0073] From the results shown in Table 6, it can be seen that it is possible to obtain the polyimide film having excellent mechanical properties and transmittance as well as excellent thermal resistance such as a thermal decomposition temperature of 400 °C or higher, or even 500 °C or higher.

Claims

1. A polyimide precursor solution consisting of a polyimide precursor and an amide-based organic solvent, wherein the organic solvent has a positive distribution coefficient (Log P) at 25 °C, a density of 1 g / cm3 or less, the amide-based organic solvent is diethylpropionamide (DEPA), the polyimide precursor solution has a solid content of 12% by weight, and a viscosity of 7000 mPa*s (cP) or less, the polyimide precursor is a polyamic acid consisting of a repeating structure of the following formula 1: wherein, X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 4,4'-oxydiphthalic anhydride, N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide] or 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, and Y consists of a divalent organic derived from 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine, wherein, when X is a tetravalent organic group derived from 9,9'-bis(3,4-dicarboxyphenyl)fluorene dianhydride, the viscosity is 2900 mPa*s (cP) and the weight average molecular weight is 72,000 g / mol; when X is a tetravalent organic group derived from 4,4'-oxydiphthalic anhydride, the viscosity is 3250 mPa*s (cP) and the weight average molecular weight is 95,000 g / mol; when X is a tetravalent organic group derived from N,N'-(9H-Fluoren-9-ylidenedi-4,1-phenylene)bis[1,3-dihydro-1,3-dioxo-5-isobenzofurancarboxamide], the viscosity is 3020 mPa*s (cP) and the weight average molecular weight is 91,200 g / mol; and when X is a tetravalent organic group derived from 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, the viscosity is 3050 mPa*s (cP) and the weight average molecular weight is 81,000 g / mol, the distribution coefficient (Log P) is calculable using an ACD / Log P module of ACD / Percepta platform from ACD / Labs, wherein the ACD / Log P module uses an algorithm based on QSPR (Quantitative Structure-Property Relationship) methodology using 2D molecular structures, the density is as measured according to Standard ASTM D1475, the viscosity is as measured at 25 °C with a Brookfield rotational viscometer, and the weight average molecular weight is as measured by using Gel Permeation Chromatography (GPC) and eluent THF: DMF 50:50 by volume.

2. A method for forming a polyimide film comprising applying the polyimide precursor solution of claim 1 onto a substrate; and heat treating the applied polyimide precursor solution to form the polyimide film.