A transparent film, including an optical material of the transparent film and a display device including the transparent film

Abstract

The present invention provides a transparent film, an optical material including the transparent film, and a display device including the transparent film.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10 - 2019 - 0097130, filed with the Korean Intellectual Property Office on August 9, 2019, the entire disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The following invention relates to a transparent film, an optical material including the transparent film, and a display device including the transparent film. More specifically, the following invention relates to a transparent film having excellent optical properties (such as visibility), an optical material including the transparent film, and a display device including the transparent film. Background Art

[0004] The substrate material for a flexible display that exhibits superiority as a next - generation display device should be lightweight, shatter - resistant, and bendable, and is not limited by its form due to its processability.

[0005] In addition, glass substrates are currently used as substrate materials for display devices. However, glass substrates may be easily broken due to external impacts, making glass substrates liable to damage when used in display devices. In addition, glass substrates do not have flexibility, making it difficult to use glass substrates in flexible displays. Therefore, as the most suitable material for realizing flexible displays, polymers, as lightweight, shatter - resistant, and film - manufacturable materials, exhibit superiority.

[0006] When manufacturing a polymer film by solution casting using such a polymer, a mura phenomenon occurs in which mura is generated during projection. When the above - mentioned mura phenomenon occurs, the image of the display device is distorted, which results in a decrease in the visual clarity of the display.

[0007] Therefore, depending on a specific angle or light, the visibility of the polymer film is poor, and in order to improve the display device, it is necessary to improve the appearance quality. Summary of the Invention

[0008] Embodiments of the present invention are directed to providing a transparent film and a display device including the transparent film, the transparent film being capable of suppressing chromaticity - uneven mura phenomena that are color - uneven and look like stains.

[0009] Another embodiment of the present invention is directed to providing a high - quality display device having excellent visibility due to clear image quality.

[0010] In one general aspect, there is provided a transparent film in which when a polarized light passes through the film, light in which two waves are mixed with each other is generated, and the generated light satisfies the following relational expression 1 and relational expression 2:

[0011] [Relational expression 1]

[0012] 0.90A ≤ T ≤ 1.10A

[0013] [Relational expression 2]

[0014] θ ≤ 30

[0015] Wherein

[0016] T is the sum of the amplitudes of the amplitudes of two waves of transmitted light when interfering with each other, A is the amplitude of the wave of polarized light, and θ is the retardation angle (°) of the wave traveling to the slow axis of the transparent film among the two waves.

[0017] The transparent film can be a polyimide-based film.

[0018] The transparent film can have an average amplitude of the surface roughness curve of 1 to 1000 nm.

[0019] The transparent film can have a bent area of 70% or less with respect to the total surface area.

[0020] The transparent film can have a thickness of 20 μm to 200 μm.

[0021] The transparent film can have a residual solvent content within 3% by weight with respect to the total weight of the film.

[0022] When drying the transparent film, drying can be performed in a drying area composed of any one or more nozzles selected from an impinged nozzle, a parallel flow nozzle, and a Venturi-type nozzle.

[0023] In another general aspect, an optical material including the above transparent film is provided.

[0024] In another general aspect, a display device including the above optical material is provided.

[0025] Through the following detailed description and the accompanying drawings, other features and aspects of the present invention will become apparent. Description of the Drawings

[0026] Figure 1 is a photograph obtained by visually observing the appearance when projected onto the transparent films of the examples and comparative examples according to the present invention. Detailed Description of the Invention

[0027] Hereinafter, the present invention will be described in more detail with reference to embodiments, examples, and the accompanying drawings. The following specific examples and illustrations are only for describing the present invention in detail and are not intended to limit it, and it can be implemented in various forms.

[0028] In addition, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. The terms used herein are only for effectively describing certain embodiments and are not intended to limit the present invention.

[0029] In the description of the present invention in the specification, unless there is a clear contrary description, "including" any element will be understood to imply further including other elements rather than excluding other elements.

[0030] In addition, unless otherwise indicated in the context, the singular forms used in the detailed description are intended to include the plural forms.

[0031] The inventors found that when observing by transmitting light through an optically transparent film, if the phase relationship between two waves satisfies a specific range, the moiré phenomenon due to chromaticity non-uniformity does not occur, and thus the appearance quality and visibility may be greatly improved. Thereby, the present invention was completed.

[0032] To achieve the above object, in the transparent film according to the present invention, when one kind of polarized light transmits through the film, light in which two waves are mixed with each other is generated, and the generated light satisfies the following relational expression 1 and relational expression 2:

[0033] [Relational Expression 1]

[0034] 0.90A ≤ T ≤ 1.10A

[0035] [Relational Expression 2]

[0036] θ ≤ 30

[0037] Wherein

[0038] T is the sum of the amplitudes of the two waves when the transmitted light interferes with each other, A is the amplitude of the wave of the polarized light, and θ is the retardation angle (°) of the wave that travels to the slow axis of the transparent film among the two waves.

[0039] Specifically, in the transparent film according to the present invention, when two waves having the same wavelength and amplitude from one light source have reached a point through different paths, the phase relationship generated by the path difference satisfies the above relational expression 1 and relational expression 2.

[0040] In addition, according to the present invention, T can specifically be the sum of the amplitude (W1) when two waves of transmitted light interfere constructively with each other and the amplitude (W2) when two waves of transmitted light interfere destructively with each other.

[0041] In addition, specifically, two waves transmitted through the optically anisotropic film have two traveling speeds according to the polarization direction, resulting in a phase difference. The polarization direction with a faster light speed is called the fast axis, and the polarization direction with a slower light speed, that is, the axis perpendicular to the fast axis, is called the slow axis. Based on this, θ represents the retardance angle of the wave traveling along the slow axis.

[0042] The transparent film according to the present invention can have optical anisotropy. Due to the optical anisotropy of the transparent film as described above, a phase difference can exist between two waves of light generated during light transmission through the transparent film, and the above-mentioned relational expressions 1 and 2 can be satisfied.

[0043] The transparent film according to the present invention satisfies the above-mentioned relational expressions 1 and 2, so that when white light is projected onto the transparent film, the moiré phenomenon of generating moiré due to chromaticity non-uniformity is minimized, and the appearance quality is excellent. Therefore, the optical material and the display device manufactured therefrom can have excellent visibility.

[0044] According to one aspect of the present invention, the above-mentioned relational expression 1 can preferably satisfy 0.92A to 1.08A, and more preferably satisfy 0.95A to 1.05A. The above-mentioned relational expression 2 can preferably be less than 30°, and more preferably be less than 29°. If the relational expressions 1 and 2 are satisfied as described above, when white light is projected onto the transparent film, the moiré phenomenon of generating moiré due to chromaticity non-uniformity hardly occurs, thereby preventing image distortion and achieving excellent visibility.

[0045] According to one aspect of the present invention, the transparent film has transparency. Specifically, it can have a total transmittance of 70% to 99% measured based on a thickness of 30 μm to 80 μm according to the ASTM E313 standard and using model 300 manufactured by Nippon Denshoku, preferably 80% to 99%, and more preferably 90% to 99%.

[0046] According to one aspect of the present invention, the transparent film can be a polyimide-based film. The polyimide-based film has excellent transparency, heat resistance, chemical resistance, mechanical properties, electrical properties, and dimensional stability, and is beneficial for light weight, thinning, and flexibility, making it very suitable for flexible display devices.

[0047] According to one aspect of the present invention, the polyimide-based film may be provided as a polyimide film, which comprises a polyimide resin obtained by polymerizing a monomer mixture including a dianhydride and a diamine. There is no particular limitation on the dianhydride and the diamine, as long as they are commonly used known materials.

[0048] According to one aspect of the present invention, the diamine may be, for example, any one or a mixture of two or more selected from aliphatic diamines and aromatic diamines.

[0049] More specifically, there is no particular limitation on the aromatic diamine, but it may be, for example, any one or a mixture of two or more selected from bis(trifluoromethyl)benzidine (TFDB), bis(3-aminophenyl)sulfone (3DDS), bis(4-aminophenyl)sulfone (4DDS), diaminophenyl ether, o-phenylenediamine (o-PDA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 4,4'-oxydianiline (ODA), methylenedianiline (MDA), bis(aminophenyl)hexafluoropropane (HFDA), and 1,3-bis(4-aminophenoxy)benzene (TPE-R).

[0050] There is no particular limitation on the aliphatic diamine, but it may be, for example, any one or a mixture of two or more selected from 1,4-diaminocyclohexane, 1,4-cyclohexanebis(methylamine), 4,4'-diaminodicyclohexylmethane (MCA), 4,4'-methylenebis(2-methylcyclohexylamine) (MMCA), ethylenediamine (EN), 1,3-diaminopropane (13DAP), tetramethylenediamine, 1,6-hexamethylenediamine (16DAH), and 1,12-diaminododecane (112DAD).

[0051] According to one aspect of the present invention, the dianhydride may be any one or a mixture of two selected from aliphatic dianhydrides and aromatic dianhydrides.

[0052] More specifically, according to one aspect of the present invention, there is no particular limitation on the aromatic dianhydride, but it may be, for example, any one or a mixture of two or more selected from 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA), 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), benzophenonetetracarboxylic dianhydride (BTDA), 4,4'-oxybisphthalic anhydride (ODPA), and bis(dicarboxyphenoxy)diphenylsulfide dianhydride (BDSDA).

[0053] The aliphatic dianhydride is not particularly limited, but may be, for example, any one or a mixture of two or more selected from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexene-1,2-dicarboxylic dianhydride (DOCDA), bicyclooctene-2,3,5,6-tetracarboxylic dianhydride (BODA), 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA), 1,2,4-tricarboxy-3-methylcarboxycyclopentane dianhydride, and 1,2,3,4-tetracarboxycyclopentane dianhydride.

[0054] According to one aspect of the present invention, relative to 100 moles of diamine, the dianhydride may be copolymerized in an amount of 5 moles to 80 moles, preferably 10 moles to 70 moles.

[0055] When the dianhydride is included within the above range, a polyimide film having excellent visibility and optical properties can be provided.

[0056] The polyimide film produced from the above composition can suppress microscopic non-uniformity on the surface to minimize the mottling phenomenon in which mottles are generated on the film.

[0057] According to one aspect of the present invention, a polyamideimide film based on polyimide can be provided by further including an aromatic diacid dichloride in the monomer mixture to improve mechanical properties.

[0058] The aromatic diacid dichloride is not particularly limited, but may be, for example, any one or a mixture of two or more selected from terephthaloyl chloride (TPC), isophthaloyl chloride (IPC), 1,1'-biphenyl-4,4'-dicarbonyl chloride (BPC), 1,4-naphthalenedicarbonyl chloride (1,4-NaDC), 2,6-naphthalenedicarbonyl chloride (2,6-NaDC), 1,5-naphthalenedicarbonyl chloride (1,5-NaDC), etc. Preferably, it includes any one or a mixture of two or more selected from terephthaloyl chloride and isophthaloyl chloride.

[0059] In addition, the aromatic diacid dichloride is not limited to being used as the above-exemplified compounds and may be further used in combination with other acyl halide compounds, but preferably the aromatic diacid dichloride is used alone.

[0060] According to one aspect of the present invention, relative to 100 moles of diamine, the aromatic diacid dichloride may be copolymerized in an amount of 20 moles to 95 moles, preferably 30 moles to 90 moles.

[0061] The polyamideimide film made from the above composition can not only have excellent mechanical properties and heat resistance, but also provide excellent display quality even at high temperatures, and can provide optical properties that maintain high transparency.

[0062] According to one aspect of the present invention, the dianhydride and the aromatic diacyl chloride may be included in a molar ratio of 5:95 to 80:20, preferably in a molar ratio of 10:90 to 70:30.

[0063] According to one aspect of the present invention, when measuring the surface roughness in a 5 mm × 5 mm area by an optical microscope, the average amplitude of the surface roughness curve of the transparent film may be 1 nm to 1000 nm. The average amplitude of the surface roughness curve of the transparent film may preferably be 1 nm to 100 nm, more preferably 1 nm to 80 nm, and most preferably 1 nm to 30 nm. When the transparent film has the average amplitude of the surface roughness curve as described above, the mottles (such as light spots) on the film surface can be significantly reduced according to the light or angle, and thus excellent surface quality can be provided.

[0064] According to one aspect of the present invention, the transparent film may have a curved area of 70% or less with respect to the total surface area of 5 mm × 5 mm. Here, the curved area is an area where the average step (Ra value) of the surface flexure exceeds 30 nm when the step of the flat plane without curvature measured by a confocal microscope is zero (0). Specifically, the transparent film may have a curved area of 0.1% to 70% with respect to the total surface area. The transparent film may preferably have a curved area of 1 to 65%, more preferably 1 to 62% with respect to the total surface area. When the curved area satisfies the above range, the projected mottles caused by the curved area can be reduced. In addition, when the average amplitude of the surface roughness curve also satisfies the above range, the mottle phenomenon can be reduced and the display quality can be improved due to the improved visibility.

[0065] According to one aspect of the present invention, the transparent film may have a residual solvent content of 3% by weight or less with respect to the total weight of the film. Specifically, the transparent film may have a residual solvent content of 0.01 to 3% by weight, preferably 0.01 to 2% by weight with respect to the total weight of the film. Here, the residual solvent content is measured with respect to the weight before drying, excluding the solid components in the coating liquid. The transparent film has the residual solvent content as described above, so that the deterioration of physical properties can be prevented, and swelling or shrinkage does not occur due to the external environment. Therefore, the reliability of the quality can be further improved.

[0066] According to one aspect of the present invention, a transparent film can be manufactured by adjusting the drying rate during the drying process after the solution casting process. During the drying process, the drying rate can be adjusted in two or more steps, preferably three or more steps, to manufacture the film. For example, the drying process can be carried out through the following steps: measuring the residual solvent content in the solution for preparing the transparent film, then drying at a first drying rate in step 1, drying at a second drying rate in step 2, and then drying at a third drying rate in step 3, using different drying rates in each step. The first to third drying rates can be the same as or different from each other. Preferably, in order to prevent the occurrence of mottling phenomenon where mottles are generated on the film surface, the first to third drying rates can all be different from each other. Here, during the drying process, steps 1 to 3 are carried out in sequence.

[0067] More specifically, the first drying rate can be 100 to 900 g / m 2 ·min, the second drying rate can be 10 to 400 g / m 2 ·min, and the third drying rate can be 1 to 200 g / m 2 ·min. Preferably, the first drying rate can be 300 to 900 g / m 2 ·min, the second drying rate can be 10 to 350 g / m 2 ·min, and the third drying rate can be 10 to 150 g / m 2 ·min. When drying in the drying section at the above drying rates according to the residual solvent content after drying, the transparent film can satisfy the above relational expression 1 and relational expression 2, and basically no mottling phenomenon will occur on the film surface. Therefore, excellent appearance quality can be achieved.

[0068] Preferably, on the other hand, during the drying process, the drying rate can be adjusted in four or more steps to manufacture the film. For example, the drying process can be carried out through the following steps: measuring the residual solvent content in the solution for preparing the transparent film, then drying at a first drying rate in step 1, drying at a second drying rate in step 2, drying at a third drying rate in step 3, and then drying at a fourth drying rate in step 4, using different drying rates in each step. The first to fourth drying rates can be the same as or different from each other. Here, during the drying process, steps 1 to 4 are carried out in sequence.

[0069] Preferably, in order to prevent the occurrence of mottling phenomenon where mottles are generated on the film surface, the first to fourth drying rates can be different from each other in three or more steps. More preferably, the first to fourth drying rates can be different from each other in three or more steps, and the different drying rates can gradually slow down in sequence.

[0070] More specifically, the first drying rate can be 100 to 900 g / m 2 ·min, the second drying rate can be 10 to 400 g / m 2 ·min, the third drying rate can be 1 to 200 g / m 2 ·min, and the fourth drying rate can be 1 to 150 g / m 2 ·min. Preferably, the first drying rate can be 360 to 900 g / m 2 ·min, the second drying rate can be 10 to 350 g / m 2 ·min, the third drying rate can be 10 to 150 g / m 2 ·min, and the fourth drying rate can be 10 to 130 g / m 2 ·min. When drying in the drying section at the above drying rates according to the residual solvent content after drying, the transparent film can satisfy the above relational expression 1 and relational expression 2, and basically no mottling phenomenon appears on the film surface, so more excellent appearance quality can be achieved.

[0071] According to one aspect of the present invention, when adjusting the drying rate in three or more steps, the drying rate can satisfy the following formula 1 and formula 2:

[0072] [Formula 1]

[0073] 50 ≤ V1 - V2 ≤ 600

[0074] [Formula 2]

[0075] 10 ≤ V2 - V3 ≤ 300

[0076] Wherein

[0077] V1 is the first drying rate (g / m 2 ·min) during the drying process of the transparent film, V2 is the second drying rate (g / m 2 ·min) during the drying process of the transparent film, and V3 is the third drying rate (g / m 2 ·min) during the drying process of the transparent film.

[0078] Preferably, formula 1 can satisfy 50 to 550, and formula 2 can satisfy 20 to 280.

[0079] On the other hand, when adjusting the drying rate in four or more steps, the drying rate can satisfy the above formula 1 and formula 2 and the following formula 3:

[0080] [Formula 3]

[0081] 0 ≤ V3 - V4 ≤ 150

[0082] Wherein

[0083] V3 is the third drying rate (g / m 2 ·min) during the drying process of the transparent film, and V4 is the fourth drying rate (g / m 2 ·min) during the drying process of the transparent film.

[0084] Preferably, Equation 3 can satisfy 0 to 100.

[0085] If the drying rate satisfies the above Equation 1 and Equation 2, or Equation 1 to Equation 3 as described above, the transparent film can satisfy Relational Expression 1 and Relational Expression 2, prevent the mottling phenomenon on the film surface to achieve excellent appearance quality, and prevent the increase of haze under harsh environments.

[0086] On the other hand, when adjusting the drying rate in more than three steps, the drying rate can satisfy the following Equation 4:

[0087] [Equation 4]

[0088] 80 ≤ ΔV1 - ΔV2 ≤ 400

[0089] where

[0090] ΔV1 is the difference (V1 - V2) between the first drying rate (g / m 2 ·min) and the second drying rate (g / m 2 ·min) during the drying process of the transparent film, and ΔV2 is the difference (V2 - V3) between the second drying rate (g / m 2 ·min) and the third drying rate (g / m 2 ·min) during the drying process of the transparent film.

[0091] Preferably, Equation 4 can satisfy 85 to 350, preferably satisfy 90 to 320.

[0092] If the drying rate satisfies the above Equation 4, the appearance quality can be further improved, such that there is almost no mottling, while significantly reducing the microscopic non-uniformity of the surface.

[0093] According to one aspect of the present invention, the drying process can adjust the drying temperature, the drying air volume, the drying air speed, the support temperature, and the drying method in order to achieve the drying rate in each step.

[0094] According to one aspect of the present invention, more than one, preferably more than two, more preferably more than three, and most preferably more than four drying zones can be provided during the drying process of the transparent film. Herein, the drying temperature, the drying air volume, the drying air speed, the support temperature, and the drying method in each drying zone can be changed to match the drying rate.

[0095] Specifically, according to one aspect, if more than three drying zones are configured, each zone can dry at a different drying rate, such that the film can be dried while moving each zone. When adjusting the drying rate according to the zones as described above, the drying rate can be adjusted without a section where the rate increases or decreases according to the change in the drying rate, thereby further improving the appearance quality while significantly reducing the microscopic non-uniformity of the surface. According to an exemplary embodiment of the present invention, the transparent film can be dried in a drying zone during the drying process, and the drying zone is composed of any one or more than two nozzles selected from impact nozzles, co-current nozzles, and Venturi-type nozzles. Various nozzles can be configured to adjust the drying rate as described above, and the drying rate can be adjusted to minimize the moiré phenomenon. Accordingly, the transparent film can be used as an optical material having excellent appearance quality and improved visibility.

[0096] According to one aspect of the present invention, the transparent film can have a thickness of 20 to 200 μm. The transparent film preferably has a thickness of 20 to 150 μm, and more preferably has a thickness of 20 to 100 μm. The transparent film can have the thickness as described above to satisfy Relational Expression 1 and Relational Expression 2 and achieve excellent visibility without the moiré phenomenon.

[0097] Another aspect of the present invention is an optical material including the above-described transparent film.

[0098] In the transparent film according to the present invention, the moiré phenomenon in which moiré is generated during projection is suppressed, and thus the film can have excellent appearance quality and excellent visibility, and is therefore very suitable for use as an optical material.

[0099] The transparent film according to the present invention can be applied to a wide range of fields, such as heat-resistant high-tech materials, such as automotive materials, aerospace materials, and spacecraft materials; electronic materials, such as insulating coating agents, insulating films, semiconductors, and electrode protection films for TFT-LCDs, but is preferably used as an optical material in a display field such as a liquid crystal display device.

[0100] Specifically, the transparent film according to the present invention can be applied to the display field as various optical materials selected from a cover window film, a protective film, a retardation film, a diffusion film, a flexible substrate, an encapsulant, a polarizing plate, and a touch panel substrate.

[0101] Another aspect of the present invention is a display device including the above-described optical material. The display device can include the above-described transparent film as an optical material, thereby minimizing the fatigue of the user's eyes due to good visibility.

[0102] According to one aspect of the present invention, the display device is not particularly limited as long as it is in a field that requires excellent optical performance, and can be applied to fields selected from, for example, liquid crystal displays, organic EL displays, and electronic papers. In addition, according to the application field, the display panel can be selected and set. Specific examples may include, but are not limited to, any one or more selected from a touch sensor panel, an organic light-emitting display panel, and a liquid crystal display panel.

[0103] The transparent film according to the present invention, the optical material including the transparent film, and the display device including the transparent film will be described in more detail through examples. The following examples are only for reference to describe the present invention in detail. The present invention is not limited thereto and can be implemented in various forms.

[0104] In addition, unless otherwise defined, all technical terms and scientific terms have the same meaning as those commonly understood by those skilled in the art to which the present invention pertains. The terms used herein are only for effectively describing certain exemplary embodiments and are not intended to limit the present invention.

[0105] In addition, unless otherwise described herein, the unit of the additive can be wt%.

[0106] The physical properties of the present invention are measured as described below.

[0107] (1) Observation of the projected appearance (mottling)

[0108] The projected appearance is observed by irradiating with a HID (high-intensity discharge) lamp projected vertically onto a film having a size of 500 mm × 500 mm. The projected image is projected onto a white backboard, and the projected image is visually recognized. Here, the positions of the lamp-film-back board are adjusted to focus the projected image. When mottling is recognized, it is determined as bad / good (NG / OK) according to the size and recognition degree of the mottling. If the width of the mottling is 2 mm or more and has a clear shape during visual observation, it is considered bad (NG).

[0109] (2) T and θ

[0110] T and θ are obtained by measuring the transmittance value and the linear retardation value of the polarized light generated in a polarization state generator (PSG) using an Axoscan device manufactured by Axometrics. In particular, when the transmittance of a film having a linear retardation of 1° or less and the same refractive index is calculated as 100%, T is measured as the relative transmittance.

[0111] (3) Average amplitude and bending region of the surface roughness curve

[0112] The transparent films of the examples and comparative examples were observed using a 3D foam tester (Keyence, VK-X1050). The surface roughness (Ra) value was measured within a 5 mm × 5 mm area corresponding to the film. Here, the measurement was carried out after fixing the film so that it would not be lifted from the substrate during measurement. The surface roughness values in 15 adjacent areas were measured using the same method to calculate the average value. For the 5 mm × 5 mm area of the film, when the step of the flat plane without bending measured by a confocal microscope was zero (0), the area (%) of the bent portion was obtained by calculation using the following Equation 5:

[0113] [Equation 5]

[0114] {Area of the region where the roughness value is 80% to 120% of the average roughness (Ra)} / {Area of the total region} × 100%

[0115] (4) Measurement of drying rate

[0116] The drying rate in the dryer was calculated by measuring the residual solvent in the film passing through each drying region. Using thermogravimetric analysis (TGA) (Discovery purchased from TA), the weight A at 150 °C 150 was subtracted from the weight A at 370 °C 370 , and the obtained value was determined as the residual solvent content in the film. Here, under the measurement conditions of heating to 400 °C at a heating rate of 30 °C / min, the weight change in the range of 150 °C to 370 °C was measured.

[0117] [Preparation Example 1]

[0118] [Molar ratio of TFMB:6FDA:TPC = 100:14:86]

[0119] Dimethylacetamide (DMAc) and 2,2'-bis(trifluoromethyl)benzidine (TFMB) were added to a reactor under a nitrogen atmosphere and stirred well, then 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA) was added thereto and stirred well until dissolved. Thereafter, terephthaloyl chloride (TPC) was added thereto, the mixture was dissolved and reacted by stirring for 6 hours to prepare a polyamic acid resin composition. Here, the amounts of the respective monomers were such that the molar ratio of TFMB:6FDA:TPC = 100:14:86, and the temperature of the reactor was maintained at 30 °C. The finally obtained polyamic acid resin composition had a viscosity of 33000 cps. Then, pyridine and acetic anhydride were added to the polyamic acid resin composition in a molar amount 2.5 times the total amount of the dianhydride added, and stirred at 60 °C for 1 hour to prepare Polymer Solution 1.

[0120] Thereafter, the polymerization solution 1 was precipitated in an excess of methanol, filtered, and then the obtained solid was vacuum-dried at 50 °C for more than 6 hours to obtain a polyamideimide. The finally obtained polyamideimide had a weight-average molecular weight of 106,000 g / mol.

[0121] [Preparation Example 2]

[0122] [Molar ratio of TFMB:CBDA:IPC = 100:60:40]

[0123] Under a nitrogen atmosphere, dimethylacetamide (DMAc) and 2,2'-bis(trifluoromethyl)benzidine (TFMB) were added to a reactor and stirred well. Then, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) was added thereto and stirred well until dissolved. Thereafter, isophthaloyl chloride (IPC) was added thereto, and the mixture was dissolved and reacted by stirring for 6 hours to prepare a polyamic acid resin composition. Here, the amounts of the respective monomers were such that the molar ratio of TFMB:CBDA:IPC was 100:60:40, and the temperature of the reactor was maintained at 30 °C. The finally obtained polyamic acid resin composition had a viscosity of 90,000 cps. Then, pyridine and acetic anhydride were added to the polyamic acid resin composition in a molar amount 2.5 times the total amount of the dianhydride added, and stirred at 60 °C for 1 hour to prepare a polymerization solution 2.

[0124] Thereafter, the polymerization solution 2 was precipitated in an excess of methanol, filtered, and then the obtained solid was vacuum-dried at 50 °C for more than 6 hours to obtain a polyamideimide. The finally obtained polyamideimide had a weight-average molecular weight of 310,000 g / mol.

[0125] [Preparation Example 3]

[0126] [Molar ratio of TFMB:CBDA:IPC = 100:40:60]

[0127] Under a nitrogen atmosphere, dimethylacetamide (DMAc) and 2,2'-bis(trifluoromethyl)benzidine (TFMB) were added to a reactor and stirred well. Then, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) was added thereto and stirred well until dissolved. Thereafter, isophthaloyl chloride (IPC) was added thereto, and the mixture was dissolved and reacted by stirring for 6 hours to prepare a polyamic acid resin composition. Here, the amounts of the respective monomers were such that the molar ratio of TFMB:CBDA:IPC was 100:40:60, and the temperature of the reactor was kept at 30 °C. The finally obtained polyamic acid resin composition had a viscosity of 68,000 cps. Then, pyridine and acetic anhydride were added to the polyamic acid resin composition in a molar amount 2.5 times the total amount of the dianhydride added, and stirred at 60 °C for 1 hour to prepare a polymerization solution 3.

[0128] Thereafter, the polymerization solution 3 was precipitated in excess methanol, filtered, and then the obtained solid was dried in vacuo at 50 °C for more than 6 hours to obtain a polyamideimide. The finally obtained polyamideimide had a weight-average molecular weight of 180,000 g / mol.

[0129] The obtained polymerization solution was cast on a glass substrate using a slot die and then dried under the following drying conditions to obtain a transparent film with a thickness of 30 to 80 μm. The examples and comparative examples described in Table 1 show the configurations according to the type of polymerization solution used, the structure of the drying nozzle, and the drying rate.

[0130] [Table 1]

[0131]

[0132]

[0133] The physical properties of the examples and comparative examples described in Table 1 were measured and are shown in Table 2 below.

[0134] [Table 2]

[0135]

[0136] As shown in Table 2, it can be confirmed that the transparent film according to the present invention satisfies Relational Expression 1 and Relational Expression 2, and thus has excellent appearance characteristics, that is, almost no color unevenness and transmittance unevenness occur. In addition, this can be achieved by satisfying a certain range of drying rates. Further, when the drying rate satisfies the above Expressions 1 to 3, Relational Expression 1 and Relational Expression 2 are satisfied, so that the bending region is reduced, and the streaks such as optical patterns on the film surface are significantly reduced according to light or angle, and thus excellent surface quality can be provided.

[0137] In addition, it has been confirmed that in Examples 4, 6, 7, and 8 that satisfy all of the above Expressions 1 to 4, almost no streak phenomenon occurs on the film surface, and thus more excellent appearance quality can be achieved compared with other examples.

[0138] Therefore, the transparent film according to the present invention can minimize the streak phenomenon and can be used as an optical material having excellent appearance quality and improved visibility.

[0139] The advantage of the transparent film according to the present invention is that it has excellent appearance quality and does not generate streaks.

[0140] In addition, the advantage of the transparent film according to the present invention is that the optical pattern can be reduced to provide a display device having excellent visibility.

[0141] In addition, the advantages of the transparent film according to the present invention are that it not only has excellent display quality, but also can provide a flexible display device due to its flexibility.

[0142] In the foregoing, although the present invention has been described through specific matters, only limited embodiments are provided to help a more comprehensive understanding of the present invention. Therefore, the present invention is not limited to the exemplary embodiments. Those skilled in the art to which the present invention pertains can make various modifications and changes based on this specification.

[0143] Therefore, the spirit of the present invention should not be limited to the above embodiments, but all modifications that are the same as or equivalent to the specification are intended to fall within the scope and spirit of the present invention.

Claims

1. A transparent polyimide-based film, wherein when a polarized light transmits through the film, light in which two waves are mixed with each other is generated, and the generated light satisfies the following relational expression 1 and relational expression 2: [Relational expression 1] 0.90A ≤ T ≤ 1.10A [Relational expression 2] θ ≤ 30° where T is the sum of the magnitudes of the amplitudes when the two waves of the transmitted light interfere with each other, A is the amplitude of the wave of the polarized light, and θ is the retardation angle of the wave that travels to the slow axis of the transparent film among the two waves, The transparent polyimide-based film is manufactured by using a drying process after solution casting, the drying process includes two or more drying steps, and the drying conditions in at least two drying steps are different from each other.

2. The polyimide-based film according to claim 1, wherein, The polyimide-based film has an average amplitude of a surface roughness curve of 1 nm to 1000 nm.

3. The polyimide-based film according to claim 1, wherein, The polyimide-based film has a bending area of 70% or less with respect to the total surface area.

4. The polyimide-based film according to claim 1, wherein, The polyimide-based film has a thickness of 20 μm to 200 μm.

5. The polyimide-based film according to claim 1, wherein, The polyimide-based film has a residual solvent content of within 3% by weight with respect to the total weight of the film.

6. The polyimide-based film according to claim 1, wherein when drying the polyimide-based film, drying is performed in a drying area composed of any one or two or more nozzles selected from an impact nozzle, a co-current nozzle, and a Venturi-type nozzle.

7. The polyimide-based film according to claim 1, wherein, The drying process includes three or more drying steps, the drying conditions in at least two of the drying steps are different, and the drying rate according to the residual solvent content in the first drying step among the drying steps is the highest.

8. The polyimide-based film according to claim 7, wherein, The three or more drying steps satisfy the following formula 1 and formula 2, [Formula 1] 50 ≤ V1 - V2 ≤ 600 [Formula 2] 10 ≤ V2 - V3 ≤ 300 In the Formula 1 and the Formula 2, the V1 is the first drying rate in the drying process of the transparent polyimide-based film, with the unit of g / m 2 ·min, the V2 is the second drying rate in the drying process of the transparent polyimide-based film, with the unit of g / m 2 ·min, and the V3 is the third drying rate in the drying process of the transparent polyimide-based film, with the unit of g / m 2 ·min.

9. The polyimide-based film according to claim 1, wherein, The polyimide-based film contains a polyimide-based resin prepared by polymerizing a monomer mixture including a diamine, a dianhydride, and an aromatic diacyl chloride. In the monomer mixture, with respect to 100 moles of the diamine which is a mixture of any one or two or more selected from an aliphatic diamine and an aromatic diamine, the dianhydride which is a mixture of any one or two or more selected from an aliphatic dianhydride and an aromatic dianhydride is 5 - 80 moles, and the aromatic diacyl chloride is 20 - 95 moles.

10. The polyimide-based film according to claim 1, wherein, The polyimide-based film is prepared by polymerizing a monomer mixture composed of 2,2'-bis(trifluoromethyl)benzidine as the diamine, 4,4'-hexafluoroisopropylidene diphthalic anhydride or 1,2,3,4-cyclobutane tetracarboxylic dianhydride as the dianhydride, and terephthaloyl chloride or isophthaloyl chloride as the aromatic diacyl chloride.

11. An optical material, which includes the transparent polyimide-based film according to any one of claims 1 to 10.

12. A display device, which includes the optical material according to claim 11.

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