Substrate for display device or flexible display device, and display device or flexible display device

Abstract

The present invention relates to a substrate for a display device or flexible display device that achieves low dielectric properties and excellent heat resistance, and a display device or flexible display device using the same. The substrate for a display device or flexible display device includes: a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 to 60; a second polyimide resin layer having a yellowness index of less than 25; a first inorganic layer positioned between the first and second polyimide resin layers and having a thickness of 100 to 700 nm; and a second inorganic layer formed on the second polyimide resin layer and having a thickness of 100 to 500 nm, the yellowness indexes of the first and second polyimide resin layers being measured based on a thickness of 10±1 μm.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0155897, filed November 19, 2020, and Korean Patent Application No. 10-2021-0148107, filed November 1, 2021, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a substrate for a display device or a flexible display device that realizes high resistance characteristics and excellent heat resistance, and a display device or a flexible display device using the same. [Background technology]

[0003] The display device market is rapidly shifting to flat panel displays (FPDs) that can be easily made large, thin, and lightweight. Such flat panel displays include liquid crystal displays (LCDs), organic light emitting displays (OLEDs), and electrophoretic displays (EPDs).

[0004] Rigid type displays are manufactured using glass substrates as their base material, while flexible type displays are manufactured using plastic substrates as their base material.

[0005] However, flexible displays have problems such as image retention due to the use of plastic substrates, and plastic substrates have poorer heat resistance, thermal conductivity, and electrical insulation than glass substrates.

[0006] Nevertheless, active research is being conducted to apply plastic substrates, which have the advantages of being light, flexible, and capable of being manufactured in a continuous process, to mobile phones, notebook PCs, TVs, etc., instead of glass substrates.

[0007] Polyimide resins have the advantages of being easy to synthesize, being able to be produced as thin films, and being applicable to high-temperature processes. Along with the trend toward lighter and more precise electronic devices, polyimide resins are increasingly being used as integrated materials in semiconductors. In particular, much research is being conducted into the application of polyimide resins to flexible plastic display boards, which require light weight and flexibility. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention provides a substrate for a display device or a flexible display device that does not undergo film deformation even during high-temperature processes and achieves excellent heat resistance and high resistance properties.

[0009] The present invention also provides a display device or a flexible display device using the substrate for the display device or the flexible display device. [Means for solving the problem]

[0010] To achieve the above object, the present invention provides a substrate for a display device or a flexible display device, comprising: a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 to 60; a second polyimide resin layer having a yellowness index of less than 25; a first inorganic layer positioned between the first polyimide resin layer and the second polyimide resin layer and having a thickness of 100 to 700 nm; and a second inorganic layer formed on the second polyimide resin layer and having a thickness of 100 to 500 nm, wherein the yellowness indexes of the first polyimide resin layer and the second polyimide resin layer are measured based on a thickness of 10±1 μm.

[0011] The present invention also provides a display device or a flexible display device including the display device or flexible display device substrate.

[0012] Hereinafter, a substrate for a display device or a flexible display device, and a display device or a flexible display device according to specific embodiments of the invention will be described in more detail.

[0013] Unless expressly stated otherwise herein, terminology is for the purpose of referring to particular embodiments only and is not intended to be limiting of the invention.

[0014] As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates to the contrary.

[0015] As used herein, the meaning of "comprising" embodies certain properties, regions, integers, steps, operations, elements and / or components, and does not exclude the presence or addition of other certain properties, regions, integers, steps, operations, elements, components and / or groups.

[0016] In this specification, terms including ordinal numbers, such as "first" and "second," are used to distinguish one component from another, and are not limited by the terms. For example, within the scope of the present invention, a first component may be referred to as a second component, and similarly, a second component may be referred to as a first component.

[0017] In this specification, the term "(co)polymer" refers to a polymer or a copolymer, and the term "polymer" refers to a homopolymer consisting of a single repeating unit, and the term "copolymer" refers to a composite polymer containing two or more types of repeating units.

[0018] In this specification, examples of the substituents are described below, but are not limited to these.

[0019] As used herein, the term "substituted" means that another functional group is bonded in place of a hydrogen atom in a compound, and the position of substitution is not limited as long as it is a position at which a hydrogen atom is substituted, i.e., a position at which a substituent can be substituted, and when two or more substituents are substituted, the two or more substituents may be the same or different.

[0020] As used herein, the term "substituted or unsubstituted" refers to a group selected from the group consisting of deuterium, halogen, cyano, nitro, hydroxy, carbonyl, ester, imide, amino, primary amino, carboxy, sulfonic acid, sulfonamide, phosphine oxide, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkylsulfoxy, arylsulfoxy, silyl, boron, alkyl, cycloalkyl, alkenyl, aryl, aralkyl, aralkenyl, alkylaryl, alkoxysilylalkyl, arylphosphine, or a heterocyclic group containing one or more N, O, and S atoms, or a group in which two or more of the above-listed substituents are linked together. For example, a "substituent linked to two or more substituents" may be a biphenyl group. That is, a biphenyl group may be an aryl group or may be interpreted as a substituent in which two phenyl groups are linked together.

[0021] In this specification, [ka] means a bond connecting to another substituent, and a direct bond means that there is no other atom in the moiety represented by L.

[0022] In this specification, aromatic is defined as a compound that satisfies Huckel's rule. ' According to the Hückel rule, a compound is defined as aromatic if it satisfies all of the following three conditions: 1) There must be 4n+2 electrons in complete conjugation via empty p-orbitals, unsaturated bonds, electron pairs, etc. 2) The 4n+2 electrons must form a planar isomer and a ring structure. 3) All atoms of the ring must participate in the conjugation.

[0023] In this specification, an alkyl group is a monovalent functional group derived from an alkane, and may be linear or branched, and the number of carbon atoms in the linear alkyl group is not particularly limited, but is preferably 1 to 20. Furthermore, the number of carbon atoms in the branched alkyl group is 3 to 20. Specific examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, n-propyl, isopropyl, butyl, n-butyl, isobutyl, tert-butyl, sec-butyl, 1-methylbutyl, 1-ethylbutyl, pentyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 4-methyl-2-pentyl, 3,3-dimethylbutyl, 2-ethylbutyl, heptyl, n-heptyl, 1-methylhexyl, octyl, n-octyl, tert-octyl, 1-methylheptyl, 2-ethylhexyl, 2-propylpentyl, n-nonyl, 2,2-dimethylheptyl, 1-ethylpropyl, 1,1-dimethylpropyl, isohexyl, 2-methylpentyl, 4-methylhexyl, 5-methylhexyl, 2,6-dimethylheptan-4-yl, and the like. The alkyl group may be substituted or unsubstituted, and when substituted, examples of the substituents are as described above.

[0024] In the present specification, the term "haloalkyl group" refers to a functional group in which the above-mentioned alkyl group is substituted with a halogen group, and examples of the halogen group include fluorine, chlorine, bromine, and iodine. The haloalkyl group may be substituted or unsubstituted, and if substituted, examples of the substituent are as described above.

[0025] As used herein, a multivalent functional group refers to a residue in which multiple hydrogen atoms bonded to any compound have been removed, and examples thereof include a divalent functional group, a trivalent functional group, and a tetravalent functional group. For example, a tetravalent functional group derived from cyclobutane refers to a residue in which four hydrogen atoms bonded to cyclobutane have been removed.

[0026] In this specification, the electron-withdrawing group may include one or more selected from the group consisting of a haloalkyl group, a halogen group, a cyano group, a nitro group, a sulfonic acid group, a carbonyl group, and a sulfonyl group, and may preferably be a haloalkyl group such as a trifluoromethyl group (-CF).

[0027] In this specification, a direct bond or a single bond means that there is no atom or atomic group at that position and that the bond is made by a bond line, specifically, when there are no other atoms in the portions represented by L1 and L2 in the chemical formula.

[0028] The present invention will now be described in more detail.

[0029] According to one embodiment of the present invention, there is provided a substrate for a display device or a flexible display device, comprising: a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 to 60; a second polyimide resin layer having a yellowness index of less than 25; a first inorganic layer located between the first and second polyimide resin layers and having a thickness of 100 to 700 nm; and a second inorganic layer formed on the second polyimide resin layer and having a thickness of 100 to 500 nm.

[0030] The present inventors have newly developed a substrate for a display device or a flexible display device, in which a first inorganic layer having a predetermined thickness is positioned between a colored polyimide resin layer having excellent heat resistance properties and a transparent polyimide resin layer having excellent high resistance properties, and a second inorganic layer is positioned on the transparent polyimide resin layer.The inventors have experimentally confirmed that such a substrate for a display device or a flexible display device does not significantly change in optical properties such as yellowness index and transmittance, does not significantly deform in the shape of individual layers or the shape of the laminate structure, and does not significantly change in physical properties even after undergoing a process at a high temperature of 430°C or more, and have completed the invention based on this finding.

[0031] In particular, the substrate for a display device or a flexible display device according to the embodiment has a structure in which a first inorganic layer having a predetermined thickness is sandwiched between a first polyimide resin layer and a second polyimide resin layer having the above-described characteristics. Therefore, even in processes in which high temperatures of 400°C or higher are applied, particularly in the manufacturing process of a device for a display device or a flexible display device, the substrate does not suffer from significant degradation of optical properties or deformation of shape. Furthermore, the substrate has high surface resistance, which can prevent image retention and malfunction of the panel unit on the display panel. Therefore, a high-quality device for a display device or a flexible display device can be provided.

[0032] Specifically, the substrate for a display device or a flexible display device has a Td 1% of 570°C or higher and includes a first polyimide resin layer having a yellowness index of 25 to 60. This allows the first polyimide resin to have high packing density, ensuring mechanical properties, dimensional stability, and excellent heat resistance at high temperatures. Therefore, deformation and denaturation of the resin layer are significantly reduced even after high-temperature processes at 430°C or higher.

[0033] In particular, the first polyimide resin layer, which has excellent mechanical properties and heat resistance at high temperatures, is subjected to a greater amount of heat history. However, the substrate for a display device or a flexible display device according to one embodiment having the laminate structure significantly reduces deformation and denaturation of the substrate for a display device or a flexible display device even after undergoing a high-temperature process at 430°C or higher.

[0034] If the Td 1% of the first polyimide resin layer included in the substrate for a display device or a flexible display device is less than 570°C, the heat resistance and mechanical properties of the substrate for a display device or a flexible display device are significantly deteriorated, and deformation and denaturation of the film occur significantly at high temperatures.

[0035] In addition, by including a second polyimide resin layer having a yellowness index of less than 25, high resistance characteristics are realized due to the wide polymer chain spacing of the second polyimide resin layer, reducing electrical influences, which may help improve image retention when applied to display devices or flexible display devices.

[0036] When the second polyimide resin layer included in the substrate for a display device or a flexible display device has a high yellowness, the transmittance of the substrate for a display device or a flexible display device is reduced or the yellowness is increased, resulting in a decrease in high resistance characteristics.

[0037] Specifically, a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 to 60 is excellent in terms of shape stability at high temperatures, while a second polyimide resin layer having a yellowness index of less than 25 is excellent in high resistance characteristics.

[0038] More specifically, in the manufacturing process, the first polyimide resin layer, which is laminated and then further subjected to a heat treatment process, includes a first polyimide resin layer having excellent dimensional stability and a yellowness index of 25 to 60. Furthermore, the layer closer to the thin film transistor is more susceptible to electrical influences. Therefore, the second polyimide resin layer, which is the layer closer to the thin film transistor, includes a second polyimide resin layer having excellent high resistance characteristics and a yellowness index of less than 25. This makes it possible to achieve excellent dimensional stability and excellent high resistance characteristics in the substrate for a display device or a flexible display device according to one embodiment.

[0039] Meanwhile, the substrate for a display device or a flexible display device according to the embodiment may include a first inorganic layer positioned between the first polyimide resin layer and the second polyimide resin layer, and having a thickness of 100 nm to 700 nm, 200 nm to 600 nm, or 300 nm to 500 nm.

[0040] The first inorganic layer is located between the first polyimide resin layer and the second polyimide resin layer, and can achieve excellent device stability by blocking air and moisture. It can also provide a high-quality display device or flexible display device substrate by minimizing electrical influences caused by thin film transistors.

[0041] If the first inorganic layer is not included or its thickness is less than 100 nm, air and moisture may not be blocked, which may reduce device stability and increase electrical influence on thin film transistors, thereby reducing the quality of display devices or flexible display device substrates.

[0042] Furthermore, if the thickness of the first inorganic layer exceeds 700 nm, the thickness of the first inorganic layer becomes too thick, making it vulnerable to external changes and stresses, and also reducing productivity due to increased process costs and deposition time, making it unsuitable for use as a substrate for a display device or a flexible display device.

[0043] The inorganic layer may include one or more inorganic materials selected from the group consisting of silicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride (SiON), aluminum oxide (AlO), and aluminum oxynitride (AlON).

[0044] The first inorganic layer may be formed by a conventional method for forming inorganic materials in the technical field to which the present invention pertains, such as chemical vapor deposition (CVD).

[0045] In the substrate for a display device or a flexible display device according to the embodiment, the first polyimide resin layer may have a Td 1% of 570°C or higher, 570°C or higher and 650°C or lower, or 570°C or higher and 600°C or lower.

[0046] The Td 1% means the temperature (°C) at which the weight loss rate is 1% based on the mass of the sample at 100°C. There are no particular limitations on the method for measuring the Td 1%, and it can be measured, for example, using a TGA device. More specifically, it can be measured using a Discovery TGA device manufactured by TA Instruments.

[0047] In the substrate for a display device or a flexible display device according to the embodiment, the first polyimide resin layer exhibits excellent heat resistance because its Td 1% is 570°C or higher, and therefore does not deform even at high temperatures, ensuring mechanical properties and dimensional stability. Specifically, deformation and denaturation of the resin layer are significantly reduced even during processes at high temperatures of 430°C or higher during the manufacture of flexible displays.

[0048] In the substrate for a display device or a flexible display device according to the embodiment, the first polyimide resin layer has a yellowness index of 25 or more and 60 or less, 25 or more and 50 or less, or 25 or more and 30 or less.

[0049] The yellowness index of the first polyimide resin layer is measured for a first polyimide resin layer sample having a thickness of 10±1 μm. The method for measuring the yellowness index is not particularly limited, and the yellowness index may be measured using, for example, a color meter. More specifically, the yellowness index may be measured using a Color-Eye 7000A manufactured by GRETAGMACBETH.

[0050] Specifically, the yellowness of the first polyimide resin layer may be a value measured at the center of a first polyimide resin layer thin film having a thickness of 10±1 μm, which is obtained by coating a 10 cm×10 cm glass substrate with a composition for forming a first polyimide resin layer.

[0051] As described above, in a substrate for a display device or a flexible display device, the first polyimide resin layer laminated on the base material has a yellowness index of 25 to 60, which ensures mechanical properties, dimensional stability, and excellent heat resistance at high temperatures due to high packing density. Therefore, deformation and denaturation of the resin layer are significantly reduced even when subjected to high-temperature processes at 430°C or higher.

[0052] Meanwhile, in the substrate for a display device or a flexible display device according to the embodiment, the second polyimide resin layer has a yellowness index of less than 25, 20 or less, 1 to 20, 5 to 20, or 5 to 15.

[0053] The yellowness of the second polyimide resin layer is measured for a second polyimide resin layer sample having a thickness of 10±1 μm. The method for measuring the yellowness is not particularly limited, and the yellowness can be measured using, for example, a color meter. More specifically, the yellowness can be measured using a Color-Eye 7000A manufactured by GRETAGMACBETH.

[0054] Specifically, the yellowness index of the second polyimide resin layer may be a value measured at the center of a thin film of the second polyimide resin layer after forming a first polyimide resin layer by coating a 10 cm x 10 cm glass substrate with a composition for forming a first polyimide resin layer, forming a first inorganic layer on the first polyimide resin layer by plasma enhanced chemical vapor deposition, and forming a second polyimide resin layer on the first inorganic layer.

[0055] Since the second polyimide resin layer has a yellowness index of less than 25, excellent low dielectric properties and high resistance properties can be realized due to the wide polymer chain spacing of the second polyimide resin layer, and electrical influences can be reduced, which may be useful for improving image retention when applied to a display device or a flexible display device.

[0056] Meanwhile, in the substrate for a display device or a flexible display device according to the embodiment, the difference in yellowness between the first polyimide resin layer and the second polyimide resin layer may be 5 or more, 5 to 50 or less, 5 to 30 or less, 5 to 15 or less, or 5 to 10 or less.

[0057] In addition, the substrate for a display device or a flexible display device according to the embodiment may further include a second inorganic layer formed on the second polyimide resin layer and having a thickness of 100 nm to 500 nm, or 200 nm to 400 nm.

[0058] That is, the substrate for a display device or a flexible display device according to the embodiment may have a structure in which a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 to 60 is laminated on a base material, a first inorganic layer having a thickness of 100 nm to 700 nm is laminated on the first polyimide resin layer, a second polyimide resin layer having a yellowness index of less than 25 is laminated on the first inorganic layer, and the second inorganic layer having a thickness of 100 nm to 500 nm or 200 nm to 400 nm is laminated on the second polyimide resin layer having a yellowness index of 20 or less.

[0059] The second inorganic layer is located on the second polyimide resin layer and secondarily blocks air and moisture, thereby achieving excellent device stability and minimizing electrical influences caused by thin film transistors, thereby providing a substrate for a high-quality display device or a flexible display device.

[0060] If the second inorganic layer is not included or its thickness is less than 100 nm, air and moisture are not blocked, which may reduce device stability and increase electrical influence on thin film transistors, thereby reducing the quality of display devices or flexible display device substrates.

[0061] Furthermore, if the thickness of the second inorganic layer exceeds 500 nm, the thickness of the second inorganic layer becomes too thick, making it vulnerable to external changes and stresses, and also reducing productivity due to increased process costs and deposition time, making it unsuitable for use as a substrate for a display device or a flexible display device.

[0062] The inorganic layer may include one or more inorganic materials selected from the group consisting of silicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride (SiON), aluminum oxide (AlO), and aluminum oxynitride (AlON).

[0063] The second inorganic layer may be formed by a conventional inorganic material forming method in the technical field to which the present invention pertains, such as chemical vapor deposition (CVD).

[0064] Meanwhile, in the substrate for a display device or a flexible display device according to the embodiment, the yellowness index of the entire substrate for a display device or a flexible display device may be 36 or less, 10 to 36, 20 to 36, 24 to 36, or 24 to 30.

[0065] As described above, by including the first polyimide resin layer having a yellowness index of 25 to 60 and the second polyimide resin layer having a yellowness index of 20 or less, the yellowness index of the entire substrate for a display device or flexible display device satisfies 36 or less.

[0066] If the yellowness of the entire substrate for a display device or a flexible display device becomes high, the substrate is unsuitable for use as a substrate for a highly transparent display. Also, if the yellowness of the entire substrate for a display device or a flexible display device becomes high, a problem caused by an internal laminate structure occurs, and in particular, the dielectric constant becomes excessively high, resulting in poor image retention, making the substrate unsuitable for use as a substrate for a display device or a flexible display device.

[0067] The method for measuring the yellowness is not particularly limited, and for example, it can be measured using a color meter, more specifically, it can be measured using Color-Eye 7000A manufactured by GRETAGMACBETH.

[0068] The yellowness index can be measured from a substrate sample for a display device or flexible display device having a thickness of 16±1 μm. When the thickness of the sample increases or decreases by a certain value, the measured physical property can change by a certain value.

[0069] Specifically, the yellowness index of the entire substrate for a display device or a flexible display device may be a value measured at the center of a thin film of the second inorganic material layer after coating a 10 cm x 10 cm glass substrate with a composition for forming a first polyimide resin layer to form a first polyimide resin layer, forming a first inorganic material layer on the first polyimide resin layer by a plasma-enhanced chemical vapor deposition method, forming a second polyimide resin layer on the first inorganic material layer, and then forming a second inorganic material layer on the second polyimide resin layer by a plasma-enhanced chemical vapor deposition method.

[0070] Meanwhile, in the substrate for a display device or a flexible display device according to the embodiment, the entire substrate for a display device or a flexible display device may have a yellowness index of 50 or less, 25 to 50 or less, 30 to 50 or less, 33 to 50 or less, or 33 to 40 or less after heat treatment in which the substrate is heated to 430°C for 2 hours, isothermally maintained at 430°C for 2 hours, and then cooled to 50°C for 2 hours.

[0071] The method for measuring the yellowness is not particularly limited, and for example, it can be measured using a color meter, more specifically, it can be measured using Color-Eye 7000A manufactured by GRETAGMACBETH.

[0072] The yellowness index can be measured from a substrate sample for a display device or flexible display device having a thickness of 16±1 μm. When the thickness of the sample increases or decreases by a certain value, the measured physical property can change by a certain value.

[0073] Even after heat treatment, which involves heating to 430°C for 2 hours, maintaining an isothermal temperature at 430°C for 2 hours, and then cooling to 50°C for 2 hours, the yellowness index of the substrate for a display device or a flexible display device is 50 or less, thereby ensuring excellent optical properties such as transmittance and yellowness index, making it suitable for use as a substrate for a display device or a flexible display device.

[0074] Meanwhile, in one embodiment, the initial yellowness of the entire substrate for a display device or a flexible display device after heat treatment, which involves heating to 430°C for 2 hours, maintaining an isothermal temperature at 430°C for 2 hours, and then cooling to 50°C for 2 hours, may be 130% or less, or 100% to 130%, or 110% to 130%.

[0075] In one embodiment, a substrate for a display device or flexible display device having excellent optical properties even after a high-temperature process at 430° C. can be provided, where the yellowness ratio after a heat treatment, in which the substrate is heated to 430° C. for 2 hours, isothermally maintained at 430° C. for 2 hours, and then cooled to 50° C. for 2 hours, is 130% or less relative to the initial yellowness of the entire substrate for a display device or flexible display device. The yellowness ratio after a heat treatment, in which the substrate is heated to 430° C. for 2 hours, isothermally maintained at 430° C. for 2 hours, and then cooled to 50° C. for 2 hours, is 130% or less relative to the initial yellowness of the entire substrate for a display device or flexible display device. As described above, the substrate for a display device or flexible display device of the embodiment has a Td 1% of 570° C. or more and includes a first polyimide resin layer having a yellowness ratio of 25 to 60 and a second polyimide resin layer having a yellowness ratio of less than 25.

[0076] Meanwhile, the thickness of each of the first polyimide resin layer and the second polyimide resin layer is determined in consideration of the thickness and physical properties of each of the first inorganic layer, the second inorganic layer, and the substrate for a display device or a flexible display device or the entire substrate. For example, each of the first polyimide resin layer and the second polyimide resin layer has a thickness of 0.5 μm to 20 μm.

[0077] At this time, the yellowness of the first polyimide resin layer and the second polyimide resin layer varies depending on the thickness, but as described above, the yellowness of each of the first polyimide resin layer and the second polyimide resin layer is measured and defined based on a thickness of 10±1 μm.

[0078] More specifically, the first polyimide resin layer has a thickness of 1 μm to 10 μm, 2 μm to 10 μm, or 6 μm to 10 μm, and the second polyimide resin layer has a thickness of 0.5 μm to 6 μm, 1 μm to 6 μm, or 3 μm to 6 μm.

[0079] Alternatively, the first polyimide resin layer has a thickness of 1 μm to 6 μm, 1 μm to 5 μm, or 2 μm to 5 μm, and the second polyimide resin layer has a thickness of 6 μm to 15 μm, 8 μm to 15 μm, or 10 μm to 15 μm.

[0080] Meanwhile, the Td 1% of the second polyimide resin layer may be 500°C or more and 560°C or less, 500°C or more and 555°C or less, 520°C or more and 555°C or less, or 545°C or more and 555°C or less.

[0081] As described above, the Td 1% refers to the temperature (°C) at which the weight loss rate is 1% based on the mass of the sample at 100°C. There are no particular limitations on the method for measuring the Td 1%, and it can be measured, for example, using a TGA device. More specifically, it can be measured using a Discovery TGA device manufactured by TA Instruments.

[0082] In the substrate for a display device or a flexible display device according to the embodiment, the second polyimide resin layer has a Td 1% of 500°C or more and 560°C or less, whereby the first polyimide resin layer has a Td 1% of 570°C or more and has excellent heat resistance, and at the same time, the second polyimide resin layer has high resistance and excellent optical properties.

[0083] Specifically, the first polyimide resin layer and the second polyimide resin layer have a light transmittance of 50% or more, 50% to 100%, and 65% to 95% at a wavelength of 470 nm, respectively, and can exhibit a haze of 1.0% or less, 0.1% to 1.0%, and 0.3% to 0.8%.

[0084] The haze measurement method and device of the embodiment are not particularly limited, and various methods used in conventional haze measurement can be applied without limitation. For example, haze can be measured according to ASTM D1003 using a haze meter (model: NDH7000, Nippon Denshoku Co., Ltd.).

[0085] The method and apparatus for measuring the transmittance of the present embodiment are not particularly limited, and various methods used in conventional transmittance measurements can be applied without limitation. For example, the transmittance (T) of a 470 nm wavelength light can be measured using a UV-vis spectroscopy (Agillent, UV8453) device.

[0086] The transmittance and haze may be values ​​measured for a sample of the first polyimide resin layer and the second polyimide resin layer, each having a thickness of 10 μm. When the thickness of the resin layer increases or decreases by a certain value, the transmittance and haze may also change by a certain value.

[0087] The first polyimide resin layer and the second polyimide resin layer each have a light transmittance of 50% or more at a wavelength of 470 nm and a haze of 1.0% or less, thereby ensuring excellent optical properties such as light transmittance and yellowness index, and thus providing a substrate for a display device or a flexible display device that is suitable for use as a substrate for a display device or a flexible display device.

[0088] In this specification, the polyimide resin refers to both polyimide and its precursor polymers, polyamic acid and polyamic acid ester. That is, the polyimide resin may contain one or more repeating units selected from the group consisting of polyamic acid repeating units, polyamic acid ester repeating units, and polyimide repeating units. That is, the polyimide resin may contain one type of polyamic acid repeating unit, one type of polyamic acid ester repeating unit, one type of polyimide repeating unit, or a copolymer containing a mixture of two or more of these repeating units.

[0089] One or more repeating units selected from the group consisting of polyamic acid repeating units, polyamic acid ester repeating units, and polyimide repeating units can form the main chain of the polyimide resin.

[0090] Specifically, the second polyimide resin layer may contain a polyimide resin containing a repeating unit derived from a diamine substituted with a fluorine-based functional group.

[0091] More specifically, the second polyimide resin layer may include a reaction product of an aromatic tetracarboxylic acid or an anhydride thereof with a diamine substituted with a fluorine-based functional group, or a polyimide resin including a repeating unit derived therefrom.

[0092] When the second polyimide resin layer contains a reaction product of an aromatic tetracarboxylic acid or an anhydride thereof with a diamine substituted with a fluorine-based functional group, or a polyimide resin containing a repeating unit derived therefrom, the second polyimide resin layer can achieve a yellowness index of less than 25.

[0093] More specifically, the second polyimide resin layer may include a polyimide resin including a repeating unit represented by the following Chemical Formula 1: [Chemical formula 1] [ka] In the above Chemical Formula 1, X1 is an aromatic tetravalent functional group, and Y1 is an aromatic divalent functional group substituted with at least one fluorine-based functional group.

[0094] In the above formula 1, X1 is a functional group derived from a tetracarboxylic dianhydride compound used in the synthesis of polyimide resins.

[0095] In Formula 1, Y1 is an aromatic divalent functional group substituted with at least one fluorine-based functional group, and may be a functional group derived from a diamine compound used in the synthesis of polyimide-based resins.

[0096] Substitution with a fluorine-based functional group, such as a highly electronegative trifluoromethyl group (-CF3), can enhance the effect of suppressing the formation of charge transfer complexes (CTCs) of Pi-electrons present within the polyimide resin chain, thereby ensuring improved transparency. In other words, it can reduce packing within the polyimide structure or between chains, and weaken the electrical interaction between color sources through steric hindrance and electrical effects, thereby demonstrating high transparency in the visible light region.

[0097] Specifically, the aromatic divalent functional group Y1 substituted with at least one fluorine-based functional group may include a functional group represented by the following chemical formula 3-1. [Chemical formula 3-1] [ka] In the above chemical formula 3-1, P is an integer of 0 or more and 5 or less, and preferably an integer of 0 or more and 2 or less.

[0098] More specifically, the polyimide resin is formed by the reaction of the terminal anhydride group (-OC-O-CO-) of tetracarboxylic acid dianhydride with the terminal amino group (-NH2) of an aromatic diamine substituted with at least one fluorine-based functional group, forming a bond between the nitrogen atom of the amino group and the carbon atom of the anhydride group.

[0099] The second polyimide resin layer may be prepared by reacting two or more different diamine compounds with a tetracarboxylic acid dianhydride compound. The two diamine compounds may be added simultaneously to synthesize a random copolymer or sequentially to synthesize a block copolymer.

[0100] Specifically, the second polyimide resin layer may include a polyimide resin further including a repeating unit represented by the following Chemical Formula 1-1 in addition to the repeating unit represented by Chemical Formula 1: [Chemical formula 1-1] [ka] In the above Chemical Formula 1-1, X2 is an aromatic tetravalent functional group, and Y2 is an aromatic divalent functional group different from Y1 in the above Chemical Formula 1.

[0101] Meanwhile, the first polyimide resin layer may include a polyimide resin including a repeating unit derived from a diamine in which a fluorine-based functional group is not substituted.

[0102] Specifically, the first polyimide resin layer may include a reaction product of a tetracarboxylic acid or an anhydride thereof with a diamine in which a fluorine-based functional group is not substituted, or a polyimide resin including a repeating unit derived therefrom.

[0103] When the first polyimide resin layer contains a reaction product of a tetracarboxylic acid or an anhydride thereof with a diamine in which a fluorine-based functional group is not substituted, or a polyimide resin containing a repeating unit derived therefrom, the first polyimide resin layer can achieve a Td 1% of 570°C or higher and a yellowness index of 25 to 60.

[0104] More specifically, the first polyimide resin layer may include a polyimide resin including a repeating unit represented by the following Chemical Formula 4: [Chemical formula 4] [ka] In Chemical Formula 4, X3 is a tetravalent functional group represented by Chemical Formula 5 below, and Y3 is an aromatic divalent functional group unsubstituted with a fluorine-based functional group.

[0105] [Chemical formula 5] [ka] In the above Chemical Formula 5, R1 to R6 are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms; L1 and L2 are the same or different and each independently represent one of -COO- or -OCO-; L3 is a single bond, -O-, -CO-, -COO-, -S-, -SO-, -SO2-, -CR7R8-, or -(CH2)t -, -O(CH2) t O-, -COO(CH2) t R7 and R8 are each independently one of hydrogen, an alkyl group having 1 to 10 carbon atoms, or a haloalkyl group having 1 to 10 carbon atoms; and t is an integer of 1 to 10.

[0106] Specific examples of the functional group represented by Chemical Formula 5 include the functional group represented by Chemical Formula 8-1 below. [Chemical formula 8-1] [ka] In the above formula 4, X3 is a functional group derived from a tetracarboxylic dianhydride compound used in the synthesis of polyimide resins.

[0107] In Formula 4, Y3 is an aromatic divalent functional group unsubstituted with a fluorine-based functional group, and may be a functional group derived from a diamine compound used in the synthesis of a polyimide-based resin.

[0108] Specifically, the aromatic divalent functional group Y3 in which a fluorine-based functional group is not substituted may include a functional group represented by the following chemical formula 9-1. [Chemical formula 9-1] [ka]

[0109] Meanwhile, in the substrate for a display device or a flexible display device, the first polyimide resin layer and the second polyimide resin layer may each have a thermal expansion coefficient of 5 ppm / °C or less in the range of 100°C to 460°C.

[0110] The thermal expansion coefficient was measured by measuring the change in thermal expansion during a first heating process in which a tensile force of 0.01 N to 0.1 N or 0.01 N to 0.05 N was applied to the polyimide resin layer sample, and the sample was then cooled at a rate of 1°C / min to 10°C / min or 4°C / min to 6°C / min in the temperature range of 100°C to 460°C, using a TMA (Q400 manufactured by TA Corporation).

[0111] The thermal expansion coefficient can be measured from a polyimide resin layer sample having a thickness of 10±1 μm. When the thickness of the polyimide resin layer increases or decreases by a certain value, the physical properties measured on the polyimide resin layer can change by a certain value.

[0112] In the substrate for a display device or a flexible display device, the first polyimide resin layer and the second polyimide resin layer have a thermal expansion coefficient of 5 ppm / °C or less in the temperature range of 100°C to 460°C. This ensures sufficient heat resistance even for a substrate for a display device or a flexible display device obtained by high-temperature curing. When this is used as a plastic substrate, when a metal layer formed on the plastic substrate is heat-treated, the plastic substrate can be prevented from being damaged by heat and warpage can be suppressed in the metal thin film formed on the plastic substrate.

[0113] The weight average molecular weight (measured by GPC) of the polyimide resin is not particularly limited, but may be, for example, 1,000 g / mol to 200,000 g / mol, or 10,000 g / mol to 200,000 g / mol.

[0114] The substrate for a display device or a flexible display device according to the present invention can exhibit excellent colorless and transparent properties while maintaining properties such as heat resistance and mechanical strength due to its rigid structure, and is used in various fields such as substrates for devices, cover substrates for displays, optical films, circuit boards, IC (integrated circuit) packages, adhesive films, multi-layer FRC (flexible printed circuits), tapes, touch panels, and protective films for optical discs.

[0115] Meanwhile, the substrate for a display device or a flexible display device according to the embodiment may include a cured product of the polyimide-based resin cured at a temperature of 400° C. or higher. The cured product refers to a material obtained by curing a resin composition containing the polyimide-based resin, and the curing process is performed at a temperature of 400° C. or higher, or 400° C. to 500° C.

[0116] More specifically, the method for synthesizing the polyimide resin layer of the substrate for a display device or a flexible display device is not particularly limited, and for example, a manufacturing method including a step of applying a resin composition containing the polyimide-based resin to a substrate to form a coating film (Step 1), a step of drying the coating film (Step 2), and a step of heat-treating the dried coating film to harden it (Step 3) can be used.

[0117] Step 1 is a step of forming a coating film by applying a resin composition containing the polyimide resin to a substrate. The method of applying the resin composition containing the polyimide resin to a substrate is not particularly limited, and methods such as screen printing, offset printing, flexographic printing, and inkjet printing may be used.

[0118] The resin composition containing the polyimide resin may be dissolved or dispersed in an organic solvent. For example, if the polyimide resin is synthesized in an organic solvent, the solution may be the reaction solution itself, or the reaction solution diluted with another solvent. Furthermore, if the polyimide resin is obtained as a powder, the solution may be obtained by dissolving the powder in an organic solvent.

[0119] Specific examples of the organic solvent include toluene, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, 2-pyrrolidone, N-ethylpyrrolidone, N-vinylpyrrolidone, dimethyl sulfoxide, tetramethylurea, pyridine, dimethyl sulfone, hexamethyl sulfoxide, γ-butyrolactone, 3-methoxy-N,N-dimethylpropanamide, 3-ethoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, 1,3-dimethyl-imidazolidinone, ethyl amyl ketone, methyl nonyl ketone, methyl ethyl ketone, methyl isoamyl ketone, methyl isopropyl alcohol, methyl methyl ketone ... Examples of the alkyl ether include isopropyl ketone, cyclohexanone, ethylene carbonate, propylene carbonate, diglyme, 4-hydroxy-4-methyl-2-pentanone, ethylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol monopropyl ether, ethylene glycol monopropyl ether acetate, ethylene glycol monoisopropyl ether, ethylene glycol monoisopropyl ether acetate, ethylene glycol monobutyl ether, and ethylene glycol monobutyl ether acetate. These may be used alone or in combination.

[0120] The resin composition containing the polyimide-based resin may contain a solid content in an amount that provides an appropriate viscosity in consideration of processability, such as coatability during a film formation process. For example, the content of the composition may be adjusted so that the total resin content is 5% by weight to 25% by weight, or 5% by weight to 20% by weight.

[0121] The polyimide-based resin composition may further contain other components in addition to the organic solvent. Non-limiting examples include additives that improve the uniformity of the film thickness or the surface smoothness when the polyimide-based resin composition is applied, improve adhesion to the substrate, change the dielectric constant or conductivity, or increase the density. Examples of such additives include surfactants, silane-based compounds, dielectric or crosslinking compounds, etc.

[0122] The step 2 is a step of applying the resin composition containing the polyimide resin to a substrate and drying the resulting coating.

[0123] The drying step of the coating film can be performed by a heating means such as a hot plate, a hot air circulating oven, or an infrared oven at a temperature of 50°C or higher and 150°C or lower, or 50°C or higher and 100°C or lower.

[0124] Step 3 is a step of curing the dried coating by heat treatment, which can be performed using a heating means such as a hot plate, a hot air circulating oven, or an infrared oven at a temperature of 400°C or higher, or 400°C to 500°C.

[0125] The thickness of the substrate for a display device or a flexible display device is not particularly limited, and may be freely adjusted within a range of, for example, 0.01 μm to 1000 μm. When the thickness of the substrate for a display device or a flexible display device increases or decreases by a certain value, physical properties measured from the substrate for a display device or a flexible display device may also change by a certain value.

[0126] The substrate for a display device or a flexible display device according to the above embodiment may include a silicon oxide layer formed between the polyimide resin layer and the cured layer in order to further improve solvent resistance, moisture permeability, and optical properties, and the silicon oxide layer is produced by curing polysilazane.

[0127] Specifically, the silicon oxide layer is formed by coating and drying a solution containing polysilazane on at least one surface of the polyimide resin layer prior to forming a coating layer, and then curing the coated polysilazane.

[0128] According to yet another embodiment of the present invention, there is provided a display device or a flexible display device including the substrate for a display device or a flexible display device of the other embodiment. The content relating to the substrate for a display device or a flexible display device may include all of the content described above in the above embodiment.

[0129] The display device or flexible display device may be configured and manufactured by techniques known in the art, except that the display device or flexible display device substrate is used for the above-mentioned purposes.

[0130] The display device or flexible display device includes a curved, bendable, flexible, rollable, or foldable type mobile communication terminal, a touch panel of a smartphone or tablet PC, and various displays.

[0131] An example of the display device or flexible display device is a flexible light-emitting element display device.

[0132] For example, the organic light emitting diode (OLED) display may have a cover window of the display device or flexible display device located at an outer corner in a direction in which light or a screen emerges, and may include a cathode that provides electrons, an electron transport layer, an emission layer, a hole transport layer, and an anode that provides holes, which are sequentially formed.

[0133] The OLED display may further include a hole injection layer (HIL) and an electron injection layer (EIL).

[0134] In order for the organic light emitting diode (OLED) display to function and operate as a flexible display, the negative and positive electrodes and each component can be made of a material having a predetermined elasticity.

[0135] Another example of the display device or flexible display device is a rollable display or a foldable display.

[0136] The rollable display device may have various structures depending on the application field and specific form, and may have a structure including, for example, a cover plastic window, a touch panel, a polarizing plate, a barrier film, a light-emitting element (such as an OLED element), a transparent substrate, etc. [Effects of the Invention]

[0137] According to the present invention, there is provided a substrate for a display device or a flexible display device, which has high resistance and low moisture absorption, is not denatured even in a high-temperature heat treatment process, and has excellent optical properties, and a display device or a flexible display device using the same. DETAILED DESCRIPTION OF THE INVENTION

[0138] The present invention will be described in more detail with reference to the following examples, but the following examples are merely illustrative and are not intended to limit the scope of the present invention.

[0139] Manufacturing Example 1 A 500 mL four-necked round-bottom flask (reactor) equipped with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller, and a condenser was charged with 200 g of N-methyl-2-pyrrolidone while slowly passing nitrogen through it. The temperature of the reactor was adjusted to 60°C, and then 133 mmol of p-phenylenediamine (PDA) was added and completely dissolved.

[0140] While maintaining the temperature of the solution at 60°C, 132 mmole of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 120 g of N-methyl-2-pyrrolidone were added to the reactor, followed by stirring and dilution for 48 hours to obtain polyimide precursor composition P-1 (solids content 11 wt%).

[0141] Manufacturing Example 2 A 500 mL four-neck round-bottom flask (reactor) equipped with a stirrer, nitrogen injector, dropping funnel, temperature controller, and condenser was filled with 100 g of diethylacetamide (DEAC) while nitrogen was slowly passed through it. 33 mmol of p-phenylene diamine (PDA) and 33 mmol of 2,2'-bis(trifluoromethyl)benzidine (TFMB) were then added and completely dissolved.

[0142] While maintaining the temperature of the solution at room temperature, 51 mmol of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 15 mmol of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) were added to the reactor together with 100 g of DEAC, and the mixture was stirred and diluted for 48 hours to obtain polyimide precursor composition P-2 (solids content 13 wt%).

[0143] Manufacturing Example 3 A 500 mL four-necked round-bottom flask (reactor) equipped with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller, and a condenser was filled with 100 g of diethylacetamide (DEAC) while nitrogen was slowly passed through the flask, and 35 mmol of diamine represented by the following chemical formula A was added and completely dissolved. [Chemical formula A] [ka]

[0144] While maintaining the temperature of the solution at 40°C, 27 mmol of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 8 mmol of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) were added to the reactor together with 60 g of DEAC, and the mixture was stirred and diluted for 48 hours to obtain polyimide precursor composition P-3 (solids content 10 wt%).

[0145] Manufacturing Example 4 A 500 mL four-necked round-bottom flask (reactor) equipped with a stirrer, a nitrogen injector, a dropping funnel, a temperature controller, and a condenser was filled with 100 g of diethylacetamide (DEAC) while nitrogen was slowly passing through it, and 35 mmol of diamine represented by the following chemical formula B was added and completely dissolved. [Chemical formula B] [ka]

[0146] While maintaining the temperature of the solution at 40°C, 27 mmol of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 8 mmol of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) were added to the reactor together with 60 g of DEAC, and the mixture was stirred and diluted for 48 hours to obtain polyimide precursor composition P-4 (solids content 5 wt%).

[0147] <Example>

[0148] Example 1 The polyimide precursor composition prepared in Preparation Example 1 was spin-coated on a glass substrate. The glass substrate coated with the polymer solution was placed in an oven, heated to 470°C at a rate of 3°C / min, and maintained at 470°C for 10 minutes to form a first polyimide resin layer (thickness 10 μm).

[0149] A first inorganic layer (SiO2, thickness 5,000 Å) was formed on the first polyimide resin layer by plasma enhanced chemical vapor deposition.

[0150] The polyamide precursor composition according to Preparation Example 1 was spin-coated on the first inorganic layer, and then placed in an oven, heated to 470°C at a rate of 3°C / min, and maintained at 470°C for 10 minutes to form a second polyimide resin layer (thickness 6 μm).

[0151] Then, a second inorganic layer (SiO2, thickness 3,000 Å) was formed on the second polyimide resin layer by plasma enhanced chemical vapor deposition.

[0152] Example 2 The second polyimide resin layer was prepared in the same manner as in Example 1, except that the polyimide precursor composition of Preparation Example 2 was used instead of the polyimide precursor composition of Preparation Example 1.

[0153] Example 3 The second polyimide resin layer was prepared in the same manner as in Example 1, except that the polyimide precursor composition of Preparation Example 3 was used instead of the polyimide precursor composition of Preparation Example 1.

[0154] Example 4 The first polyimide resin layer was formed to a thickness of 2 μm, and the second polyimide resin layer was formed to a thickness of 10 μm using the polyimide precursor composition of Preparation Example 2 instead of the polyimide precursor composition of Preparation Example 1, and was prepared in the same manner as in Example 1.

[0155] Example 5 The first polyimide resin layer was formed to a thickness of 5 μm, and the second polyimide resin layer was formed to a thickness of 10 μm using the polyimide precursor composition of Preparation Example 2 instead of the polyimide precursor composition of Preparation Example 1, and was prepared in the same manner as in Example 1.

[0156] <Comparative Example>

[0157] Comparative Example 1 The first polyimide resin layer was prepared in the same manner as in Example 1, except that the polyimide precursor composition of Preparation Example 2 was used instead of the polyimide precursor composition of Preparation Example 1.

[0158] Comparative Example 2 The same method as in Example 1 was used to form the first polyimide resin layer, except that the polyimide precursor composition of Preparation Example 2 was used instead of the polyimide precursor composition of Preparation Example 1, and the polyimide precursor composition of Preparation Example 3 was used instead of the polyimide precursor composition of Preparation Example 1 to form the second polyimide resin layer.

[0159] Comparative Example 3 The same method as in Example 1 was used to form the first polyimide resin layer, except that the polyimide precursor composition of Preparation Example 3 was used instead of the polyimide precursor composition of Preparation Example 1, and the second polyimide resin layer was used instead of the polyimide precursor composition of Preparation Example 1.

[0160] Reference example 1 The polyimide precursor composition prepared in Preparation Example 1 was spin-coated on a glass substrate. The glass substrate coated with the polymer solution was placed in an oven, heated to 470°C at a rate of 3°C / min, and maintained at 470°C for 10 minutes to form a first polyimide resin layer (thickness 10 μm).

[0161] The polyamide precursor composition of Preparation Example 2 was spin-coated on the first polyimide resin layer, and then placed in an oven, heated to 470°C at a rate of 3°C / min, and maintained at 470°C for 10 minutes to form a second polyimide resin layer (thickness 6 μm).

[0162] <Experimental Example>

[0163] The physical properties of the substrates for display devices or flexible display devices obtained in the examples and comparative examples were measured by the following methods. The results are shown in Table 1.

[0164] 1. Coefficient of Thermal Expansion (CTE) The polyimide precursor compositions prepared in the Preparation Examples and Comparative Preparation Examples were spin-coated onto glass substrates, and the glass substrates coated with the polymer solutions were heated to 470°C at a rate of 3°C / min and maintained at 470°C for 10 minutes to form polyimide resin layers (10 μm thick).

[0165] The polyimide resin layer test specimens were prepared to a size of 5 mm x 20 mm and loaded using an accessory. The length of the polyimide resin layer to be measured was 16 mm. The pulling force was set to 0.02 N, and the first heating process was performed at a heating rate of 5°C / min in the temperature range of 100°C to 460°C in Preparation Example 1, and in the temperature range of 100°C to 430°C in Preparation Examples 2 and 3. The thermal expansion change during cooling at a cooling rate of 4°C / min was measured using a TMA (TA Q400).

[0166] 2. Dielectric constant The saturated charging voltage of the display device or flexible display device substrates of the examples and comparative examples was measured at a temperature of 25°C and a humidity of 40 to 50% using an H-0110 Honestmeter manufactured by Shishido Electrostatic Corporation, based on the JIS L1094 standard measurement method.

[0167] A voltage of 10 kV was applied to each of the manufactured display device or flexible display device substrates, with the distance from the tip of the needle electrode of the application unit to the rotating disk surface set to 20 mm and the distance from the electrode plate of the power receiving unit to the rotating disk surface set to 15 mm. While rotating the rotating disk, the 10 kV voltage application was stopped 100 seconds after the start of application, and the time until the charging voltage decayed to half was measured while the rotating disk was still rotating. The half-life was determined by measuring the time from the point at which the high voltage application was stopped until the potential value decreased by 50% of the saturated charging voltage value. The measured saturated charging voltages and their half-lives are shown in Table 1 below.

[0168] For reference, corona discharge is a discharge phenomenon that occurs when gas particles on the electrode surface are excited and ionized by the high voltage applied between two electrodes. This phenomenon occurs more frequently when the air density, voltage, temperature, and humidity are high. When corona discharge occurs, the gas is excited and ionized by the electric field near the wire, causing localized dielectric breakdown, resulting in the flow of current and discharge. Electrons that gain energy from the electric field collide with gas molecules, causing continuous ionization of the gas, releasing the electrons that the gas molecules were carrying.

[0169] A DC voltage is applied to the test piece in the form of a corona discharge. After the detected value reaches saturation, the high voltage is stopped and the decay state of the potential on the surface of the test piece is continuously detected to measure the electrostatic properties of the material. After charging the test piece in a corona discharge field, the time until the charged voltage decays by half (half-life) is measured.

[0170] 3.Heat resistance (Td 1%, ℃) The temperature (° C.) at which the weight loss rate of the polyimide resin layer test piece reached 1% was measured under a nitrogen atmosphere using a Discovery TGA device manufactured by TA Instruments.

[0171] Specifically, the sample was isothermally maintained at 50°C for 5 minutes, then heated to 200°C and maintained isothermally for 30 minutes. After that, the sample was cooled to 50°C and stabilized, and then heated to 600°C at a rate of 10°C / min. After stabilization, the temperature at which the weight loss rate was 1% was calculated based on the mass at 100°C.

[0172] 4. Hayes The haze was measured using a haze meter (model name: NDH7000, manufactured by Nippon Denshoku Co., Ltd.) according to the measurement method of ASTM D1003.

[0173] 5. Translucency The transmittance (T) of light with a wavelength of 470 nm was measured using a UV-vis spectroscopy (Agillent, UV8453) device. [Table 1]

[0174] 6.Yellowness (YI) In the above examples and comparative examples, after each layer was formed, the yellowness was measured using a color meter (Color-Eye 7000A manufactured by GRETAGMACBETH) and the results are shown in Table 2 below.

[0175] Specifically, after forming a first polyimide resin layer on a glass substrate, the yellowness index was measured and shown in "First PI resin layer" in Table 2 below.

[0176] A first inorganic layer was formed on the first polyimide resin layer by plasma enhanced chemical vapor deposition, and the yellowness index was measured and shown in "First inorganic layer" in Table 2 below.

[0177] After forming the second polyimide resin layer on the first inorganic layer, the yellowness index was measured and shown in "Second PI resin layer" in Table 2 below.

[0178] A second inorganic layer was formed on the second polyimide resin layer by plasma enhanced chemical vapor deposition, and the yellowness index was measured and shown in "Second inorganic layer" in Table 2 below.

[0179] Then, the temperature was raised to 430°C for 2 hours, and the sample was isothermally maintained at 430°C for 2 hours, followed by cooling to 50°C for 2 hours. The yellowness after the heat treatment was measured and shown in "Post-heat treatment" in Table 2 below.

[0180] The change in YI (%) was calculated using the following formula and is shown in Table 2 below. Formula Change in YI (%) = Yellowness after heat treatment (heated to 430°C for 2 hours, isothermed at 430°C for 2 hours, and then cooled to 50°C for 2 hours) / Yellowness before heat treatment (heated to 430°C for 2 hours, isothermed at 430°C for 2 hours, and then cooled to 50°C for 2 hours) x 100

[0181] 7. Evaluation of film floating phenomenon The substrates for display devices or flexible display devices obtained in the examples and comparative examples were subjected to a heat treatment process at 430°C for 2 hours by increasing the temperature at a rate of 3°C / min, and then the occurrence of film lifting was evaluated with the naked eye according to the following criteria. ○: Film floating phenomenon occurs X: No film floating occurs

[0182] [Table 2]

[0183] As shown in Tables 1 and 2 above, the substrates for display devices or flexible display devices of the examples exhibit excellent heat resistance and optical properties, and have long half-lives, resulting in high resistance properties. In particular, it was confirmed that optical properties such as yellowness index and transmittance do not change significantly even after undergoing high-temperature processes at 430°C or higher, and that the shape of individual layers or the shape of the laminate structure does not change significantly.

[0184] In contrast, the substrates for display devices or flexible display devices of the comparative examples were found to have inferior optical properties and heat resistance compared to the examples, and in particular, yellowness increased significantly after high-temperature treatment or film lifting occurred, resulting in defects in the internal laminate structure.

[0185] In addition, the substrate for a display device or a flexible display device in Reference Example 1 does not have an inorganic layer, and a second polyimide resin layer is formed on a first polyimide resin layer. Therefore, although the yellowness index is at a level equivalent to that of the Examples, the gas barrier property and moisture permeability are low, and air and moisture are not blocked, resulting in a decrease in the stability of the element, and it was therefore confirmed that the substrate is unsuitable as a substrate for a display device or a flexible display device.

Claims

1. A substrate for a display device or a flexible display device, a first polyimide resin layer having a Td 1% of 570°C or higher and a yellowness index of 25 or higher and 60 or lower; a second polyimide resin layer having a yellowness index of less than 25; a first inorganic layer located between the first polyimide resin layer and the second polyimide resin layer and having a thickness of 100 nm or more and 700 nm or less; and a second inorganic layer formed on the second polyimide resin layer and having a thickness of 100 nm to 500 nm; the yellowness index of the entire substrate for a display device or a flexible display device is 36 or less; The yellowness index of each of the first polyimide resin layer and the second polyimide resin layer is measured based on a thickness of 10±1 μm. A substrate for a display device or a flexible display device.

2. 2. The substrate for a display device or a flexible display device according to claim 1, wherein the substrate for a display device or a flexible display device has a yellowness index of 50 or less after heat treatment in which the substrate is heated to 430°C for 2 hours, isothermally maintained at 430°C for 2 hours, and then cooled to 50°C for 2 hours.

3. 3. The substrate for a display device or a flexible display device according to claim 2, wherein the substrate has a yellowness index of 30 to 50 after heat treatment comprising heating to 430°C for 2 hours, maintaining an isothermal temperature at 430°C for 2 hours, and then cooling to 50°C for 2 hours.

4. 4. The substrate for a display device or a flexible display device according to claim 2 or 3, wherein the ratio of yellowness after a heat treatment in which the temperature is raised to 430°C for 2 hours, the temperature is maintained at 430°C for 2 hours, and then the heat treatment is cooled to 50°C for 2 hours, is 130% or less relative to the initial yellowness of the entire substrate for a display device or a flexible display device.

5. the first polyimide resin layer has a thickness of 1 μm to 10 μm; The substrate for a display device or a flexible display device according to any one of claims 1 to 4, wherein the second polyimide resin layer has a thickness of 0.5 μm to 6 μm.

6. the first polyimide resin layer has a thickness of 1 μm to 6 μm; The substrate for a display device or a flexible display device according to any one of claims 1 to 4, wherein the second polyimide resin layer has a thickness of 6 μm to 15 μm.

7. The substrate for a display device or a flexible display device according to claim 1 , wherein the second polyimide resin layer has a Td 1% of 500° C. or more and 560° C. or less.

8. The first polyimide resin layer and the second polyimide resin layer are Each has a transmittance of 50% or more at a wavelength of 470 nm, 8. The substrate for a display device or a flexible display device according to claim 1, having a haze of 1.0% or less.

9. The substrate for a display device or a flexible display device according to claim 1 , wherein the second polyimide resin layer comprises a polyimide resin containing a repeating unit derived from a diamine substituted with a fluorine-based functional group.

10. 10. The substrate for a display device or a flexible display device according to claim 1, wherein the second polyimide resin layer comprises a polyimide resin having a repeating unit represented by the following Chemical Formula 1: [Chemical formula 1] 【Chemistry 1】 In the above Chemical Formula 1, X １ is an aromatic tetravalent functional group, Y １ is an aromatic divalent functional group substituted with at least one fluorine-based functional group.

11. 11. The substrate for a display device or a flexible display device according to claim 10, wherein the second polyimide resin layer comprises a polyimide resin further comprising a repeating unit represented by the following Chemical Formula 1-1: [Chemical formula 1-1] 【Chemistry 2】 In the above chemical formula 1-1, X ２ is an aromatic tetravalent functional group, Y ２ is Y in the above formula 1 １ is an aromatic divalent functional group different from

12. The Y １ The aromatic divalent functional group substituted with at least one fluorine-based functional group includes a functional group represented by the following chemical formula 3-1: [Chemical formula 3-1] 【Transformation 3】 In the above chemical formula 3-1, P is an integer of 0 to 5.

13. The substrate for a display device or a flexible display device according to claim 1 , wherein the first polyimide resin layer comprises a polyimide resin containing a repeating unit derived from a diamine in which a fluorine-based functional group is not substituted.

14. A display device or flexible display device comprising a display device or flexible display device substrate according to any one of claims 1 to 13.

Images