TW202317675A - Polyester film, laminate sheet and flexible display apparatus comprising same - Google Patents

Abstract

In the polyester film according to an embodiment, the transmittance for a specific wavelength and the strain for a tensile load to certain ranges when exposed to UV light, whereby it is possible to enhance the UV durability and flexibility at the same time. Accordingly, the polyester film and the laminated sheet comprising the same can be applied to a cover of a flexible display device, in particular, a foldable display device to prevent the poor or deformed appearance or the occurrence of device defects during repeated folding and exposure to UV light for long-term use.

Description

Polyester film, laminated sheet and flexible display device containing same

field of invention

Embodiments relate to a polyester film having enhanced UV durability and flexibility, a laminated sheet, and a flexible display device comprising the same.

Background of the invention

Along with the development of IT devices, display technology is constantly developing driven by demand. Technologies related to curved displays and bent displays have been commercialized. In recent years, in the field of mobile devices that require both larger screens and portability, flexible display devices that can be flexibly bent or folded in response to external forces are preferred. Specifically, a great advantage of a foldable display device is that when not in use, it is folded into a smaller size to enhance its portability, and when in use, it is unfolded to form a larger screen.

These flexible display devices mainly use transparent polyimide film or ultra-thin glass (UTG) as the cover window. Transparent polyimide films are easily scratched from the outside, and ultra-thin glass has a problem of poor anti-scattering characteristics; therefore, a protective film is applied to the surface thereof. In a film applied to a foldable display device, a tensile load is continuously applied to the film in a folded state. If the film is deformed in this state, the individual layers can be delaminated from one another.

In order to prevent the above problems, a polymer film made of a soft material such as an elastomer that is not easily deformed even when a certain load is applied for a long period of time may be used as a protective film. However, the resulting problem is that the elastomer-based polymer has a sticky characteristic, which makes process control difficult; due to the presence of gel, it is difficult to prepare a flawless transparent film, which is different from the outer cover window. A uniform feeling; it is difficult to prepare a film; and it is prone to deformation due to external impact such as pressing.

In addition, since the display devices used in mobile devices, such as mobile phones, may be exposed to ultraviolet light, the protective film needs to have the function of preventing deformation of internal components. [Prior Art Literature] (Patent Document 1) Korean Laid-Open Patent Publication No. 2017-0109746

Summary of Invention Technical Problem

In recent years, a polyethylene terephthalate (PET) film has been applied to the outer cover of the flexible display device as a protective film. However, it has a problem of weak elastic recovery ability, that is, weak flexibility required to be applied to a flexible display device. In addition, since general polyester films do not have UV durability, when they are used as protective films for display devices, UV light may cause yellowing or impair the performance of display panels.

Meanwhile, if a large amount of UV blocking agent is added, or if the film is modified to simply increase its flexibility to solve this problem, the mechanical properties and appearance characteristics or processability of the polyester film may be impaired.

According to the research conducted by the present inventors, when exposed to UV light, by adjusting the transmittance for a specific wavelength and the strain for tensile load to certain ranges, it is possible to provide both enhanced UV durability and Flexible polyester film.

In addition, according to research conducted by the present inventors, by laminating two polyester films on both sides of the transparent substrate and adding a certain amount of UV blocking agent to the film placed on the outside of the display, and if necessary, Adding a UV blocking agent to another film, thereby adjusting the characteristics of each film and the transmittance for a certain wavelength, can also provide a laminated sheet with enhanced UV durability and flexibility at the same time.

Therefore, an object of the embodiments is to provide a polyester film, a laminated sheet, and a flexible display device including the same having suppressed deformation even after exposure to UV light and repeated folding. problem solution

According to one embodiment, there is provided a polyester film having a total transmittance of 5% or less for light having a wavelength of 370 nm and after being irradiated with UV-B light for 24 hours, when viewed in a first direction in a plane It has an ultimate elongation of 3% or less when a load of N2% is applied for 1 hour. Here, the load of N2% is the load that makes the film extend 2% relative to the initial state in the first direction, and the UV-B light has a peak wavelength within 310 nm to 315 nm, a wavelength of 310 nm The irradiance of 0.66 W/m ² and the total irradiance of 31.62 W/m ² in the wavelength band from 250 nm to 400 nm.

According to another embodiment, there is provided a laminate comprising a transparent substrate; a first polyester film disposed on one side of the transparent substrate; and a second polyester film disposed on the other side of the transparent substrate , wherein the first polyester film comprises (i) 0.5 parts by weight to 2.0 parts by weight of a UV blocking agent relative to 100 parts by weight of the polyester resin contained in the first polyester film, and the first The polyester film has (ii) a total transmittance of 2% to 5.5% for light at a wavelength of 370 nm, (iii) a total transmittance of 9.5% to 22% for light at a wavelength of 380 nm, (iv) a total transmittance of 65% to 85% total transmission for light having a wavelength of 390 nm and (v) 85% to 95% total transmission for light having a wavelength of 550 nm.

According to yet another embodiment, a flexible display device is provided, which includes a flexible display panel; and a polyester film disposed on the flexible display panel.

According to yet another embodiment, there is provided a flexible display device, which includes a flexible display panel; and a laminated sheet disposed on the flexible display panel, wherein the laminated sheet includes a A transparent substrate on the display panel; a first polyester film placed on the transparent substrate; and a second polyester film placed under the transparent substrate, wherein the first polyester film comprises (i) 0.5 parts by weight to 2.0 parts by weight of a UV blocking agent, which is relative to 100 parts by weight of the polyester resin contained in the first polyester film, and the first polyester film has (ii) 2 for light with a wavelength of 370 nm % to 5.5% total transmittance, (iii) 9.5% to 22% total transmittance for light at 380 nm wavelength, (iv) 65% to 85% total transmittance for light at 390 nm wavelength and (v) a total transmittance of 85% to 95% for light having a wavelength of 550 nm. Advantageous effect of the present invention

According to an embodiment, polyesters having both enhanced UV durability and flexibility can be provided by tuning the transmittance for specific wavelengths and the strain for tensile loads into certain ranges when exposed to UV light film.

In addition, according to this embodiment, by adding a certain amount of UV blocking agent to the film placed on the outer side of the display in the two polyester films laminated on both sides of the transparent substrate, and if necessary, in another Adding a UV blocking agent to a film, thereby adjusting the characteristics of each film and the transmittance for a certain wavelength, can also provide a laminate with enhanced UV durability and flexibility at the same time.

Therefore, the polyester film and laminated sheet can be applied to flexible display devices, particularly foldable display device outer covers, to prevent defects or deformation during repeated folding and exposure to UV light in long-term use. Appearance or device defects occur.

Detailed Description of the Preferred Embodiment

In the following description about the embodiments, when it is mentioned that one element is formed "on" or "under" another element, it does not only mean that one element is directly formed "on" or "under" another element. "Under" also means that one element is indirectly formed on or under another element with other elements interposed therebetween.

The size of individual elements in the drawings may be depicted exaggeratedly for descriptive purposes and may be different from the actual size.

Throughout this specification, unless specifically stated otherwise, when a component is said to "comprise" an element, it will be understood that other elements may be included rather than excluded.

Additionally, unless otherwise indicated, all numbers expressing physical characteristics of elements, dimensions and the like used herein are to be understood as modified by the term "about".

In this specification, unless otherwise stated, an expression in the singular should be understood to encompass an expression in the singular or in the plural interpreted in the context.

In recent years, foldable display devices of the fold-in type (1a), the fold-out type (1b) and similar types as shown in FIGS. 1a and 1b have been developed. The polyester film applied to the outer cover (10) of these display devices has a high modulus at room temperature, thus making deformation possible near the point of repeated folding (p1, p2). In addition, since general polyester film does not have UV durability, as shown in FIG. 2a, when it is applied to the display device outer cover (10), UV light is absorbed inside the display panel (20) during long-term use. , which may cause problems during operation.

The embodiments described below provide polyester films that have both enhanced UV durability and flexibility when exposed to UV light by tuning the transmittance for specific wavelengths and the strain in tensile load within certain ranges , a laminated sheet and a flexible display device comprising the same. Features of polyester film

When the polyester film according to one embodiment is applied to the outer cover of a display device, it advantageously prevents deformation by protecting its internal components from UV light.

FIG. 3 shows the spectra of the transmittance versus the wavelength of light for Examples and Comparative Examples. As shown in FIG. 3 , Examples and Comparative Examples generally show low transmittance in the UV wavelength band, but the curves of Examples and Comparative Examples differ in transmittance at specific wavelengths.

Specifically, the curves of the examples maintain very low transmission until the end of the UV region (eg 370 nm). Then, the transmittance increases rapidly from the boundary (eg, 380 nm), thereby showing a relatively high level of transmittance at the beginning of the visible region (eg, 390 nm). On the other hand, the curves of the comparative examples show that the transmittance gradually increases until the end of the UV region (e.g. 370 nm), reaches a certain level at the border (e.g. 380 nm), and it increases gently until the visible region Start (eg 390 nm).

Therefore, the polyester film can show a specific value or lower transmittance at wavelengths of 370 nm and 380 nm, and can show a specific value or higher transmittance at visible light wavelengths such as 390 nm.

For example, a polyester film may have 10% or less, 7% or less, 5.5% or less, 5% or less, 4.5% or less, or 4% or less for light at a wavelength of 370 nm. Lower total transmittance. The polyester film according to one embodiment has a total transmittance of 5% or less for light having a wavelength of 370 nm.

In addition, the polyester film may have 30% or less, 25% or less, 22% or less, 20% or less, 15% or less, or 10% or less for light having a wavelength of 380 nm the total transmittance. Specifically, the polyester film may have a total transmittance of 20% or less for light having a wavelength of 380 nm.

In addition, the polyester film may have a total transmittance of 45% or higher, 50% or higher, 55% or higher, 60% or higher, or 65% or higher for light having a wavelength of 390 nm. For example, the polyester film may have a total transmission of 50% to 90%, 55% to 85%, or 60% to 80% for light having a wavelength of 390 nm. In addition, the polyester film may have a total transmittance of 75% or higher, 80% or higher, 85% or higher, 90% or higher, or 95% or higher for light having a wavelength of 550 nm. As a specific example, the polyester film may have a total transmittance of 55% or higher for light having a wavelength of 390 nm and 85% or higher for light having a wavelength of 550 nm.

As a specific example, a polyester film may have a total transmittance of 2% to 5.5% for light at a wavelength of 370 nm, 9.5% to 22% for light at a wavelength of 380 nm, and a total transmittance of 9.5% to 22% for light at a wavelength of 390 nm. The light has a total transmittance of 65% to 85% and has a total transmittance of 85% to 95% for light with a wavelength of 550 nm.

The polyester film can have an average change rate of total transmittance (%) of 0.1%/nm to 1%/nm at a wavelength of 370 nm to 375 nm and 3%/nm to 10 at a wavelength of 385 nm to 390 nm The average change rate of total transmittance (%) in %/nm.

Furthermore, in the polyester film according to one embodiment, even after exposure to UV light, the strain against tensile load was adjusted within a certain range. Therefore, the polyester film can maintain its original characteristics when applied to the outer cover of the flexible display device and undergoes multiple repeated foldings after exposure to UV light. The UV light can be, for example, UV-B light. A specific wavelength range for UV-B light may be about 280 nm to 315 nm. Specifically, UV-B light has a peak wavelength within 310 nm to 315 nm, an irradiance of about 0.66 W/m ² (for example, 0.6 to 0.7 W/m ² ) at a wavelength of 310 nm, and an irradiance of about 0.6 to 0.7 W/m 2 at a wavelength of 250 nm to Total irradiance of about 31.62 W/m ² (eg, 31.6 to 31.7 W/m ² ) in the wavelength band of 400 nm.

The polyester film according to one embodiment has a final stretch of 3% or less when a load of N2% is applied in the first direction in the plane for 1 hour after being irradiated with UV-B light for 24 hours. Here, the N2% load is the load that makes the film stretch 2% relative to the initial state in the first direction. Specifically, the stretching rate can be at room temperature, using the initial dimension of the film in the first direction of 50 mm (the initial distance between the points where the tensile load is applied) and the The dimension measurement is 15 mm in the vertical direction.

More specifically, (1) When a polyester film having an initial dimension of 50 mm in a first direction in a plane and a dimension of 15 mm in a direction perpendicular to the first direction is stretched in the first direction , first measure the load (N2%) when the size is increased by 2% relative to the initial size, and (2) when the load (N2%) is continuously applied to the film in the first direction for 1 hour, The ratio of the dimension increase relative to the initial dimension, that is, the final elongation is 3% or less.

As an example, after 24 hours of irradiation with UV-B light, the polyester film may have 3% or less, 2.7% or less, 2.5% or less, or 2.3% or lower final elongation. For example, after 24 hours of irradiation with UV-B light, when a load of N2% is applied for 1 hour, the polyester film can have 1.5% to 3% in the first direction and the second direction perpendicular to each other in the plane, respectively. , 2% to 3%, 2.3% to 3%, 2% to 2.7% or 2% to 2.5% of the final stretch rate. Specifically, after irradiating with UV-B light for 24 hours, when a load of N2% is applied for 1 hour, the polyester film can have 2% to 3% in the first direction and the second direction perpendicular to each other in the plane, respectively. the final elongation.

As another example, after 24 hours of irradiation with UV-B light, the polyester film may have a final stretch of 2% or less when a load of N1% is applied in the first direction in the plane for 1 hour. Here, the load of N1% is the load that makes the film stretch 1% relative to the initial state in the first direction. For example, after 24 hours of irradiation with UV-B light, a polyester film may have a final N of 1.7% or less, 1.5% or less, or 1.3% or less when a load of N1% is applied for 1 hour. Stretch rate. Specifically, after 24 hours of irradiation with UV-B light, when a load of N1% is applied for 1 hour, the polyester film can have 1.1% to 2% in the first direction and the second direction perpendicular to each other in the plane, respectively. , 1.3% to 2%, 1.5% to 2%, 1.1% to 1.7% or 1.1% to 1.5% final elongation. Stretch can be measured at room temperature using an initial dimension of the film of 50 mm in the first direction and a dimension of 15 mm in a direction perpendicular to the first direction.

As yet another example, after 24 hours of irradiation with UV-B light, the polyester film may have a final stretch of 7% or less when a load of N3% is applied in the first direction in the plane for 1 hour . Here, the load of N3% is the load that makes the film stretch 3% relative to the initial state in the first direction. For example, after 24 hours of irradiation with UV-B light, when a load of N3% is applied for 1 hour, the polyester film can have 6.7% or less, 6.5% or less, 6.3% or less, 6% or lower, or 5.7% or lower final elongation. Specifically, after irradiating with UV-B light for 24 hours, when a load of N3% is applied for 1 hour, the polyester film can have 3% to 7% in the first direction and the second direction perpendicular to each other in the plane, respectively. , 5% to 7%, 5% to 6%, 5.5% to 6.5% or 6% to 7% of the final stretch rate. Stretch can be measured at room temperature using an initial dimension of the film of 50 mm in the first direction and a dimension of 15 mm in a direction perpendicular to the first direction.

As a specific example, after 24 hours of irradiation with UV-B light, the polyester film can have a final stretch of 2% or less when a load of N1% is applied in the first direction in the plane for 1 hour and in the plane In the first direction, it has a final elongation of 7% or lower when a load of N3% is applied for 1 hour. Here, the load of N1% or N3% means that the film is stretched by 1% or 3% relative to the initial state in the first direction.

The first direction can be any direction in the plane of the polyester film, and the second direction can be defined in a direction perpendicular to the first direction in the plane. For example, the first direction may be the longitudinal direction (MD) or the transverse direction (TD) of the film, and the second direction may be the transverse direction (TD) or the longitudinal direction (MD) perpendicular thereto. Specifically, the first direction may be the machine direction (MD) of the film and the second direction may be the transverse direction (TD) of the film.

Fig. 6 shows a graph of elongation (%) versus load (N) applied to a polyester film. The loads of N1%, N2% and N3% can be respectively determined from the curves of elongation versus load. Specifically, for the polyester film, the load of N1% may be 28 N to 32 N, the load of N2% may be 50 N to 55 N, and the load of N3% may be 64 N to 68 N.

7 and 8 respectively show graphs of elongation (%) versus time (seconds) at a certain load (N) applied to the polyester films of Examples and Comparative Examples after irradiation with UV light. As described above, the polyester film of the example has a final stretch of 2% or less when a load of N1% is applied for 1 hour, and a final stretch of 3% or less when a load of N2% is applied for 1 hour rate and has a final stretch rate of 7% or lower when a load of N3% is applied for 1 hour, while the polyester film of the comparative example does not satisfy these stretch characteristics.

The ratio of N2% load to N1% load may be 1.6:1 to 2.1:1. Specifically, the ratio of the load of N2% to the load of N1% may be 1.75:1 to 1.95:1. In addition, the ratio of N3% load to N2% load may be 1.2:1 to 1.5:1. Specifically, the ratio of the load of N3% to the load of N2% may be 1.20:1 to 1.35:1.

In addition, since polyester films have excellent UV blocking ability, when two sheets of polyester films are overlapped and UV-B light is irradiated on the surface of one film for 24 hours, the The physical properties of another thin film do not deteriorate.

As an example, when two polyester films are overlapped and UV-B light is irradiated on one film for 24 hours, the other film can be loaded with N2% in the first direction in the plane for 1 hour Have a final elongation of 3% or less.

As another example, when two polyester films are superimposed and UV-B light is irradiated on one film for 24 hours, the other film may have 80% or higher, such as 85% or higher, 100 % or higher, 120% or higher, 125% or higher, or 135% or higher, specifically, an elongation at break of 80% to 200%, 100% to 180%, or 135% to 160%. More specifically, when two polyester films are superimposed and UV-B light is irradiated on one film for 24 hours, the other film may have a breakage of 125% or more in the machine direction (MD) Elongation and have an elongation at break of 85% or higher in the transverse direction (TD). Here, the elongation at break refers to the elongation at break of the film at room temperature. Stretch can be measured at room temperature using an initial dimension of the film of 50 mm in the stretching direction and a dimension of 15 mm in a direction perpendicular to the stretching direction.

In addition, after 24 hours of irradiation with UV-B light, the polyester film can withstand 100 or more, 1,000 or more, 10,000 or more, 50,000 times with a radius of curvature of 1.5 mm or more, 100,000 or more, 150,000 or more, or 200,000 or more repeated folds until whitening or cracking occurs. Specifically, the polyester film can withstand 200,000 or more repeated foldings until whitening or cracking occurs. Within the above range, it can be favorably applied to a flexible display device because it does not deform even when repeatedly folded when exposed to UV light. A protective film can be applied to the flexible display device, in particular the outer cover of the foldable display device due to these features.

From the viewpoint of flexibility and elastic recovery, the polyester film is preferably a biaxially stretched film. In such cases, the ratio between the respective extension ratios in the two directions may be 1:0.5 to 1:1.5, 1:0.7 to 1:1.3 or 1:0.8 to 1:1.2. Specifically, the polyester film may be biaxially stretched, and the ratio between the respective stretching ratios in the two directions may be 1:0.8 to 1:1.2. If the ratio of the elongation ratio is within the above range, it may be advantageous for flexibility, and in this case, deformation will not occur even if a certain load is maintained for a long period of time.

The polyester film can be biaxially stretched in a first direction in a plane and in a second direction perpendicular to the first direction. In this case, the extension ratio in the first direction may be 2.0 to 5.0, specifically 2.8 to 3.5 or 3.3 to 3.5. In addition, the extension ratio in the second direction may be 2.0 to 5.0, specifically 2.9 to 3.7 or 3.5 to 3.8. Specifically, the polyester film may be biaxially stretched at a stretch ratio of 3.3 to 3.5 in the longitudinal direction and at a stretch ratio of 3.5 to 3.8 in the transverse direction. In addition, the ratio (d2/d1) of the extension ratio (d2) in the second direction to the extension ratio (d1) in the first direction may be 1.2 or lower. For example, it may be 1.0 to 1.2, 1.0 to 1.1, 1.0 to 1.15 or 1.05 to 1.1.

Polyester films are available in thicknesses from 10 µm to 500 µm, 10 µm to 300 µm, 10 µm to 100 µm, 10 µm to 80 µm, 20 µm to 80 µm, 30 µm to 80 µm, 40 µm to 60 µm or 10 µm to 30 µm. For example, when a polyester film is applied on the outer cover of a display device as a protective film, it may have a thickness of 10 μm to 80 μm. As an example, when a polyester film is applied as a front protective film of an outer cover, it may have a thickness of 20 μm to 80 μm. As another example, when a polyester film is applied as a rear protective film of a transparent substrate, it may have a thickness of 30 μm or less, specifically, 10 μm to 30 μm. Composition of polyester film

The polyester film contains polyester resin.

The polyester resin may be a homopolymer resin or a copolymer resin obtained by polycondensation of dicarboxylic acid and diol. In addition, the polyester resin may be a blended resin in which a homopolymer resin or a copolymer resin is mixed.

Examples of dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 2,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, Dicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4 - Cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2 , 2-Dimethylglutaric acid, adipic acid, 2-methyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecanedicarboxylic acid and their analogs.

In addition, examples of diols include ethylene glycol, propylene glycol, hexanediol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decanediol, 1,3- Propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)pyridine and its analogues.

Preferably, the polyester resin may be an aromatic polyester resin having excellent crystallinity. For example, it may have polyethylene terephthalate (PET) resin as a main component.

As an example, the polyester film may comprise a polyester resin, specifically a PET resin, in an amount of at least about 85%, more specifically, at least 90%, at least 95%, or at least 99% by weight. As another example, the polyester film may further contain a polyester resin other than PET resin. Specifically, the polyester film may further include up to about 15% by weight of polyethylene naphthalate (PEN) resin. More specifically, the polyester film may further include a PEN resin in an amount of about 0.1 wt % to 10 wt % or about 0.1 wt % to 5 wt %.

The polyester film having the above composition can have increased crystallinity and enhanced mechanical properties such as tensile strength and the like in the process of preparing it through heating, stretching and the like.

The polyester film according to one embodiment includes a UV blocking agent.

The content of the UV blocking agent in the polyester film may be 0.5 parts by weight or higher, 0.7 parts by weight or higher, 0.9 parts by weight or higher, 1.1 parts by weight or higher, or 1.3 parts by weight relative to 100 parts by weight of the polyester resin. parts by weight or higher. In addition, the content of the UV blocking agent in the polyester film may be 2.4 parts by weight or less, 2.2 parts by weight or less, 2.0 parts by weight or less, 1.8 parts by weight or less, based on 100 parts by weight of the polyester resin. or 1.6 parts by weight or less.

As an example, the polyester film includes 100 parts by weight of polyester resin; and 0.7 to 2.0 parts by weight of UV blocking agent. Within the above range, the problem of poor appearance due to migration of UV additives to the surface during or after preparation of a film having UV region blocking ability is advantageously prevented.

Meanwhile, if the UV blocking agent does not have sufficient heat resistance, it may be largely decomposed when it is mixed with polyester resin and extruded at high temperature, which may adversely affect the UV blocking ability of the film. Therefore, preferably, the UV blocking agent has a weight reduction rate at a certain level or lower at the melting temperature of the polyester resin. For example, the UV blocking agent may have 20% or less, 15% or less, Or a weight reduction rate of 10% or less. Specifically, the UV blocking agent may have a weight loss rate of 15% or less when maintained at 280° C. in an air atmosphere for 1 hour under isothermal conditions of a thermogravimetric analyzer (TGA). More specifically, the weight reduction rate may be 1% to 15%, 5% to 15%, or 10% to 15%.

The specific kind of UV blocking agent is not particularly limited. For example, it may be at least one UV blocking agent selected from the group consisting of benzophenones, benzotriazoles, trismethacones, malonates, benzoates and benzophenones.

As an example, the UV blocking agent may be a benzophenone-based UV blocking agent. Specifically, it may have one, two or more benzoxone groups. More specifically, it may have two benzoxone groups attached around an unsaturated hydrocarbon chain or an aromatic ring.

As an example, the UV blocking agent may be at least one compound having the structure of Formula 1 below: [Formula 1]

In the above formula, R ₁ and R ₂ are each independently hydrogen, alkyl or alkenyl, and L is a single bond or a hydrocarbon chain, hydrocarbon ring or heterocycle having one or more unsaturated bonds.

The alkyl group may be an alkyl group having 1 to 6 carbon atoms, and the alkenyl group may be an alkenyl group having 2 to 6 carbon atoms. The hydrocarbon chain can have 2 to 10 carbon atoms, the hydrocarbon ring can have 6 to 20 carbon atoms, and the heterocycle can be a 5 to 20 membered ring containing one or more heteroatoms selected from N, S, and O. In addition, a hydrocarbon ring or a heterocycle may also be an aromatic ring. For example, the hydrocarbon ring can be a benzene ring, and the heterocycle can be a pyridine ring.

L may be a group that forms a conjugated structure with the two benzoxalone groups connected to it. Method for preparing polyester film

Such a polyester film can be prepared by a method including biaxial stretching at a specific temperature at an adjusted stretch ratio and heat treatment using a resin composition mixed with a UV blocking agent. Specifically, the method for producing a polyester film comprises forming a film from a resin composition, and subjecting it to biaxial stretching and heat treatment to obtain a final elongation of 3% or more when a load of N2% is applied for 1 hour Low film. Here, the composition and process conditions are adjusted so that the polyester film finally manufactured by the above method satisfies the tensile characteristics described above. Specifically, in order to make the final polyester film meet the above characteristics, the extrusion and pouring temperature of the polyester resin are adjusted, the preheating temperature during stretching, the stretching ratio in each direction, the stretching temperature, and the conveying speed And similar conditions are adjusted, or heat treatment and relaxation are performed after stretching, and the heat treatment temperature and relaxation rate are adjusted at the same time.

Each step is described in more detail below.

First, a UV blocking agent is added to a polyester resin to prepare a resin composition. In this case, the composition of the polyester resin and the type and content of the UV blocking agent are as exemplified above. Thereafter, the resin composition is melted and extruded to be cast into a film. Extrusion may be performed at a temperature of 230°C to 300°C or 250°C to 280°C.

The film may be preheated at a certain temperature prior to stretching. Based on the glass transition temperature (Tg) of the polyester resin, the preheating temperature satisfies the range of Tg+5°C to Tg+50°C, and at the same time it is confirmed that it satisfies the range of 70°C to 90°C. Within the above range, the film may be soft enough to be easily stretched, and also effectively prevented from breaking during its stretching.

The extension is performed by biaxial extension. For example, it can be biaxially stretched in the machine direction (or machine direction; MD) and in the transverse direction (or tenter direction; TD) by simultaneous biaxial stretching or sequential biaxial stretching. Preferably, it is performed by sequential biaxial extension, wherein extension is first performed in one direction, and then extension is performed in a direction perpendicular to this direction. The extension speed may be 6.5 m/min to 8.5 m/min, but it is not particularly limited.

The extension ratio in the longitudinal direction may be 2.0 to 5.0, specifically 2.8 to 3.5 or 3.3 to 3.5. In addition, the elongation ratio in the lateral direction may be 2.0 to 5.0, specifically 2.9 to 3.7 or 3.5 to 3.8. Within the above preferred range, a uniform thickness can be more favorably obtained. In addition, in order to balance the longitudinal direction (MD) and the transverse direction (TD), it is preferable to adjust the load applied in each direction during stretching while measuring the refractive index, thereby minimizing the difference in the refractive index in each direction. difference. In addition, the ratio (d2/d1) of the stretch ratio (d2) in the transverse direction to the stretch ratio (d1) in the longitudinal direction may be 1.2 or less. For example, it may be 1.0 to 1.2, 1.0 to 1.1, 1.0 to 1.15 or 1.05 to 1.1. The extension ratio (d1, d2) refers to the ratio of the length after extension to the length before extension, and the length before extension is 1.0.

Afterwards, heat treatment is performed on the stretched film. The heat treatment may be performed at a temperature of 180°C or higher, specifically 195°C or higher, more specifically 195°C to 230°C. The heat treatment may be performed for 0.2 minutes to 1 minute, more specifically, for 0.4 minutes to 0.7 minutes.

Furthermore, after the initial heat treatment, the film can be relaxed in the machine direction and/or in the transverse direction, and the temperature of relaxation can be in the range of 150°C to 250°C. Relaxation may be performed at a relaxation rate of 1% to 10%, 2% to 7%, or 3% to 5%. Furthermore, the relaxation may be performed for 1 second to 1 minute, 2 seconds to 30 seconds, or 3 seconds to 10 seconds.

In addition, the film can be cooled after heat treatment. Cooling may be performed at a temperature lower than the heat treatment temperature by 50°C to 150°C. laminate

As shown in Fig. 2a and Fig. 2b, the outer cover (10) of the display device (1) is placed on the display panel (20). In the display device outer cover (10), the protective films (100a, 100b) are respectively laminated on two sides of the transparent substrate (200) through the adhesive layer (300). As the display device outer cover (10), a sheet in which a polyester film is laminated on both sides of a transparent substrate can be used.

A laminated sheet according to one embodiment includes a transparent substrate; a first polyester film disposed on one side of the transparent substrate; and a second polyester film disposed on the other side of the transparent substrate. In the laminate, the first polyester film contains a UV blocking agent and the second polyester film may or may not contain a UV blocking agent.

In addition, the laminated sheet may further include an adhesive layer between the transparent substrate and the first polyester film and between the transparent substrate and the second polyester film.

Since the laminated sheet comprises the UV blocking polyester film described above, it may have the same or similar UV blocking ability. That is, the laminated sheet may exhibit a specific value or less of transmittance at wavelengths of 370 nm and 380 nm and may exhibit a specific value or higher of transmittance at a wavelength of visible light such as 390 nm.

Therefore, using the laminated sheet according to one embodiment as an outer cover of a display device will help prevent deformation by protecting its internal components from UV light.

For example, the laminate can have a total transmission of 10% or less, 7% or less, 5% or less, 4.5% or less, or 4% or less for light at a wavelength of 370 nm . In addition, the laminate can have a total transmission of 30% or less, 25% or less, 20% or less, 15% or less, or 10% or less for light having a wavelength of 380 nm. As a specific example, the laminate may have a total transmittance of 5% or less for light having a wavelength of 370 nm and a total transmittance of 20% or less for light having a wavelength of 380 nm.

In addition, the laminate can have a total transmission of 45% or higher, 50% or higher, 55% or higher, 60% or higher, or 65% or higher for light having a wavelength of 390 nm. For example, the laminate can have a total transmission of 50% to 90%, 55% to 85%, or 60% to 80% for light having a wavelength of 390 nm. In addition, the laminate can have a total transmission of 75% or higher, 80% or higher, 85% or higher, 90% or higher, or 95% or higher for light having a wavelength of 550 nm. As a specific example, the laminate may have a total transmittance of 55% or higher for light having a wavelength of 390 nm and 85% or higher for light having a wavelength of 550 nm.

The laminated sheet may have an average change rate of total transmittance (%) of 0.1%/nm to 1%/nm at a wavelength of 370 nm to 375 nm and 3%/nm to 10 at a wavelength of 385 nm to 390 nm The average change rate of total transmittance (%) in %/nm.

In addition, after 24 hours of irradiation with UV-B light, the laminated sheet can withstand 100 or more, 1,000 or more, 10,000 or more, 50,000 times with a radius of curvature of 1.5 mm The folding is repeated one or more times, 100,000 or more times, 150,000 or more times, or 200,000 or more times until delamination occurs. Specifically, the laminated sheet can withstand 200,000 or more repeated foldings until delamination occurs. Within the above range, it can be favorably applied to a flexible display device because it is not deformed even after repeated folding while being exposed to UV light.

In addition, the total content of the UV blocking agent in the laminated sheet may be 0.5 parts by weight or more, 0.7 parts by weight or more, 0.9 parts by weight or more, relative to 100 parts by weight of the polyester resin used in the laminated sheet, 1.1 parts by weight or more, or 1.3 parts by weight or more. In addition, the total content of the UV blocking agent in the laminated sheet may be 2.4 parts by weight or less, 2.2 parts by weight or less, 2.0 parts by weight or less, relative to 100 parts by weight of the polyester resin used in the laminated sheet, 1.8 parts by weight or less, or 1.6 parts by weight or less. Specifically, the total content of the UV blocking agent contained in the first polyester film and the second polyester film can be 0.7 wt. parts to 2.0 parts by weight.

When applying the laminate to the display device, the first polyester film can be positioned on the front side (outside) and the second polyester film can be positioned on the back side (inside). That is, as shown in FIGS. 2a and 2b, when the laminated sheet (10) is applied to the display device (1), the second polyester film (100b) may be applied to face the display panel (20). As described above, the first polyester film can be used as the front protective film, and the second polyester film can be used as the back protective film.

Since the front protective film must protect the display device from external stimuli, a greater thickness is advantageous. However, since the front protective film in the flexible display device is the part where the most tensile or compressive force is applied, it is advantageous to have a low modulus of flexibility. Meanwhile, since the back protection film is not directly stimulated by the outside, it can be thinner. Even if the modulus of flexibility exceeds a certain level, it will not be significantly weakened due to the thinner thickness.

Therefore, the laminated sheet can satisfy the following relations (1) and (2): 1.5 ≤ T1/T2 ... (1) 0.8 ≤ M2/M1 ≤ 1.2 ... (2)

In the above relationship, T1 is the thickness of the first polyester film, T2 is the thickness of the second polyester film, M1 is the modulus (GPa) of the first polyester film in the transverse direction (TD), and M2 is the modulus (GPa) of the second polyester film in the transverse direction (TD).

T1/T2 may be 1.5 or greater, 2.0 or greater, 2.5 or greater, or 3.0 or greater, eg, 1.5 to 5.0 or 2.0 to 4.0. As an example, T1 may be 45 μm to 80 μm, and T2 may be 10 μm to 30 μm. M2/M1 may be 0.8 or greater, 1.0 or greater, greater than 1.0, 1.05 or greater, or 1.1 or greater, and it may be 1.2 or less, or 1.15 or less, such as 1.05 to 1.2. The first polyester film and the second polyester film

The first polyester film and the second polyester film contain polyester resin. Specific types of polyester resins and examples of monomers (dicarboxylic acids and diols) are the same as described above for polyester films.

The first polyester film includes a UV blocking agent and the second polyester film may or may not include a UV blocking agent. The specific type, chemical structure and properties such as heat resistance of the UV blocking agent are the same as described above for the polyester film.

As an example, both the first polyester film and the second polyester film may include a UV blocking agent. In this case, the content of the UV blocking agent in the first polyester film or the second polyester film may be 0.5 parts by weight or more, 0.7 parts by weight relative to 100 parts by weight of the polyester resin used in the respective polyester films. parts by weight or higher, 0.9 parts by weight or higher, 1.1 parts by weight or higher, or 1.3 parts by weight or higher. In addition, the content of the UV blocking agent in the first polyester film or the second polyester film may be 2.4 parts by weight or less, 2.2 parts by weight or more relative to 100 parts by weight of the polyester resin used in the respective polyester films. Low, 2.0 parts by weight or less, 1.8 parts by weight or less, or 1.6 parts by weight or less.

As another example, the second polyester film may contain no or trace amounts of UV blocking agents. In this case, the content of the UV blocking agent contained in the second polyester film may be less than 0.5 parts by weight or less than 0.1 parts by weight relative to 100 parts by weight of the polyester resin used in the second polyester film.

According to one embodiment, the content of the UV blocking agent in the first polyester film may be 0.5 parts by weight to 2.0 parts by weight relative to 100 parts by weight of the polyester resin used in the first polyester film. The content of the UV blocking agent contained in the second polyester film may be less than 0.1 parts by weight relative to 100 parts by weight of polyester resin used in the second polyester film.

When a polyester film comprising a UV blocker is applied to the outer cover of a display device, it advantageously prevents deformation by protecting its internal components from UV light.

The UV light transmittance of the first polyester film and the second polyester film may be the same as the UV light transmittance of the polyester film exemplified above.

According to one embodiment, the first polyester film has a total transmittance of 2% to 5.5% for light having a wavelength of 370 nm, a total transmittance of 9.5% to 22% for light having a wavelength of 380 nm, and a total transmittance of 9.5% to 22% for light having a wavelength of 390 nm. It has a total transmission of 65% to 85% for light at a wavelength of 550 nm and 85% to 95% for light at a wavelength of 550 nm. Within the above range, when the laminated sheet including the first polyester film is applied to the outer cover of the display device, it advantageously prevents deformation by protecting its internal components from UV light.

In addition, the first polyester film may have an average change rate of total transmittance (%) of 0.1%/nm to 1%/nm at a wavelength of 370 nm to 375 nm and 3% at a wavelength of 385 nm to 390 nm. The average change rate of the total transmittance (%) from /nm to 10%/nm.

Furthermore, in the first polyester film, the strain against tensile load was adjusted within a specific range even after exposure to UV light. Therefore, the laminated sheet including the first polyester film can maintain its original characteristics when applied to the outer cover of the flexible display device and undergoes multiple repeated foldings after being exposed to UV light from the outside. The UV light can be, for example, UV-B light. A specific wavelength range of UV-B light may be from 280 nm to 315 nm. Specifically, UV-B light has a peak wavelength within 310 nm to 315 nm, an irradiance of about 0.66 W/m ² (for example, 0.6 to 0.7 W/m ² ) at a wavelength of 310 nm, and an irradiance of about 0.6 to 0.7 W/m 2 at a wavelength of 250 nm to Total irradiance of about 31.62 W/m ² (eg, 31.6 to 31.7 W/m ² ) in the wavelength band of 400 nm.

According to one embodiment, after 24 hours of irradiation with UV-B light, when a load of N2% is applied in the first direction in the plane for 1 hour, the first polyester film may have a final stretch of 3% or less Rate. Here, the N2% load is the load that makes the film stretch 2% relative to the initial state in the first direction. Specifically, the stretching rate can be at room temperature, using the initial dimension of the film in the first direction of 50 mm (the initial distance between the points where the tensile load is applied) and the The dimension measurement is 15 mm in the vertical direction.

More specifically, (1) when a first polyester film having an initial dimension of 50 mm in a first direction in a plane and a dimension of 15 mm in a direction perpendicular to the first direction is When extending upwards, first measure the load (N2%) when the size is increased by 2% relative to the initial size, and (2) when the film is continuously applied in the first direction for 1 hour The load (N2%) , the ratio of the size increase relative to the initial size, that is, the final elongation is 3% or less.

As an example, after 24 hours of irradiation with UV-B light, when a load of N2% is applied for 1 hour, the first polyester film may have 3% or less, 2.7% or less, 2.5% or less, Or 2.3% or lower final elongation. For example, after 24 hours of irradiation with UV-B light, when a load of N2% is applied for 1 hour, the film can have 1.5% to 3%, 2% in the first direction and the second direction perpendicular to each other in the plane, respectively. % to 3%, 2.3% to 3%, 2% to 2.7%, or 2% to 2.5% final stretch. Specifically, after irradiating with UV-B light for 24 hours, when a load of N2% is applied for 1 hour, the film can have a final density of 2% to 3% in the first direction and the second direction perpendicular to each other in the plane, respectively. Stretch rate.

As another example, after 24 hours of irradiation with UV-B light, the first polyester film may have a final stretch of 2% or less when a load of N1% is applied in the first direction in the plane for 1 hour Rate. Here, the load of N1% is the load that makes the film stretch 1% relative to the initial state in the first direction. For example, after 24 hours of irradiation with UV-B light, the film can have a final stretch of 1.7% or less, 1.5% or less, or 1.3% or less when a load of N1% is applied for 1 hour Rate. Specifically, after 24 hours of irradiation with UV-B light, when a load of N1% is applied for 1 hour, the film can have 1.1% to 2%, 1.3% in the first direction and the second direction perpendicular to each other in the plane, respectively. % to 2%, 1.5% to 2%, 1.1% to 1.7%, or 1.1% to 1.5% final stretch. Stretch can be measured at room temperature using an initial dimension of the film of 50 mm in the first direction and a dimension of 15 mm in a direction perpendicular to the first direction.

As yet another example, after 24 hours of irradiation with UV-B light, the first polyester film may have a final tensile strength of 7% or less when a load of N3% is applied in the first direction in the plane for 1 hour. Elongation. Here, the load of N3% is the load that makes the film stretch 3% relative to the initial state in the first direction. For example, after 24 hours of irradiation with UV-B light, when a load of N3% is applied for 1 hour, the film can have 6.7% or less, 6.5% or less, 6.3% or less, 6% or more Low, or 5.7% or lower final elongation. Specifically, after 24 hours of irradiation with UV-B light, when a load of N3% is applied for 1 hour, the film can have 3% to 7% and 5% in the first direction and the second direction perpendicular to each other in the plane, respectively. % to 7%, 5% to 6%, 5.5% to 6.5%, or 6% to 7% final stretch. Stretch can be measured at room temperature using an initial dimension of the film of 50 mm in the first direction and a dimension of 15 mm in a direction perpendicular to the first direction.

As a specific example, after 24 hours of irradiation with UV-B light, the first polyester film may have a final stretch of 2% or less when a load of N1% is applied in the first direction in the plane for 1 hour and Have an ultimate elongation of 7% or less when a load of N3% is applied for 1 hour in the first direction in the plane. Here, the load of N1% or N3% means that the film is stretched by 1% or 3% relative to the initial state in the first direction.

Additionally, the second polyester film may have the same stretch characteristics as the first polyester film.

The first direction can be any direction in the plane of the film, and the second direction can be defined in a direction perpendicular to the first direction in the plane. For example, the first direction may be the longitudinal direction (MD) or the transverse direction (TD) of the film, and the second direction may be the transverse direction (TD) or the longitudinal direction (MD) perpendicular thereto. Specifically, the first direction may be the machine direction (MD) of the film and the second direction may be the transverse direction (TD) of the film.

From the viewpoint of flexibility and elastic recovery, each of the first polyester film and the second polyester film is preferably a biaxially stretched film. In such cases, the ratio between the respective extension ratios in the two directions may be 1:0.5 to 1:1.5, 1:0.7 to 1:1.3 or 1:0.8 to 1:1.2. Specifically, the first polyester film and the second polyester film may be biaxially stretched, and the ratio between the respective stretching ratios in the two directions may be 1:0.8 to 1:1.2. If the ratio of the elongation ratio is within the above range, it may be advantageous for flexibility, and in this case, deformation will not occur even if a certain load is maintained for a long period of time.

The first polyester film and the second polyester film may be biaxially stretched in a first direction in a plane and a second direction perpendicular to the first direction. In this case, the extension ratio in the first direction may be 2.0 to 5.0, specifically 2.8 to 3.5 or 3.3 to 3.5. In addition, the extension ratio in the second direction may be 2.0 to 5.0, specifically 2.9 to 3.7 or 3.5 to 3.8. Specifically, the first polyester film and the second polyester film may be biaxially stretched at a stretch ratio of 3.3 to 3.5 in the longitudinal direction and at a stretch ratio of 3.5 to 3.8 in the transverse direction. In addition, the ratio (d2/d1) of the extension ratio (d2) in the second direction to the extension ratio (d1) in the first direction may be 1.2 or lower. For example, it may be 1.0 to 1.2, 1.0 to 1.1, 1.0 to 1.15 or 1.05 to 1.1.

The first polyester film and the second polyester film may each have a thickness of 10 µm to 500 µm, 10 µm to 300 µm, 10 µm to 100 µm, 10 µm to 80 µm, 20 µm to 80 µm, 30 µm to 80 µm, 40 µm to 60 µm or 10 µm to 30 µm thickness. As an example, when the first polyester film and the second polyester film are respectively applied to the front protective film and the rear protective film of the outer cover, the thickness of the first polyester film may be 20 μm to 80 μm, specifically 60 μm to 80 μm, and the thickness of the second polyester film may be 30 μm or less, specifically 10 μm to 30 μm. transparent substrate

The transparent substrate can be the window of the outer cover of the display device.

The transparent substrate can be a polymer film or a glass substrate. Specifically, the transparent substrate may be polyimide film or ultra-thin glass (UTG).

As one example, the transparent substrate may include polyimide resin. Specifically, the transparent outer cover can be a film mainly made of polyimide.

The polyimide-based film comprises a polyimide-based polymer prepared by polymerizing a diamine compound, a dianhydride compound, and optionally a dicarbonyl compound.

Polyimide-based polymers are polymers containing imide repeating units. In addition, polyimide-based polymers may optionally contain amide repeat units. The polyimide-based polymer can be prepared by simultaneously or sequentially reacting reactants including a diamine compound and a dianhydride compound. Specifically, a polyamide-based polymer is prepared by polymerizing a diamine compound and a dianhydride compound. Alternatively, a polyimide-based polymer is prepared by polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound. Here, the polyimide-based polymer includes imide repeating units obtained by polymerization of diamine compounds and dianhydride compounds and amide repeat units obtained by polymerization of diamine compounds and dicarbonyl compounds.

The diamine compound may be, for example, an aromatic diamine compound containing an aromatic structure. Specifically, the diamine compound may include 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB), but it is not limited thereto.

The dianhydride compound may be, for example, an aromatic dianhydride compound containing an aromatic structure or an alicyclic dianhydride compound containing an alicyclic structure. Specifically, the dianhydride compound may include 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA), but it is not limited thereto.

The dicarbonyl compound may be an aromatic dicarbonyl compound containing an aromatic structure. The dicarbonyl compound may include terephthaloyl chloride (TPC), 1,1'-biphenyl-4,4'-dicarbonyldichloride (BPDC), isophthaloyl chloride (IPC), or combinations thereof. But it is not limited to this.

The diamine compound and the dianhydride compound can be polymerized to form polyamic acid. Subsequently, the polyamic acid can be converted into polyimide through a dehydration reaction, and the polyimide includes imide repeating units.

For example, polyimide may include a repeating unit represented by the following formula A-1, but it is not limited thereto. [Formula A-1]

In Formula A-1, n is an integer of 1 to 400, for example.

In addition, diamine compounds and dicarbonyl compounds may be polymerized to form amide repeating units represented by the following formulas B-1 to B-3. [Formula B-1]

In Formula B-1, x is an integer of 1 to 400, for example. [Formula B-2]

In Formula B-2, y is an integer of, for example, 1 to 400. [Formula B-3]

In Formula B-3, y is an integer of 1 to 400, for example.

The thickness of the transparent substrate can be 20 µm to 500 µm, 30 µm to 300 µm or 40 µm to 100 µm.

The transparent substrate may have a surface hardness of HB or higher and a light transmittance of 80% or greater at a wavelength of 550 nm. In addition, the transparent substrate may have a yellowness index of 5 or less and a haze of 2% or less based on a thickness of 50 μm.

The transparent substrate can have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. Within the above range, it can be favorably applied to a flexible display device because no whitening occurs although deformation occurs after frequent folding. Strain refers to the ratio of the change in dimension of the film to the initial dimension. Here, the changed size may be an increased size or a decreased size. Specifically, the protective film and the transparent substrate may each have a strain of 10% or more until whitening occurs. Adhesive layer and coating

The laminated sheet may further include an adhesive layer between the transparent substrate and the first polyester film and between the transparent substrate and the second polyester film.

The adhesive layer may have a thickness of 1 μm to 50 μm, 3 μm to 30 μm, 5 μm to 20 μm, 5 μm to 15 μm, 7 μm to 12 μm, or 8 μm to 12 μm. Specifically, the adhesive layer may have a thickness of 5 μm to 15 μm. Within the above preferable thickness range, it can more favorably suppress curling and have excellent interfacial adhesion.

The adhesive layer includes an adhesive resin and may further include a curing agent and/or a photoinitiator. The adhesive resin is not particularly limited. For example, it can be a resin that can be cured by UV light irradiation, so the adhesive layer can be prepared by UV curing. In addition, the adhesive resin may be a resin that does not turn yellow under UV light and has good UV absorber dispersibility. For example, the adhesive resin may be polyester resin, acrylic resin, alkyd resin, amino resin or the like. The adhesive resin may be used alone or in the form of a copolymer or a mixture of two or more types thereof. Among them, acrylic resins are preferable because they have excellent optical characteristics, weather resistance, adhesion to substrates, and the like. In addition, the adhesive resin may be an optical clear adhesive (OCA).

The curing agent is not particularly limited as long as it is a substance capable of curing the adhesive resin. Specifically, it may be at least one selected from the group consisting of isocyanate curing agent, epoxy curing agent and aziridine curing agent, and these curing agents will not turn yellow under UV light. In addition, the amount of the curing agent may be 0.2 to 0.5% by weight, 0.3 to 0.5% by weight, 0.3 to 0.45% by weight or 0.35 to 0.45% by weight based on the weight of each adhesive layer.

為主之化合物、以α-羥基酮為主之化合物、以酮為主之化合物、以乙醛酸苯酯為主之化合物及以丙烯醯基氧化膦為主之化合物。該光引發劑之用量基於各別黏著劑層之重量計可為0.1至5.0重量%。A photoinitiator is required for UV curing. For example, it may be at least one selected from the group consisting of benzophenone-based compounds, 9-oxosulfur

Compounds based on α-hydroxy ketones, compounds based on ketones, compounds based on phenyl glyoxylate and compounds based on acrylphosphine oxide. The photoinitiator can be used in an amount of 0.1 to 5.0% by weight based on the weight of each adhesive layer.

In addition, the laminated sheet may further comprise at least one coating disposed on the first polyester film or the second polyester film. The coating can be placed between the polyester film and the transparent substrate or on the outside of the protective film. The coating can be a functional coating for hardness enhancement, antistatic, anti-scattering, adjustment of refractive index, surface protection, or the like. Method for preparing laminated sheet

A laminated sheet can be prepared by preparing a polyester film and then laminating it on both sides of a transparent substrate using an adhesive.

A method for preparing a laminated sheet according to one embodiment includes preparing a first polyester film and a second polyester film; forming an adhesive layer on one side of the first polyester film and the second polyester film, respectively and laminating the first polyester film and the second polyester film on which the adhesive layer is formed on one side and the other side of the transparent substrate respectively.

The first polyester film and the second polyester film can be produced using the resin composition by a method including biaxial stretching and heat treatment at a specific temperature at an adjusted stretching ratio. Here, a UV blocking agent may be added to the resin composition. The amount thereof can be adjusted as exemplified above. Specifically, the method for producing the first polyester film and the second polyester film includes forming a film from a resin composition, and biaxially stretching and heat-treating it to obtain Films with a final stretch of 3% or less. Here, the composition and process conditions are adjusted so that the film finally produced by the above method satisfies the tensile characteristics described above. Specifically, in order to make the final film meet the above characteristics, the extrusion and casting temperatures of the polyester resin are adjusted, the preheating temperature during stretching, the stretching ratio in each direction, the stretching temperature, the conveying speed and the like are adjusted. Adjust the conditions, or perform heat treatment and relaxation after stretching, and adjust the heat treatment temperature and relaxation rate at the same time.

The specific methods and conditions used in each step of preparing the first polyester film and the second polyester film can be respectively compared with the specific methods and conditions in each step of the method for preparing the polyester film as described above. same.

Adhesives are applied to one side of the first and second polyester films prepared in this way, respectively, which can then be laminated on both sides of the transparent substrate. flexible display device

A flexible display device according to one embodiment includes the polyester film or laminated sheet described above in an outer cover.

A flexible display device according to one embodiment includes a flexible display panel; and a polyester film disposed on the flexible display panel. A flexible display device according to another embodiment includes a flexible display panel; and a laminated sheet disposed on the flexible display panel.

The polyester film or laminate used in the flexible display device has substantially the same configuration and features as the polyester film or laminate described above.

Specifically, the polyester film has a total transmittance of 5% or less for light having a wavelength of 370 nm and after irradiating with UV-B light for 24 hours, when N2% is applied for 1 hour in the first direction in the plane It has an ultimate elongation of 3% or less under load. Here, the load of N2% is the load that makes the film extend 2% relative to the initial state in the first direction, and the UV-B light has a peak wavelength within 310 nm to 315 nm, a wavelength of 310 nm The irradiance of 0.66 W/m ² and the total irradiance of 31.62 W/m ² in the wavelength band from 250 nm to 400 nm.

In addition, the laminated sheet includes a transparent substrate placed on the display panel; a first polyester film placed on the transparent substrate; and a second polyester film placed under the transparent substrate, wherein the first polyester film Including (i) 0.5 parts by weight to 2.0 parts by weight of UV blocking agent, which is relative to 100 parts by weight of polyester resin contained in the first polyester film, and the first polyester film has (ii) for 2% to 5.5% total transmission for light at 370 nm, (iii) 9.5 to 22% total transmission for light at 380 nm, (iv) 65% for light at 390 nm % to 85% total transmittance and (v) 85% to 95% total transmittance for light having a wavelength of 550 nm.

According to the above embodiments, a UV blocking agent is added to at least one of the two polyester films provided in the laminate, thereby preventing undesirable or deformed appearance or occurrence of devices during exposure to UV light in long-term use. defect.

The flexible display device can be a foldable display device. Specifically, the foldable display device can be folded in or folded out depending on the folding direction. Referring to FIG. 1a and FIG. 1b, such foldable display devices are presented as an inward-folding type (1a), wherein the screen is located inside the folding direction; and an outward-folding type (1b), wherein the screen is located outside the folding direction.

As important as flexibility is in materials applied to flexible display devices, original features do not deteriorate despite frequent bending or folding. When a conventional material is fully folded and then unfolded, marks remain and it is almost impossible to return to its original state. Therefore, the development of materials applied to flexible display devices should be accompanied by features that address this limitation.

Specifically, due to the deformation caused by the load applied to the point (p1) of the inward fold (1a) as shown in Fig. 1a and due to the deformation applied to the Deformation, whitening or cracks caused by the load of the outwardly folded point (p2) in the outwardly folded type (1b) may appear in the outer cover (10), thereby deteriorating its characteristics. Such whitening and cracking can generally be resolved when the modulus of the protective film is small at room temperature. In general, the conventional polyester film or the outer cover with the film has a relatively large modulus at room temperature, thereby making it prone to whitening and cracking problems when applied to a flexible display device.

However, the polyester film according to one embodiment can achieve the desired characteristics of a flexible display cover by adjusting the strain to a tensile load after UV light irradiation to a specific range. Therefore, the polyester film according to one embodiment and the laminate sheet comprising the same can maintain its original characteristics when applied to the outer cover of a flexible display device and undergoes multiple repeated foldings after exposure to UV light.

In addition, the laminated sheet according to one embodiment can achieve the requirements of the flexible display cover by adjusting the mechanical properties of the polyester film and other constituent layers used therein to a specific range after exposure to UV light. of flexibility. Therefore, the laminated sheet according to one embodiment can maintain its original characteristics when applied to the outer cover of the flexible display device and undergoes repeated folding.

In the flexible display device, the flexible display panel may specifically be an organic light emitting display (OLED) panel. Fig. 9 schematically shows an example of a cross-sectional view of an organic light emitting display device (1'), the organic light emitting display device including an organic light emitting display panel (20-2). Referring to Fig. 9, an organic light emitting display (1') includes a front polarizer (20-1) and an organic light emitting display panel (20-2). A front polarizer may be disposed on the front side of the organic light emitting display panel. In more detail, the front polarizer can be bonded to one side of the organic light emitting display panel for displaying images. The organic light emitting display panel displays images by self-emission of pixel units. The organic light emitting display panel includes an organic light emitting substrate and a driving substrate. The organic light emitting substrate includes a plurality of organic light emitting units corresponding to respective pixels. The organic light emitting units each include a cathode, an electron transport layer, a light emitting layer, a hole transport layer and an anode. The driving substrate is operatively connected to the organic light emitting substrate. That is, the driving substrate may be coupled to the organic light emitting substrate to apply a driving signal, such as a driving current. More specifically, the driving substrate may drive the organic light emitting substrate by applying current to each organic light emitting unit. Mode of the invention

Hereinafter, more specific examples will be described, but the scope of the embodiments is not limited thereto. Evaluation example of UV blocking agent

Thermogravimetric analysis (TGA) evaluations were performed to select UV blockers with excellent processing efficiency even at the processing temperatures of polyethylene terephthalate (PET) resins. If using the Q500 equipment from TA Instruments, after maintaining in air atmosphere under isothermal conditions of 280°C for 1 hour, the weight loss does not exceed 15%, then the applicability of the UV blocking agent is determined to be qualified, otherwise it is determined to be unqualified . [Table 1] name TGA assessment results UV blocker 1 Benzophenone-3 unqualified UV blocker 2 2-(2H-Benzotriazol-2-yl)-4-methylphenol unqualified UV blocker 3 2,2'-(1,4-phenylene)bis(4H-3,1-benzo㗁𠯤-4-one) qualified Preparation example of polyester film

At 280°C, the composition was extruded and then cast into a film shape through a T-die, in which composition UV blocking agents as shown in the table below were added to polyterephthalic acid as shown in Table 2 Ethylene ester (PET) resin. The cast film was preheated at 100°C and stretched 3.0 times in the machine direction (MD) and 3.3 times in the transverse direction (TD). Here, the stretching temperature is 130°C. After that, the stretched film was heat-set at 200°C, relaxed by 3%, and cooled to prepare a polyester film having a thickness of 60 µm. Also, as a comparative example, a polyester film was prepared in the same manner as above without a UV blocking agent. [Table 2] PET resin composition UV blocker type Content of UV blocking agent (parts by weight)* Film A1 Comparative Example 1 - - Film A2 Comparative example 2 UV blocker 1 1.3 Film B1 Example 1 UV blocker 3 1.1 Film B2 Example 2 UV blocker 3 1.6 Film B3 Example 3 UV blocker 3 0.9 Film B4 Example 4 UV blocker 3 0.8 Film B5 Example 5 UV blocker 3 0.8 *The content of UV blocking agent: the weight part of the solid content of UV blocking agent added per 100 parts by weight of PET resin Test example of total transmittance

The total transmittance of the polyester film samples was measured as follows. - Measuring equipment: Ultrascan pro colorimeter from Hunterlab - Measurement procedure: ASTM D1003 - Light source: D65/10 - Diffusion angle: 8°

The results are shown in the table below. [table 3] Total transmittance (%) 370 nm 380 nm 390 nm 550 nm Film A1 Comparative Example 1 75 81 87.8 93.2 Film A2 Comparative example 2 5.8 18.5 72.1 92.9 Film B1 Example 1 3.81 11.5 70.7 93.2 Film B2 Example 2 3.5 9.7 71.3 92.9 Film B3 Example 3 3.9 16.5 68.7 92.9 Film B4 Example 4 3.98 19.1 70.9 93.1 Film B5 Example 5 3.97 19.6 71.1 93.2 Test example of stretch rate

The tensile ratio of the polyester film was measured as follows. Referring to Fig. 5, four adjacent rectangular samples (101) with a width (w) of 15 mm were cut from the polyester film (100). Fix the two ends of the sample (101) to the fixture (21) of the testing device (2). Afterwards, the elongation rate was measured according to the following conditions and procedures. - Initial dimension in the stretching direction: 50 mm - Dimensions in the direction perpendicular to the stretching direction: 15 mm - Test temperature: room temperature (25°C) - Test equipment: UTM 5566A of Instron - Tensile speed: 50 mm/min - Stretching direction: longitudinal direction (MD) or transverse direction (TD) of polyester film (1) Measure the load according to the initial stretch ratio

First, the respective loads to elongate the specimen by 1%, 2% and 3% relative to the original dimension were measured. (2) Measure the ultimate elongation under continuous load

After that, the load measured above was applied to the sample, kept for 1 hour, and the final elongation (%) relative to the initial dimension of the sample was measured.

(3) Carry out the procedures of (1) and (2) above for 4 samples, and obtain the average value. The results are shown in the table below. Test example of elongation after irradiation with UV light

The elongation rate after irradiation with UV light was measured under the following conditions. (a) UV irradiation - UV equipment: QUV tester from Q-lab - UV lamp: UVB-313 - UV dose: 0.66 W/ ^m2 at 310 nm - Total UV dose: 31.62 W/m2 at 250 to 400 nm m ² - UV exposure time: 24 hours

(b) The elongation of the UV-irradiated sample was measured in the same manner as above.

The results are shown in the table below. Test example of elongation at break of laminated sheet with two layers after UV irradiation

Laminate two identical sheets of mylar together. One side thereof was irradiated with UV light, and the elongation at break of the film on the opposite side was measured. Here, the UV irradiation was performed in the same manner as above, and the elongation at break was measured by measuring the elongation at break when the test sample was elongated under the same equipment and conditions as described above for measuring elongation. Measurement. The results are shown in the table below. [Table 4] Initial elongation (%) Elongation after 1 hour (%) Elongation at break of laminated sheet with two layers after UV irradiation Before UV irradiation After UV irradiation MD TD MD TD MD TD Film A1 Comparative Example 1 1 1.2 1.21 1.67 2.11 7 8 2 2.4 2.38 3.1 3.57 3 5.80 5.91 7.13 7.9 Film A2 Comparative example 2 1 1.33 1.41 1.71 1.99 132 83 2 2.15 2.67 2.81 3.21 3 4.97 5.93 6.37 7.13 Film B1 Example 1 1 1.11 1.15 1.17 1.21 141 97 2 2.37 2.69 2.38 2.66 3 5.11 6.7 5.36 6.91 Film B2 Example 2 1 1.31 1.32 1.31 1.35 138 101 2 2.48 2.91 2.49 2.91 3 5.31 6.72 5.33 6.71 Film B3 Example 3 1 1.16 1.17 1.23 1.25 140 96 2 2.31 2.67 2.47 2.82 3 4.09 5.11 4.66 5.93 Film B4 Example 4 1 1.16 1.23 1.19 1.24 129 89 2 2.3 2.35 2.53 2.78 3 4.6 5.7 5.21 6.37 Film B5 Example 5 1 1.11 1.18 1.21 1.67 139 93 2 2.2 2.45 2.57 2.91 3 5.31 6.21 6.27 6.91 Preparation example of laminated sheet

Films having a thickness of 60 µm were prepared in the same manner as the above polyester films A1 to B5, respectively. Each of them was used as the first polyester film. Furthermore, in the same manner as in Film A1 without adding a UV blocking agent, a film having a thickness of 15 to 20 μm was prepared. It is used as the second polyester film. An optically clear adhesive (OCA) was coated on one side of the first polyester film and one side of the second polyester film to a thickness of 10 μm, respectively. It is laminated on both sides of the cover window. A transparent polyimide film (see preparation example below) with a thickness of 50 μm was used as the cover window. Thus, a laminated sheet having a structure of first polyester film/OCA layer/cover window/OCA layer/second polyester film was obtained. [table 5] first polyester film Second Mylar Preparation UV Blocker Content* Thickness (µm) UV Blocker Content* Thickness (µm) Laminate A1 Comparative Example 1 Film A1 0 60 0 20 Laminate A2 Comparative example 2 Film A2 1.3 60 0 20 Laminate B1 Example 1 Film B1 1.1 60 0 20 Laminate B2 Example 2 Film B2 1.6 60 0 15 Laminate B3 Example 3 Film B3 0.9 60 0 15 Laminate B4 Example 4 Film B4 0.8 60 0 17 Laminate B5 Example 5 Film B5 0.8 60 0 17 * The content of UV blocking agent: parts by weight of the solid content of UV blocking agent added per 100 parts by weight of PET resin Preparation example of polyimide film

At 20° C. under nitrogen atmosphere, 563.3 g of dimethylacetamide (DMAc) as an organic solvent was charged into a 1-liter glass reactor equipped with a temperature-controlled double-layer jacket. Next, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) and 4,4'-oxydiphenylamine (ODA) were slowly added thereto to be dissolved. Subsequently, 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) was slowly added thereto, and the mixture was stirred for 2 hours. Next, isophthaloyl chloride (IPC) was added, followed by stirring for 2 hours. And terephthaloyl chloride (TPC) was added, followed by stirring for 3 hours, thereby preparing a polymer solution.

The polymer solution thus obtained was applied to a glass plate, and then dried with hot air at 80° C. for 30 minutes. Afterwards, the xerogel sheet was fixed on the frame while extending 1.01 times in the first direction and 1.03 times in the second direction perpendicular to the first direction. Afterwards, the xerogel sheet was fixed on the plug frame while it was cured in an atmosphere heated at a rate of 2°C/min at a temperature ranging from 80°C to 300°C. After that, it was cooled to obtain a polyimide film having a thickness of 50 μm. Test example of laminate modulus

Samples were prepared by cutting each of the first polyester film, the cover window, and the second polyester film constituting the laminate prepared above. The modulus is measured, namely Young's modulus. - Initial dimension in the stretching direction: 50 mm - Dimension in the direction perpendicular to the stretching direction: 15 mm - Test temperature: room temperature (25°C) - Test equipment: Instron UTM 5566A - Tensile speed: 50 mm/min - Tensile direction: longitudinal direction (MD) or transverse direction (TD) [Table 6] Modulus (GPa) (25℃) first polyester film Cover window Second Mylar MD TD MD TD MD TD Laminate A1 Comparative Example 1 4.1 4.6 5.6 5.8 3.9 5.1 Laminate A2 Comparative example 2 3.9 4.5 5.8 5.7 4.1 4.6 Laminate B1 Example 1 4.3 4.7 6.3 6.6 3.9 5.2 Laminate B2 Example 2 4.1 4.6 5.9 6.1 4.0 5.1 Laminate B3 Example 3 4.2 4.7 5.7 6.3 4.1 5.2 Laminate B4 Example 4 4.3 4.8 6.9 6.7 3.9 5.2 Laminate B5 Example 5 4.2 4.7 6.7 6.6 4.1 5.3 Folding features after UV irradiation

The above-prepared laminated sheet was attached to one side of the display panel to prepare a display module, which was then subjected to UV irradiation and folding durability tests, as shown in FIG. 4 . (1) UV irradiation - UV equipment: QUV tester from Q-lab - UV lamp: UVB-313 - UV dose: 0.66 W/ ^m2 at 310 nm - Total UV dose: 31.62 W/m2 at 250 to 400 nm m ² - UV irradiation time: 24 hours (2) Folding test - Test equipment: MIT-D0A of Toyoseiki - Radius of curvature (R): 1.5 mm - Folding test procedure: MIT folding test according to ASTM D 2176 and TAPPI T 511

- Standard: If delamination, drive error, whitening, or other defects were observed after repeated folding 200,000 times, it was rated as X. Otherwise, it is rated as ○.

The results are shown in the table below. [Table 7] folding durability Laminate A1 Comparative Example 1 x Laminate A2 Comparative example 2 x Laminate B1 Example 1 ○ Laminate B2 Example 2 ○ Laminate B3 Example 3 ○ Laminate B4 Example 4 ○ Laminate B5 Example 5 ○

As can be seen from the above table, the display module comprising the laminated sheet A1 or A2 had defects during 200,000 or more repeated foldings after UV irradiation. In contrast, in the display module including any of the laminated sheets B1 to B5, after UV irradiation, such factors as delamination, driving error, and whitening were not observed during repeated folding of 200,000 times or more. Such defects; therefore, UV durability and flexibility are excellent.

1: Flexible display device 1': organic light emitting display device 1a and 1b: Inward-folding and outward-folding flexible display devices 2: Test equipment 10: Cover/Laminate 11,101: Sample 20: Display panel 20-1: Front polarizer 20-2: Organic Light Emitting Display Panel 21: Fixture 22: Dynamometer 30: frame 100: polyester film 100a: Front protective film 100b: back protection film/second polyester film 200: cover window/transparent substrate 300: Adhesive layer A, A': cutting line p1, p2: folding points R: Curvature w: distance

FIG. 1a shows an example of an in-folding type display device. Figure 1b shows an example of an out-folding type display device. Fig. 2a is a perspective view of the display device with the lower outer cover removed. Figure 2b shows an example of a cross-sectional view of an outer cover of a display device. FIG. 3 shows the spectra of the transmittance versus the wavelength of light for Examples and Comparative Examples. Fig. 4 shows the method of durability test on repeated folding. Figure 5 shows the method for tensile testing of polyester film. Figure 6 shows a graph of stretch ratio versus load applied to the polyester film. Fig. 7 shows a graph of elongation (%) versus time (seconds) at a certain load applied to the polyester film of the example after UV light irradiation. FIG. 8 shows a graph of elongation (%) versus time (seconds) at a certain load applied to the polyester film of Comparative Example after UV light irradiation. FIG. 9 shows an example of a cross-sectional view of an organic light emitting display device.

Claims (10)

A polyester film which has a total transmittance of 5% or less for light having a wavelength of 370 nm, and which, after being irradiated with UV-B light for 24 hours, when loaded with N2% in a plane When applied in the first direction for 1 hour, have a final stretch rate of 3% or less, wherein the load of N2% is a load that makes the film stretch 2% in the first direction relative to the initial state, The UV-B light has a peak wavelength within 310 nm to 315 nm, an irradiance of 0.66 W/ ^m2 at a wavelength of 310 nm, and 31.62 W/ ^m2 in a wavelength band of 250 nm to 400 nm One of the total irradiance. The polyester film as claimed in claim 1, which comprises 100 parts by weight of a polyester resin; and 0.7 to 2.0 parts by weight of a UV blocking agent. The polyester film as claimed in claim 2, wherein the UV blocking agent has 15% or more when maintained in an air atmosphere at 280° C. for 1 hour under isothermal conditions of a thermogravimetric analyzer (TGA). One of the lowest weight loss rates. Such as the polyester film of claim 1, after being irradiated with UV-B light for 24 hours, it has one of 2% or less when a load of N1% is applied in a first direction in a plane for 1 hour and having an ultimate elongation of 7% or less when a load of N3% is applied in a first direction in a plane for 1 hour, Wherein the load of N1% or N3% means that the film is stretched by 1% or 3% relative to the initial state in the first direction. A laminate comprising a transparent substrate; a first polyester film disposed on one side of the transparent substrate; and a second polyester film disposed on the other side of the transparent substrate, Wherein the first polyester film comprises (i) 0.5 parts by weight to 2.0 parts by weight of a UV blocking agent, which is relative to 100 parts by weight of polyester resin used in the first polyester film, and the first polyester film The ester film has (ii) a total transmittance of 2% to 5.5% for a light with a wavelength of 370 nm, (iii) a total transmittance of 9.5% to 22% for a light with a wavelength of 380 nm, ( iv) a total transmittance of 65% to 85% for a light having a wavelength of 390 nm and (v) a total transmittance of 85% to 95% for a light of a wavelength of 550 nm. The laminated sheet according to claim 5, wherein the UV blocking agent contained in the second polyester film is contained in an amount of less than 0.1 parts by weight relative to 100 parts by weight of the polyester resin used in the second polyester film . Such as the laminated sheet of claim 5, which satisfies the following relations (1) and (2): 1.5 ≤ T1/T2 ... (1) 0.8 ≤ M2/M1 ≤ 1.2 ... (2) In the above relationship, T1 is the thickness of the first polyester film, T2 is the thickness of the second polyester film, M1 is the modulus (GPa) of the first polyester film in the transverse direction (TD), and M2 is the modulus (GPa) of the second polyester film in the transverse direction (TD). The laminated sheet according to claim 5, wherein the first polyester film has a thickness of 20 µm to 80 µm, and the second polyester film has a thickness of 30 µm or less. A flexible display device comprising a flexible display panel; and the polyester film according to claim 1 arranged on the flexible display panel. A flexible display device comprising a flexible display panel; and a laminated sheet disposed on the flexible display panel, Wherein the laminate comprises a transparent substrate disposed on the flexible display panel; a first polyester film disposed on the transparent substrate; and a second polyester film disposed under the transparent substrate, Wherein the first polyester film comprises (i) 0.5 parts by weight to 2.0 parts by weight of a UV blocking agent, which is relative to 100 parts by weight of polyester resin contained in the first polyester film, and the first The polyester film has (ii) a total transmittance of 2% to 5.5% for a light having a wavelength of 370 nm, (iii) a total transmittance of 9.5% to 22% for a light of a wavelength of 380 nm, (iv) a total transmittance of 65% to 85% for a light having a wavelength of 390 nm and (v) a total transmittance of 85% to 95% for a light of a wavelength of 550 nm.

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