CN118185254A - Polyester film, protective film and preparation method thereof - Google Patents

Abstract

The present invention relates to a polyester film having excellent durability, transparency and visibility, which comprises a first polyester resin and a second polyester resin containing a specific component and a specific content of a diol and a dicarboxylic acid, has excellent mechanical properties such as tensile strength and impact strength, and also has excellent fingerprint recognition rate, and can be applied to display devices such as smart phones, tablet computers, notebook computers, and the like, and a method for preparing the same. Further, examples relate to a protective film excellent in scratch resistance, flexibility, durability, transparency and visibility, the protective film including a curable resin layer interposed between a first substrate layer and a second substrate layer thinner than the first substrate layer, the first substrate layer and the second substrate layer forming a curved surface portion when the curable resin layer is cured, the protective film being capable of being fully attached to the curved surface in the case of being applied to a display device having a curved surface shape, and exhibiting excellent characteristics in the case of being applied to the display device having a curved surface shape, and a method for producing the protective film.

Description

Polyester film, protective film and preparation method thereof

This patent application is a divisional application of the invention patent application with application date of December 02, 2021, application number 2021114603682, publication number CN114591604A, and invention name “Polyester film, protective film and preparation method thereof”.

Technical Field

Examples relate to polyester films, protective films and methods for their preparation.

Background Art

Polyester film has excellent dimensional stability and optical transparency, and is widely used as a material for not only display devices but also various industrial materials. However, although polyester film has excellent transparency, its durability and heat resistance are lower than those of polyimide film. Therefore, research has been ongoing to improve the durability and heat resistance of polyester film without reducing its dimensional stability and optical transparency.

In addition, a variety of display devices having three-dimensional curved shapes that can achieve gorgeous beauty or excellent visual effects are being used recently. In particular, in the case of portable mobile devices such as smartphones and tablet computers that are frequently used in daily life, products having three-dimensional curved shapes in which the curvature of the side portions of the display panel is greater than that of the center portion have been introduced.

When such a display with a three-dimensional curved surface is attached with a conventional protective film, the protective film cannot completely adhere to the curved surface due to the restoring force caused by the elasticity of the protective film itself, and the protective film may warp. Therefore, we continue to research protective films that can completely adhere to the curved surface and have excellent scratch resistance, durability, transparency, etc.

On the other hand, as e-commerce, online banking and other services are becoming more common through display devices such as smartphones, laptops and tablets, research on using information that can identify living organisms to strengthen security is ongoing.

Fingerprint recognition methods are widely used as one of the above-mentioned methods of using living body information. Such fingerprint recognition methods include optical, ultrasonic, capacitive, electric field measurement, thermal induction, etc. Among them, the optical fingerprint recognition method uses the following principle: light is irradiated by a light source such as a light emitting diode (LED) inside the device, and the light reflected by the fingerprint is sensed by a sensor. That is, since it is a method of obtaining a fingerprint image reflected by light and comparing it with the previously registered fingerprint information, in order to improve the fingerprint recognition rate of the optical fingerprint recognition method, there should be no distortion of the reflected light after sensing the fingerprint.

However, display devices such as smartphones, laptops, and tablets are attached with protective films to improve durability, but light distortion occurs due to the protective films, which reduces the fingerprint recognition rate. In particular, the thickness of the protective film can vary depending on the application and needs, so the reliability of the fingerprint recognition rate may also decrease as the thickness of the film increases. Therefore, research on how to improve the fingerprint recognition rate while ensuring excellent durability and transparency is also ongoing.

As an example, Korean Patent Publication No. 2020-0125466 discloses a protective film that improves the fingerprint recognition rate by reducing the in-plane phase difference to below 25nm, but in order to reduce the phase difference to this level, a highly controlled stretching process is required, which increases the process cost of the film and reduces productivity.

As another example, Korean Patent No. 2014-0013155 discloses a protective cover having a curved surface formed by multiple printed layers. However, this protective cover requires the preparation of multiple printed layers, so the preparation process is complicated, and the curved surface is formed according to the number of printed layers stacked, so it is difficult to correctly deal with a variety of products with different curved surface shapes.

Prior art literature

Patent Literature

Patent Document 1: Korean Patent Publication No. 2020-0125466

Patent document 2: Korean Patent Publication No. 2014-0013155.

Summary of the invention

Therefore, the examples provide a polyester film excellent in durability, transparency, and visibility, and a method for producing the same.

Also, the present invention provides a protective film that is in full contact with a curved surface portion and is excellent in scratch resistance, softness, durability, transparency, and visibility, and a method for preparing the same.

A polyester film of one example comprises: a first polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component; and a second polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component, at least 70 mol% of ethylene glycol as a diol component, and at least 10 mol% of a C ₃ to C ₁₅ alcohol. The in-plane retardation of the polyester film is 3000 nm or more.

A method for preparing a polyester film in another embodiment includes: preparing a mixture of a first polyester resin and a second polyester resin; preparing an unstretched sheet by melt-extruding the mixture; preparing a stretched film by stretching the unstretched sheet by 1 to 1.5 times in a first direction and by 3 to 5 times in a second direction perpendicular to the first direction at a temperature of 70° C. to 125° C.; and preparing a polyester film by heat-fixing the stretched film at a temperature of 160° C. to 230° C., wherein the first polyester resin contains more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component, and the second polyester resin contains more than 95 mol% of terephthalic acid as a dicarboxylic acid component, at least 70 mol% of ethylene glycol as a diol component, and at least 10 mol% of a C ₃ to C ₁₅ alcohol, and the in-plane retardation of the polyester film is greater than 3000 nm.

A protective film of another example comprises: a polyester film; and a first curable resin layer located on one side of the polyester film, wherein the polyester film comprises: a first polyester resin comprising more than 95 mol percent of terephthalic acid as a dicarboxylic acid component and more than 95 mol percent of ethylene glycol as a diol component; and a second polyester resin comprising more than 95 mol percent of terephthalic acid as a dicarboxylic acid component, at least 70 mol percent of ethylene glycol as a diol component, and at least 10 mol percent of a C ₃ to C ₁₅ alcohol, wherein the in-plane phase difference of the polyester film is greater than 3000 nm.

A protective film in another example includes: a first substrate layer; a second substrate layer located on the first substrate layer; and a second curable resin layer located between the first substrate layer and the second substrate layer, wherein the thickness of the first substrate layer is thinner than that of the second substrate layer, and when the curable resin layer is cured, the first substrate layer and the second substrate layer form a curved portion.

Another example of a method for preparing a protective film includes: the steps of melt-extruding a third polyester resin and a fourth polyester resin to prepare a first sheet and a second sheet, respectively; stretching the first sheet and the second sheet by 1 to 1.5 times in a first direction and 3 to 5 times in a second direction perpendicular to the first direction at a temperature of 70°C to 125°C to prepare a first substrate layer and a second substrate layer, respectively; applying a curable resin composition on one side of the first substrate layer to form a second curable resin layer; and laminating the second substrate layer on one side of the second curable resin layer, wherein the thickness of the first substrate layer is thinner than that of the second substrate layer, and when the second curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface portion.

The polyester film of the example includes the first polyester resin and the second polyester resin containing diol and dicarboxylic acid in specific components and contents, and therefore has excellent mechanical properties such as tensile strength and impact strength.

Furthermore, the orientation angle, haze, transmittance, orientation angle, in-plane phase difference, and luminous flux of the polyester film of the example are all within the preferred range, which can ensure not only excellent transparency and durability but also excellent visibility.

Furthermore, the protective film of the example includes a second curable resin layer between a first substrate layer having a thickness thicker than that of the first substrate layer and a second substrate layer having a thickness thinner than that of the first substrate layer. Therefore, when the second curable resin layer is cured, the first substrate layer and the second substrate layer form a curved portion, thereby being completely attached to the front of the curved display device, and almost no warping phenomenon and whitening or cracking caused by the phenomenon occur.

Furthermore, the orientation angle, luminous flux, light transmittance and moisture permeability of the protective film of the example are all within the preferred range, and excellent visibility, transparency and durability can be ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a protective film.

FIG. 2 shows still another example of a protective film.

FIG. 3 shows a method for measuring the luminous flux of a polyester film.

FIG. 4 shows a display device of an example.

FIG. 5 is a cross-sectional view showing the display device of FIG. 3 cut along line XX′.

FIG. 6 is a cross-sectional view showing an in-folding type flexible display device.

FIG. 7 is a cross-sectional view illustrating an out-folding type flexible display device.

FIG. 8 shows another example of a protective film.

FIG. 9 shows still another example of a protective film.

Description of Reference Numerals

1: Display device

2: Inward-folding flexible display device

3: Outward-folding flexible display device

10: Optical power meter

21: First polarizer

22: Second polarizing plate

30: Lux meter

a: curved surface

b: Flat surface

c: direction of light

d: Optical axis

R: Radius of curvature

W: bending angle

Q: Winding axis

100: Polyester film

200: Protective film

210: First substrate layer

220: Second base material layer

231: First curable resin layer

232: Second curable resin layer

240: Hard coating

250: Adhesive layer

260: Release layer

300: Cover window

400: Display panel

DETAILED DESCRIPTION

The present invention will be described in detail below by way of examples. The examples are not limited to the contents disclosed below, and can be modified into various forms without changing the gist of the present invention.

In the present specification, when a certain component is expressed as “including”, this does not mean that other components are excluded, and other components may be included unless otherwise stated.

All numbers and expressions indicating the amounts of structural components, reaction conditions, etc. described in the present specification should be understood as being modified using the term "about" unless otherwise specified.

The terms "first", "second", "first time", "second time" and the like in this specification are used to describe various structural elements, and the above structural elements are not limited by the above terms. The above terms are used to distinguish a certain structural element from other structural elements.

In this specification, when each thin film or layer is described as being formed "on" or "under" each film or layer, "on" or "under" includes all cases where they are formed "directly" or "indirectly with other structural elements interposed therebetween."

Polyester film

A polyester film of one example comprises: a first polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component; and a second polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component, at least 70 mol% of ethylene glycol as a diol component, and at least 10 mol% of a C ₃ to C ₁₅ alcohol. The in-plane retardation of the polyester film is 3000 nm or more.

The first polyester resin contains more than 95 mol % of terephthalic acid as a dicarboxylic acid component and more than 95 mol % of ethylene glycol as a diol component.

Specifically, based on the total molar number of dicarboxylic acid components, the first polyester resin may contain greater than 95 mol percent of terephthalic acid, more than 95 mol percent of terephthalic acid, more than 97 mol percent of terephthalic acid, more than 98 mol percent of terephthalic acid, more than 99 mol percent of terephthalic acid, or 100 mol percent of terephthalic acid.

Furthermore, based on the total molar number of the glycol component, the first polyester resin may contain greater than 95 mole percent of ethylene glycol, more than 95 mole percent of ethylene glycol, more than 97 mole percent of ethylene glycol, more than 98 mole percent of ethylene glycol, more than 99 mole percent of ethylene glycol, or 100 mole percent of ethylene glycol.

The first polyester resin may be a mixture of terephthalic acid and ethylene glycol in a molar ratio of 1:1.

When the contents of the diol component and the dicarboxylic acid component of the first polyester resin satisfy the above ranges, transparency can be improved.

The second polyester resin comprises more than 95 mol % of terephthalic acid as a dicarboxylic acid component, at least 70 mol % of ethylene glycol as a diol component, and at least 10 mol % of a C ₃ to C ₁₅ alcohol.

Specifically, based on the total molar number of dicarboxylic acid components, the second polyester resin may contain greater than 95 mol percent of terephthalic acid, more than 95 mol percent of terephthalic acid, more than 97 mol percent of terephthalic acid, more than 98 mol percent of terephthalic acid, more than 99 mol percent of terephthalic acid, or 100 mol percent of terephthalic acid.

Furthermore, based on the total molar number of the diol component, the second polyester resin may contain more than 70 mole percent of ethylene glycol, more than 73 mole percent, more than 75 mole percent or more than 76 mole percent of ethylene glycol, and may contain 70 to 90 mole percent of ethylene glycol, 73 to 88 mole percent of ethylene glycol, 75 to 85 mole percent of ethylene glycol, or 76 to 80 mole percent of ethylene glycol.

Based on the total molar number of diol components, the second polyester resin may contain more than 10 mol percent of _C3 to _C15 alcohol, more than 13 mol percent of _C3 to _C15 alcohol, more than ₁₅ mol percent of _C3 to C15 alcohol, more than 16 mol percent or more than 18 mol percent of _C3 to _C15 alcohol, and may contain 10 to 30 mol percent of _C3 to _C15 alcohol, ₁₃ to 28 mol percent of C3 to _C15 alcohol, 15 to 25 mol percent of _C3 to _C15 alcohol, 15 to 23 mol percent or 16 to 20 mol percent of _C3 to _C15 alcohol.

The C ₃ to C ₁₅ alcohol may include cyclohexanedimethanol or neopentyl glycol.

Furthermore, the second polyester resin may contain less than 10 mol % of diethylene glycol. For example, based on the total molar number of the diol component, the second polyester resin may contain less than 10 mol %, less than 8 mol %, or less than 6 mol % of diethylene glycol, and may contain 0.5 mol % to 10 mol % of diethylene glycol, 1 mol % to 8 mol % or 3 mol % to 6 mol % of diethylene glycol.

The second polyester resin contains the diol component and the dicarboxylic acid component, and the contents of the diol component and the dicarboxylic acid component satisfy the above ranges, thereby improving mechanical properties such as tensile strength and impact strength, and thus has excellent durability.

Furthermore, the polyester film of the example may contain 0.5 to 20 weight percent of the second polyester resin. Specifically, based on the total weight of the polyester film, the polyester film may contain 0.5 to 20 weight percent of the second polyester resin, 0.5 to 18 weight percent of the second polyester resin, 0.7 to 15 weight percent of the second polyester resin, 0.7 to 13 weight percent of the second polyester resin, 1 to 10 weight percent of the second polyester resin, 1.5 to 8 weight percent, or 2 to 6 weight percent of the second polyester resin.

When the content of the second polyester resin satisfies the above range, mechanical properties such as tensile strength and impact strength can be improved without reducing transparency.

Also, the polyester film may have a first elongation composite strength (ECT1) represented by the following Formula 1 of 17 kgf/mm ² to 30 kgf/mm ² .

Formula 1: ECT1 = EL1 × TS1

In the above formula 1, ECT1 is the first elongation composite strength (Kg/mm ² ), EL1 is the elongation in the first direction (%), and TS1 is the tensile strength in the first direction (Kg/mm ² ).

For example, the first elongation composite strength may be 17kgf/ ^mm2 to 30kgf/ ^mm2 , 18kgf/ ^mm2 to 27kgf/ ^mm2 , 18kgf/ ^mm2 to 25kgf/ ^mm2 , or 20kgf/ ^mm2 to 23kgf/ ^mm2 . When the first elongation composite strength satisfies the above range, mechanical properties such as tensile strength and impact strength can be improved, thereby having excellent durability.

In this specification, the first direction may be a width direction (TD) or a length direction (MD). Specifically, the first direction may be a length direction (MD), and a second direction perpendicular to the first direction may be a width direction (TD). More specifically, the second direction may be a main shrinkage direction.

Also, the polyester film may have a second elongation composite strength (ECT2) represented by the following Formula 2 of 0.95 kgf/mm ² to 2 kgf/mm ² .

Formula 2: ECT2 = EL2 × TS2

In the above formula 2, ECT2 is the second elongation composite strength (Kg/mm ² ), EL2 is the elongation in the second direction perpendicular to the first direction (%), and TS2 is the tensile strength in the second direction perpendicular to the first direction (Kg/mm ² ).

For example, the second elongation composite strength may be 0.95kgf/ ^mm2 to 2kgf/ ^mm2 , 0.95kgf/ ^mm2 to 1.8kgf/ ^mm2 , 1kgf/ ^mm2 to 1.6kgf/ ^mm2 , 1kgf/ ^mm2 to 1.5kgf/ ^mm2 , or 1.05kgf/ ^mm2 to 1.3kgf/ ^mm2 . When the second elongation composite strength satisfies the above range, mechanical properties such as tensile strength and impact strength can be improved, thereby having excellent durability.

Furthermore, the ratio of the first elongation composite strength to the second elongation composite strength may be 1:0.035 to 1:0.117. For example, the ratio of the first elongation composite strength to the second elongation composite strength may be 1:0.035 to 1:0.117, 1:0.035 to 1:0.11, 1:0.038 to 1:0.105, 1:0.038 to 1:0.1, 1:0.04 to 1:0.09, 1:0.04 to 1:0.08, or 1:0.04 to 1:0.07. The ratio of the first elongation composite strength to the second elongation composite strength satisfies the above range, so that mechanical properties such as tensile strength and impact strength can be maximized without reducing transparency.

The haze of the polyester film may be less than 10%. For example, the haze of the polyester film may be less than 10%, less than 10%, less than 8%, less than 6%, less than 4%, less than 3%, or less than 2.5%.

When heat-treated at 150° C. for 30 minutes, the thermal shrinkage rate in the first direction of the polyester film of the example may be 15% or less. For example, when heat-treated at 150° C. for 30 minutes, the thermal shrinkage rate in the first direction of the polyester film may be 15% or less, 13% or less, 11% or less, 10% or less, or 9% or less.

Furthermore, when heat-treated at 150° C. for 30 minutes, the thermal shrinkage rate of the polyester film in the second direction perpendicular to the first direction may be 10% or less. For example, when heat-treated at 150° C. for 30 minutes, the thermal shrinkage rate of the polyester film in the second direction perpendicular to the first direction may be 10% or less, 8% or less, 6% or less, 5% or less, or 4.5% or less.

When heat-treated at 85° C. for 24 hours, the thermal shrinkage rate in the first direction of the polyester film of the example may be 5% or less. For example, when heat-treated at 85° C. for 24 hours, the thermal shrinkage rate in the first direction of the polyester film may be 5% or less, 4% or less, 3% or less, 2% or less, 1.5% or less, 1% or less, 0.7% or less, 0.6% or less, or 0.5% or less.

Furthermore, when heat-treated at 85° C. for 24 hours, the heat shrinkage rate of the polyester film in the second direction perpendicular to the first direction may be 3% or less. For example, when heat-treated at 85° C. for 24 hours, the heat shrinkage rate of the polyester film in the second direction perpendicular to the first direction may be 3% or less, 2% or less, 1.5% or less, 1.3% or less, 1% or less, 0.8% or less, 0.6% or less, 0.4% or less, 0.15% or less, or 0.1% or less.

The above-mentioned thermal shrinkage rate can be calculated by the following formula 3 and formula 4.

Formula 3:

Formula 4:

In the above formula 3 and formula 4, T _MD is the heat shrinkage rate in the MD direction (%), L _MD1 is the length of the initial film in the MD direction (mm), and L _MD2 is the length of the MD direction after heat shrinkage (mm). _{T TD} is the heat shrinkage rate in the TD direction (%), L _TD1 is the length of the initial film in the TD direction (mm), and L _TD2 is the length of the TD direction after heat shrinkage (mm).

On the other hand, the optical fingerprint recognition method is a method of obtaining a fingerprint image reflected by light and comparing it with the previously registered fingerprint information. In order to improve the fingerprint recognition rate of the optical fingerprint recognition method, there should be no distortion of the reflected light after the fingerprint is recognized. Therefore, the lower the orientation angle and orientation angle deviation of the protective film attached to the surface of the display device such as a smartphone, the higher the fingerprint recognition rate and the effect of preventing fingerprint recognition errors.

The polyester film of the example not only has excellent durability and transparency, but also has an excellent fingerprint recognition rate and the effect of preventing fingerprint recognition errors.

Specifically, the orientation angle change rate of the polyester film in the width direction may be 3°/10 cm or less. More specifically, when the polyester film is cut at intervals of 10 cm in the width direction and each orientation angle is measured, the orientation angle change rate may be 3°/10 cm or less, 2.5°/10 cm or less, 2.3°/10 cm or less, 2°/10 cm or less, 1.5°/10 cm or less, 1.3°/10 cm or less, 1°/10 cm or less, 0.8°/10 cm or less, 0.6°/10 cm or less, 0.5°/10 cm or less, 0.3°/10 cm or less, or 0.2°/10 cm or less. The orientation angle change rate satisfies the above range, so that the film can ensure excellent visibility at any position, thereby making the reliability of visibility very excellent.

Furthermore, the orientation angle deviation of the polyester film relative to the full width may be within ±5°. Specifically, the orientation angle deviation based on the average orientation angle measured for the full width of the film may be within ±5°, within ±4.5°, within ±4°, within ±3.5°, within ±3°, within ±2.8°, within ±2.5°, within ±2°, within ±1.5°, within ±1.2°, within ±1°, within ±0.9°, or within ±0.7°. The orientation angle deviation relative to the full width satisfies the above range, so that the film can ensure excellent visibility at any position, thereby making the reliability of visibility very excellent.

The orientation angle deviation relative to the width direction within ±2000 mm from the central axis of the polyester film may be within ±2.5°. For example, the orientation angle deviation relative to the width direction within ±2000 mm from the central axis of the polyester film may be within ±2.5°, within ±2°, within ±1.5°, within ±1.2°, within ±1°, within ±0.9°, or within ±0.7°.

Furthermore, the orientation angle deviation relative to the width direction from the central axis of the polyester film to more than ±2000 mm may be within ±5°. For example, the orientation angle deviation relative to the width direction from the central axis of the polyester film to more than ±2000 mm may be within ±5°, within ±4.5°, within ±4°, within ±3.5°, within ±3°, within ±2.8°, within ±2.5°, within ±2°, within ±1.5°, within ±1.2°, within ±1°, within ±0.9°, or within ±0.7°.

The difference (θ ₁ - θ ₂ ) between the orientation angle (θ ₁ ) at an arbitrary point of the polyester film and the orientation angle (θ ₂ ) at a point located within ±2000 mm from the arbitrary point may be within ±5°. For example, the difference (θ 1 -θ 2 ) between the orientation angle (θ ₁ ) at any point of the polyester _film and the orientation angle (θ ₂ ) at a point located within ±2000 mm, ±1800 mm, ±1500 mm, ±1300 mm, ±1000 mm, ±800 mm, ±500 mm, ±300 mm, ±100 mm or ± ₅₀ mm from the arbitrary point may be within ±5°, within ±4.5°, within ±4°, within ±3.5°, within ±3°, within ±2.8°, within ±2.5°, within ±2°, within ±1.5°, within ±1.2°, within ±1°, within ±0.9°, within ±0.7°, within ±0.5°, within ±0.4°, within ±0.2°, within ±0.1° or within ±0.05°.

And, the total width of the polyester film is 50 cm to 6000 cm. For example, the total width of the polyester film can be 50 cm to 6000 cm, 50 cm to 5500 cm, 50 cm to 5000 cm, 50 cm to 4000 cm, 50 cm to 3000 cm, 50 cm to 2500 cm, 50 cm to 2300 cm, 50 cm to 2000 cm, 50 cm to 1800 cm, 50 cm to 1500 cm, 50 cm to 1300 cm, 50 cm to 1000 cm, 50 cm to 800 cm, 70 cm to 800 cm or 90 cm to 700 cm.

In 90% or more, 95% or more, 98% or more, 99% or more, or 100% of the total width, the orientation angle of the polyester film may be within ±5°, within ±4°, within ±3.8°, within ±3.5°, within ±3.3°, within ±3°, within ±2.8°, or within ±2.5°, based on the width direction. The orientation angle of the polyester film satisfies the above range, thereby improving visibility and reliability, thereby achieving excellent fingerprint recognition rate and fingerprint recognition error prevention effects.

Even if the polyester film of the example has a wide width of 50cm to 6000cm, in more than 90% of the above total width, the orientation angle is within ±5° based on the above width direction, thereby having an excellent fingerprint recognition rate and the effect of preventing fingerprint recognition errors.

The in-plane phase difference (Re, 550nm) of the polyester film is at least 3000nm. For example, the in-plane phase difference (Re) of the polyester film may be 3000nm or more, 3500nm or more, 4000nm or more, 5000nm or more, 5500nm or more, 6000nm or more, or 7000nm or more, and may be 3000nm to 13000nm, 3500nm to 12500nm, 4000nm to 12000nm, 5000nm to 12000nm, 5500nm to 11500nm, 6000nm to 11000nm, 7000nm to 10000nm, 7500nm to 9500nm, or 7500nm to 9000nm. The in-plane phase difference satisfies the above range, which not only improves durability but also maximizes the difference in refractive index in a first direction and a second direction perpendicular to the first direction to avoid sensing distortion of light, thereby ensuring excellent visibility.

Specifically, the above-mentioned in-plane phase difference (Re) is a parameter defined as the product of the anisotropy (△Nxy=|Nx-Ny|) of the refractive index of two axes perpendicularly intersecting in the plane of the film and the thickness d (nm) of the film, and is a measure of optical isotropy or anisotropy. More specifically, the above-mentioned in-plane phase difference (Re) can be calculated by mathematical formula A.

Mathematical formula A:

Re＝ΔNxy×d

In the above mathematical formula A, d is the thickness of the film, ΔNxy is the absolute value of the difference between Nx and Ny (ΔNxy=|Nx-Ny|), Nx is the refractive index in the slow axis direction in the plane, and Ny is the refractive index in the fast axis direction in the plane. Specifically, Nx can be the refractive index in the length direction (MD), and Ny can be the refractive index in the width direction (TD).

The refractive indices (Nx, Ny) of the two axes can be measured using a refractometer (RETS-100, measuring wavelength 550 nm) manufactured by Otsuka Corporation, but are not limited thereto.

And, the in-plane phase difference (Re) deviation of the above-mentioned polyester film can be 600nm/m or less. For example, the in-plane phase difference (Re) deviation of the above-mentioned polyester film can be 600nm/m or less, 500nm/m or less, 400nm/m or less, 300nm/m or less or 200nm/m or less, and can be 5nm/m to 600nm/m, 5nm/m to 500nm/m, 10nm/m to 400nm/m, 10nm/m to 350nm/m, 10nm/m to 300nm/m or 10nm/m to 200nm/m. The in-plane phase difference deviation satisfies the above-mentioned range, so that not only the durability can be improved, but also the refractive index difference in the first direction and the second direction perpendicular to the above-mentioned first direction can be maximized to avoid sensing the distortion of light, so that excellent visibility can be ensured.

On the other hand, the phase difference in the thickness direction (Rth, 550nm) is calculated as follows: the average value of the values obtained by multiplying the two birefringence values ΔNxz (=|Nx-Nz|) and ΔNyz (=|Ny-Nz|) when viewed in the cross section of the film thickness direction by the film thickness d (nm) is taken. Specifically, the phase difference in the thickness direction (Rth) can be calculated by the following mathematical formula B.

Mathematical formula B:

d is the thickness of the film, ΔNxz is the absolute value of the difference between Nx and Nz (ΔNxz = |Nx-Nz|), and ΔNyz is the absolute value of the difference between Ny and Nz (ΔNyz = |Ny-Nz|). The above Nx is the refractive index in the slow axis direction of the plane, and the above Ny is the refractive index in the fast axis direction of the plane. Specifically, the above Nx can be the refractive index in the length direction (MD), and the above Ny can be the refractive index in the width direction (TD).

The phase difference (Rth) in the thickness direction of the polyester film may be 8000nm to 14000nm. For example, at a wavelength of 550nm, the phase difference (Rth) in the thickness direction of the polyester film may be 8000nm to 14000nm, 8000nm to 13500nm, 8500nm to 13000nm or 8500nm to 12800nm. The phase difference in the thickness direction satisfies the above range, thereby not only improving durability, but also maximizing the difference in refractive index in the first direction and the second direction perpendicular to the first direction to avoid sensing distortion of light, thereby ensuring excellent visibility.

Furthermore, the light transmittance of the polyester film at a wavelength of 380 nm may be greater than 80%. For example, the light transmittance of the polyester film at a wavelength of 380 nm may be greater than 85%, greater than 85%, greater than 90%, 80% to 99%, 80% to 95%, 85% to 95%, or 85% to 92%. The light transmittance at a wavelength of 380 nm satisfies the above range, thereby ensuring excellent transparency.

Furthermore, the luminous flux of the polyester film according to the following formula 5 may be 90% or more.

Formula 5:

In the above formula 5, A is the brightness (lux) when 530nm light is passed through two parallel polarizing plates, and B is the brightness (lux) when 530nm light is passed through after the above polyester film is placed between the above two polarizing plates. In this case, the width direction (TD) of the above polyester film is at an angle of 45° relative to the optical axis d of the above two polarizing plates.

The optical fingerprint recognition method uses a light source such as a light emitting diode to illuminate the device and uses an image sensor to sense the light reflected by the fingerprint and compare it with the previously registered fingerprint information. Therefore, the amount of light irradiated and reflected by the device can be increased sufficiently and the distortion of the irradiated and reflected light can be avoided to improve the fingerprint recognition rate.

The luminous flux of the polyester film of the example satisfies 90% or more according to the above formula 5, so that the amount of irradiated and reflected light can be sufficiently ensured, thereby achieving excellent visibility. Therefore, when the polyester film is used as a protective film for display devices such as smartphones, tablet computers, notebook computers, etc. and optical sensors such as barcode readers, the product information such as barcodes and fingerprint recognition rates can be improved due to excellent visibility.

For example, the luminous flux of the polyester film according to the above formula 5 can be 90% or more, 90.5% or more, 91% or more, 91.2% or more, 91.5% or more, 92% or more, or 90% to 99%, 90.5% to 98%, 91% to 98%, 91% to 96%, 91% to 95%, 91% to 93%, 91.2% to 93%, or 92% to 93%. According to formula 5, the luminous flux satisfies the above range, so that the amount of light irradiated and reflected by the film can be fully ensured, thereby improving visibility.

The luminous flux can be measured by an illuminometer. For example, the illuminometer may be a device that places two polarizing plates in parallel at a specific distance and places a polyester film between the two polarizing plates, supplies light and transmits the light to measure the brightness of the light.

FIG. 3 shows a method for measuring the luminous flux of a polyester film.

Specifically, as shown in Fig. 3, an optical power meter 10 is disposed at the lower end, and a first polarizing plate 21 and a second polarizing plate 22 are placed in parallel and spaced apart from each other on the upper portion of the optical power meter. In this case, the distance between the optical power meter 10 and the first polarizing plate 21 may be shorter than the distance between the optical power meter 10 and the second polarizing plate 22, but is not limited thereto.

For example, the distance between the optical power meter 10 and the first polarizing plate 21 may be 1 cm to 10 cm, 1.2 cm to 8 cm, 1.4 cm to 6.5 cm, 1.5 cm to 6 cm, 1.8 cm to 5.5 cm or 2 cm to 5 cm, and the distance between the optical power meter 10 and the second polarizing plate 22 may be 5 cm to 30 cm, 7 cm to 28 cm, 8 cm to 25 cm, 9 cm to 23 cm or 10 cm to 20 cm.

Then, the optical power meter 10 can be used to supply 530nm light (c: direction of light) at a voltage of 12V and pass the light to measure the brightness (lux) before and after placing the polyester film 100 between the first polarizing plate 21 and the second polarizing plate 22, and then the luminous flux can be calculated according to the above formula 5.

Furthermore, the polyester film may be placed at an angle of rotation within 180°. In this specification, the luminous flux is measured when the width direction (TD) of the film forms an angle of 45° with the optical axis b of the two polarizing plates.

The thickness of the polyester film may be 30 μm to 150 μm. For example, the thickness of the polyester film may be 30 μm to 150 μm, 40 μm to 150 μm, 45 μm to 145 μm, 50 μm to 140 μm, 55 μm to 135 μm or 55 μm to 130 μm. The thickness of the polyester film may be selected within the above range according to the requirement of improving formability or durability. Specifically, if the thickness of the polyester film is less than 30 μm, it may have excellent formability but low durability, and if it is greater than 150 μm, it may have excellent durability but low formability, resulting in poor quality when used as a protective film.

In particular, the orientation angle, orientation angle change rate, and orientation angle deviation of the polyester film are not affected by the film thickness, and excellent visibility can be ensured without reducing properties such as transparency, formability, and durability.

Furthermore, the thickness deviation of the polyester film may be 5 μm or less. For example, the thickness deviation of the polyester film may be 4 μm or less, 3 μm or less, 2.5 μm or less, 2 μm or less, or 1.8 μm or less, and may be 0.05 μm to 5 μm, 0.1 μm to 4 μm, 0.1 μm to 3 μm, 0.3 μm to 2 μm, or 0.3 μm to 1.8 μm. The thickness deviation satisfies the above range, so that uniform visibility can be achieved with appropriate phase difference deviation.

The difference (D1-D2) between the thickness (D1) at any point of the polyester film and the thickness (D2) at a point located within ±2000 mm from the arbitrary point may be within ±4 μm. For example, the difference (D1-D2) between the thickness (D1) at any point of the polyester film and the thickness (D2) at a point located within ±2000 mm, ±1800 mm, ±1500 mm, ±1300 mm, ±1000 mm, ±800 mm, ±500 mm, ±300 mm, ±100 mm or ±50 mm from the arbitrary point may be within ±4 μm, within ±3.5 μm, within ±3 μm, within ±2.5 μm, within ±2.3 μm, within ±2 μm, within ±1.8 μm, within ±1 μm or within ±0.8 μm.

Furthermore, the moisture permeability of the polyester film may be 20 g/m ² .day or less. For example, the moisture permeability of the polyester film may be 20 g/m ² .day or less, 18 g/m ² .day or less, 15 g/m ² .day or less, 12 g/m ² .day or less, or 0.1 g/m ² .day to 20 g/m ² .day, 0.5 g/m ² .day to 18 g/m ² .day, 1 g/m ² .day to 15 g/m ² .day, 3 g/m ² .day to 13 g/m ² .day, 4 g/m ² .day to 11 g/m ² .day, 4.5 g/m ² .day to 10 g/m ² .day, or 4.8 g/m ² .day to 10 g/m ² .day. When the moisture permeability satisfies the ^above range, excellent durability can be ensured. Specifically, the polyester film having the above-mentioned moisture permeability range has significantly better moisture permeability properties than the triacetate (TAC) film conventionally used as a protective film, and when the above-mentioned polyester film is used as a protective film for a display device, the display device can be effectively protected from an external moisture environment.

The difference in refractive index between the first direction and the second direction perpendicular to the first direction of the polyester film may be 0.08 to 0.14. For example, the difference in refractive index between the first direction and the second direction perpendicular to the first direction of the polyester film may be 0.08 to 0.14, 0.08 to 0.13, 0.08 to 0.125, 0.083 to 0.115, or 0.085 to 0.11. The difference in refractive index between the first direction and the second direction satisfies the above range, thereby avoiding distortion of light, thereby ensuring excellent visibility.

The polyester film has an ultraviolet durability (TSM _UV ) in the machine direction (MD) of 80% or more according to the following mathematical formula C.

Mathematical formula C:

In the above mathematical formula C, _TSMUV is the ultraviolet durability in the MD direction (%), TSM1 is the initial tensile strength in the MD direction, and TSM2 is the tensile strength in the MD direction measured after exposure to ultraviolet light with a power of 0.68 W/ ^m2 for 48 hours.

For example, according to the above mathematical formula C, the ultraviolet durability (TSM _UV ) may be 80% or more, or 82% or more, or 80% to 100% or 80% to 95%.

Alternatively, the polyester film may have an ultraviolet durability (TST _UV ) in the width direction (TD) of 80% or more according to the following mathematical formula D.

Mathematical formula D:

In the above mathematical formula D, TST _UV is the ultraviolet durability in the TD direction (%), TST1 is the initial tensile strength in the TD direction, and TST2 is the tensile strength in the TD direction measured after exposure to ultraviolet light with a power of 0.68 W/m ² for 48 hours.

For example, the ultraviolet durability (TST _UV ) according to the above mathematical formula D may be 80% or more, 85% or more, or 88% or more, and may be 80% to 100% or 80% to 95%.

Specifically, the ultraviolet durability is evaluated based on the tensile strength. Since the polyester film is a stretched film, it may have different ultraviolet durability in different directions.

The polyester film of the example has an MD direction UV durability (TSM _UV ) and a TD direction UV durability (TST _UV ) of more than 80%, and thus can maintain excellent durability even under repeated strong UV light.

Preparation method of polyester film

A method for preparing a polyester film in another embodiment includes: preparing a mixture of a first polyester resin and a second polyester resin; preparing an unstretched sheet by melt-extruding the mixture; preparing a stretched film by stretching the unstretched sheet by 1 to 1.5 times in a first direction and by 3 to 5 times in a second direction perpendicular to the first direction at a temperature of 70° C. to 125° C.; and preparing a polyester film by heat-fixing the stretched film at a temperature of 160° C. to 230° C., wherein the first polyester resin contains more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component, and the second polyester resin contains more than 95 mol% of terephthalic acid as a dicarboxylic acid component, at least 70 mol% of ethylene glycol as a diol component, and at least 10 mol% of a C ₃ to C ₁₅ alcohol, and the in-plane retardation of the polyester film is greater than 3000 nm.

The polyester film prepared above has substantially the same structure and properties as the polyester film in the above example.

The polyester film prepared by the above method can satisfy the above-described properties (orientation angle, phase difference, etc.) by adjusting the combination and process conditions. Specifically, in order to make the final polyester film satisfy the above-described properties, the combination of polyester resins can be adjusted, and the extrusion temperature, preheating temperature during stretching, stretching ratios in different directions, stretching speed, etc. can be adjusted, or the heat treatment temperature and relaxation rate can be adjusted during heat treatment and relaxation after stretching.

Hereinafter, each step will be described in more detail.

First, a mixture of a first polyester resin and a second polyester resin is prepared.

The descriptions of the first polyester resin and the second polyester resin are the same as those described above.

Then, the above mixture is melt-extruded to prepare an unstretched sheet.

Specifically, the mixture may be melt-extruded at a temperature of 260° C. to 300° C. or 270° C. to 290° C. and then cooled to prepare an unstretched sheet.

Then, the unstretched sheet is transferred to pass through a roller. In this case, the thickness of the desired film can be adjusted by adjusting the speed and discharge amount of the unstretched sheet.

Then, the unstretched sheet is stretched at a temperature of 70°C to 125°C.

According to another example, a step of preheating the unstretched sheet may be included before the stretching step.

The preheating temperature ranges from Tg+5°C to Tg+50°C based on the glass transition temperature (Tg) of the polyester resin, and can be set to 70°C to 90°C. The preheating temperature satisfies the above range, thereby effectively preventing breakage during stretching while ensuring flexibility that allows for easy stretching.

On the other hand, the stretching may be performed at 70 to 125° C., 75 to 120° C., 80 to 110° C., 85 to 100° C., or 80 to 100° C. If the stretching temperature is outside the above range, breakage may occur.

More specifically, the stretching temperature in the first direction may be 75 to 90° C. or 75 to 85° C., and the stretching temperature in the second direction may be 80 to 110° C. or 80 to 120° C. If the stretching temperature exceeds the above range, breakage may occur.

And, the stretching speed may be 5 m/min to 20 m/min, 7 m/min to 18 m/min, or 10 m/min to 18 m/min.

The stretching can be carried out along the first direction at a stretching ratio of 1 to 1.5 times or 1 to 1.45 times, and can be carried out along the second direction perpendicular to the first direction at a stretching ratio of 3 to 5 times, 3.3 to 4.8 times, 3.5 to 4.8 times, 4 to 4.8 times or 4.2 to 4.5 times.

The ratio of the stretching ratio in the first direction to the second direction may be 1:1.5 to 1:5.5. For example, the ratio of the stretching ratio in the first direction to the second direction may be 1:2 to 1:5, 1:2.5 to 1:4.5, or 1:3.5 to 1:4.5. The ratio of the stretching ratio in the first direction to the second direction satisfies the above range, thereby further improving durability and uniformity of curvature.

Furthermore, the coating process may be performed after the stretching. Specifically, the coating process may be performed before the stretching in the first direction, or after the stretching in the first direction and before the stretching in the second direction. More specifically, a coating process may be performed to form a promotion layer capable of imparting antistatic and other functionalities on the film. The coating process may be performed by spin coating or in-line coating, but is not limited thereto.

Thereafter, the stretched film was heat-set at a temperature of 160° C. to 230° C. to prepare a polyester film.

Specifically, the thermal fixation may be annealing, which may be performed at a temperature of 165° C. to 210° C., 170° C. to 200° C., 170° C. to 190° C., or 175° C. to 185° C. for 0.5 to 8 minutes, 0.5 to 5 minutes, 0.5 to 3 minutes, or 1 to 2 minutes. After the thermal fixation is completed, the temperature may be gradually lowered.

A relaxation step may be further included after the above-mentioned stretching step.

The relaxation may be performed along the first direction or in a second direction perpendicular to the first direction. Specifically, the relaxation may be performed at a temperature of 60°C to 180°C, 80°C to 150°C, 80°C to 120°C or 90°C to 110°C with a relaxation rate of 5% or less. For example, the relaxation rate may be 5% or less, 4% or less or 3% or less, and may be 0.1% to 5%, 0.5% to 4% or 1% to 3%.

Protective film

A protective film of another example comprises: a polyester film; and a first curable resin layer located on one side of the polyester film, wherein the polyester film comprises: a first polyester resin comprising more than 95 mol % of terephthalic acid as a dicarboxylic acid component and more than 95 mol % of ethylene glycol as a diol component; and a second polyester resin comprising more than 95 mol % of terephthalic acid as a dicarboxylic acid component, at least 70 mol % of ethylene glycol as a diol component, and at least 10 mol % of a C ₃ to C ₁₅ alcohol, wherein the in-plane phase difference of the polyester film is at least 3000 nm.

The polyester film is as described above.

The protective film of the example includes a first curable resin layer on one side of the polyester film, thereby having an advantageous effect of absorbing impact.

FIG1 shows an example of a protective film. Specifically, FIG1 shows a protective film 200 including a polyester film 100 and a first curable resin layer 231 located on one side of the polyester film 100.

Specifically, the first curable resin layer may include a photocurable resin or a thermosetting resin. For example, the photocurable resin may include a polyurethane acrylate oligomer, an epoxy acrylate oligomer, or a mixture thereof, and the thermosetting resin may include a polyurethane acrylate polyol, a melamine acrylate polyol, an epoxy acrylate polyol, or a mixture thereof. For example, the curable resin layer may include a polyurethane acrylate resin.

Furthermore, the first curable resin layer may further include one or more additives selected from the group consisting of a crosslinking agent, an antistatic agent, and a defoaming agent. For example, the crosslinking agent may be a silane crosslinking agent, which may be an alkoxysilane such as vinylethoxysilane, vinyl-tri-(β-methoxyethoxy)silane, methacrylpropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, and trimethoxysilane; epoxysilane such as triepoxysilane; aminosilane such as butylaminosilane and epoxy-aminosilane; and alkylsilane, silane, or disilane such as methylsilane, dimethylsilane, vinylmethyldimethylcyclotrisiloxane, dimethylsilane-oxycyclopentane, cyclohexylsilane, and cyclohexyldisilane, but is not limited thereto.

The thickness of the first curable resin layer may be 10 nm to 200 nm. For example, the thickness of the first curable resin layer may be 20 nm to 200 nm, 35 nm to 180 nm, 50 nm to 150 nm, 50 nm to 130 nm, 60 nm to 120 nm, or 80 nm to 100 nm.

Furthermore, the protective film may further include one or more selected from the group consisting of a hard coating layer, an adhesive layer, and a release layer as required.

Fig. 2 shows another example of a protective film. Specifically, the protective film 200 shown in Fig. 2 comprises a polyester film 100, a first curable resin layer 231 located on one side of the polyester film, a hard coating layer 240 located on one side of the first curable resin layer, an adhesive layer 250 located on the other side of the polyester film, and a release layer 260 located on one side of the adhesive layer.

The hard coating layer may include a photocurable resin. The protective film includes the hard coating layer, thereby improving the hardness of the film surface and having excellent scratch resistance.

The above-mentioned photocurable resin may be a compound having one or more unsaturated bonds, such as a compound having an acrylate photoenergy group. The compound having one unsaturated bond may be, for example, ethyl (meth)acrylate, ethylhexyl (meth)acrylate, styrene, methylstyrene, N-vinyl pyrrolidone, etc. The compound having two or more unsaturated bonds may be, for example, polymethylolpropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol penta(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, etc. In this specification, "(meth)acrylate" refers to methacrylate and acrylate.

The above-mentioned adhesive layer may include a binder resin. The above-mentioned binder resin may be formed by polymerization of one or more selected from, for example, acrylic acid monomers and unsaturated monomers having a carboxyl group. Acrylic acid monomers include, for example, methyl (meth)acrylate, butyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, isobutyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, glycerol (meth)acrylate, poly (ethylene glycol) methyl ether (meth)acrylate, methoxytripropylene glycol (meth)acrylate, dicyclopentyl (meth)acrylate, etc. In addition, unsaturated monomers containing a carboxyl group include, for example, acrylic acid, methacrylic acid, itaconic acid, maleic acid, etc.

The release layer may be a polyester film such as polyethylene terephthalate film, polyethylene naphthalate film, polypropylene terephthalate film, polybutylene terephthalate film, polypropylene naphthalate film, but is not limited thereto.

A protective film of another example includes: a first substrate layer; a second substrate layer located on the first substrate layer; and a second curable resin layer located between the first substrate layer and the second substrate layer, wherein the thickness of the first substrate layer is thinner than that of the second substrate layer, and when the curable resin layer is cured, the first substrate layer and the second substrate layer form a curved portion.

Fig. 4 shows a display device of an example. Specifically, the display device 1 shown in Fig. 4 includes a cover window 300 located on the display panel and a protective film 200 located on one side of the cover window. The protective film 200 may be located in front of the cover window 300 and may include a curved surface portion a having a curved surface shape and a flat surface portion b having no curved surface shape.

Fig. 5 is a cross-sectional view of the display device of Fig. 3 cut along line XX'. Specifically, Fig. 5 illustrates a display device 1 having a structure in which a display panel 400, a cover window 300, and a protection film 200 are sequentially stacked.

Specifically, when the protective film 200 is disposed on the cover window 300 and the second curable resin layer of the protective film is cured, the protective film 200 is bent along the arrow direction with respect to the winding axis Q to form a curved portion a. In this case, the direction of the winding axis Q refers to the direction of the Z axis.

More specifically, the display device 1 may be an in-folding type or an out-folding type according to the direction in which the display panel 400 forms a curved surface. FIG. 6 is a cross-sectional view showing an in-folding type flexible display device 2, and FIG. 7 is a cross-sectional view showing an out-folding type flexible display device 3.

As shown in Figures 6 and 7, whether the protective film of the example is used as a protective film 200 for the inward-folding flexible display device 2 or as a protective film 200 for the outward-folding flexible display device 3, the protective film is bent along the direction of the arrow with the winding axis Q as a reference, and can completely adhere to the curved surface without warping.

Fig. 8 shows another example of a protective film. Specifically, the protective film 200 shown in Fig. 8 includes a first substrate layer 210, a second curable resin layer 232 located on the first substrate layer, and a second substrate layer 220 located on the second curable resin layer.

The protective film of the example may have a curved portion a (refer to FIGS. 4 to 7). Specifically, the protective film includes a second curable resin layer between a first substrate layer and a second substrate layer thinner than the first substrate layer, so that the first substrate layer and the second substrate layer may form a curved portion when the second curable resin layer is cured. More specifically, the curved portion may be formed on the entire protective film or only on the end of the protective film.

The first substrate layer and the second substrate layer

The first base layer may include a third polyester resin, and the second base layer may include a fourth polyester resin.

Specifically, the third polyester resin may be a homopolymer resin or a copolymer resin obtained by polycondensation of dicarboxylic acid and diol. Furthermore, the third polyester resin may be a blended resin obtained by mixing the homopolymer resin and the copolymer resin. More specifically, the third polyester resin may be a mixture of dicarboxylic acid and diol in a molar ratio of 1:1.

The above-mentioned dicarboxylic acid can be 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, diphenyl carboxylic acid, diphenoxyethane dicarboxylic acid, diphenyl sulfone carboxylic acid, anthracene dicarboxylic acid, 1,3-cyclopentane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyl adipic acid, trimethyl adipic acid, pimelic acid, azelaic acid, sebacic acid or suberic acid, dodecanedicarboxylic acid.

Furthermore, the diol may be ethylene glycol, propylene glycol, hexylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane or bis(4-hydroxyphenyl)sulfone.

Preferably, the third polyester resin may be an aromatic polyester resin with excellent crystallinity, and specifically, may be mainly composed of polyethylene terephthalate (PET) resin. For example, the first substrate layer may be a polyethylene terephthalate film.

For example, the first substrate layer may contain 85 weight percent or more of polyethylene terephthalate resin as the third polyester resin, and more specifically, may contain 90 weight percent or more, 95 weight percent or more, or 99 weight percent or more of polyethylene terephthalate resin as the third polyester resin.

Alternatively, in addition to the polyethylene terephthalate resin, the first substrate layer may include other polyester resins. Specifically, the first substrate layer may include less than 15 weight percent of polyethylene naphthalate (PEN) resin. More specifically, the first substrate layer may include 0.1 weight percent to 10 weight percent or 0.1 weight percent to 5 weight percent of polyethylene naphthalate resin.

By satisfying the above combination and content, the mechanical properties such as tensile strength of the protective film can be improved when it undergoes heating, stretching and other processes.

According to an example, the fourth polyester resin may be the same as the third polyester resin.

The thickness of the first substrate layer is thinner than that of the second substrate layer. Specifically, when the second curable resin layer between the first substrate layer and the second substrate layer is cured, the difference in the thickness of the first substrate layer and the second substrate layer causes a difference in the degree of shrinkage of the first substrate layer and the second substrate layer. Therefore, the protective film is bent toward the direction of the first substrate layer to form a curved portion. As shown in FIGS. 5 and 6 , the protective film 200 is bent toward the direction where the first substrate layer 210 is located with the winding axis Q as a reference.

The thickness ratio of the first substrate layer to the second substrate layer may be 1:1.2 to 1:3. For example, the thickness ratio of the first substrate layer to the second substrate layer may be 1:1.2 to 1:2.8, 1:1.3 to 1:2.5, or 1:1.3 to 1:2.2. The thickness ratio of the first substrate layer to the second substrate layer satisfies the above range, thereby further improving the durability and visual quality of the protective film.

Furthermore, the thickness of the first substrate layer may be 5 μm to 20 μm, and the thickness of the second substrate layer may be 15 μm to 40 μm. For example, the thickness of the first substrate layer may be 5 μm to 18 μm, 8 μm to 15 μm, or 10 μm to 15 μm. The thickness of the second substrate layer may be 15 μm to 35 μm, 18 μm to 30 μm, or 20 μm to 28 μm.

The first substrate layer and the second substrate layer each have a main stretching direction, and the main stretching direction of the first substrate layer may correspond to the main stretching direction of the second substrate layer.

Specifically, the first substrate layer and the second substrate layer each have a main stretching direction, and the main stretching direction can be the width direction (TD) or the length direction (MD). More specifically, the main stretching direction of the first substrate layer and the second substrate layer can both be the width direction (TD).

Second curable resin layer

The second curable resin layer is interposed between the first base layer and the second base layer.

Specifically, the second curable resin layer may include a photocurable resin or a thermosetting resin. For example, the photocurable resin may include a polyurethane acrylate oligomer, an epoxy acrylate oligomer, or a mixture thereof, and the thermosetting resin may include a polyurethane acrylate polyol, a melamine acrylate polyol, an epoxy acrylate polyol, or a mixture thereof. For example, the curable resin layer may include a polyurethane acrylate resin.

Furthermore, the second curable resin layer may further include one or more additives selected from the group consisting of a crosslinking agent, an antistatic agent, and a defoaming agent. For example, the crosslinking agent may be a silane crosslinking agent, which may be an alkoxysilane such as vinylethoxysilane, vinyl-tri-(β-methoxyethoxy)silane, methacrylpropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, and trimethoxysilane; epoxysilane such as triepoxysilane; aminosilane such as butylaminosilane and epoxy-aminosilane; and alkylsilane, silane, or disilane such as methylsilane, dimethylsilane, vinylmethyldimethylcyclotrisiloxane, dimethylsilane-oxycyclopentane, cyclohexylsilane, and cyclohexyldisilane, but is not limited thereto.

The thickness of the second curable resin layer may be 10 μm to 100 μm. For example, the thickness of the second curable resin layer may be 10 μm to 100 μm, 15 μm to 80 μm, 20 μm to 70 μm, 25 μm to 50 μm, 10 μm to 50 μm, or 20 μm to 35 μm. The thickness of the second curable resin layer satisfies the above range, thereby further improving the uniformity of the curved surface.

The protective film of the example may be formed into a curved portion having various curvature radii and bending angles according to the thickness difference between the first base layer and the second base layer.

The curvature radius R of the curved surface portion may be 5 mm to 30 mm. For example, the curvature radius R of the curved surface portion may be 5 mm to 30 mm, 7 mm to 25 mm, 7 mm to 20 mm, 9 mm to 18 mm, or 10 mm to 15 mm.

Furthermore, the bending angle W of the curved portion may be 5° to 45°. For example, the bending angle W of the curved portion may be 5° to 45°, 5° to 35°, 7° to 30°, or 10° to 30° (see FIGS. 6 and 7 ).

The curvature radius R and the bending angle W of the curved surface portion satisfy the above ranges, so that the anti-warping effect of the protective film can be maximized.

Furthermore, the protective film may further include one or more selected from the group consisting of a hard coating layer, an adhesive layer, and a release layer as required.

The descriptions of the hard coating layer, adhesive layer and release layer are as described above.

Fig. 9 shows another example of a protective film. Specifically, the protective film 200 shown in Fig. 9 includes a first substrate layer 210, a curable resin layer 230 located on one side of the first substrate layer, a second substrate layer 220 located on one side of the curable resin layer, a hard coating layer 240 located on one side of the second substrate layer, an adhesive layer 250 located on the other side of the first substrate layer, and a release layer 260 located on one side of the adhesive layer.

On the other hand, the optical fingerprint recognition method is a method of obtaining a fingerprint image reflected by light and comparing it with the previously registered fingerprint information. In order to improve the fingerprint recognition rate of the optical fingerprint recognition method, there should be no distortion of the reflected light after the fingerprint is recognized. Therefore, the lower the orientation angle and orientation angle deviation of the protective film attached to the surface of the display device such as a smartphone, the higher the fingerprint recognition rate and the effect of preventing fingerprint recognition errors.

The protective film of the example not only has excellent durability and transparency, but also has an excellent fingerprint recognition rate and the effect of preventing fingerprint recognition errors.

The descriptions of the orientation angle characteristics, in-plane phase difference characteristics and thickness direction phase difference characteristics, light transmittance, luminous flux, and ultraviolet light durability of the above-mentioned protective film are the same as those of the above-mentioned polyester film.

The thickness of the protective film may be 30 μm to 200 μm. For example, the thickness of the protective film may be 30 μm to 200 μm, 35 μm to 180 μm, 40 μm to 160 μm, 50 μm to 150 μm, or 55 μm to 130 μm.

The thickness of the protective film can be selected within the above range according to requirements such as improving formability or durability. Specifically, if the thickness of the protective film is less than 30 μm, the formability may be excellent but the durability may be low, and if it is greater than 200 μm, the durability may be excellent but the formability may be low, resulting in poor quality.

In particular, the orientation angle, orientation angle change rate and orientation angle deviation of the protective film are not affected by the protective thickness, and excellent visibility can be ensured without reducing the transparency, formability, durability and other characteristics.

Furthermore, the thickness deviation of the protective film may be 5 μm or less. For example, the thickness deviation of the protective film may be 4 μm or less, 3 μm or less, 2.5 μm or less, 2 μm or less, or 1.8 μm or less, and may be 0.05 μm to 5 μm, 0.1 μm to 4 μm, 0.1 μm to 3 μm, 0.3 μm to 2 μm, or 0.3 μm to 1.8 μm. The thickness deviation satisfies the above range, so that uniform visibility can be achieved with appropriate phase difference deviation.

The difference (D1-D2) between the thickness (D1) at any point of the protective film and the thickness (D2) at a point located within ±2000 mm from the arbitrary point may be within ±4 μm. For example, the difference (D1-D2) between the thickness (D1) at any point of the protective film and the thickness (D2) at a point located within ±2000 mm, ±1800 mm, ±1500 mm, ±1300 mm, ±1000 mm, ±800 mm, ±500 mm, ±300 mm, ±100 mm or ±50 mm from the arbitrary point may be within ±4 μm, within ±3.5 μm, within ±3 μm, within ±2.5 μm, within ±2.3 μm, within ±2 μm, within ±1.8 μm, within ±1 μm or within ±0.8 μm.

Furthermore, the moisture permeability of the protective film may be 20 g/m ² .day or less. For example, the moisture permeability of the protective film may be 20 g/m ² .day or less, 18 g/m ² .day or less, 15 g/m ² .day or less, 12 g/m ² .day or less, or 10 g/m ² .day or less, and may be 0.1 g/m ² .day to 20 g/m ² .day, 0.5 g/m ² .day to 18 g/m ² .day, 1 g/m ² .day to 15 g/m ² .day, 3 g/m ² .day to 13 g/m ² .day, 4 g/m ² .day to 11 g/m ² .day, 4.5 g/m ² .day to 10 g/m ² .day, or 4.8 g/m ² .day to 10 g/m ² .day.

The moisture permeability satisfies the above range, thereby ensuring excellent durability. Specifically, the protective film having the moisture permeability in the above range has significantly better moisture permeability than the triacetate film used as a protective film in the past, and when the protective film is applied to a protective film of a display device, the display device can be effectively protected from an external moisture environment.

The difference in refractive index between the first direction and the second direction perpendicular to the first direction of the protective film may be 0.08 to 0.14. For example, the difference in refractive index between the first direction and the second direction perpendicular to the first direction of the protective film may be 0.08 to 0.14, 0.08 to 0.13, 0.08 to 0.125, 0.083 to 0.115, or 0.085 to 0.11. The difference in refractive index between the first direction and the second direction satisfies the above range, thereby avoiding distortion of light, thereby ensuring excellent visibility.

In still another example, the first direction in the plane of the protective film satisfies the following formula A.

Formula A: 0.5≤|S ₁ –S ₂ |≤3.1

In the above formula A, the above _S1 represents the final stretching rate (%) after continuing N1 _% for 20 minutes, and the above _S2 represents the final stretching rate (%) after continuing N2 _% for 20 minutes. In this case, N1 _% represents the load for stretching the above protective film by 1% along the above first direction, and N2 _% represents the load for stretching the above protective film by 2% along the above first direction.

The elongation ratio and the tensile load are the results of measuring a test piece (50 mm long×10 mm wide) of the protective film at room temperature and a tensile speed of 50 mm/min.

Specifically, _S1 is the final stretching rate (%) of the protective film of the example after the first direction in the plane is stretched at N1 _% for 20 minutes, and _S2 is the final stretching rate (%) of the polyester film of the example after the first direction in the plane is stretched at N2 _% for 20 minutes.

More specifically, a method of measuring the above-mentioned _S1 is as follows.

First, (1) a test piece of the protective film having a length of 50 mm and a width of 10 mm is stretched in a first direction at room temperature and a stretching speed of 50 mm/min to obtain a stretching rate curve according to the load. (2) The load (N _1% ) at the time point when the length increases by 1% compared to the initial length is obtained from the stretching rate curve. (3) When the load N _1% is continuously applied to the test piece in the first direction for 20 minutes, the length ratio of the test piece to the initial length is the final stretching rate (%), that is, the above S ₁ .

In this specification, the first direction may be a width direction (TD) or a length direction (MD). Specifically, the first direction may be a length direction (MD), and a second direction perpendicular to the first direction may be a width direction (TD). More specifically, the second direction may be a main shrinkage direction.

And, a method of measuring the above-mentioned _S2 is as follows.

First, (1) a test piece of the protective film having a length of 50 mm and a width of 10 mm is stretched in a first direction at room temperature and a stretching speed of 50 mm/min to obtain a stretching rate curve according to the load. (2) The load (N _2% ) at the time point when the length increases by 2% compared to the initial length is obtained from the stretching rate curve. (3) The ratio of the length increase of the test piece compared to the initial length when the load of N _2% is continuously applied to the test piece in the first direction for 20 minutes is the final stretching rate (%), that is, the above S ₂ .

According to the above formula, the value of A can be 0.5 to 3.1, 0.7 to 2.8, 0.9 to 2.5, 1 to 2, or 1.2 to 1.8. When the above formula 2 is satisfied, the protective film can be applied as a protective film for a foldable display device without whitening or cracking due to the warping phenomenon even if it is folded tens of thousands of times.

The above _S1 may be 0.1 to 2.5, and the above _S2 may be 1.5 to 4.5. For example, the above _S1 may be 0.1 to 2.5, 0.1 to 2.3, 0.3 to 1.8, 0.5 to 1.6, or 0.8 to 1.2, and the above _S2 may be 1.5 to 4.5, 1.5 to 4, 1.8 to 3.5, 1.8 to 3, 2 to 3, or 2.2 to 2.7. _S1 and _S2 satisfy the above ranges, so that when the above protective film is applied as a protective film for a foldable display device, even if it is folded tens of thousands of times, whitening or cracks caused by the warping phenomenon will hardly occur.

Furthermore, N _{1 %} in the first direction may be 10N to 25N, and N _2% in the first direction may be 28N to 50N. For example, N 1% in the first direction may be 10N to 25N, 28N to 45N, 30N to 43N, or 33N to 40N, and N _2% in the first direction may be 28N to 50N, 28N to 45N, 30N to 43N, or 33N to 40N.

Also, the second direction of the polyester film of the example, which is perpendicular to the first direction, may satisfy the following formula B.

Formula B: 0.5≤|S ₃ –S _4| ≤5.2

In the above formula B, the above _S3 represents the final stretching rate (%) after continuing N1 _% for 20 minutes, and the above _S4 represents the final stretching rate (%) after continuing N2 _% for 20 minutes. In this case, N1 _% represents the load for stretching the above protective film by 1% along the above second direction, and N2 _% represents the load for stretching the above protective film by 2% along the above second direction.

The method for measuring the above _S3 and _S4 is the same as the method for measuring _S1 and _S2 except that the second direction is used instead of the first direction.

The value of the above formula B can be 0.5 to 5.2, 0.7 to 5, 0.7 to 4.5, 1 to 4, 1.2 to 3.3, 1.5 to 2.8, 1.7 to 2.5 or 2 to 2.3. The above formula B is satisfied, so that when the above protective film is applied as a protective film for a foldable display device, even if it is folded tens of thousands of times, whitening or cracks caused by the warping phenomenon will hardly occur.

The above _S3 may be 0.8 to 2.4, and the above _S4 may be 2.3 to 7.5. For example, the above _S3 may be 0.8 to 2.4, 1 to 2.4, 1.2 to 2.4, 1.6 to 2.2, or 1.8 to 2.2, and the above _S4 may be 2.3 to 7.5, 2.8 to 7, 2.8 to 6.5, 3 to 6, 3.5 to 5.8, or 4.1 to 5.2. _S3 and _S4 satisfy the above ranges, so that when the above protective film is applied as a protective film for a foldable display device, even if it is folded tens of thousands of times, whitening or cracks caused by the warping phenomenon will hardly occur.

Furthermore, N _{1 %} in the second direction may be 25N to 45N, and N _2% in the second direction may be 50N to 70N. For example, N 1% in the second direction may be 25N to 45N, 28N to 45N, 30N to 43N, 33N to 40N, or 33N to 38N, and N _2% in the second direction may be 50N to 70N, 50N to 65N, 52N to 63N, or 57N to 63N.

Furthermore, the third direction of the protective film of the example, which is 45° with respect to the first direction, satisfies the following formula C.

Formula C: 0.5≤|S ₅ –S ₆ |≤7.2

In the above formula C, the above _S5 represents the final stretching rate (%) after continuing N1 _% for 20 minutes, and the above _S6 represents the final stretching rate (%) after continuing N2 _% for 20 minutes. In this case, N1 _% represents the load for stretching the above polyester film by 1% along the above third direction, and N2 _% represents the load for stretching the above polyester film by 2% along the above third direction.

The method for measuring the above _S5 and _S6 is the same as the method for measuring _S1 and _S2 , except that a third direction which is 45° with respect to the first direction is used instead of the first direction.

The value of the above formula C can be 0.5 to 7.2, 0.7 to 6.5, 0.7 to 5.8, 0.9 to 5, 0.9 to 4, 1.1 to 3.5, 1.1 to 2.8, 1.1 to 2.3, 1.2 to 1.8 or 1.2 to 1.6. When the above formula C is satisfied, when the above polyester film is applied as a protective film for a foldable display device, even if it is folded tens of thousands of times, whitening or cracking caused by the warping phenomenon hardly occurs.

The above _S5 may be 0.1 to 5.5, and the above _S6 may be 1.5 to 12.5. For example, the above _S5 may be 0.1 to 5.5, 0.1 to 5, 0.1 to 4.5, 0.2 to 4.3, 0.2 to 4, 0.5 to 3.3, 0.5 to 2.8, 0.7 to 2.3, 0.7 to 1.8, 0.9 to 1.6, or 0.9 to 1.3, and the above _S6 may be 1.5 to 12.5, 1.5 to 10, 1.5 to 8.5, 1.8 to 7, 1.8 to 6.5, 2 to 6, 2 to 5, 2 to 4, 2.2 to 3.3, 2.2 to 3, or 2.2 to 2.7. _S5 and _S6 satisfy the above ranges, so that when the above protective film is applied as a protective film for a foldable display device, even if it is folded tens of thousands of times, whitening or cracks caused by the warping phenomenon will hardly occur.

Furthermore, N _{1 %} in the third direction may be 10N to 25N, and N _2% in the third direction may be 28N to 50N. For example, N 1% in the third direction may be 10N to 25N, 28N to 45N, 30N to 43N, or 33N to 40N, and N _2% in the third direction may be 28N to 50N, 28N to 45N, 30N to 43N, or 33N to 40N.

The ratio of _S1 to _S3 may be 0.4:1 to 0.7:1, and the ratio of _S2 to _S4 may be 0.4:1 to 0.7:1. For example, the ratio of _S1 to _S3 may be 0.4:1 to 0.7:1, 0.45:1 to 0.65:1, or 0.45:1 to 0.6:1, and the ratio of S2 to _S4 may be 0.4: ₁ to 0.7:1, 0.45:1 to 0.65:1, or 0.45:1 to 0.6:1. The ratio of _S1 to _S3 and the ratio of _S2 to _S4 satisfy the above ranges, respectively, so that the effect of preventing warping can be improved, and therefore, even if folded tens of thousands of times, whitening or cracking caused by the warping phenomenon hardly occurs.

The ratio of _S3 to _S5 may be 1:0.4 to 1:0.7, and the ratio of _S4 to _S6 may be 0.4:1 to 0.7:1. For example, the ratio of _S3 to _S5 may be 0.4:1 to 0.7:1, 0.45:1 to 0.65:1, or 0.45:1 to 0.6:1, and the ratio of _S4 to _S6 may be 0.4:1 to 0.7:1, 0.45:1 to 0.65:1, or 0.45:1 to 0.6:1. The ratio of _S3 to _S5 and the ratio of _S4 to _S6 satisfy the above ranges, respectively, so that the effect of preventing warping can be improved. Therefore, even if folded tens of thousands of times, whitening or cracking caused by the warping phenomenon hardly occurs.

The ratio of S1 to _S5 may be 1:0.8 to 1:1.4, and the ratio of _S2 to _S6 may be ₁ :0.8 to 1:1.4. For example, the ratio of _S1 to _S5 may be 1:0.8 to 1:1.4, 1:0.85 to 1:1.3, 1:0.9 to 1:1.2, or 1:0.95 to 1:1.1, and the ratio of _S2 to _S6 may be 1:0.8 to 1:1.4, 1:0.85 to 1:1.3, 1:0.9 to 1:1.2, or 1:0.95 to 1:1.1. The ratio of _S1 to _S5 and the ratio of _S2 to _S6 satisfy the above ranges, respectively, so that the effect of preventing warping can be improved, and therefore, even if folded tens of thousands of times, whitening or cracking caused by the warping phenomenon hardly occurs.

The N _1% :N _2% in the first direction may be 1:1.5 to 1:3. For example, the N _1% :N _2% in the first direction may be 1:1.5 to 1:2.8, 1:1.5 to 1:2.3 or 1:1.6 to 1:2.1.

The N1 _% : _{N2 %} in the second direction may be 1:1.1 to 1:2.5. For example, the N1 _% :N2% in the second direction may be 1:1.1 to 1:2.5, 1:1.2 to 1:2.3, 1:1.3 to 1:2.1 or 1:1.5 to 1:2.

The N _1% :N _2% in the third direction may be 1:1.5 to 1:3. For example, the N _1% :N _2% in the third direction may be 1:1.5 to 1:2.8, 1:1.5 to 1:2.5, or 1:1.6 to 1:2.2.

When the ratios of N _1% and N _2% from the first direction to the third direction satisfy the above ranges, the effect of preventing warping can be improved. Therefore, even if the film is folded tens of thousands of times, whitening or cracking due to warping will hardly occur.

Method for preparing protective film

A method for preparing a protective film in another embodiment includes: preparing a polyester film; and forming a first curable resin layer on one side of the polyester film, wherein the polyester film comprises a first polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component; and a second polyester resin comprising more than 95 mol% of terephthalic acid as a dicarboxylic acid component, at least 70 mol% of ethylene glycol as a diol component, and at least 10 mol% of a _C3 to _C15 alcohol. The in-plane retardation of the polyester film is at least 3000 nm.

The above-mentioned polyester film and its preparation method are as described above.

Specifically, the step of forming the first curable resin layer includes the step of applying a primer composition on one side of the polyester film. Specifically, the primer composition may include a photocurable resin and a thermosetting resin, and may also include one or more selected from the group consisting of a crosslinking agent, an antistatic agent, and a defoaming agent.

The descriptions of the above-mentioned photocurable resin, thermosetting resin and additives are as described above.

The above-mentioned coating may be performed by a roll coating method, a gravure coating method, a spray coating method, etc., but is not limited thereto.

A method for preparing a protective film in another embodiment includes: the steps of preparing a first sheet and a second sheet by melt-extruding a third polyester resin and a fourth polyester resin, respectively; the steps of preparing a first substrate layer and a second substrate layer by stretching the first sheet and the second sheet by 1 to 1.5 times in a first direction and 3 to 5 times in a second direction perpendicular to the first direction at a temperature of 70°C to 125°C, respectively; the steps of applying a curable resin composition on one side of the first substrate layer to form a second curable resin layer; and the steps of laminating the second substrate layer on one side of the second curable resin layer, wherein the thickness of the first substrate layer is thinner than that of the second substrate layer. When the second curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface.

The protective film can be prepared by biaxial stretching with an adjusted stretching ratio and a process including heat treatment at a specific temperature. Specifically, in order to make the prepared protective film satisfy the above-mentioned stretching ratio, orientation angle relative to the full width and other characteristics, the melt extrusion temperature of the third polyester resin and the fourth polyester resin, the preheating temperature during stretching, the stretching ratio in each direction, the stretching temperature, the heat fixing temperature and the relaxation rate can be adjusted respectively.

The following is a more detailed description of the steps.

First, the third polyester resin and the fourth polyester resin are melt-extruded to prepare a first sheet and a second sheet. The first sheet and the second sheet may be unstretched sheets.

Specifically, the third polyester resin and the fourth polyester resin are melt-extruded at a temperature of 260° C. to 300° C. or 270° C. to 290° C., respectively, and then cooled to prepare the first sheet and the second sheet.

Then, the first sheet and the second sheet are transferred to pass through the rollers. In this case, the thickness of the film can be adjusted to a desired value by adjusting the speed and discharge amount of the first sheet and the second sheet.

Then, the first sheet and the second sheet are stretched 3 to 5 times in a first direction and 1 to 1.5 times in a second direction perpendicular to the first direction at a temperature of 70° C. to prepare a first substrate layer and a second substrate layer.

For example, the stretching may be performed at 70 to 125° C., 75 to 120° C., 80 to 110° C., 85 to 100° C., or 80 to 100° C. If the stretching temperature exceeds the above range, breakage may occur.

More specifically, the stretching temperature in the first direction may be 75 to 90° C. or 75 to 85° C., and the stretching temperature in the second direction may be 80 to 110° C. or 80 to 120° C. If the stretching temperature exceeds the above range, breakage may occur.

The stretching can be carried out along a first direction at a stretching ratio of 1 to 1.5 times or 1 to 1.45 times, and can be carried out along a second direction perpendicular to the first direction at a stretching ratio of 3 to 5 times, 3.3 to 4.8 times, 3.5 to 4.8 times, 4 to 4.8 times or 4.2 to 4.5 times.

The ratio of the stretching ratio in the first direction to the second direction may be 1:1.5 to 1:5.5. For example, the ratio of the stretching ratio in the first direction to the second direction may be 1:2 to 1:5, 1:2.5 to 1:4.5, or 1:3.5 to 1:4.5. The ratio of the stretching ratio in the first direction to the second direction satisfies the above range, thereby further improving durability and uniformity of curvature.

According to another example, a step of preheating the first sheet and the second sheet may be included before the stretching step.

Specifically, the first sheet and the second sheet may be preheated by being conveyed through the chamber at a speed of 10 to 110 m/min, 25 to 90 m/min, 40 to 80 m/min, or 50 to 60 m/min.

The preheating temperature ranges from Tg+5°C to Tg+50°C based on the glass transition temperature (Tg) of the third polyester resin and the fourth polyester resin, and can be set to 70°C to 90°C. The preheating temperature is within the above range, thereby ensuring the softness that is easy to stretch and effectively preventing the phenomenon of breakage during stretching.

According to another example, the coating process can be performed after the stretching. Specifically, the coating process can be performed before the stretching in the first direction, or before the stretching in the second direction after the stretching in the first direction. More specifically, a coating process can be performed to form a promotion layer that can impart antistatic and other functionalities on the film. The coating process can be performed by spin coating or in-line coating, but is not limited thereto.

Then, the first base layer and the second base layer are thermally fixed respectively.

Specifically, the thermal fixation may be annealing, which may be performed at a temperature of 165° C. to 210° C., 170° C. to 200° C., 170° C. to 190° C., or 175° C. to 185° C. for 0.5 to 8 minutes, 0.5 to 5 minutes, 0.5 to 3 minutes, or 1 to 2 minutes. After the thermal fixation is completed, the temperature may be gradually lowered.

According to yet another example, a relaxation step may be further included after the stretching step.

The relaxation may be performed in a first direction or in a second direction perpendicular to the first direction. Specifically, the relaxation may be performed at a temperature of 60°C to 180°C, 80°C to 150°C, 80°C to 120°C or 90°C to 110°C with a relaxation rate of 5% or less. For example, the relaxation rate may be 5% or less, 4% or less or 3% or less, and may be 0.1% to 5%, 0.5% to 4% or 1% to 3%.

Then, the second curable resin composition is applied on one side of the prepared first base layer to form a second curable resin layer.

Specifically, the step of forming the second curable resin layer includes applying a second curable resin composition to one side of the first substrate layer. Specifically, the second curable resin composition may include a photocurable resin or a thermosetting resin, and may also include one or more additives selected from the group consisting of a crosslinking agent, an antistatic agent, and a defoaming agent.

The descriptions of the above-mentioned photocurable resin, the above-mentioned thermosetting resin and the above-mentioned additives are as described above.

The above-mentioned coating may be performed by a roll coating method, a gravure coating method, a spray coating method, etc., but is not limited thereto.

Finally, the second base material layer is laminated on the second curable resin layer.

Display device

A display device of another embodiment includes: a display panel; and a protective film located on one side of the display panel. The protective film includes: a first substrate layer; a second substrate layer located on the first substrate layer; and a second curable resin layer located between the first substrate layer and the second substrate layer, wherein the first substrate layer is thinner than the second substrate layer, and when the curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface.

The description of the above-mentioned protective film is as described above.

Specifically, the protective film can be sized according to the conditions of the preparation process, so as to achieve the characteristics required as a protective film for a display device, especially a flexible display device. More specifically, when the protective film is applied to a curved flexible display device, it can be completely attached to the front of the curved surface without whitening or cracking caused by warping, while maintaining the characteristics of durability, transparency and visibility.

Mode for Carrying Out the Invention

The above contents will be described in more detail by way of examples. However, the following examples are only for illustrating the present invention, and the scope of the examples is not limited to the following examples.

Preparation of polyester film

Example 1-1

97 weight percent of a first polyester resin and 3 weight percent of a second polyester resin were mixed. The first polyester resin was a polyethylene terephthalate resin (manufacturer: SKC Corporation) obtained by mixing ethylene glycol and terephthalic acid at a molar ratio of 1:1, and the second polyester resin was prepared by mixing 77 mole percent of ethylene glycol, 18 mole percent of neopentyl glycol, and 5 mole percent of diethylene glycol as diol components and 100 mole percent of terephthalic acid as a dicarboxylic acid component, based on the total molar number of the diol components.

The mixture of the first polyester resin and the second polyester resin was melted at 270°C and extruded using a T-die, and then cooled using a casting roll at 35°C to prepare an unstretched sheet. The unstretched sheet was transferred at a speed of 16.5 m/min and preheated to 95°C, stretched 1.1 times in the MD direction at 85°C, stretched 4.3 times in the TD direction, and then heat-fixed at 200°C for 90 seconds. Then, the polyester film with a thickness of 80 μm was prepared by relaxing at 130°C at a relaxation rate of 2% in the TD direction.

Example 1-2

A polyester film was prepared in the same manner as in Example 1-1 except that 95 weight percent of the first polyester resin and 5 weight percent of the second polyester resin were mixed.

Comparative Example 1-1

A polyethylene terephthalate resin (manufacturer: SKC Corporation) prepared by melting ethylene glycol and terephthalic acid at a molar ratio of 1:1 at 280°C and extruding it using a T-die, was cooled using a casting roll at 35°C to prepare an unstretched sheet. The unstretched sheet was transferred at a speed of 13 m/min and preheated to 95°C, stretched 1.1 times in the MD direction and 4.3 times in the TD direction at 85°C, and then heat-fixed at 200°C for 90 seconds. Then, the film was relaxed at 130°C at a relaxation rate of 2% in the TD direction to prepare a polyester film having a thickness of 85 μm.

Experimental example

Experimental Example 1-1: Elongation and Composite Strength

The polyester films prepared in the above-mentioned Example 1-1, Example 1-2 and Comparative Example 1-1 were cut into pieces of 100 mm in length and 15 mm in width. According to ASTM D 882, a universal testing machine (4206-001, manufacturer: UTM) of INSTRON was used to set the spacing between the chucks to 50 mm and then conduct the experiment at a tensile speed of 100 mm/min. The tensile strength was measured by the built-in program of the equipment.

In addition, the polyester film prepared in the above-mentioned Example 1-1, Example 1-2 and Comparative Example 1-1 was cut into pieces with a length of 4 cm and a width of 1 cm, and the maximum deformation before rupture was measured at a speed of 50 mm/min using a universal testing machine (4206-001, manufacturer: UTM) from INSTRON. The ratio of the maximum deformation to the initial length was calculated as the elongation.

Then, the elongation composite strength was calculated using the above result values according to the following formulas 1 and 2.

Formula 1: ECT1 = EL1 × TS1

Formula 2: ECT2 = EL2 × TS2

In the above formulas 1 and 2, ECT1 is the first elongation composite strength (Kg/mm ² ), EL1 is the elongation in the TD direction (%), TS1 is the tensile strength in the TD direction (Kg/mm ² ), ECT2 is the second elongation composite strength (Kg/mm ² ), EL2 is the elongation in the MD direction (%), and TS2 is the tensile strength in the MD direction (Kg/mm ² ).

Experimental Example 1-2: Thermal Shrinkage

The polyester film prepared in the above-mentioned Example 1-1, Example 1-2 and Comparative Example 1-1 was cut into pieces of 300 mm in length and 15 mm in width, and immersed in a water tank preheated to 150°C for 30 minutes, or immersed in a water tank preheated to 85°C for 24 hours, and then the moisture was removed at room temperature, and the thermal shrinkage rate was calculated according to the following Formulas 3 and 4.

Formula 3:

Formula 4:

In the above formula 3 and formula 4, T _MD is the heat shrinkage rate in the MD direction (%), L _MD1 is the length of the initial film in the MD direction (mm), and L _MD2 is the length of the MD direction after heat shrinkage (mm). _{T TD} is the heat shrinkage rate in the TD direction (%), L _TD1 is the length of the initial film in the TD direction (mm), and L _TD2 is the length of the TD direction after heat shrinkage (mm).

Experimental Example 1-3: Haze

The haze of the polyester films prepared in Example 1-1, Example 1-2 and Comparative Example 1-1 was measured using a haze meter (model: SEP-H) manufactured by Nihon Semitsu Kogaku Co., Ltd. (Japan) using a C-light source.

Experimental Example 1-4: Light Transmittance

The light transmittance of the polyester films prepared in the above Example 1-1, Example 1-2 and Comparative Example 1-1 was measured using a spectrophotometer (UV2600, measuring wavelength: 380 nm) produced by Shimadzu Corporation.

Experimental Example 1-5: Orientation Angle

The full-width orientation angles of the polyester films prepared in Example 1-1, Example 1-2 and Comparative Example 1-1 were measured using a refractometer (RETS-100, measuring wavelength: 550 nm) manufactured by Shimadzu Corporation.

Experimental Example 1-6: In-plane phase difference

The in-plane phase differences of the polyester films prepared in the above-mentioned Example 1-1, Example 1-2 and Comparative Example 1-1 were measured.

Specifically, for the above-mentioned polyester film, the refractive index (Nx, Ny) of two perpendicular axes was measured using a refractometer (RETS-100, measuring wavelength: 550 nm) produced by Shimadzu Corporation, and the thickness d (nm) of the film was measured using an electric micrometer (Millitron 1245D, manufacturer: Pineloop Corporation) and the unit was converted to nm.

The in-plane retardation (Re) is calculated by multiplying the measured ΔNxy (=|Nx-Ny|) by the thickness d (nm) of the film according to the following mathematical formula A.

Mathematical formula A:

Re△Nxy×d

Experimental Example 7: Luminous Flux

The luminous flux of the polyester films prepared in the above Example 1-1, Example 1-2 and Comparative Example 1-1 was measured using a TES digital illuminometer (trade name: TES-1334A, manufacturer: TES Corporation).

Specifically, as shown in Fig. 3, an optical power meter 10 is disposed at the lower end, and a first polarizing plate 21 and a second polarizing plate 22 are placed in parallel and spaced apart from each other on the upper portion of the optical power meter. In this case, the distance between the optical power meter 10 and the first polarizing plate 21 is about 2.5 cm, and the distance between the optical power meter 10 and the second polarizing plate 22 is about 20 cm.

The optical power meter 10 supplies 530 nm light (a: direction of light) at a voltage of 12 V and allows the light to pass through to measure the brightness (lux) before and after the polyester film 100 is placed between the first polarizing plate 21 and the second polarizing plate 22, and then calculates the luminous flux according to the following formula 5.

Formula 5:

In the above formula 5, A is the brightness (lux) when 530nm light is passed through two parallel polarizing plates, and B is the brightness (lux) when 530nm light is passed through after the above polyester film is placed between the above two polarizing plates. In this case, the width direction (TD) of the above polyester film is at an angle of 45° relative to the optical axis b of the above two polarizing plates.

Experimental Example 8: Impact Strength

For the polyester films prepared in the above-mentioned Example 1-1, Example 1-2 and Comparative Example 1-1, three identical films were stacked and arranged using a Film Impact Tester (manufacturer: Toyoseiki Co., Ltd.). A pendulum tip with a diameter of 1 inch was used from a height of 30 cm above the upper surface of the arranged films to evaluate whether there was penetration.

X: The pendulum tip does not penetrate the film.

○: The pendulum tip penetrates the film.

Table 1

As shown in Table 1, the polyester films of Examples 1-1 and 1-2 showed excellent results in durability, transparency and visibility compared with the film of Comparative Example 1-1.

Specifically, the tensile strength, elongation, and heat shrinkage of the polyester films of Examples 1-1 and 1-2 all satisfy the preferred ranges, and the impact strength results are also excellent, so it can be seen that the durability is excellent.

Furthermore, the haze, light transmittance, orientation angle, in-plane phase difference, and luminous flux of the polyester films of Examples 1-1 and 1-2 all satisfy the preferred ranges, and therefore it is understood that the durability, transparency, and visibility are excellent.

Example

Example 2-1

A polyethylene terephthalate resin (manufacturer: SKC Corporation) prepared by mixing ethylene glycol and terephthalic acid at a molar ratio of 1:1 was extruded through an extruder at 280° C. and then cooled using a casting roll at 35° C. to prepare a first sheet and a second sheet.

The first sheet and the second sheet were stretched 1.1 times in the MD direction and 4.3 times in the TD direction at 95° C., and then heat-fixed at 180° C. for 90 seconds to prepare the first substrate layer and the second substrate layer. In this case, the thickness ratio of the first substrate layer and the second substrate layer was adjusted to 1:2.08 by adjusting the process time.

Then, a polyurethane acrylate resin ( A second curable resin layer having a thickness of 30 μm was formed by using a second curable resin composition of Henkel (manufacturer: Henkel Corporation). The second base material layer was laminated on the second curable resin layer to prepare a protective film.

Examples 2-2 to 2-4 and Comparative Examples 2-1 to 2-3

According to the process conditions described in Table 2 below, a protective film was prepared in the same manner as in Example 2-1 except that the protective film was prepared at the thickness ratio of the first substrate layer and the second substrate layer described in Table 2 below.

Table 2

Experimental example

Experimental Example 2-1: Thickness Deviation

The thickness (μm) of the protective films prepared in the above Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 was measured using a refractometer (RETS, measuring wavelength: 550 nm) manufactured by Shimadzu Corporation, and the relevant thickness deviation was calculated.

Experimental Example 2-2: In-plane phase difference and thickness direction phase difference

The in-plane phase difference and the thickness direction phase difference of the protective films prepared in the above-mentioned Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 were measured.

Specifically, for the above-mentioned protective film, the refractive index (Nx, Ny) of two perpendicular axes was measured using a refractometer (RETS-100, measurement wavelength: 550nm) produced by Shimadzu Corporation, and the thickness d (nm) of the film was measured using an electric micrometer (Millitron 1245D, manufacturer: Pineloop Corporation) and the unit was converted to nm.

According to the following mathematical formulas A and B, the in-plane phase difference (Re) is calculated by multiplying the measured △Nxy (＝|Nx-Ny|) by the thickness d (nm) of the film, and the average value of the values obtained by multiplying the measured △Nxz (＝|Nx-Nz|) and △Nyz (＝|Ny-Nz|) by the thickness d (nm) of the film is calculated as the thickness direction phase difference (Rth).

Mathematical formula A:

Re△Nxy×d

Mathematical formula B:

Experimental Example 2-3: Warping phenomenon

After the protective films prepared in Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 were attached to a curved display, it was checked whether bubbles were generated due to incomplete attachment or whether the film was warped.

○: Complete adhesion without any bubbles or the like.

X: Bubbles or partial lifting occurs.

Experimental Example 2-4: Curvature Radius and Bending Angle

After the protective films prepared in the above-mentioned Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 were attached to a curved display, the curvature radius and the bending angle of the curved portion of the protective film were measured with the winding axis as a reference.

Experimental Example 2-5: Orientation Angle

The orientation angles relative to the full width of the protective films prepared in Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 were measured using a refractometer (RETS-100, measuring wavelength: 550 nm) manufactured by Shimadzu Corporation.

Experimental Example 2-6: Luminous Flux

The luminous flux of the protective films prepared in the above-mentioned Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 was measured using an illuminometer (1334A) of TES Company.

Experimental Example 2-7: Light Transmittance

The light transmittance of the protective films prepared in the above Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 was measured using a spectrophotometer (UV2600, measuring wavelength: 380 nm) manufactured by Shimadzu Corporation.

Experimental Example 2-8: Moisture Permeability

The moisture permeability of the protective films prepared in the above Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-3 was measured using a moisture permeability tester (PERMATRAN_W) produced by Mocon.

Table 3

Table 4

As shown in Tables 3 and 4, the protective films of Examples 2-1 to 2-4 had no warping phenomenon and showed excellent results in durability, transparency and visibility compared with the protective films of Comparative Examples 2-1 to 2-3.

Specifically, the protective films of Examples 2-1 to 2-4 do not warp at all, so they can be tightly attached to display devices with curved shapes, which shows that they have excellent quality. In addition, the thickness deviation, phase difference, orientation angle, luminous flux, light transmittance and moisture permeability of the protective films of Examples 2-1 to 2-4 all meet the preferred ranges, so it can be seen that they have excellent durability, transparency and visibility.

Claims (5)

1. A protective film is characterized in that,

Comprising:

a first substrate layer;

A second substrate layer on the first substrate layer; and

A second curable resin layer interposed between the first substrate layer and the second substrate layer,

The thickness of the first base material layer is thinner than the thickness of the second base material layer,

The first base material layer and the second base material layer form curved surfaces when the curable resin layer is cured.

2. The protective film according to claim 1, wherein the first substrate layer comprises a third polyester resin and the second substrate layer comprises a fourth polyester resin.

3. The protective film according to claim 1, wherein,

The first substrate layer and the second substrate layer have a main stretching direction,

The main stretching direction of the first substrate layer corresponds to the main stretching direction of the second substrate layer,

The thickness ratio of the first substrate layer to the second substrate layer is 1:1.2 to 1:3.

4. The protective film according to claim 1, wherein,

The first substrate layer and the second substrate layer are curved to form a curved surface portion with respect to a winding axis,

The winding axis corresponds to the main stretching direction of the first substrate layer,

The curvature radius R of the curved surface portion is 5mm to 30mm,

The curved surface portion has a curved angle W of 5 ° to 45 °.

5.A preparation method of a protective film is characterized in that,

Comprising the following steps:

A step of preparing a first sheet and a second sheet by melt-extruding a third polyester resin and a fourth polyester resin, respectively;

A step of stretching the first sheet and the second sheet 1 to 1.5 times in a first direction and 3 to 5 times in a second direction perpendicular to the first direction at a temperature of 70 to 125 ℃ to prepare a first base material layer and a second base material layer, respectively;

a step of forming a second curable resin layer by applying a curable resin composition to one surface of the first base material layer; and

A step of laminating the second base material layer on one surface of the second curable resin layer,

The thickness of the first base material layer is thinner than the thickness of the second base material layer,

The first base material layer and the second base material layer form curved surfaces when the second curable resin layer is cured.