CN114591604A

CN114591604A - Polyester film, protective film and preparation method thereof

Info

Publication number: CN114591604A
Application number: CN202111460368.2A
Authority: CN
Inventors: 柳雅林; 李世喆; 李善基; 金建旭; 崔相珉; 朴珍奭; 刘仲原
Original assignee: SKC Co Ltd
Current assignee: Aisikai Mcwoo Co ltd
Priority date: 2020-12-07
Filing date: 2021-12-02
Publication date: 2022-06-07

Abstract

The present invention relates to a polyester film comprising a first polyester resin and a second polyester resin containing a diol and a dicarboxylic acid in specific components and amounts, which is excellent in mechanical properties such as tensile strength and impact strength, has an excellent fingerprint recognition rate, and can exhibit excellent characteristics when applied to display devices such as smart phones, tablet computers, and notebook computers, and a method for preparing the same. The present invention relates to a protective film having excellent scratch resistance, flexibility, durability, transparency, and visibility, and a method for manufacturing the same, wherein the protective film includes a curable resin layer interposed between a first substrate layer and a second substrate layer having a thickness smaller than that of the first substrate layer, and when the curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface portion, and when the protective film is applied to a display device having a curved surface shape, the protective film can be completely attached to the curved surface, and the protective film can exhibit excellent characteristics when applied to the curved surface shape.

Description

Polyester film, protective film and preparation method thereof

Technical Field

Examples relate to polyester films, protective films, and methods of making the same.

Background

Polyester films are excellent in dimensional stability and optical transparency, and are widely used not only as materials for display devices but also as various industrial materials. However, although the polyester film has excellent transparency, the durability and heat resistance thereof are lower than those of the polyimide film. Therefore, studies have been continuously conducted to improve durability and heat resistance without lowering dimensional stability and optical transparency of the polyester film.

Also, various display devices having a three-dimensional curved surface shape capable of realizing a luxurious aesthetic feeling or an excellent screen feeling have been recently used. In particular, in the case of portable mobile devices such as smartphones and tablet computers which are frequently used in daily life, there has been a product which has a three-dimensional curved surface shape in which the curvature of the side portion of the display panel is larger than that of the center portion.

In such a display having a three-dimensional curved surface shape, when a conventional protective film is attached, the protective film cannot be completely adhered to the curved surface portion due to the restoring force caused by the elasticity of the protective film itself, and the protective film is lifted. Therefore, a protective film having excellent scratch resistance, durability, transparency, and the like while being completely adhered to a curved surface portion is continuously studied.

On the other hand, with the standardization of business such as electronic commerce and internet banking using a display device such as a smartphone, a notebook computer, or a tablet computer, there is a continuing research for enhancing security by using information that can identify living bodies.

As one of the above-described methods using biological information, a fingerprint recognition method is widely used, and such a fingerprint recognition method is used by an optical method, an ultrasonic method, a capacitive method, an electric field measurement method, a thermal induction method, or the like. The optical fingerprint identification method utilizes the following principle: inside the device, Light is irradiated by a Light source such as a Light Emitting Diode (LED), and Light reflected by a fingerprint is sensed by a sensor. That is, since the method of obtaining the fingerprint image reflected by the light and comparing the fingerprint image with the previously registered fingerprint information, there is no distortion of the reflected light after sensing the fingerprint in order to improve the fingerprint recognition rate of the optical fingerprint recognition method.

However, although a protective film is attached to a display device such as a smartphone, a notebook computer, or a tablet computer to improve durability, the protective film causes light distortion, which causes a problem of lowering a fingerprint recognition rate. In particular, the thickness of the protective film may be varied according to the use and needs, and thus the reliability of the fingerprint recognition rate may be lowered as the thickness of the film is increased. Therefore, studies have been continued on how to improve the fingerprint recognition rate while achieving excellent durability and transparency.

As an example, korean laid-open patent No. 2020 and 0125466 disclose a protective film for improving fingerprint recognition rate by reducing in-plane retardation to 25nm or less, but in order to reduce the retardation to such a degree, a highly controlled stretching process is required, and thus the process cost of the film is increased, thereby reducing productivity.

As another example, korean laid-open patent publication No. 2014-0013155 discloses a protective cover having a curved surface portion formed by a plurality of printed layers, but the protective cover requires a plurality of printed layers, thus complicating the manufacturing process, and has difficulty in accurately coping with various products having curved surfaces with different shapes because the curved surface portion is formed according to the number of printed layers.

Documents of the prior art

Patent document

Patent document 1: korean laid-open patent No. 2020-0125466

Patent document 2: korean laid-open patent No. 2014-0013155.

Disclosure of Invention

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

Also, examples provide a protective film which is completely in contact with a curved surface portion and is excellent in scratch resistance, flexibility, durability, transparency, and visibility, and a method of preparing the same.

An example polyester film comprises: a first polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component and greater than 95 mole percent ethylene glycol as a diol component; and a second polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component, at least 70 mole percent ethylene glycol as a diol component, and at least 10 mole percent C₃To C₁₅The alcohol of (1). The polyester film has an in-plane retardation of 3000nm or more.

A method of preparing a polyester film according to yet another embodiment includes: a step of preparing a mixture of a first polyester resin and a second polyester resin; a step of melt-extruding the mixture to prepare an unstretched sheet; stretching the unstretched sheet at a temperature of 70 ℃ to 125 ℃ by 1 to 1.5 times in a first direction and in a second direction perpendicular to the first directionA step of preparing a stretched film by 3 to 5 times; and a step of heat-fixing the stretched film at a temperature of 160 ℃ to 230 ℃ to produce a polyester film, the first polyester resin containing more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component, the second polyester resin containing 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 C₃To C₁₅The alcohol of (3), wherein the in-plane retardation of the polyester film is 3000nm or more.

Another example protective film comprises: a polyester film; and a first curable resin layer on one surface of the polyester film, the polyester film including: a first polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component and greater than 95 mole percent ethylene glycol as a diol component; and a second polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component, at least 70 mole percent ethylene glycol as a diol component, and at least 10 mole percent C₃To C₁₅The alcohol of (3), wherein the in-plane retardation of the polyester film is 3000nm or more.

A further example of a protective film comprises: a first base material layer; a second base material layer positioned on the first base material layer; and a second curable resin layer interposed between the first substrate layer and the second substrate layer, wherein the first substrate layer has a thickness smaller than that of the second substrate layer, and the first substrate layer and the second substrate layer form a curved surface portion when the curable resin layer is cured.

A method of preparing a protective film of another embodiment includes: 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 preparing a first base material layer and a second base material layer by stretching the first sheet and the second sheet in a first direction by 1 to 1.5 times and in a second direction perpendicular to the first direction by 3 to 5 times at a temperature of 70 to 125 ℃, 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, wherein the thickness of the first base material layer is smaller than that of the second base material layer, and the first base material layer and the second base material layer form a curved surface portion when the second curable resin layer is cured.

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

The polyester film of the example has an orientation angle, haze, light transmittance, orientation angle, in-plane retardation, and luminous flux in preferred ranges, and ensures excellent visibility as well as excellent transparency and durability.

Further, since the protective film of the example includes the second curable resin layer interposed between the first substrate layer and the second substrate layer thinner than the first substrate layer, the first substrate layer and the second substrate layer form a curved surface portion when the second curable resin layer is cured, and are completely attached to the front surface of the curved display device, and the lift-up phenomenon and whitening or cracking due to the lift-up phenomenon are hardly generated.

Also, the orientation angle, luminous flux, light transmittance and moisture permeability of the protective film of the example are all in the preferred ranges, and excellent visibility, transparency and durability can be secured.

Drawings

Fig. 1 shows an example of a protective film.

Fig. 2 shows a further example of a protective film.

Fig. 3 shows a method of measuring the light flux of the polyester film.

Fig. 4 shows an example display device.

Fig. 5 is a sectional view showing the display device of fig. 3 cut along X-X'.

Fig. 6 is a sectional view illustrating an in-folding type flexible display device.

Fig. 7 is a sectional view showing an out-folding type flexible display device.

Fig. 8 shows a protective film of another example.

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

Description of reference numerals

1: display device

2: inward folding type flexible display device

3: eversion type flexible display device

10: optical power meter

21: a first polarizing plate

22: the second polarizing plate

30: illuminometer

a: curved surface part

b: plane part

c: direction of light

d: optical axis

R: radius of curvature

W: bending angle

Q: winding shaft

100: polyester film

200: protective film

210: first base material layer

220: second base material layer

231: a 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

Hereinafter, the present invention is described in detail by way of examples. The examples are not limited to the following disclosure, and may be modified into various forms without changing the gist of the present invention.

In the present specification, when a certain structural element is referred to as being "included", unless otherwise specified, it is not intended to exclude other structural elements, and other structural elements may be included.

All numbers and expressions referring to amounts of structural components, reaction conditions, and the like described in the specification are to be understood as modified by the term "about" unless otherwise specified.

In the present specification, terms such as "first", "second", and the like are used to describe various components, and the components are not limited to the terms. The above terms are used for the purpose of distinguishing one structural element from another.

In the present specification, when each thin film, layer, or the like is described as being formed on "upper (on)" or "lower (under)" of each film, layer, or the like, "upper (on)" or "lower (under)" includes all cases where "direct" or "other structural elements are formed with (indirect) intervening therebetween".

Polyester film

The first polyester resin comprises greater than 95 mole percent terephthalic acid as a dicarboxylic acid component and greater than 95 mole percent ethylene glycol as a diol component.

Specifically, the first polyester resin may contain more than 95 mol% of terephthalic acid, 95 mol% or more of terephthalic acid, 97 mol% or more of terephthalic acid, 98 mol% or more of terephthalic acid, 99 mol% or more of terephthalic acid, or 100 mol% of terephthalic acid, based on the total number of moles of the dicarboxylic acid component.

The first polyester resin may contain more than 95 mol% of ethylene glycol, 95 mol% or more of ethylene glycol, 97 mol% or more of ethylene glycol, 98 mol% or more of ethylene glycol, 99 mol% or more of ethylene glycol, or 100 mol% of ethylene glycol, based on the total number of moles of the glycol component.

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

When the content of the diol component and the dicarboxylic acid component in the first polyester resin satisfies the above range, the transparency can be improved.

And the second polyester resin comprises more than 95 mole percent of terephthalic acid as a dicarboxylic acid component, at least 70 mole percent of ethylene glycol as a diol component, and at least 10 mole percent of C₃To C₁₅The alcohol of (1).

Specifically, the second polyester resin may contain more than 95 mole percent of terephthalic acid, 95 mole percent or more of terephthalic acid, 97 mole percent or more of terephthalic acid, 98 mole percent or more of terephthalic acid, 99 mole percent or more of terephthalic acid, or 100 mole percent of terephthalic acid, based on the total moles of the dicarboxylic acid component.

The second polyester resin may contain 70 mol% or more of ethylene glycol, 73 mol% or more of ethylene glycol, 75 mol% or more of ethylene glycol, or 76 mol% or more of ethylene glycol, 70 mol% to 90 mol% of ethylene glycol, 73 mol% to 88 mol% of ethylene glycol, 75 mol% to 85 mol% of ethylene glycol, or 76 mol% to 80 mol% of ethylene glycol, based on the total number of moles of the diol component.

The second polyester resin may contain 10 mol% or more of C based on the total number of moles of the diol component₃To C₁₅Alcohol of (2), C of 13 mol% or more₃To C₁₅Alcohol, C of 15 mol% or more₃To C₁₅16 mol% or more or 18 mol% or more of C₃To C₁₅May contain 10 to 30 mole percent of C₃To C₁₅13 to 28 mole percent of C₃To C₁₅15 to 25 mole percent of C₃To C₁₅15 to 23 mole percent or 16 to 20 mole percent of C₃To C₁₅The alcohol of (1).

C above₃To C₁₅The alcohol of (c) may comprise cyclohexanedimethanol or neopentyl glycol.

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

The second polyester resin contains the diol component and the dicarboxylic acid component in such amounts that the diol component and the dicarboxylic acid component satisfy the above ranges, and thus mechanical properties such as tensile strength and impact strength can be improved, thereby having excellent durability.

Also, the mylar of the example may include 0.5 weight percent to 20 weight percent of the second polyester resin described above. Specifically, the mylar may include 0.5 to 20 weight percent of the second mylar resin, 0.5 to 18 weight percent of the second mylar resin, 0.7 to 15 weight percent of the second mylar resin, 0.7 to 13 weight percent of the second mylar resin, 1 to 10 weight percent of the second mylar resin, 1.5 to 8 weight percent, or 2 to 6 weight percent of the second mylar resin, based on the total weight of the mylar.

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

The polyester film may have a first elongation composite strength (ECT1) of 17kgf/mm, which is represented by formula 1 below²To 30kgf/mm²。

Formula 1: ECT 1-EL 1 XTS 1

In the above formula 1, ECT1 is the first elongation composite Strength (Kg/mm)²) EL1 is the elongation (%) in the first direction, TS1 is the tensile strength (Kg/mm) in the first direction²)。

For example, the first elongation composite strength may be 17kgf/mm²To 30kgf/mm²、18kgf/mm²To 27kgf/mm²、18kgf/mm²To 25kgf/mm²Or 20kgf/mm²To 23kgf/mm². The first elongation composite strength satisfies the above range, and mechanical properties such as tensile strength and impact strength can be improved, thereby having excellent durability.

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

The polyester film may have a second elongation composite strength (ECT2) of 0.95kgf/mm, represented by the following formula 2²To 2kgf/mm²。

Formula 2: ECT 2-EL 2 XTS 2

In the above formula 2, ECT2 represents the second elongation composite strength (Kg/mm)²) EL2 is the elongation (%) in the second direction perpendicular to the first direction, TS2 is the tensile strength (Kg/mm) in the second direction perpendicular to the first direction²)。

For example, the second elongation composite strength may be 0.95kgf/mm²To 2kgf/mm²、0.95kgf/mm²To 1.8kgf/mm²、1kgf/mm²To 1.6kgf/mm²、1kgf/mm²To 1.5kgf/mm²Or 1.05kgf/mm²To 1.3kgf/mm². The second elongation composite strength satisfies the above range, and mechanical properties such as tensile strength and impact strength can be improved, thereby having excellent durability.

And, 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 and second composite strengths satisfies the above range, so that mechanical properties such as tensile strength and impact strength can be maximally improved without deteriorating 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%, 10% or less, 8% or less, 6% or less, 4% or less, 3% or less, or 2.5% or less.

The heat shrinkage rate of the mylar of the example in the first direction may be 15% or less when heat treated at a temperature of 150 c for 30 minutes. For example, the heat shrinkage rate of the mylar film in the first direction may be 15% or less, 13% or less, 11% or less, 10% or less, or 9% or less when heat-treated at a temperature of 150 ℃ for 30 minutes.

And, when heat-treated at a temperature of 150 ℃ for 30 minutes, the heat shrinkage rate of the polyester film in a second direction perpendicular to the first direction may be 10% or less. For example, the heat shrinkage rate of the polyester film in a 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 a temperature of 150 ℃ for 30 minutes.

The heat shrinkage rate of the mylar of the example in the first direction may be 5% or less when heat treated at a temperature of 85 c for 24 hours. For example, the mylar film may have a heat shrinkage in the first direction of 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 when heat-treated at a temperature of 85 ℃ for 24 hours.

And, when heat-treated at a temperature of 85 ℃ for 24 hours, the mylar film may have a thermal shrinkage rate of 3% or less in a second direction perpendicular to the first direction. For example, the mylar film may have a heat shrinkage rate in a second direction perpendicular to the first direction of 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 when heat-treated at a temperature of 85 ℃ for 24 hours.

The thermal shrinkage can be calculated by the following formulas 3 and 4.

Formula 3:

formula 4:

in the above formulas 3 and 4, T_MDHeat shrinkage (%) in MD, L_MD1Is the MD-directional length (mm) of the original film, L_MD2The length (mm) in the MD direction after heat shrinkage. T is_TDHeat shrinkage (%) in TD direction, L_TD1Length in TD direction (mm), L, of the starting film_TD2The length (mm) in the TD direction after heat shrinkage.

On the other hand, in the optical fingerprint recognition method, in order to increase the fingerprint recognition rate of the optical fingerprint recognition method, the reflected light cannot be distorted after the fingerprint is recognized, as a method for comparing the fingerprint image reflected by the light with the previously registered fingerprint information. Therefore, the lower the orientation angle and the deviation of the orientation angle of the protective film attached to the surface of the display device such as a smartphone are, the higher the fingerprint recognition rate and the effect of preventing a fingerprint recognition error can be obtained.

The polyester film of the example is excellent not only in durability and transparency but also in fingerprint recognition rate and in preventing erroneous fingerprint recognition.

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

The deviation of the orientation angle of the polyester film with respect to the full width may be within ± 5 °. Specifically, the deviation of the orientation angle based on the average value of the orientation angle measured for the full width of the above-mentioned 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 deviation of the orientation angle with respect to the full width satisfies the above range, so that the film can secure excellent visibility regardless of the position, and thus reliability of visibility is excellent.

The deviation of the orientation angle in the width direction from the central axis of the polyester film to within. + -. 2000mm may be within. + -. 2.5 ℃. For example, the deviation of the orientation angle from the width direction within + -2000 mm from the central axis of the polyester film may be within + -2.5 DEG, within + -2 DEG, within + -1.5 DEG, within + -1.2 DEG, within + -1 DEG, within + -0.9 DEG or within + -0.7 deg.

Further, the deviation of the orientation angle from the central axis of the polyester film to the width direction of more than. + -. 2000mm may be within. + -. 5 ℃. For example, the deviation of the orientation angle from the central axis of the polyester film to a width direction of. + -. 2000mm or more may be within. + -. 5 DEG, within. + -. 4.5 DEG, within. + -. 4 DEG, within. + -. 3.5 DEG, within. + -. 3 DEG, within. + -. 2.8 DEG, within. + -. 2.5 DEG, within. + -. 2 DEG, within. + -. 1.5 DEG, within. + -. 1.2 DEG, within. + -. 1 DEG, within. + -. 0.9 DEG or within. + -. 0.7 deg.

Orientation angle (. theta.) of any point of the above polyester film₁) Orientation angle (theta) to a point located within + -2000 mm from any of the above points₂) Difference of (theta)₁-θ₂) Can be within + -5 deg. For example, the orientation angle (. theta.) at any point of the above polyester film₁) An orientation angle (θ) with a point located within ± 2000mm, ± 1800mm, ± 1500mm, ± 1300mm, ± 1000mm, ± 800mm, ± 500mm, ± 300mm, ± 100mm or ± 50mm from any of the above points₂) Difference of (theta)₁-θ₂) May be within + -5 DEG, within + -4.5 DEG, within + -4 DEG, within + -3.5 DEG, within + -3 DEG, within + -2.8 DEG, within + -2.5 DEG, within + -2 DEG, within + -1.5 DEG, within + -1.2 DEG, within + -1 DEG, within + -0.9 DEG, within + -0.7 DEG, within + -0.5 DEG, within + -0.4 DEG, within + -0.2 DEG, within + -0.1 DEG or within + -0.05 deg.

And the total width of the polyester film is 50cm to 6000 cm. For example, the total width of the above polyester film may be 50cm to 6000cm, 50cm to 5500cm, 50cm to 5000cm, 50cm to 4000cm, 50cm to 3000cm, 50cm to 2500cm, 50cm to 2300cm, 50cm to 2000cm, 50cm to 1800cm, 50cm to 1500cm, 50cm to 1300cm, 50cm to 1000cm, 50cm to 800cm, 70cm to 800cm, or 90cm 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 ° with respect to the width direction. The orientation angle of the polyester film satisfies the above range, so that visibility and reliability thereof can be improved, thereby making fingerprint recognition rate and fingerprint recognition error prevention effects excellent.

Even though the mylar film of the example has a wide width of 50cm to 6000cm, the orientation angle satisfies within ± 5 ° with respect to the width direction in 90% or more of the total width, thereby having an excellent fingerprint recognition rate and an effect of preventing a fingerprint recognition error.

The polyester film has an in-plane retardation (Re, 550nm) of at least 3000 nm. For example, the above polyester film may have an in-plane retardation (Re) of 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, 5500 to 11500nm, 6000nm to 11000nm, 7000nm to 10000nm, 7500 to 9500nm, or 7500 to 9000 nm. The in-plane phase difference satisfies the above range, so that not only durability can be improved, but also a difference in refractive index between a first direction and a second direction perpendicular to the first direction can be maximized to avoid sensing distortion of light, and thus excellent visibility can be ensured.

Specifically, the in-plane retardation (Re) is a parameter defined as a product of anisotropy (Δ Nxy ═ Nx-Ny |) of refractive indices (Nx, Ny) of biaxial perpendicular intersection in a plane of the film and the thickness d (nm) of the film, and is a measure representing optical isotropy or anisotropy. More specifically, the in-plane retardation (Re) can be calculated by the equation a.

The mathematical formula A:

Re＝ΔN×y×d

in the above formula a, d is the thickness of the thin film, Δ Nxy is the absolute value of the difference between Nx and Ny (Δ Nxy | -Nx-Ny |), the Nx is the refractive index in the in-plane slow axis direction, and the Ny is the refractive index in the in-plane fast axis direction. Specifically, Nx may be a refractive index in a longitudinal direction (MD), and Ny may be a refractive index in a width direction (TD).

The biaxial refractive indices (Nx, Ny) can be measured using a refractometer (RETS-100, measurement wavelength 550nm) from tsukamur corporation, but are not limited thereto.

Further, the variation of the in-plane retardation (Re) of the polyester film may be 600nm/m or less. For example, the above polyester film may have a variation in-plane retardation (Re) of 600nm/m or less, 500nm/m or less, 400nm/m or less, 300nm/m or less or 200nm/m or less, and may 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 200 nm/m. The in-plane phase difference deviation satisfies the above range, so that not only durability can be improved, but also a difference in refractive index between a first direction and a second direction perpendicular to the first direction can be maximized to avoid sensing distortion of light, and thus excellent visibility can be secured.

On the other hand, the retardation in the thickness direction (Rth, 550nm) is calculated as follows: the average value of values obtained by multiplying Δ Nxz (═ Nx-Nz |) and Δ Nyz (═ Ny-Nz |) of the two birefringence values as viewed in a cross section in the film thickness direction by the film thickness d (nm) is obtained. Specifically, the retardation in the thickness direction (Rth) can be calculated by the following formula B.

The mathematical formula B:

d is the thickness of the film, Δ Nxz is the absolute value of the difference between Nx and Nz (Δ Nxz | -Nx-Nz |), and Δ Nyz is the absolute value of the difference between Ny and Nz (Δ Nyz | -Ny-Nz |). Nx is a refractive index in an in-plane slow axis direction, and Ny is a refractive index in an in-plane fast axis direction. Specifically, Nx may be a refractive index in a longitudinal direction (MD), and Ny may be a refractive index in a width direction (TD).

The retardation (Rth) in the thickness direction of the polyester film may be 8000nm to 14000 nm. For example, the above polyester film may have a retardation in the thickness direction (Rth) of 8000nm to 14000nm, 8000nm to 13500nm, 8500nm to 13000nm, or 8500nm to 12800nm at a wavelength of 550 nm. The phase difference in the thickness direction satisfies the above range, so that not only durability can be improved, but also a difference in refractive index between the first direction and the second direction perpendicular to the first direction can be maximized to avoid sensing distortion of light, and thus excellent visibility can be ensured.

And, the transmittance of the polyester film to a wavelength of 380nm may be more than 80%. For example, the above polyester film may have a light transmittance of more than 85%, 85% or more, 90% or more, 80% to 99%, 80% to 95%, 85% to 95%, or 85% to 92% at a wavelength of 380 nm. The light transmittance for a wavelength of 380nm satisfies the above range, so that excellent transparency can be secured.

The polyester film may have a luminous flux of 90% or more according to the following formula 5.

Formula 5:

in the above formula 5, a is the luminance (lux) when 530nm light is transmitted through two polarizers placed in parallel, and B is the luminance (lux) when 530nm light is transmitted after the polyester film is placed between the two polarizers, in which case the width direction (TD) of the polyester film is at an angle of 45 ° with respect to the optical axes d of the two polarizers.

The optical fingerprint recognition method is to irradiate light with a light source such as a light emitting diode inside the device and to compare the light reflected by the fingerprint sensed by an image sensor with previously registered fingerprint information. Therefore, the amount of light irradiated and reflected by the apparatus can be made sufficiently large and distortion of the irradiated and reflected light can be avoided to improve the fingerprint recognition rate.

The light flux of the mylar film of the example according to the above formula 5 satisfies 90% or more, so that the amount of light irradiated and reflected can be sufficiently secured, thus making the visibility excellent. Therefore, when the polyester film is used as a protective film for a display device such as a smart phone, a tablet computer, a notebook computer, or the like, and an optical sensor such as a barcode reader, the polyester film has excellent visibility, and thus can improve product information such as a barcode and fingerprint recognition rate.

For example, the light flux according to formula 5 of the polyester film may be 90% or more, 90.5% or more, 91% or more, 91.2% or more, 91.5% or more, 92% or more, or 92.5% or more, and may be 90% to 99%, 90.5% to 98%, 91% to 96%, 91% to 95%, 91% to 93%, 91.2% to 93%, or 92% to 93%. The luminous flux satisfies the above range according to equation 5, so that the amount of light irradiated and reflected by the thin film can be sufficiently secured, and thus the visibility can be improved.

The above-mentioned luminous flux can be measured with a luminometer. For example, the illuminometer may be as follows: after two polarizing plates were placed in parallel at a specific interval and a polyester film was placed between the two polarizing plates, light was supplied and transmitted to measure the brightness of light.

Fig. 3 shows a method of measuring the light flux of the polyester film.

Specifically, as shown in fig. 3, the optical power meter 10 is disposed at the lower end, and the first polarizing plate 21 and the second polarizing plate 22 are disposed in parallel at an interval above 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 1cm to 10cm, 1.2cm to 8cm, 1.4cm to 6.5cm, 1.5cm to 6cm, 1.8cm to 5.5cm, or 2cm to 5cm, and the distance between the optical power meter 10 and the second polarizing plate 22 may be 5cm to 30cm, 7cm to 28cm, 8cm to 25cm, 9cm to 23cm, or 10cm to 20 cm.

Then, the light power meter 10 was used to supply light of 530nm (c: direction of light) at a voltage of 12V and to pass the light, and then the luminance (lux) before and after the polyester film 100 was placed between the first polarizing plate 21 and the second polarizing plate 22 was measured, and the luminous flux was calculated according to the above equation 5.

The polyester film may be placed at an angle of 180 ° or less, and in this specification, the light flux is measured when the width direction (TD) of the film forms an angle of 45 ° with the optical axes b of the two polarizing plates.

The thickness of the polyester film may be 30 to 150 μm. For example, the thickness of the above 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 demand for improvement in moldability or durability. Specifically, if the thickness of the polyester film is less than 30 μm, the polyester film may have excellent moldability but low durability, and if it exceeds 150 μm, the polyester film may have excellent durability but low moldability, thereby causing a problem of poor quality when used as a protective film.

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

The thickness variation of the polyester film may be 5 μm or less. For example, the thickness variation of the above 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 it is possible to have uniform visibility with an appropriate phase difference deviation.

The difference (D1-D2) between the thickness (D1) of any point of the polyester film and the thickness (D2) of a point located within + -2000 mm of the above-mentioned any point may be within + -4 μm. For example, the difference (D1-D2) between the thickness (D1) of any point of the polyester film and the thickness (D2) of 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 of the point may be within + -4 μm, + -3.5 μm, + -3 μm, + -2.5 μm, + -2.3 μm, + -2 μm, + -1.8 μm, + -1 μm or + -0.8 μm.

The polyester film may have a moisture permeability of 20g/m²Day or less. For example, the moisture permeability of the polyester film may be 20g/m²Under day, 18g/m²Day, below, 15g/m²Day or less, 12g/m²Day or less or 10g/m²May be 0.1g/m or less than day²Day to 20g/m².day、0.5g/m²Day to 18g/m².day、1g/m²Day to 15g/m².day、3g/m²Day to 13g/m².day、4g/m²Day to 11g/m².day、4.5g/m²Day to 10g/m²Day or 4.8g/m²Day to 10g/m²Day. The moisture permeability satisfies the above range, so that excellent durability can be secured. Specifically, the polyester film having a moisture permeability in the above range has significantly excellent moisture permeability characteristics as compared with a triacetyl cellulose (TAC) film conventionally used as a protective film, and when the polyester film is applied to a protective film for a display device, the display device can be effectively protected from an external moisture environment.

The difference between the refractive index of the polyester film in a first direction and the refractive index of the polyester film in a second direction perpendicular to the first direction may be 0.08 to 0.14. For example, the difference in refractive index between the first direction of the polyester film and the second direction perpendicular to the first direction 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, so that distortion of the sensing side to light can be avoided, and thus excellent visibility can be ensured.

Ultraviolet light durability (TSM) in the longitudinal direction (MD) of the polyester film according to the following formula C_UV) Is more than 80%.

The mathematical formula C:

in the above formula C, TSM_UVUV durability in MD (%), TSM1 tensile Strength in initial MD, TSM2 Exposure to Power of 0.68W/m²Tensile strength in the MD direction measured after 48 hours in ultraviolet light.

For example, the ultraviolet light durability (TSM) according to the above formula C_UV) May be 80% or more or 82% or more, and may be 80% to 100% or 80% to 95%.

Or, ultraviolet light durability (TST) in a width direction (TD) according to the following equation D of the polyester film_UV) Is more than 80%.

The mathematical formula D:

in the above equation D, TST_UVFor UV durability (%) in TD, TST1 is tensile strength in the original TD, TST2 is exposure to power of 0.68W/m²Tensile strength in TD after 48 hours in ultraviolet light.

For example, ultraviolet light durability (TST) according to the above equation D_UV) 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, and the polyester film is a stretched film, and thus, the ultraviolet durability can be different in different directions.

MD ultraviolet light durability (TSM) of the example mylar film_UV) And TD Direction ultraviolet light durability (TST)_UV) All of which satisfy 80% or more, so that excellent durability can be maintained even in repeated strong ultraviolet light.

Method for preparing polyester film

A method of preparing a polyester film according to yet another embodiment includes: a step of preparing a mixture of a first polyester resin and a second polyester resin; a step of melt-extruding the mixture to prepare an unstretched sheet; a step of stretching the above unstretched sheet at a temperature of 70 ℃ to 125 ℃ by 1 to 1.5 times in a first direction and 3 to 5 times in a second direction perpendicular to the above first direction to prepare a stretched film; and a step of heat-fixing the stretched film at a temperature of 160 ℃ to 230 ℃ to produce a polyester film, the first polyester resin containing more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a diol component, the second polyester resin containing 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 C₃To C₁₅Alcohol of (2) toThe polyester film has an in-plane retardation of 3000nm or more.

The polyester film prepared above has substantially the same structure and characteristics as those of the polyester film of the above example.

The polyester film produced by the above method satisfies the above-described characteristics (orientation angle, retardation, etc.) by adjusting the combination and process conditions. Specifically, in order to satisfy the above-described characteristics of the final polyester film, the combination of the polyester resins may be adjusted, and the extrusion temperature, the preheating temperature at the time of stretching, the stretching ratios in different directions, the stretching speed, and the like may be adjusted, or the heat treatment temperature and the relaxation rate may be adjusted at the time of heat treatment and relaxation after stretching.

Hereinafter, each step will be described more specifically.

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

The first polyester resin and the second polyester resin are explained in the same manner as described above.

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

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

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

Then, the above-mentioned unstretched sheet is stretched at a temperature of 70 ℃ to 125 ℃.

According to another example, a step of preheating the non-stretched sheet may be further included before the above-described stretching step.

The ranges of the preheating temperatures described above may be set to satisfy the ranges of Tg +5 ℃ to Tg +50 ℃ based on the glass transition temperature (Tg) of the polyester resin, respectively, and at the same time, may be set to satisfy the ranges of 70 ℃ to 90 ℃. The preheating temperature satisfies the above range, so that the phenomenon of breakage during stretching can be effectively prevented while flexibility for easy stretching is ensured.

On the other hand, the above stretching may be performed at a temperature of 70 ℃ to 125 ℃, 75 ℃ to 120 ℃, 80 ℃ to 110 ℃, 85 ℃ to 100 ℃, or 80 ℃ to 100 ℃. 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 ℃ or 75 to 85 ℃, and the stretching temperature in the second direction may be 80 to 110 ℃ or 80 to 120 ℃. If the stretching temperature is outside the above range, breakage may occur.

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

The above stretching may be performed at a stretching ratio of 1 to 1.5 times or 1 to 1.45 times in a first direction, and may be performed 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 in a second direction perpendicular to the above first direction.

The ratio of the first direction to the second direction may be 1: 1.5 to 1: 5.5. For example, the ratio of the stretch ratio in the first direction to the stretch ratio in 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 stretch ratios in the first direction and the second direction satisfies the above range, and the durability and the uniformity of curvature can be further improved.

Further, the coating step may be performed after the stretching. Specifically, the coating step may 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 step of forming a promoting layer capable of providing a function such as static electricity prevention on the thin film may be performed. The coating step may be performed by spin coating or in-line coating, but is not limited thereto.

Thereafter, the above stretched film is heat-set at a temperature of 160 ℃ to 230 ℃ to prepare a polyester film.

Specifically, the above heat-fixing may be annealing, and may be performed at a temperature of 165 ℃ to 210 ℃, 170 ℃ to 200 ℃, 170 ℃ to 190 ℃, or 175 ℃ to 185 ℃ for 0.5 minutes to 8 minutes, 0.5 minutes to 5 minutes, 0.5 minutes to 3 minutes, or 1 minute to 2 minutes. After the thermal fixing is completed, the temperature can be gradually lowered.

A relaxation step may also be included after the stretching step.

The relaxation may be performed in a first direction or a second direction perpendicular to the first direction. Specifically, the above relaxation may be performed at a relaxation rate of 5% or less at a temperature of 60 ℃ to 180 ℃, 80 ℃ to 150 ℃, 80 ℃ to 120 ℃, or 90 ℃ to 110 ℃. For example, the above 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 further example of a protective film comprises: a polyester film; and a first curable resin layer on one surface of the polyester film, the polyester film including: a first polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component and greater than 95 mole percent ethylene glycol as a diol component; and a second polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component, at least 70 mole percent ethylene glycol as a diol component, and at least 10 mole percent C₃To C₁₅The alcohol of (3), wherein the in-plane retardation of the polyester film is at least 3000 nm.

The above polyester film is described in the above.

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

Fig. 1 shows an example of a protective film. Specifically, fig. 1 illustrates a protective film 200 including a polyester film 100 and a first curable resin layer 231 on one surface of the polyester film 100.

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

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 an antifoaming agent. For example, the crosslinking agent may be a silane-based crosslinking agent, and may be an alkoxysilane such as vinylethoxysilane, vinyl-tris- (β -methoxyethoxy) silane, methacryloxypropyltrimethoxysilane, γ -aminopropyltriethoxysilane, γ -mercaptopropyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, or trimethoxysilane; epoxy silanes such as triepoxy silane; aminosilanes such as butylaminosilane and epoxy-aminosilane; and alkylsilanes, silanes or disilanes such as methylsilane, dimethylsilane, vinylmethylcyclotrisiloxane, dimethylsilane-oxocyclopentane, cyclohexylsilane and cyclohexyldisilane, but not limited thereto.

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

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

Fig. 2 shows a further example of a protective film. Specifically, the protective film 200 illustrated in fig. 2 includes a polyester film 100, a first curable resin layer 231 on one surface of the polyester film, a hard coat layer 240 on one surface of the first curable resin layer, an adhesive layer 250 on the other surface of the polyester film, and a release layer 260 on one surface of the adhesive layer.

The hard coat layer may contain a photocurable resin. The protective film includes the hard coating layer, so that the hardness of the surface of the film can be increased, thereby having excellent scratch resistance.

The photocurable resin may be a compound having one or more unsaturated bonds, such as a compound having an acrylate optical group. The compound having one unsaturated bond may be, for example, ethyl (meth) acrylate, ethylhexyl (meth) acrylate, styrene, methylstyrene, N-vinylpyrrolidone, etc. The compound having two or more unsaturated bonds may be, for example, polyhydroxymethylpropane 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, or the like. In the present specification, "(meth) acrylate" means methacrylate and acrylate.

The adhesive layer may contain an adhesive resin. The binder resin may be formed by polymerizing at least one selected from the group consisting of an acrylic monomer and an unsaturated monomer having a carboxyl group. Examples of the acrylic monomer include 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, methoxy tripropylene glycol (meth) acrylate, and dicyclopentyl (meth) acrylate. Examples of the unsaturated monomer having a carboxyl group include acrylic acid, methacrylic acid, itaconic acid, and maleic acid.

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

A protective film of yet another example comprises: a first base material layer; a second base material layer positioned on the first base material layer; and a second curable resin layer interposed between the first substrate layer and the second substrate layer, wherein the first substrate layer has a thickness smaller than that of the second substrate layer, and the first substrate layer and the second substrate layer form a curved surface portion when the curable resin layer is cured.

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

Fig. 5 is a sectional view showing the display device of fig. 3 cut along X-X'. Specifically, fig. 5 illustrates the display device 1 having a structure in which the display panel 400, the cover window 300, and the protective 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 in the direction of the arrow with reference to the winding axis Q to form the curved surface 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 (in-folding) type or an out-folding (out-folding) type according to a direction in which the display panel 400 forms a curved surface. Fig. 6 is a sectional view showing an in-folding type flexible display device 2, and fig. 7 is a sectional view showing an out-folding type flexible display device 3.

As shown in fig. 6 and 7, in the case where the protective film of the example is used as the protective film 200 of the fold-in type flexible display device 2 or as the protective film 200 of the flip-out type flexible display device 3, the protective film is bent in the arrow direction with respect to the winding axis Q, and can be completely adhered to the curved portion without causing the lifting phenomenon.

Fig. 8 shows a protective film of yet another example. Specifically, the protective film 200 illustrated in fig. 8 includes a first base material layer 210, a second curable resin layer 232 on the first base material layer, and a second base material layer 220 on the second curable resin layer.

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

First base material layer and second base material layer

The first substrate layer may include a third polyester resin, and the second substrate 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 a dicarboxylic acid and a diol. The third polyester resin may be a blend resin obtained by mixing the homopolymer resin and the copolymer resin. More specifically, the third polyester resin can be formed by mixing a dicarboxylic acid and a diol in a molar ratio of 1: 1.

The dicarboxylic acid may be terephthalic acid, isophthalic acid, phthalic acid, 2, 5-naphthalenedicarboxylic acid, 2, 6-naphthalenedicarboxylic acid, 1, 4-naphthalenedicarboxylic acid, 1, 5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracenedicarboxylic acid, 1, 3-cyclopentanedicarboxylic acid, 1, 3-cyclohexanedicarboxylic acid, 1, 4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3-diethylsuccinic acid, glutaric acid, 2-dimethylglutaric acid, adipic acid, 2-methylhexanedioic acid, trimethyladipic acid, pimelic acid, azelaic acid, sebacic acid, or suberic acid, dodecanedicarboxylic acid.

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-bis (4-hydroxyphenyl) propane or bis (4-hydroxyphenyl) sulfone.

Preferably, the third polyester resin may be an aromatic polyester resin having excellent crystallinity, and specifically, may have a polyethylene terephthalate (PET) resin as a main component. For example, the first substrate layer may be a polyethylene terephthalate film.

For example, the first base material layer may include 85 weight percent or more of polyethylene terephthalate resin as the third polyester resin, and more specifically, may include 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, the first substrate layer may contain other polyester resins in addition to the polyethylene terephthalate resin. Specifically, the first base material layer may include polyethylene naphthalate (PEN) resin in an amount of 15 wt% or less. More specifically, the above-described first base material layer may contain 0.1 to 10 weight percent or 0.1 to 5 weight percent of polyethylene naphthalate resin.

By satisfying the above combination and content, the mechanical properties such as tensile strength can be improved when the protective film is subjected to heating, stretching, and other steps.

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

The thickness of the first base material layer is smaller than that of the second base material layer. Specifically, when the second curable resin layer interposed between the first substrate layer and the second substrate layer is cured, a difference in the degree of shrinkage between the first substrate layer and the second substrate layer occurs due to a difference in thickness between the first substrate layer and the second substrate layer. Therefore, the protective film is bent in the direction of the first base material layer to form a curved surface portion. As shown in fig. 5 and 6, the protective film 200 is bent in a direction in which the first base material layer 210 is located with respect to the winding axis Q.

The thickness ratio of the first base material layer to the second base material 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 base material layer and the second base material layer satisfies the above range, so that the durability and the picture feeling of the protective film can be further improved.

The thickness of the first base material layer may be 5 to 20 μm, and the thickness of the second base material layer may be 15 to 40 μm. For example, the thickness of the above-described 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 to 35 μm, 18 to 30 μm, or 20 to 28 μm.

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

Specifically, each of the first substrate layer and the second substrate layer may have a main stretching direction, and the main stretching direction may be a width direction (TD) or a length direction (MD). More specifically, the primary stretching direction of the first base material layer and the secondary stretching direction of the second base material layer may be the width direction (TD).

Second curable resin layer

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

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

The second curable resin layer may further contain one or more additives selected from the group consisting of a crosslinking agent, an antistatic agent, and an antifoaming agent. For example, the crosslinking agent may be a silane-based crosslinking agent, and may be an alkoxysilane such as vinylethoxysilane, vinyl-tris- (. beta. -methoxyethoxy) silane, methacryloxypropyltrimethoxysilane, gamma-aminopropyltriethoxysilane, gamma-mercaptopropyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, or trimethoxysilane; epoxy silanes such as triepoxy silane; aminosilanes such as butylaminosilane and epoxy-aminosilane; and alkylsilanes, silanes or disilanes such as methylsilane, dimethylsilane, vinylmethylcyclotrisiloxane, dimethylsilane-oxocyclopentane, cyclohexylsilane and cyclohexyldisilane, but not limited thereto.

The thickness of the second curable resin layer may be 10 μm to 100 μm. For example, the thickness of the above 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, and the uniformity of the curved surface portion can be further improved.

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

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

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

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

The hard coat layer, the adhesive layer and the release layer are as described above.

Fig. 9 shows a protective film of still another example. Specifically, the protective film 200 illustrated in fig. 9 includes a first base material layer 210, a curable resin layer 230 on one surface of the first base material layer, a second base material layer 220 on one surface of the curable resin layer, a hard coat layer 240 on one surface of the second base material layer, an adhesive layer 250 on the other surface of the first base material layer, and a release layer 260 on one surface of the adhesive layer.

On the other hand, in the optical fingerprint recognition method, in order to increase the fingerprint recognition rate of the optical fingerprint recognition method, the reflected light cannot be distorted after the fingerprint is recognized, as a method for comparing the fingerprint image reflected by the light with the previously registered fingerprint information. Therefore, the lower the orientation angle and the orientation angle deviation of the protective film attached to the surface of the display device such as a smart phone, the higher the fingerprint recognition rate and the effect of preventing the fingerprint recognition error.

The protective film of the example is excellent not only in durability and transparency but also in fingerprint recognition rate and in preventing erroneous fingerprint recognition.

The characteristics of the orientation angle, the in-plane retardation and the thickness direction retardation, the light transmittance, the light flux and the 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 above 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 may be selected within the above range according to the requirements such as improvement in moldability or durability. Specifically, if the thickness of the protective film is less than 30 μm, moldability may be excellent but durability is low, and if it exceeds 200 μm, durability may be excellent but moldability is low, so that a problem of poor quality may occur.

In particular, the orientation angle variation rate, and the orientation angle deviation of the above-described protective film are not affected by the protective thickness, and excellent visibility can be ensured without deteriorating the characteristics such as transparency, moldability, and durability.

The thickness variation of the protective film may be 5 μm or less. For example, the thickness variation of the above 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 it is possible to have uniform visibility with an appropriate phase difference deviation.

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

The protective film may have a moisture permeability of 20g/m²Day or less. For example, the protective film may have a moisture permeability of 20g/m²Under day, 18g/m²Day, below, 15g/m²Day or less, 12g/m²Under day or 10g/m²May be 0.1g/m or less than day²Day to 20g/m².day、0.5g/m²Day to 18g/m².day、1g/m²Day to 15g/m².day、3g/m²Day to 13g/m².day、4g/m²Day to 11g/m².day、4.5g/m²Day to 10g/m²Day or 4.8g/m²Day to 10g/m².day。

The moisture permeability satisfies the above range, so that excellent durability can be secured. Specifically, the protective film having a moisture permeability in the above range has significantly excellent moisture permeability characteristics as compared with a triacetyl cellulose film conventionally used as a protective film, and when the protective film is applied to a protective film for a display device, the display device can be effectively protected from an external moisture environment.

The difference between the refractive indices of the protective film in a first direction and a second direction perpendicular to the first direction may be 0.08 to 0.14. For example, the difference in refractive index between the first direction of the protective film and the second direction perpendicular to the first direction 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, so that distortion of the side-to-side light can be avoided, and thus excellent visibility can be ensured.

The in-plane first direction of the protective film of yet another example satisfies the following formula a.

Formula A: s is more than or equal to 0.5₁–S₂|≤3.1

In the formula A, S is₁Indicates a duration of N_1％Final elongation (%) after 20 minutes, S as described above₂Indicates a duration of N_2％Final elongation (%) after 20 minutes, in this case, N_1％N represents a load of stretching the protective film by 1% in the first direction_2％The load is a load of 2% for stretching the protective film in the first direction.

The above-mentioned stretching ratio and stretching load were measured on a test piece (length: 50 mm. times. width: 10mm) of the above-mentioned protective film at room temperature and at a stretching speed of 50 mm/min.

Specifically, S is as defined above₁For the in-plane first direction of the example protective film with N_1％Final elongation (%) after 20 min, S₂For the first in-plane direction of the mylar of the example, N_2％Final tensile (%) after 20 minutes.

More specifically, the above S is measured₁The method of (3) is as follows.

First, (1) a test piece of the above-mentioned protective film having a length of 50mm and a width of 10mm was stretched in a first direction at a stretching speed of 50mm/min at room temperature to obtain a tensile rate curve according to a load. (2) The load (N) at the time point when the length increased by 1% from the initial length was obtained from the elongation curve_1％). (3) Continuously applying the N to the test piece along a first direction_1％The ratio of the length of the test piece increased from the initial length at a load time of 20 minutes is the final elongation (%), that is, the S₁。

And, the above S is measured₂The method of (3) is as follows.

First, (1) a test piece of the above-mentioned protective film having a length of 50mm and a width of 10mm was stretched in a first direction at a stretching speed of 50mm/min at room temperature to obtain a tensile rate curve according to a load. (2) The load (N) at the time point when the length increased by 2% from the initial length was obtained from the elongation curve_2％). (3) Continuously applying the N to the test piece along a first direction_2％The ratio of the length of the test piece increased from the initial length at a load of 20 minutes is the final elongation (%), that is, S₂。

The value according to formula a above may 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. Satisfying the above formula 2, the protective film can be applied to a protective film for a foldable display device, and can be folded tens of thousands of times with little occurrence of whitening or cracking due to a lift-off phenomenon.

S above₁Can be from 0.1 to 2.5, S above₂May be 1.5 to 4.5. For example, S mentioned above₁Can 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, S, above₂May 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. S₁And S₂Satisfying the above range, the whitening or cracking due to the lift-off phenomenon can hardly occur even when the above protective film is folded several tens of thousands times in the case of applying the above protective film to a protective film for a foldable display device.

And N in the first direction_1％May be 10N to 25N, N in the first direction_2％May be 28N to 50N. For example, N in the first direction_1％May be 10N to 25N, 28N to 45N, 30N to 43N, or 33N to 40N, N of the first direction_2％May be 28N to 50N, 28N to 45N, 30N to 43N, or 33N to 40N.

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

Formula B: s is more than or equal to 0.5₃–S_4|≤5.2

In the formula B, S is₃Indicates a duration of N_1％Final elongation (%) after 20 minutes, S as described above₄Indicates a duration of N_2％Final elongation (%) after 20 minutes, in this case, N_1％N represents a load of stretching the protective film by 1% in the second direction_2％The load of the protective film stretched in the second direction by 2% is shown.

Measure the above S₃And S₄In addition to using the second direction instead of the first direction, with the above-mentioned measurement S₁And S₂The same method is used.

Values according to formula B above may 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. Satisfying the formula B, the protective film can be applied to a protective film for a foldable display device, and can be folded tens of thousands of times with little occurrence of whitening or cracking due to a lift-off phenomenon.

S above₃May be 0.8 to 2.4, S above₄And may be 2.3 to 7.5. For example, S mentioned above₃Can 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, S, above₄May 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. S₃And S₄Satisfying the above range, the protective film can be applied to a protective film for a foldable display device, and can be hardly whitened or cracked due to a lift-off phenomenon even when folded several tens of thousands times.

And N in the second direction_1％May be 25N to 45N, N in the second direction_2％May be 50N to 70N. For example, N in the second direction_1％May be 25N to 45N, 28N to 45N, 30N to 43N, 33N to 40N, or 33N to 38N, N in the second direction_2％Can be 50N to 70N, 50N to 65N, 52N to 63N or 57N to 63N.

Also, the third direction of the protective film of the example at 45 ° with respect to the first direction satisfies the following formula C.

Formula C: s is more than or equal to 0.5₅–S₆|≤7.2

In the formula C, S is₅Indicates a duration of N_1％Final elongation (%) after 20 minutes, S as described above₆Indicates a duration of N_2％Final elongation (%) after 20 minutes, in this case, N_1％N represents a load of stretching the polyester film by 1% in the third direction_2％The weight of the polyester film stretched in the third direction was 2%.

Measuring the above-mentioned S₅And S₆In addition to using a third direction at 45 DEG with respect to the first direction instead of the first direction, the method of (1) is similar to the above-mentioned measurement S₁And S₂The same method is used.

The value according to formula C above may 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. Satisfying the formula C, the polyester film is hardly whitened or cracked due to the lift-off phenomenon even when it is folded several tens of thousands times when it is used as a protective film for a foldable display device.

S above₅May be 0.1 to 5.5, S above₆And may be 1.5 to 12.5. For example, S mentioned above₅May 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, S as described above₆May 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. S₅And S₆Satisfying the above range, the whitening or cracking due to the lift-off phenomenon can hardly occur even when the above protective film is folded several tens of thousands times in the case of applying the above protective film to a protective film for a foldable display device.

And N in the third direction_1％May be 10N to 25N, the third mentionedDirection N_2％May be 28N to 50N. For example, N in the third direction_1％May be 10N to 25N, 28N to 45N, 30N to 43N or 33N to 40N, N of the third direction_2％May be 28N to 50N, 28N to 45N, 30N to 43N, or 33N to 40N.

S above₁The above-mentioned S₃Can be from 0.4: 1 to 0.7: 1, S as defined above₂The above-mentioned S₄Can be from 0.4: 1 to 0.7: 1. For example, S mentioned above₁The above-mentioned S₃May be in the range of 0.4: 1 to 0.7: 1, 0.45: 1 to 0.65: 1 or 0.45: 1 to 0.6: 1, S as defined above₂The above-mentioned S₄May be from 0.4: 1 to 0.7: 1, from 0.45: 1 to 0.65: 1 or from 0.45: 1 to 0.6: 1. S₁And S₃Ratio of (A) to (B) and S₂And S₄The ratio of (b) satisfies the above range, respectively, and the effect of preventing lift-off can be improved, so that whitening or cracking due to the lift-off phenomenon hardly occurs even when the sheet is folded several tens of thousands times.

S above₃The above-mentioned S₅Can be 1: 0.4 to 1: 0.7, S above₄The above-mentioned S₆Can be from 0.4: 1 to 0.7: 1. For example, S mentioned above₃The above-mentioned S₅May be in the range of 0.4: 1 to 0.7: 1, 0.45: 1 to 0.65: 1 or 0.45: 1 to 0.6: 1, S as defined above₄The above-mentioned S₆May be from 0.4: 1 to 0.7: 1, from 0.45: 1 to 0.65: 1 or from 0.45: 1 to 0.6: 1. S. the₃And the above-mentioned S₅Ratio of (A) to (B) and S₄And the above S₆The ratio of (b) satisfies the above range, respectively, and the effect of preventing lift-off can be improved, so that whitening or cracking due to the lift-off phenomenon hardly occurs even when the sheet is folded several tens of thousands times.

S above₁The above-mentioned S₅Can be 1: 0.8 to 1: 1.4, S above₂The above-mentioned S₆May be 1: 0.8 to 1: 1.4. For example, S mentioned above₁The above-mentioned S₅May 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, S as defined above₂S is₆May 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. S₁And S₅Ratio of (A) to (B) and S₂And S₆The ratio of (b) satisfies the above range, respectively, and the effect of preventing lift-off can be improved, so that whitening or cracking due to the lift-off phenomenon hardly occurs even when the sheet is folded several tens of thousands times.

N in the first direction_1％∶N_2％It can be 1: 1.5 to 1: 3. For example, N in the first direction_1％∶N_2％May be 1: 1.5 to 1: 2.8, 1: 1.5 to 1: 2.3, or 1: 1.6 to 1: 2.1.

N in the second direction_1％∶N_2％May be 1: 1.1 to 1: 2.5. For example, N in the second direction_1％∶N_2％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.

N in the third direction_1％∶N_2％May be 1: 1.5 to 1: 3. For example, N in the third direction_1％∶N_2％May be 1: 1.5 to 1: 2.8, 1: 1.5 to 1: 2.5 or 1: 1.6 to 1: 2.2.

N from the first direction to the third direction_1％And N_2％The ratio of (b) satisfies the above range, respectively, and the effect of preventing lift-off can be improved, so that whitening or cracking due to the lift-off phenomenon hardly occurs even when the sheet is folded several tens of thousands times.

Preparation method of protective film

A method of preparing a protective film of another embodiment includes: a step of preparing a polyester film; and a step of forming a first curable resin layer on one surface of the polyester film, the polyester film containing a first polyester resin containing more than 95 mol% of terephthalic acid as a dicarboxylic acid component and more than 95 mol% of ethylene glycol as a glycol component; and a second polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component, at least 70 mole percent ethylene glycol as a diol component, and at least 10 mole percent C₃To C₁₅The alcohol of (1). The polyester film has an in-plane retardation of at least 3000 nm.

The polyester film and the process for producing the same are as described above.

Specifically, the step of forming the first curable resin layer includes a step of applying a primer composition to one surface of the mylar film. Specifically, the primer composition may include a photocurable resin and a thermosetting resin, and may further include one or more selected from the group consisting of a crosslinking agent, an antistatic agent, and an antifoaming agent.

The photocurable resin, the thermosetting resin, and the additive are as described above.

The coating may be performed by roll coating, gravure coating, spray coating, or the like, but is not limited thereto.

A method of preparing a protective film of another embodiment includes: 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 preparing a first base material layer and a second base material layer by stretching the first sheet and the second sheet in a first direction by 1 to 1.5 times and in a second direction perpendicular to the first direction by 3 to 5 times at a temperature of 70 to 125 ℃, 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, wherein the thickness of the first base material layer is smaller than that of the second base material layer. When the second curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface portion.

The protective film can be produced by biaxial stretching with the stretching ratio adjusted and a process including heat treatment at a specific temperature. Specifically, in order to allow the produced protective film to satisfy the above-mentioned properties such as the stretching ratio and the orientation angle with respect to the entire width, the melt extrusion temperature, the preheating temperature at the time of stretching, the stretching ratio in each direction, the stretching temperature, the heat-setting temperature, and the relaxation ratio of the third polyester resin and the fourth polyester resin may be adjusted.

Hereinafter, the procedure will be described more specifically.

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

Specifically, the first sheet and the second sheet are prepared by melt-extruding the third polyester resin and the fourth polyester resin at a temperature of 260 ℃ to 300 ℃ or 270 ℃ to 290 ℃, respectively, and then cooling.

Then, the first sheet and the second sheet are transferred through the rollers. In this case, the thickness of the film can be adjusted to a desired thickness by adjusting the speed and the 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 to 125 ℃.

For example, the stretching may be performed at a temperature of 70 ℃ to 125 ℃, 75 ℃ to 120 ℃, 80 ℃ to 110 ℃, 85 ℃ to 100 ℃, or 80 ℃ to 100 ℃. If the stretching temperature is outside the above range, breakage may occur.

The stretching may be performed in a first direction at a stretching ratio of 1 to 1.5 times or 1 to 1.45 times, and may be performed in 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.

According to still another embodiment, the stretching step may further include a step of preheating the first sheet and the second sheet.

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

The range of the preheating temperature may be set to satisfy a range of Tg +5 ℃ to Tg +50 ℃ based on the glass transition temperature (Tg) of the third polyester resin and the fourth polyester resin, respectively, and to satisfy a range of 70 ℃ to 90 ℃. The preheating temperature satisfies the above range, so that the phenomenon of breakage during stretching can be effectively prevented while flexibility for easy stretching is ensured.

According to still another embodiment, the coating step may be performed after the stretching. Specifically, the coating step may 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 step of forming a promoting layer capable of providing a function such as static electricity prevention on the thin film may be performed. The coating step may be performed by spin coating or in-line coating, but is not limited thereto.

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

According to still another example, a relaxation step may be further included after the above-described stretching step.

Then, a second curable resin composition is applied to one surface of the prepared first substrate layer to form a second curable resin layer.

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

Finally, the second substrate layer is laminated on the second curable resin layer.

Display device

A display device of another example includes: a display panel; and a protective film on one surface of the display panel. The protective film includes: a first base material layer; a second base material layer positioned on the first base material layer; and a second curable resin layer interposed between the first substrate layer and the second substrate layer, wherein the first substrate layer has a thickness smaller than that of the second substrate layer, and the first substrate layer and the second substrate layer form a curved surface portion when the curable resin layer is cured.

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

Specifically, the above protective film may be adjusted in size according to conditions on a manufacturing process, thereby realizing characteristics required as a protective film for a display device, particularly a flexible display device. More specifically, in the case where the above protective film is applied to a curved flexible display device, the characteristics of durability, transparency, and visibility can be maintained without whitening or cracking due to a lift-off phenomenon while being completely attached to the front of a curved surface.

Modes for carrying out the invention

The above-described contents are explained in more detail by examples. However, the following examples are merely illustrative of the present invention, and the scope of the examples is not limited to the following examples.

Preparation of polyester film

Examples 1 to 1

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

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

Examples 1 to 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 Co.) obtained by mixing ethylene glycol and terephthalic acid in a molar ratio of 1: 1 was melted at a temperature of 280 ℃ and extruded using a t-die, and then cooled using a casting roll at 35 ℃ to prepare an unstretched sheet. The above unstretched sheet was transferred at a speed of 13 m/min, preheated to 95 ℃, stretched 1.1 times in the MD at a temperature of 85 ℃ and 4.3 times in the TD, and then heat-set at a temperature of 200 ℃ for 90 seconds. Then, the film was relaxed in the TD direction at a relaxation rate of 2% at a temperature of 130 ℃ to prepare a polyester film having a thickness of 85 μm.

Examples of the experiments

Experimental example 1-1: elongation combined strength

After the polyester films prepared in the above examples 1-1 and 1-2 and comparative examples 1-1 were cut into a length of 100mm and a width of 15mm, the test was carried out at a drawing speed of 100mm/min according to ASTM D882 with the interval between chucks set to 50mm using an universal tester (4206-.

Further, the polyester films prepared in the above examples 1-1, 1-2 and comparative example 1-1 were cut into a length of 4cm and a width of 1cm, and the maximum deformation amount immediately before the rupture was measured at a speed of 50mm/min using an universal tester (4206-.

Then, the elongation composite strength is calculated from the following

expressions

1 and 2 using the above-described result values.

Formula 1: ECT 1-EL 1 XTS 1

Formula 2: ECT 2-EL 2 XTS 2

In the

above formulas

1 and 2, ECT1 represents the first elongation composite strength (Kg/mm)²) EL1 is the elongation (%) in the TD direction, and TS1 is the tensile strength (Kg/mm) in the TD direction²) ECT2 is the second elongation composite Strength (Kg/mm)²) EL2 denotes the elongation (%) in the MD direction, and TS2 denotes the tensile strength (Kg/mm) in the MD direction²)。

Experimental examples 1-2: thermal shrinkage rate

The polyester films prepared in examples 1-1 and 1-2 and comparative examples 1-1 were cut into a length of 300mm and a width of 15mm, and were immersed in a water bath preheated to 150 ℃ for 30 minutes or in a water bath preheated to 85 ℃ for 24 hours, and then the heat shrinkage was calculated according to the following formulas 3 and 4 after removing moisture at normal temperature.

Formula 3:

formula 4:

Experimental examples 1 to 3: haze degree

The haze of the polyester films prepared in the above examples 1-1, 1-2 and comparative example 1-1 was measured using a C-light source using a haze measuring instrument (model: SEP-H) of Nihon Semitsu Kogaku corporation (Japan).

Experimental examples 1 to 4: light transmittance

The light transmittance of the polyester films prepared in the above examples 1-1, 1-2 and comparative example 1-1 was measured by using a spectrophotometer (UV2600, measuring wavelength: 380nm) of Shimadzu corporation.

Experimental examples 1 to 5: angle of orientation

The orientation angles of the full widths of the polyester films prepared in the above examples 1-1, 1-2 and comparative example 1-1 were measured by a refractometer (RETS-100, measuring wavelength: 550nm) of Shimadzu corporation.

Experimental examples 1 to 6: in-plane retardation

The in-plane retardation of the polyester films prepared in examples 1-1 and 1-2 and comparative example 1-1 was measured.

Specifically, with respect to the above polyester film, the refractive indices (Nx, Ny) of the perpendicularly intersecting biaxial are measured by a refractometer (RETS-100, measuring wavelength: 550nm) of Shimadzu corporation, and the thickness D (nm) of the film is measured by an electric micrometer (Millitron 1245D, manufacturer: Pine loop corporation), and the unit is converted to nm.

The in-plane retardation (Re) is calculated by multiplying Δ Nxy (═ Nx-Ny) measured above by the thickness d (nm) of the film, according to the following equation a.

The mathematical formula A:

Re＝ΔNxy×d

experimental example 7: luminous flux

The light fluxes of the polyester films prepared in examples 1-1 and 1-2 and comparative example 1-1 were measured by a TES digital illuminometer (trade name: TES-1334A, manufacturer: TES Co.).

Specifically, as shown in fig. 3, the optical power meter 10 is disposed at the lower end, and the first polarizing plate 21 and the second polarizing plate 22 are disposed in parallel at an interval above the optical power meter. In this case, the distance between the optical power meter 10 and the first polarizing plate 21 is about 2.5cm, and the distance between the optical power meter 10 and the second polarizing plate 22 is about 20 cm.

The light power meter 10 was used to supply light (a: direction of light) of 530nm at a voltage of 12V and to pass the light, and then the luminance (lux) before and after the polyester film 100 was placed between the first polarizing plate 21 and the second polarizing plate 22 was measured, and the luminous flux was calculated according to the following formula 5.

Formula 5:

in the above formula 5, a is the luminance (lux) when 530nm light is transmitted through two polarizers placed in parallel, and B is the luminance (lux) when 530nm light is transmitted after the polyester film is placed between the two polarizers, in which case the width direction (TD) of the polyester film is at an angle of 45 ° with respect to the optical axes B of the two polarizers.

Experimental example 8: impact strength

Three identical films were laminated and arranged on the polyester films prepared in examples 1-1 and 1-2 and comparative example 1-1 using a Film impact Tester (Film impact Tester, manufactured by Toyoseiki). Whether or not the film was penetrated was evaluated from a height of 30cm above the film disposed as described above using a pendulum tip (pendulum tip) having a diameter of 1 inch (inch).

X: the pendulum tip does not penetrate the membrane.

O: the pendulum tip penetrates through 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 as compared with the film of comparative example 1-1.

Specifically, the polyester films of examples 1-1 and 1-2 all satisfied the preferable ranges of tensile strength, elongation, and heat shrinkage, and were excellent in impact strength results, and thus were excellent in durability.

Further, the polyester films of examples 1-1 and 1-2 were found to have excellent durability, transparency and visibility because the haze, light transmittance, orientation angle, in-plane retardation and luminous flux all satisfied the preferred ranges.

Examples

Example 2-1

After a polyethylene terephthalate resin (manufacturer: SKC) obtained by mixing ethylene glycol and terephthalic acid at a molar ratio of 1: 1 was extruded through an extruder at 280 ℃, the mixture was cooled using a casting roll at 35 ℃ 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 a temperature of 95 ℃ and then heat-fixed at a temperature of 180 ℃ for 90 seconds to prepare a first substrate layer and a second substrate layer. In this case, the thickness ratio of the first base material layer and the second base material layer was adjusted to 1: 2.08 by adjusting the process time.

Then, a layer containing urethane acrylate resin (A) is spin-coated on one surface of the first substrate layer

The manufacturer: henkel corporation) to form a second curable resin layer having a thickness of 30 μm. The second substrate layer is 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

A protective film was produced in the same manner as in example 2-1, except that a protective film was produced according to the process conditions described in table 2 below, except that the thickness ratio of the first base material layer and the second base material layer described in table 2 below was changed.

TABLE 2

Examples of the experiments

Experimental example 2-1: deviation in thickness

The thicknesses (. mu.m) of the protective films prepared in the above examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were measured using a refractometer (RETS, measuring wavelength: 550nm) of Shimadzu corporation, and then the relevant thickness deviations were calculated.

Experimental example 2-2: in-plane retardation and thickness direction retardation

The in-plane retardation and the thickness direction retardation of the protective films prepared in the above examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were measured.

Specifically, with respect to the above protective film, the refractive indices (Nx, Ny) of the two perpendicularly intersecting axes were measured by a refractometer (RETS-100, measuring wavelength: 550nm) of Shimadzu corporation, and the thickness D (nm) of the film was measured by an electrical micrometer (Millitron 1245D, manufacturer: Pine loop corporation) and converted to nm in unit.

The in-plane retardation (Re) is calculated by multiplying the measured Δ Nxy (═ Nx-Ny |) by the film thickness d (nm) according to the following equations a and B, and the average value of the values obtained by multiplying the measured Δ Nxz (═ Nx-Nz |) and Δ Nyz (═ Ny-Nz |) by the film thickness d (nm) is calculated as the thickness direction retardation (Rth).

The mathematical formula A:

Re＝ΔN×y×d

the mathematical formula B:

experimental examples 2 to 3: tilting phenomenon

After the protective films prepared in examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were attached to the curved display, it was confirmed whether bubbles were generated or the films were lifted due to incomplete attachment.

O: no bubble and the like are generated and the adhesive tape is completely attached.

X: bubbles or the like may be generated or a partial lift phenomenon may occur.

Experimental examples 2 to 4: radius of curvature and angle of curvature

After the protective films prepared in examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were attached to curved displays, the curvature radius and the bending angle were measured with respect to the winding axis for the curved surface portion of the protective film.

Experimental examples 2 to 5: angle of orientation

The orientation angles with respect to the full width of the protective films prepared in the above examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were measured by a refractometer (RETS-100, measuring wavelength: 550nm) of Shimadzu corporation.

Experimental examples 2 to 6: luminous flux

The light fluxes of the protective films prepared in the above examples 2-1 to 2-4 and comparative examples 2-1 to 2-3 were measured using a luminometer (1334A) of TES corporation.

Experimental examples 2 to 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: 380nm) of Shimadzu corporation.

Experimental examples 2 to 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) of Mocon corporation.

TABLE 3

TABLE 4

As shown in the above tables 3 and 4, the protective films of examples 2-1 to 2-4 did not have the lifting phenomenon and showed excellent results in durability, transparency and visibility, as compared to the protective films of comparative examples 2-1 to 2-3.

Specifically, the protective films of examples 2-1 to 2-4 did not cause any warpage at all, and thus, they were closely adhered to each other when applied to a display device having a curved surface shape, and thus, they were found to have excellent quality. Further, the protective films of examples 2-1 to 2-4 all satisfied preferable ranges of thickness variation, phase difference, orientation angle, luminous flux, light transmittance, and moisture permeability, and thus were found to be excellent in durability, transparency, and visibility.

Claims

1. A polyester film characterized in that,

comprises the following steps:

a first polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component and greater than 95 mole percent ethylene glycol as a diol component; and

a second polyester resin comprising greater than 95 mole percent terephthalic acid as a dicarboxylic acid component, at least 70 mole percent ethylene glycol as a diol component, and at least 10 mole percent C₃To C₁₅The alcohol (a) of (b) is,

the polyester film has an in-plane retardation of 3000nm or more.

2. The polyester film according to claim 1,

c above₃To C₁₅The alcohol of (a) comprises cyclohexane dimethanol or neopentyl glycol,

the polyester film contains 0.5 to 20 weight percent of the second polyester resin.

3. The polyester film according to claim 1,

the first elongation composite strength ECT1 represented by the following formula 1 was 17kgf/mm²To 30kgf/mm²：

Formula 1: ECT1 ═ EL1 × TS1,

in the above formula 1, ECT1 is the first elongation composite strength in Kg/mm²EL1 is elongation in the first direction, in%, TS1 is tensile strength in the first direction, in Kg/mm²。

4. The polyester film according to claim 3,

the second elongation composite strength ECT2 represented by the following formula 2 was 0.95kgf/mm²To 2kgf/mm²，

The ratio of the first elongation composite strength to the second elongation composite strength is 1: 0.035 to 1: 0.117:

formula 2: ECT2 ═ EL2 × TS2,

in the above formula 2, ECT2 is the second elongation composite strength in Kg/mm²EL2 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 directionIn units of Kg/mm²。

5. The polyester film according to claim 1,

the haze is less than 10 percent,

within + -5 deg. of orientation angle relative to the full width,

and a thermal shrinkage rate in the first direction of 15% or less when heat-treated at 150 ℃ for 30 minutes.

6. A method for preparing polyester film is characterized in that,

the method comprises the following steps:

a step of preparing a mixture of a first polyester resin and a second polyester resin;

a step of melt-extruding the mixture to prepare an unstretched sheet;

a step of stretching the above unstretched sheet at a temperature of 70 ℃ to 125 ℃ by 1 to 1.5 times in a first direction and 3 to 5 times in a second direction perpendicular to the first direction to prepare a stretched film; and

a step of heat-fixing the above stretched film at a temperature of 160 ℃ to 230 ℃ to prepare a polyester film,

the above-mentioned first polyester resin contains terephthalic acid in an amount of more than 95 mol% as a dicarboxylic acid component and ethylene glycol in an amount of more than 95 mol% as a glycol component,

the second polyester resin comprises greater than 95 mole percent terephthalic acid as the dicarboxylic acid component, at least 70 mole percent ethylene glycol as the diol component, and at least 10 mole percent C₃To C₁₅The alcohol (a) of (b) is,

the polyester film has an in-plane retardation of 3000nm or more.

7. The process for producing a polyester film according to claim 6, wherein the ratio of the stretch ratio in the first direction to the stretch ratio in the second direction is 1: 1.5 to 1: 5.5.

8. A protective film characterized in that,

comprises the following steps:

a polyester film; and

a first curable resin layer on one surface of the polyester film,

the polyester film comprises:

the polyester film has an in-plane retardation of 3000nm or more.

9. A protective film characterized in that,

comprises the following steps:

a first base material layer;

a second base material layer positioned on the first base material 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 smaller than that of the second base material layer,

when the curable resin layer is cured, the first base material layer and the second base material layer form a curved surface portion.

10. The protective film according to claim 9, wherein the first substrate layer contains a third polyester resin, and the second substrate layer contains a fourth polyester resin.

11. The protective film according to claim 9,

the first base material layer and the second base material layer each have a main stretching direction,

the main stretching direction of the first base material layer and the main stretching direction of the second base material layer correspond to each other,

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

12. The protective film according to claim 9,

the first base material layer and the second base material layer are bent with the winding axis as a reference to form a curved surface portion,

the winding shaft corresponds to the main stretching direction of the first base material layer,

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

the curved surface portion has a bending angle W of 5 DEG to 45 deg.

13. A method for preparing a protective film is characterized in that,

the method comprises 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 preparing a first base material layer and a second base material layer by stretching the first sheet and the second sheet in a first direction by 1 to 1.5 times and in a second direction perpendicular to the first direction by 3 to 5 times at a temperature of 70 to 125 ℃, 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,

when the second curable resin layer is cured, the first substrate layer and the second substrate layer form a curved surface portion.