CN114387876B

CN114387876B - Polyester film for display protection

Info

Publication number: CN114387876B
Application number: CN202011484773.3A
Authority: CN
Inventors: 高明俊; 吴东昊; 姜昌远
Original assignee: Toray Advanced Materials Korea Inc
Current assignee: Toray Advanced Materials Korea Inc
Priority date: 2020-10-20
Filing date: 2020-12-16
Publication date: 2023-10-13
Anticipated expiration: 2040-12-16
Also published as: CN114387876A; KR102472872B1; KR20220052425A

Abstract

The polyester film for display protection according to the present application includes: a biaxially stretched base film, a first undercoat layer formed on one surface of the base film, and a second undercoat layer formed on the other surface of the base film, wherein the difference in refractive index between the first undercoat layer and the second undercoat layer is 0.1 or more. Accordingly, a polyester film for display protection having high total transmittance and high fingerprint recognition rate can be provided.

Description

Polyester film for display protection

Technical Field

The present application relates to a polyester film for display protection, and a polyester film for display protection having excellent adhesion and optical and mechanical properties.

Background

Recently, interest in liquid crystal displays, organic light emitting displays, and electronic paper displays has rapidly grown, and many attempts have been made to improve durability of such display devices by forming a protective film on the outermost portion of the device.

In general, polyester films have excellent dimensional stability, thickness uniformity, and optical transparency, and thus are used in various applications including not only as display devices but also as various industrial materials. In particular, research into polyester films used as films for display protection, which require high optical properties, has been actively conducted in recent years.

When a polyester film is used as an optical protective film, high total transmittance and low optical loss are required for the optical fingerprint recognition function employed in most portable display devices. The optical loss of a polyester film is related to the physical properties expressed by the orientation angle or optical axis. Basically, polyester films have an orientation angle due to bending and crystallization during the stretching process. In the case of biaxial stretching, since two optical axes are generated, polarized light from the polarizer becomes elliptically polarized light or circularly polarized light, which may appear as color change or iridescent irregularities to the human eye.

In order to solve these problems, many applications using a polyester film have been actively made. As an example of such an application, japanese patent laid-open No. 2011-532061 discloses a technique of ensuring visibility without rainbow-like irregularities by reducing optical anisotropy caused by birefringence. However, in this application, there arises a problem that the fingerprint recognition rate on the display protection film to be subjected to the surface treatment is lowered, and there is no attempt to reduce the orientation angle of the main chain for this purpose, and thus productivity in obtaining a practically effective width is lowered, which hampers the use of the polyester film as a film for display protection.

Further, studies have been made to improve the optical characteristics of polyester films by increasing the total transmittance through a double-sided heterogeneous coating, but general double-sided heterogeneous coatings have a problem in that the orientation angle cannot be adjusted or optical loss and oligomer precipitation are suppressed.

Thus, there is a need for such polyester films for display protection: it has inherent optical characteristics of a polyester film and also reflects a design of an appropriate structure capable of minimizing stress at the interface of respective layers such as a primer layer, AG, hard coat layer, etc., and a design for suppressing the occurrence of foreign matters.

[ Prior Art literature ]

[ patent literature ]

(patent document 001) Japanese patent laid-open No. 2011-532061

Disclosure of Invention

Technical problem

The present application has been conceived to solve the above-mentioned problems, and it is an object of the present application to provide a polyester film for display protection: by adjusting the crystal angle of the main chain of the film and reducing the main orientation angle as much as possible, the polyester film can minimize rainbow-like irregularities observed when viewed from an oblique direction.

Further, it is an object of the present application to provide a polyester film for display protection, which has high total transmittance and is capable of suppressing optical loss, and which is capable of minimizing occurrence of foreign substances that may reduce fingerprint recognition rate when used as a protective film for a display device having an optical fingerprint recognition function.

The above and other objects and advantages of the present application will become apparent from the following description of the preferred embodiments.

Technical proposal

The above object is accomplished by providing a polyester film for display protection, comprising: a biaxially stretched base film, a first undercoat layer formed on one surface of the base film, and a second undercoat layer formed on the other surface of the base film, wherein the difference in refractive index between the first undercoat layer and the second undercoat layer is 0.1 or more.

Preferably, the polyester film for display protection may have a haze change of 1% or less after heat treatment at 150 ℃ for 1 hour.

Preferably, the optical loss of the polyester film for display protection according to the following equation 1 may be 11% or less:

(equation 1)

Optical loss = (maximum amount of light of polarizer in parallel nicols state-maximum amount of light after inserting film between two polarizers)/(maximum amount of light of polarizer in parallel nicols state)

Preferably, the difference between the orientation angle of the polyester film for display protection and the optical axis may be 9 degrees or less.

Preferably, the difference in orientation angle between the center portion and the edge portion of the polyester film for display protection may be 20 degrees or less.

Preferably, the orientation angle of the main chain of the polyester film for display protection may be 20 degrees or less.

Preferably, the refractive index of the first primer layer may be in the range of 1.58 to 1.62, and the refractive index of the second primer layer may be in the range of 1.45 to 1.49.

Preferably, the ratio of MD stretch ratio to TD stretch ratio may be 1:1.45 to 1:1.75.

Preferably, the total transmittance of the polyester film for display protection may be 93% or more.

Preferably, the first primer layer may comprise a polyester copolymer resin, a polyurethane-based resinAn oxazoline curing agent, a melamine-based curing agent, an anionic surfactant and inorganic particles.

Preferably, the second primer layer may include a polyacrylic resin having a carboxyl group as a functional group, a melamine-based curing agent, an anionic surfactant, and inorganic particles.

Preferably, the second primer layer may have an oligomer blocking function.

Preferably, the base film may be at least one selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, and polycarbonate.

Preferably, the thickness of the base film may be in the range of 25 μm to 250 μm.

The above object is accomplished by providing a method of manufacturing a polyester film for display protection, the method comprising: a first step of forming an unstretched sheet by melt-extruding a polyester resin; a second step of applying a coating liquid for forming a first undercoat layer on one surface of the unstretched sheet and drying it; a third step of applying a coating liquid for forming a second primer layer on the other side of the unstretched sheet and drying it; a fourth step of uniaxially stretching the unstretched sheet having the first primer layer and the second primer layer formed thereon in the machine direction MD; a fifth step of biaxially stretching the uniaxially stretched sheet in the width direction TD; and a sixth step of heat setting the biaxially stretched sheet to form a polyester film for display protection.

Preferably, the thickness of each of the first and second primer layers may be in the range of 10nm to 200 nm.

Preferably, the temperature for the heat setting of the sixth step may be in the range of 180 ℃ to 220 ℃.

Advantageous effects

The polyester film for display protection according to one embodiment of the present application can suppress optical loss by a high total transmittance. In addition, both Liquid Crystal Display (LCD) devices and Organic Light Emitting Diode (OLED) display devices use polarizers. In order for such a display device to have a high fingerprint recognition rate, the optical axis of the polyester film for display protection is aligned with the optical axis of the polarizer to minimize optical loss, whereby the polyester film for display protection according to one embodiment of the present application may have a high fingerprint recognition rate due to a small change in the optical axis.

Further, the polyester film for display protection according to one embodiment of the present application may have a high fingerprint recognition rate by suppressing foreign substances in the film and minimizing the occurrence of oligomers.

In addition, the polyester film for display protection according to one embodiment of the present application may minimize the occurrence of foreign matters during heat treatment, thereby increasing yield during pressing (pushing).

However, the effects of the present application are not limited to the above-described effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description.

Drawings

Fig. 1 is a cross-sectional view of a polyester film for display protection according to an embodiment of the present application.

Detailed Description

Hereinafter, embodiments of the present application will be described in detail with reference to the accompanying drawings so as to be easily practiced by those of ordinary skill in the art. It is to be understood that the application is not to be construed as limited to the embodiments set forth herein and may be embodied in many different forms.

The dimensions and thicknesses of elements in the drawings may be exaggerated for convenience of description. Like reference numerals refer to like elements throughout the specification. Furthermore, it will be understood that when an element such as a layer, film, region or sheet is referred to as being "on" another element, it can be directly on the other element and one or more intervening elements may also be present. In contrast, when an element such as a layer, film, region or sheet is referred to as being "directly on" another element, there are no intervening elements present.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. If a term in the present application contradicts or conflicts with a term in the incorporated reference, the term of the present application takes precedence over the conflicting term from the incorporated reference. Although methods and compositions similar or identical to those described in the present disclosure may be applied to embodiments or testing of the present application, the present disclosure provides suitable methods and compositions.

Referring to fig. 1, a polyester film for display protection includes: a base film 2, a first primer layer 1 formed on one surface of the base film 2, and a second primer layer 3 formed on the other surface of the base film 2. In this case, it is considered that the first primer layer 1 and the second primer layer 3 may be exchanged in their positions.

In one embodiment, the base film 2 preferably contains at least one polyester selected from polyethylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, and polycarbonate, and may be a biaxially stretched film.

In one embodiment, the thickness of the base film 2 is preferably in the range of 25 μm to 250 μm. When the thickness of the base film is less than 25 μm, wrinkles may be generated during the surface treatment and the lamination of the polarizer, and the yield may be reduced. When the thickness of the base film 2 exceeds 250 μm, it is difficult to control the phase difference and there is a problem that the entire thickness of the display becomes thick.

In addition, the base film 2 may further contain an additive material, for example, an ultraviolet absorber capable of absorbing ultraviolet rays, or an antioxidant for suppressing brown discoloration of the ultraviolet absorber, depending on its intended purpose.

In one embodiment, the difference in refractive index between the first undercoat layer 1 formed on the surface of the base film 2 and the second undercoat layer 3 formed on the other surface is preferably 0.1 or more. When the difference in refractive index between the first primer layer 1 and the second primer layer 3 is less than 0.1, the total transmittance decreases and the optical loss of the film increases.

In one embodiment, since the first undercoat layer 1 generally forms a hard coating layer, its refractive index is preferably in the range of 1.58 to 1.62, and more preferably in the range of 1.59 to 1.60, to control rainbow unevenness caused by interference when the hard coating layer is formed on the surface.

In one embodiment, the first primer layer 1 includes at least one base material selected from a polyester copolymer resin and a polyurethane-based resin as a binder resin. Preferably, it may comprise two base materials.

Furthermore, the first primer layer 1 comprises a material selected from the group consisting ofAt least one of an oxazoline-based material, a carbodiimide-based material, and a melamine-based material is used as a curing agent resin. In particular based on->The azoline-based curing agent inhibits penetration of moisture into the film due to moisture resistance, the melamine-based curing agent reacts with the base material and also improves the coating film by curing reaction between itself and melamineThereby preventing blocking phenomenon that may occur during double-sided coating. Preferably, it may be comprised of a base +.>An oxazoline curing agent and a melamine-based curing agent.

Furthermore, it is preferable that the first undercoat layer 1 contains an anionic surfactant as an additive.

Furthermore, the first primer layer 1 may further contain an additional additive in the coating liquid to improve coating characteristics and functionality. As described above, organic particles, inorganic particles, an antifoaming agent, or the like can be used as an additive for the first undercoat layer 1.

In the present application, the first undercoat layer 1 can ensure the running characteristics by inorganic particles, and the average particle diameter of the inorganic particles is preferably in the range of 10nm to 300 nm. When the average particle diameter of the inorganic particles exceeds 300nm, there is a problem that haze increases, and when the average particle diameter of the inorganic particles is less than 10nm, running characteristics cannot be ensured.

Further, the inorganic particles are preferably at least one selected from the group consisting of silica particles and silica-organic composite materials, and the refractive index of the inorganic particles is preferably 1.5 or more in view of the refractive index of the first undercoating layer 1.

In one embodiment, the first primer layer 1 is preferably formed by applying to the base film 2 a coating liquid containing: binder resin composed of a mixture of polyester copolymer resin and polyurethane-based resin, and a polyurethane-based resinAn oxazoline curing agent and a melamine-based curing agent, an anionic surfactant and inorganic particles.

The composition constituting the first primer layer 1 is prepared as a coating liquid in the form of an aqueous dispersion with water and applied on one surface of the base film 2. In one embodiment, it is preferable that the thickness of the first primer layer 1 applied is in the range of 10nm to 200 nm. When the thickness is applied less than 10nm, there is a problem in that a reflective rainbow may be observed after the surface treatment and the adhesion is reduced, and when the thickness exceeds 200nm, a reflective rainbow and blocking occur.

In one embodiment, it is preferable that the adhesion force and moisture-resistant adhesion force between the first primer layer 1 and the base film 2 be in the range of 95% to 100%.

Next, since the adhesive or binder is treated on the surface of the second undercoat layer 3, it is preferable to form an undercoat layer capable of realizing the treatment of the adhesive or binder, and in order to suppress occurrence of foreign substances after the heat treatment, it is preferable to have a function of oligomer blocking. Since the functional groups capable of reacting with the curing agent are increased in the base material of the second undercoat layer 3 and the curing agent having excellent reactivity with these functional groups is used, the network structure after curing of the undercoat layer becomes smaller than the size of the oligomer, so that the oligomer formed after the heat treatment can be prevented from precipitating on the undercoat layer. As described above, the polyester film for display protection according to one embodiment of the present application may minimize the occurrence of foreign matters during heat treatment, thereby increasing the yield during pressing.

In one embodiment, the refractive index of the second primer layer is preferably in the range of 1.45 to 1.49 such that the difference in refractive index between the first primer layer and the second primer layer is 0.1 or more.

In one embodiment, a polyacrylic resin and/or a polyalkylene oxide resin having a carboxyl group as a functional group is preferably used as a binder resin of the second undercoat layer 3, and a specific crosslinked structure is designed. Preferably, a polyacrylic resin having a carboxyl group as a functional group may be contained.

In addition, the second primer layer 3 comprises a material selected from the group consisting ofAt least one of an oxazoline-based material, a carbodiimide-based material, and a melamine-based material is used as a curing agent resin. In particular based on->The curing agent of oxazoline inhibits moisture penetration into the film, and the melamine-based curing agent reacts with the base material and also increases the strength of the coated film through a curing reaction between itself and melamine, thereby preventing blocking phenomenon that may occur during double-sided coating. Preferably, a melamine-based curing agent may be included.

Furthermore, the second undercoat layer 3 preferably contains an anionic surfactant as an additive.

In addition, the second primer layer 3 may further contain an additional additive in the coating liquid to improve coating characteristics and functionality. As described above, organic particles, inorganic particles, an antifoaming agent, or the like may be used as an additive for the second undercoat layer 3.

In the present application, the second undercoat layer 3 can ensure the running characteristics by inorganic particles, and the average particle diameter of the inorganic particles is preferably in the range of 10nm to 300 nm. When the average particle diameter of the inorganic particles exceeds 300nm, there is a problem that haze increases, and when the average particle diameter of the inorganic particles is less than 10nm, running characteristics cannot be ensured. Further, the inorganic particles are preferably at least one selected from silica particles and silica-organic composite materials. In this case, the inorganic particles may be the same inorganic particles as those of the first primer layer 1 described above.

The composition constituting the second primer layer 3 is prepared as a coating liquid in the form of an aqueous dispersion with water and applied on the other surface of the base film 2 (i.e., the surface to which the first primer layer is not applied). In one example, the thickness is preferably applied in the range of 10nm to 200 nm. At this time, when the thickness is applied to less than 10nm, it is difficult to secure the running characteristics, and there may be a problem of the adhesive force of the adhesive, and when the thickness exceeds 200nm, it is not preferable because the color of the film itself may appear yellow.

In one embodiment, it is preferable that the adhesion force and moisture-resistant adhesion force between the second primer layer 3 and the base film 2 be in the range of 95% to 100%.

It is preferable that the polyester film for display protection according to one embodiment of the present application is a biaxially stretched film in the machine direction MD and/or the width direction TD. In particular, since polyethylene terephthalate constituting the base film 2 has no crystallinity in an unstretched state, its mechanical strength is weak, and it is difficult to control a low-level thickness, and thus a polyester film more preferably used for display protection is a biaxially stretched film.

In a polyester film for display protection, the direction of the orientation angle in the main chain direction is determined by the crystallinity of the polyester-based material in the stretching direction when heat-set is performed after the stretching process.

The orientation angle of the main chain refers to an angle formed by crystals of the main chain with respect to the width direction of the film. The direction of the orientation angle in the main chain direction is determined by the crystallinity of the polyester-based material (polyester resin) with respect to the stretching direction in the process of heat-setting the film after the biaxial stretching process. First, after an unstretched film made of polyester is stretched in the machine direction MD, the stretched film is further stretched in the width direction TD. In this case, the orientation angle of the main chain at the end of stretching in the width direction is 0 degrees. In addition, during the heat setting, a bending phenomenon occurs due to the residual stress after the stretching. Due to the bending phenomenon, an arcuate gradient occurs in a direction opposite to the advancing direction of the film.

Since the orientation of the main chain crystals causes a change in the optical axis, if the orientation angle is not reduced, optical loss occurs after the formation of the protective film, thereby reducing the fingerprint recognition rate. In order to minimize such an influence, it is necessary to suppress the bending phenomenon so that the edge portion of the film is also set to be as similar as possible to the physical characteristics at the center of the width. In order to suppress the bending phenomenon, the film must be stretched more in the TD direction than in the MD direction, and therefore, it is preferable that the ratio of MD stretching to TD stretching is 1:1.45 or more, and the ratio is preferably adjusted to be less than or equal to 1:1.75 for process stability and thickness control.

In one embodiment, the orientation angle of the main chain of the polyester film for display protection is preferably 20 degrees or less, more preferably 17 degrees or less, even more preferably 12 degrees or less. When the orientation angle of the main chain of the polyester film for display protection exceeds 20 degrees, the deviation between the optical axis and the orientation angle increases, so that the optical loss increases.

The optical loss is a value representing the optical loss according to an angle when the film is inserted into a polarizer in a parallel nicols state, and can be represented by equation 1.

(equation 1)

In this case, in the polyester film for display protection according to one embodiment of the present application, it is preferable that the optical loss obtained by equation 1 is 11% or less. When the optical loss exceeds 11%, a problem arises in that the fingerprint recognition rate is lowered due to high optical loss.

In the polyester film for display protection according to one embodiment of the present application, the optical loss obtained by equation 1 generally has a minimum value at a central portion where the orientation angle is around 0 degrees, and tends to increase as the orientation increases. Therefore, in the polyester film for display protection according to one embodiment of the present application, the orientation angle is adjusted so that the edge portion may have physical properties as similar as possible to those at the center portion to minimize optical loss. For this reason, it is preferable that the difference in orientation angle between the center portion and the edge portion of the polyester film for display protection is 20 degrees or less. When the difference in orientation angle between the center portion and the edge portion exceeds 20 degrees, the optical loss increases.

In one embodiment, it is preferable that the difference between the orientation angle of the polyester film for display protection and the optical axis is 9 degrees or less. When the difference between the orientation angle and the optical axis is greater than 9 degrees, the difference of the optical axis between the polarizer and the polyester film in the display device increases, so that pressing along the optical axis of the polarizer is impossible, and even when pressing is performed, the fingerprint recognition rate is significantly reduced, and screen color distortion occurs, resulting in a problem that the polyester film cannot be used as a protective film.

Further, the total light transmittance of the polyester film for display protection is preferably 93% or more, and more preferably 95% or more. When the total light transmittance is less than 93%, the fingerprint recognition rate is lowered and the brightness of the display material is lowered.

Further, it is preferable that the polyester film for display protection according to one embodiment of the present application has a haze change of 1% or less after heat treatment at 150 ℃ for 1 hour. When the haze exceeds 1%, the transparency is lowered, resulting in a problem of lowering the contrast.

The method of manufacturing a polyester film for display protection according to an embodiment of the present application as described above includes: a first step of forming an unstretched sheet by melt-extruding a polyester resin; a second step of applying a coating liquid for forming a first undercoat layer on one surface of the unstretched sheet and drying it; a third step of applying a coating liquid for forming a second primer layer on the other side of the unstretched sheet and drying it; a fourth step of uniaxially stretching the unstretched sheet having the first primer layer and the second primer layer formed thereon in the machine direction MD; a fifth step of biaxially stretching the uniaxially stretched sheet in the width direction TD; and a sixth step of heat setting the biaxially stretched sheet to form a polyester film for display protection.

In one embodiment, the temperature for heat setting is preferably in the range of 180 ℃ to 220 ℃, and more preferably in the range of 180 ℃ to 200 ℃. In this case, when the temperature for heat setting is lower than 180 ℃, heat setting of the base film 2, the first primer layer 1, and the second primer layer 3 cannot be properly performed, and when the temperature exceeds 220 ℃, a bending phenomenon occurs, and the difference between the orientation angle and the optical axis may increase. In this way, by performing the heat treatment at a temperature lower than the normal heat-setting temperature, the stress difference between the stretched portion and the heat-set portion can be minimized.

In the method of manufacturing the polyester film for display protection according to one embodiment of the present application, redundant description of the above-described polyester film for display protection will be omitted.

Hereinafter, the structure of the present application and the effects obtained thereby will be described in more detail with reference to examples and comparative examples. However, examples are provided to describe the present application in more detail, and the scope of the present application is not limited to the examples.

Examples (example)

Preparation example

(coating liquid 1)

As binder resin, 12.5% by weight of base material 1 composed of 70% by weight of water and 30% by weight of polyurethane resin (H-15 of Cheil Industries) and 12.5% by weight of polyester-based copolymer resin (Takamatsu Oil) composed of 70% by weight of water and 30% by weight of polyester-based copolymer resin (Takamatsu Oil&TR620K of Fat co., ltd. 12.5 wt% of the base material 2 were mixed with each other. The following were mixed together: 20.0% by weight of the aqueous base resin dispersion thus formed, and a proportion of 40% by weight, based on the moisture resistance, of a curing agent for improving moisture resistance1.87 wt.% of hardener 1 consisting of an oxazoline hardener (Nippon Carbide Industries co., WS500 of inc.) and 60 wt.% of water, and 1.87 wt.% of hardener 2 consisting of 70 wt.% of melamine-based hardener (PM 80, DIC co., ltd.) and 30 wt.% of water. The coating liquid 1 was prepared by adding the following substances to the mixture: 1.0 wt% aqueous surfactant dispersion consisting of 90 wt% water and 10 wt% anionic surfactant (Goo Chemical co., ltd. RY-2), 0.5 wt% water-dispersible granule coating liquid consisting of 70 wt% silica particles and 30 wt% water, and the remainder water.

(coating liquid 2)

As the binder resin, a mixture of 20 wt% aqueous base resin dispersion composed of 70 wt% water and 30 wt% polyacrylic resin (Nippon Carbide Industries co., RX-7013ED of inc.) having carboxyl groups as functional groups and 10 wt% aqueous dispersion of curing agent composed of 30 wt% melamine-based curing agent (DIC co., PM80 of ltd.) and 70 wt% water was prepared, and the coating liquid 2 was prepared by adding to the mixture: 1.0 wt% aqueous surfactant dispersion consisting of 90 wt% water and 10 wt% anionic surfactant (RY-2 of Goo Chemical co., ltd.) 0.5 wt% water dispersed particle coating liquid consisting of 70 wt% silica particles and 30 wt% water, and the remainder water.

(coating liquid 3)

Coating liquid 3 was prepared in the same manner as coating liquid 1, except that a polyurethane resin (DIC co., AP-40F of ltd.) was used instead of polyurethane resin (H-15 of Cheil Industries) as a binder resin.

Examples 1 to 4

After melt-extruding a raw material sheet of polyethylene terephthalate, an unstretched sheet was manufactured on a casting roll, and then a coating liquid was applied on one surface and the other surface of the unstretched sheet using a #4 metal rod, and dried at 80 ℃ to form a first primer layer and a second primer layer each having a thickness of 100 nm. Next, after stretching the unstretched sheet in both the machine direction MD and the width direction TD, heat setting was performed at 190 ℃ to prepare a biaxially stretched polyester film for display protection.

At this time, the composition of the coating liquid, the stretching conditions and the thickness of the entire film are shown in table 1, and the refractive indices of the first primer layer and the second primer layer are shown in table 3.

Comparative examples 1 to 5

A polyester film for display protection was prepared in the same manner as in example 1, except that the composition of the coating liquid and the stretching conditions were changed as shown in table 1 below.

TABLE 1

Physical properties were measured by the following experimental examples using the polyester films according to examples 1 to 4 and comparative examples 1 to 5, and the results are shown in tables 2 and 3 below.

Experimental example

(1) Measurement of total transmittance

The total transmittance of the films was measured with a haze meter (Nippon Denshoku Industries co., NDH-4000 of ltd.).

(2) Measurement of thickness

The thickness of the film was measured using a micrometer (VL-50 aS of Mitutoyo Corporation).

(3) Measurement of optical loss

After the film was cut into pieces having a width of 5cm and a length of 5cm, two polarizing films were arranged in a 100% transmission state (parallel nicols state) to examine the maximum amount of light, and the cut film was placed between the two polarizing films. Then, the change in the maximum amount of light was measured while changing the angle of the film, and the optical loss was calculated by equation 1.

(equation 1)

(4) Measurement of orientation angle

After biaxially stretching the film, the crystallization direction of the main chain with respect to the width direction of the film was measured at the center portion and the edge portion of the film sample using an orientation angle measuring device (ssmt-4000 of NOMURA shojico., ltd.). In this case, since the orientation angle of the center portion is 0 degrees, the orientation angle in table 2 represents the orientation angle of the edge portion.

(5) Measurement of optical axis

After the film was inserted between two polarizers of the polarizers in the cross-nicol state, the angle at which the black mode was achieved was checked and measured with a protractor.

(6) Inspection of rainbow-like irregularities

An insulating tape (Nitto Corporation) was attached to the back surface of the prepared film, and visual inspection was performed under reflection of a fluorescent lamp, and then evaluated as follows.

O: no rainbow irregularities were observed

X: rainbow irregularities are observed

(7) Measurement of haze variation

For the prepared film, haze before heat treatment was measured using a haze meter (Nippon Denshoku Industries co., NDH-4000 of ltd.) and haze after heat treatment in an oven at 150 ℃ for 1 hour was measured.

(8) Measurement of adhesion force

After UV-curable acrylic resin was applied to the surface of the first primer layer by using #20 bar, cutting lines were formed on the prepared film by a cutter, and 1mm×1mm squares were placed in a 10×10 matrix. A cellophane tape (nichiband co., no.405, width: 24 mm) was then attached to the film on which the dicing lines were formed, and the tape was rubbed with velvet to firmly adhere it to the film. Thereafter, the tape was removed vertically. At this time, the area of the primer layer remaining on the coating layer formed of the UV-curable acrylic resin was substituted into the following equation 2 to calculate the adhesion force.

(equation 2)

Further, after the film prepared in the same manner as described above was left in an environment at a temperature of 60 ℃ and a humidity of 90% for 100 hours, the wet adhesion was measured in the same manner as described above.

(9) Measurement of refractive index of primer layer

After the coating liquids 1 to 3 were applied onto the glass plates, respectively, and thermally cured, a black insulating tape was attached to the back surface of the glass plate on which the coating liquid was not applied, and then the refractive index was measured using an ellipsometer (Ellipso Technology co., elli-SE of ltd.).

TABLE 2

TABLE 3

As shown in tables 2 and 3, it can be seen that examples 1 to 4 according to the present application all have excellent adhesive force as well as optical and mechanical properties, and that the total transmittance, optical loss, orientation angle, rainbow-like irregularities caused by interference, and haze change all satisfy all the properties required for the protective film.

In contrast, in comparative example 1, it can be seen that the difference in refractive index between the first primer layer and the second primer layer disposed on each side of the base film was 0.1 or more, but the orientation angle of the main chain exceeded 20, and excessive optical loss occurred due to the low stretch ratio in the MD direction and the TD direction.

Further, in comparative example 2 in which the difference in refractive index between the first undercoat layer and the second undercoat layer disposed on each side of the base film was less than 0.1 and the stretch ratio in the MD direction and the TD direction was low, it can be seen that the orientation angle of the main chain exceeded 20, the haze variation was large, and excessive optical loss occurred.

Further, in comparative examples 3 and 4 in which the first undercoat layer and the second undercoat layer disposed on each side of the base film were formed of the same coating liquid, it can be seen that the refractive index between the undercoat layers was the same, and thus the total transmittance was low, and rainbow-like irregularities or a large change in haze occurred.

Further, in comparative example 5, it can be seen that since the difference between the orientation angle and the optical axis exceeds 9 degrees, excessive optical loss occurs, which exceeds 11%.

As described above, the polyester film for display protection according to one embodiment of the present application can improve productivity during post-treatment by minimizing interference unevenness after a hard coating treatment of a surface, suppress occurrence of foreign substances by an oligomer blocking function, and increase fingerprint recognition rate by controlling optical loss according to a low orientation angle. In addition, the polyester film according to the present application can be widely used as an optical film other than a protective film due to its excellent optical characteristics.

While the application has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the application is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[ reference numerals ]

1: first primer layer 2: base film

3: and a second primer layer.

Claims

1. A polyester film for display protection comprising:

biaxially stretching the base film;

a first primer layer formed on one surface of the base film; and

a second primer layer formed on the other surface of the base film,

wherein a difference in refractive index between the first primer layer and the second primer layer is 0.1 or more,

wherein the difference between the orientation angle of the polyester film for display protection and the optical axis is 9 degrees or less.

2. The polyester film according to claim 1, wherein the polyester film for display protection has a haze change of 1% or less after heat treatment at 150 ℃ for 1 hour.

3. The polyester film according to claim 1, wherein the optical loss of the polyester film for display protection is 11% or less according to the following equation 1:

equation 1

Optical loss= (maximum amount of light of polarizer in parallel nicols state-maximum amount of light after inserting film between two polarizers)/(maximum amount of light of polarizer in parallel nicols state).

4. The polyester film according to claim 1, wherein a difference in orientation angle between a center portion and an edge portion of the polyester film for display protection is 20 degrees or less.

5. The polyester film according to claim 1, wherein the orientation angle of the main chain of the polyester film for display protection is 20 degrees or less.

6. The polyester film of claim 1 wherein the first primer layer has a refractive index in the range of 1.58 to 1.62 and the second primer layer has a refractive index in the range of 1.45 to 1.49.

7. The polyester film of claim 1 wherein the ratio of MD stretch ratio to TD stretch ratio is from 1:1.45 to 1:1.75.

8. The polyester film according to claim 1, wherein the total transmittance of the polyester film for display protection is 93% or more.

9. The polyester film of claim 1, wherein the first primer layer comprises a polyester copolymer resin, a polyurethane-based resinAn oxazoline curing agent, a melamine-based curing agent, an anionic surfactant and inorganic particles.

10. The polyester film of claim 1, wherein the second primer layer comprises a polyacrylic resin having carboxyl groups, a melamine-based curing agent, an anionic surfactant, and inorganic particles.

11. The polyester film of claim 1 wherein the second primer layer has an oligomer blocking function.

12. The polyester film according to claim 1, wherein the base film is at least one selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, and polycarbonate.

13. The polyester film of claim 1, wherein the base film has a thickness in the range of 25 μιη to 250 μιη.

14. A method of making a polyester film for display protection, the method comprising:

a first step of forming an unstretched sheet by melt-extruding a polyester resin;

a second step of applying a coating liquid for forming a first undercoat layer on one surface of the unstretched sheet and drying it;

a third step of applying a coating liquid for forming a second primer layer on the other side of the unstretched sheet and drying it;

a fourth step of uniaxially stretching the unstretched sheet on which the first primer layer and the second primer layer are formed in the machine direction MD;

a fifth step of biaxially stretching the uniaxially stretched sheet in the width direction TD; and

a sixth step of heat-setting the biaxially stretched sheet to form a polyester film for display protection,

15. The method of claim 14, wherein the first primer layer and the second primer layer each have a thickness in the range of 10nm to 200 nm.

16. The method of claim 14, wherein the temperature for the heat setting of the sixth step is in the range of 180 ℃ to 220 ℃.