CN114387876A

CN114387876A - Polyester film for display protection

Info

Publication number: CN114387876A
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: 2022-04-22
Anticipated expiration: 2040-12-16
Also published as: CN114387876B; KR102472872B1; KR20220052425A

Abstract

The polyester film for display protection according to the present invention includes: a biaxially stretched 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. Accordingly, a polyester film for display protection having a high total transmittance and a high fingerprint recognition rate can be provided.

Description

Polyester film for display protection

Technical Field

The present invention relates to a polyester film for display protection, and a polyester film for display protection having excellent adhesive force and optical and mechanical characteristics.

Background

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

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, a high total transmittance and a low optical loss are required for an optical fingerprint recognition function employed by most portable display devices. The optical loss of the polyester film is related to physical properties represented by an orientation angle or an optical axis. Basically, the polyester film has 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 or circularly polarized light, which may appear as color change or iridescent irregularity to human eyes.

In order to solve these problems, many inventions using a polyester film have been actively made. As an example of such an invention, 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 invention, a problem arises that the fingerprint recognition rate on the display protective film to be subjected to surface treatment is lowered, and no attempt is made to reduce the orientation angle of the main chain therefor, and therefore the productivity in obtaining a practical effective width is lowered, which hinders the use of the polyester film as a film for display protection.

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

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

[ Prior art documents ]

[ patent document ]

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

Disclosure of Invention

Technical problem

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

Further, it is an object of the present invention to provide a polyester film for display protection, which has a high total transmittance and is capable of suppressing optical loss, and which is capable of minimizing the occurrence of foreign substances that may reduce a 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 invention will become apparent from the following description of the preferred embodiments.

Technical scheme

The above object is accomplished by providing a polyester film for display protection, comprising: a biaxially stretched 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.

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

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

(equation 1)

Optical loss ═ (maximum amount of light of polarizer in parallel nicol state-maximum amount of light after interposing film between two polarizers)/(maximum amount of light of polarizer in parallel nicol state)

Preferably, the difference between the orientation angle and the optical axis of the polyester film for display protection 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 polyester film for display protection may have a total transmittance of 93% or more.

Preferably, the first primer layer may include a polyester copolymer resin, a polyurethane-based resin, a resin

Curing agents for oxazolines, melamine based curing agents, anionic surfactants 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 barrier 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 primer layer on one surface of an 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 primer layer and the second primer layer 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 invention can suppress optical loss by high total transmittance. In addition, both Liquid Crystal Display (LCD) devices and Organic Light Emitting Diode (OLED) display devices use polarizers. In order to allow 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 invention may have a high fingerprint recognition rate due to a small change in the optical axis.

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

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

Drawings

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

Detailed Description

Hereinafter, embodiments of the present invention 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 present invention is not to be construed as limited to the embodiments set forth herein and may be embodied in many different forms.

The size and thickness of elements in the drawings may be exaggerated for convenience of description. Like reference numerals refer to like elements throughout the specification. Further, it will be understood that when an element such as a layer, film, region or panel 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 panel 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 invention belongs. If a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference. Although methods and components similar or identical to those described in this disclosure can be applied to embodiments or tests of the invention, the disclosure provides suitable methods and components.

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

In one embodiment, the base film 2 preferably comprises at least one polyester selected from the group consisting of 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 surface treatment and lamination of the polarizer, and good 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, such as 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 primer layer 1 formed on one surface of the base film 2 and the second primer 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 primer layer 1 generally forms a hard coat 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, in order to control rainbow unevenness caused by interference when the hard coat 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 base coat layer 1 comprises a material selected from the group consisting of

At least one material of a material of oxazoline, a material of carbodiimide, and a material of melamine is used as the curing agent resin. In particular, based on

The curing agent of oxazoline inhibits moisture penetration into the film due to moisture resistance, and the melamine-based curing agent reacts with the base material and also improves the strength of the coated film through the curing reaction between its own melamine, thereby preventing a blocking phenomenon that may occur during double-sided coating. Preferably, can comprise a base

A curing agent mixture of an oxazoline curing agent and a melamine-based curing agent.

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

In addition, the first undercoat layer 1 may also contain additional additives in the coating liquid to improve coating characteristics and functionality. As described above, organic particles, inorganic particles, an antifoaming agent, and the like may be used as an additive for the first undercoat layer 1.

In the present invention, the first undercoat layer 1 may ensure running characteristics by the 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 in that haze increases, and when the average particle diameter of the inorganic particles is less than 10nm, running characteristics cannot be secured.

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

In one embodiment, the first primer layer 1 is preferably formed by applying a coating liquid containing: binder resin composed of a mixture of polyester copolymer resin and polyurethane-based resin, binder resin composed of polyurethane-based resin

A mixture of an oxazoline curing agent and a melamine based curing agent, an anionic surfactant, and inorganic particles.

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

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

Next, since the adhesive or the binder is treated on the surface of the second undercoat layer 3, it is preferable to form an undercoat layer capable of achieving the treatment of the adhesive or the binder, and to suppress the occurrence of foreign matter after the heat treatment, it is preferable to have a function of oligomer blocking. Since functional groups capable of reacting with the curing agent are increased in the base material of the second undercoat layer 3 and a 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 it is possible to prevent the oligomer formed after heat treatment from precipitating on the undercoat layer. As described above, the polyester film for display protection according to one embodiment of the present invention can minimize the occurrence of foreign substances during heat treatment, thereby increasing 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, it is preferable to use a polyacrylic resin and/or a polyalkylene oxide resin having a carboxyl group as a functional group as a binder resin of the second undercoat layer 3 and design a specific cross-linked structure. Preferably, a polyacrylic resin having a carboxyl group as a functional group may be included.

In addition, the second primer layer 3 comprises a material selected from the group consisting of

The curing agent of oxazoline inhibits moisture from penetrating into the film, and the melamine-based curing agent reacts with the base material and also improves the strength of the coated film through the curing reaction between its own melamine, thereby preventing a blocking phenomenon that may occur during double-sided coating. Preferably, a melamine based curing agent may be included.

Further, the second primer layer 3 preferably contains an anionic surfactant as an additive.

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

In the present invention, the second undercoat layer 3 may ensure running characteristics by the 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 in that haze increases, and when the average particle diameter of the inorganic particles is less than 10nm, running characteristics cannot be secured. Further, the inorganic particles are preferably at least one selected from silica particles and silica-organic composites. In this case, the inorganic particles may be the same inorganic particles as those of the first undercoat layer 1 described above.

The composition constituting the second primer layer 3 is prepared with water into a coating liquid in the form of an aqueous dispersion 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 application thickness is preferably in the range of 10nm to 200 nm. At this time, when the application thickness is 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 and wet adhesion resistance 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 invention 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 of thickness, it is more preferable that the polyester film for display protection is a biaxially stretched film.

In the 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-setting is performed after the stretching process.

The orientation angle of the main chain means an angle formed by the crystal 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 the stretching in the width direction was 0 degree. In addition, during the heat-setting process, a bending phenomenon occurs due to residual stress after the stretching. Due to the bending phenomenon, a bow gradient occurs in a direction opposite to the advancing direction of the film.

Since the orientation of the main chain crystal causes a change in the optical axis, if the orientation angle is not decreased, optical loss occurs after the protective film is formed, thereby decreasing 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 properties at the center of the width. In order to suppress the bending phenomenon, the film must be stretched in the TD direction more than in the MD direction, and therefore, the ratio of MD stretch to TD stretch is preferably 1:1.45 or more, and the ratio is preferably adjusted to 1:1.75 or less 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, and 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 loss of light 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 invention, it is preferable that the optical loss obtained from equation 1 is 11% or less. When the optical loss exceeds 11%, there arises a problem 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 invention, the optical loss obtained by equation 1 generally has a minimum value at the central portion around an orientation angle of 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 invention, 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 between the orientation angles of the central 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 central 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 and the optical axis of the polyester film for display protection is 9 degrees or less. When the difference between the orientation angle and the optical axis is greater than 9 degrees, the difference in optical axis between the polarizer and the polyester film in the display device increases, so that pressing along the optical axis of the polarizer cannot be performed, and even when pressing is performed, the fingerprint recognition rate significantly decreases, and distortion in screen color 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 decreases and the brightness of the display material decreases.

Further, it is preferable that the polyester film for display protection according to one embodiment of the present invention 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 the polyester film for display protection according to one embodiment of the present invention 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 primer layer on one surface of an 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 less than 180 ℃, heat-setting of the base film 2, the first undercoat layer 1, and the second undercoat layer 3 cannot be appropriately performed, and when the temperature exceeds 220 ℃, a bowing 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 invention, redundant description of the above polyester film for display protection will be omitted.

Hereinafter, the structure of the present invention 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 invention in more detail, and the scope of the present invention is not limited to the examples.

[ examples ]

[ preparation examples ]

(coating solution 1)

As the binder resin, 12.5% by weight of a base material 1 composed of 70% by weight of water and 30% by weight of a polyurethane resin (H-15 of chemical Industries) and 70% by weight of water and 30% by weight of a polyester-based copolymer resin (Takamatsu Oil) were added&TR620K of Fat co., ltd.) 12.5 wt% of the base material 2 are mixed with each other. Mixing together: 20.0% by weight of the aqueous base resin dispersion thus formed, and a curing agent for improving moisture resistance composed of 40% by weight of a base resin

Curing agent for oxazoline (WS 500 from Nippon Carbide Industries Co., Inc.) 1.87% by weight of curing agent 1 consisting of 60% by weight of water, and curing agent based on melamine consisting of 70% by weightAgent (PM80, DIC co., Ltd.) and 30 wt.% water 1.87 wt.% of curing agent 2. Coating solution 1 was prepared by adding to the mixture: 1.0% by weight of an aqueous surfactant dispersion consisting of 90% by weight of water and 10% by weight of an anionic surfactant (RY-2 of Goo Chemical CO., LTD.), 0.5% by weight of a water-dispersible particle coating solution consisting of 70% by weight of silica particles and 30% by weight of water, and the remainder of water.

(coating solution 2)

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

(coating solution 3)

Coating solution 3 was prepared in the same manner as coating solution 1 except that a urethane resin (DIC CO., AP-40F of LTD.) was used as a binder resin in place of the urethane resin (H-15 of Chemil Industries).

[ examples 1 to 4]

After melt-extruding a raw material sheet of polyethylene terephthalate, an unstretched sheet was produced 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 bar and dried at 80 ℃ to form a first undercoat layer and a second undercoat layer each having a thickness of 100 nm. Next, after the unstretched sheet was stretched in both the longitudinal 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, 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 the following tables 2 and 3.

[ Experimental example ]

(1) Measurement of Total Transmission

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

(2) Thickness measurement

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

(3) Measurement of optical loss

After the film was cut into a sheet having a width of 5cm and a length of 5cm, two polarizing films were arranged in a 100% transmission state (parallel nicols state) to check a 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 the film was biaxially stretched, the crystal direction of the main chain with respect to the width direction of the film was measured at the central portion and the edge portion of the film sample using an orientation angle measuring apparatus (nosura shojico., SST-4000 of ltd.). In this case, since the orientation angle of the central portion is 0 degree, 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 a polarizer in a cross-nicol state, the angle at which the black mode was achieved was checked and measured with a protractor.

(6) Inspection of rainbow 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: iridescent irregularities were not observed

X: rainbow irregularities were observed

(7) Measurement of haze Change

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

(8) Measurement of adhesion

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

(equation 2)

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

(9) Measurement of refractive index of undercoat layer

After the coating liquids 1 to 3 were applied on the glass plates, respectively, and heat cured, black insulating tapes were attached to the back surfaces of the glass plates on which the coating liquids were not applied, and then the refractive indices were measured using an ellipsometer (Elli-SE of Ellipso Technology co.

[ Table 2]

[ Table 3]

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

In contrast, in comparative example 1, it can be seen that the difference in refractive index between the first undercoat layer and the second undercoat 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 change 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 was the same between the undercoat layers, and therefore the total transmittance was low, and rainbow-like irregularities occurred or the haze change was large.

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

As described above, the polyester film for display protection according to one embodiment of the present invention can improve productivity during post-processing by minimizing interference unevenness after a hard coating process on a surface, suppress foreign matter occurrence 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 invention can be widely used as an optical film other than a protective film due to its excellent optical characteristics.

While the invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention 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 undercoat layer 2: base film

3: second base coat layer

Claims

1. A polyester film for display protection, comprising:

biaxially stretching the 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 a difference in refractive index between the first undercoat layer and the second undercoat layer is 0.1 or more.

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

3. A polyester film according to claim 1 wherein the polyester film for display protection has an optical loss of 11% or less according to the following equation 1:

equation 1

Optical loss ═ (maximum amount of light for the polarizer in the parallel nicol state-maximum amount of light after the film was inserted between the two polarizers)/(maximum amount of light for the polarizer in the parallel nicol state).

4. A polyester film according to claim 1, wherein the difference between the orientation angle and the optical axis of the polyester film for display protection is 9 degrees or less.

5. A polyester film according to claim 1 wherein the difference in orientation angle between the central portion and the edge portion of the polyester film for display protection is 20 degrees or less.

6. A 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.

7. A polyester film according to claim 1 wherein the refractive index of the first primer layer is in the range of 1.58 to 1.62 and the refractive index of the second primer layer is in the range of 1.45 to 1.49.

8. A polyester film according to claim 1 wherein the ratio of MD draw ratio to TD draw ratio is from 1:1.45 to 1: 1.75.

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

10. A polyester film as defined in claim 1, wherein the first primer layer comprisesPolyester copolymer resin, polyurethane-based resin composition

11. A polyester film according to claim 1, wherein the second primer layer comprises a polyacrylic resin having a carboxyl group, a melamine-based curing agent, an anionic surfactant, and inorganic particles.

12. A polyester film according to claim 1 wherein the second primer layer has an oligomer barrier function.

13. 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.

14. A polyester film according to claim 1 wherein the thickness of the base film is in the range 25 μm to 250 μm.

15. 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 primer 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 a 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.

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

17. The method according to claim 15, wherein the temperature for the heat-setting of the sixth step is in the range of 180 ℃ to 220 ℃.