KR102438415B1 - Optical polyester protection film - Google Patents

Abstract

The present invention relates to a polyester protective film, and more particularly, to an optical polyester protective film and a method for manufacturing the same.

Description

Optical polyester protection film

The present invention relates to a polyester protective film, and more particularly, to an optical polyester protective film and a method for manufacturing the same.

Various types of displays used in TVs, computers, mobile phones, etc. have been developed and commercialized, from light-receiving type liquid crystal displays (LCDs) to light-emitting type displays such as OLED displays that have recently been commercialized. These display devices are provided with optical films of various functions that are arranged on the optical path until the light emitted from the light source exits the front side of the viewer. , the viewing angle, brightness, etc. are improved.

Meanwhile, in recent years, display devices have been required to be able to withstand use in various environments, for example, harsher environments. For example, a display device for a mobile device such as a mobile phone or a display device for a vehicle such as a car navigation system is required to require durability even for use in a harsher environment. In addition, in large display devices such as televisions, the inside of the display device tends to be exposed to a high temperature environment due to an increase in the amount of heat generated by the enlargement and high luminance. Accordingly, various types of optical films used in display devices are required to have small characteristic changes, ie, high durability under harsh environments such as high temperature and high humidity.

On the other hand, these optical films are sometimes composed of a functional layer that performs a desired function, but in order to solve the problem that the functional layer is weak in moisture, heat, and mechanical strength, a structure in which a protective film is laminated on the functional layer is adopted. . For example, the absorption polarizer is an optical film commonly used in LCD or OLED displays. In the absorption polarizer, triacetyl cellulose (TAC) is widely used as a protective film to protect weak mechanical strength. However, when the triacetyl cellulose film is thinned, sufficient mechanical strength cannot be obtained, and moisture permeability is high, so that the polarizer is easily deteriorated. In addition, since the triacetyl cellulose film is expensive, there is a situation in which a low-cost alternative material is required.

Due to these demands, recent attempts to use a polyester film as a protective film for an optical film replacing the TAC film are continuing. Polyester films are less expensive than TAC films and have high mechanical strength.

However, the polyester film has a large birefringence property and has a large birefringence property in the in-plane and thickness directions during stretching. There is a problem in that the stain is visible. In particular, the use of a polyester film as a protective film of an optical film disposed on the viewing side of a liquid crystal cell, such as a polarizer, is limited as these rainbow spots are more easily recognized according to recent LCD high brightness and high color purity. In addition, even if it is implemented to express excellent properties such as no rainbow staining at the center of the width, problems such as rainbow stains occurring toward the end of the width direction or changes in optical properties even if such stains do not occur are still inherent. It is difficult to show physical properties, and it is difficult to obtain a large width.

Registered Patent Publication No. 688256

The present invention guarantees moisture resistance, heat resistance, impact resistance, etc., and has excellent and uniform optical properties over a wide width, significantly reduces the visibility of rainbow stains due to interference of transmitted light even in various types of light sources, and realizes excellent color reproducibility. Accordingly, an object of the present invention is to provide a polyester protective film that is very suitable as a protective film for optical films.

In order to solve the above problems, the present invention provides a step of (1) melt-extruding a polyester resin to prepare an unstretched sheet, (2) first stretching the unstretched sheet 3.0 to 3.3 times in the longitudinal direction (MD) , (3) The sheet is stretched in multiple stages in the width direction (TD) for a total of 3.3 to 3.7 times the second stretch, but at least three stages are stretched so that the draw ratio for each section becomes smaller toward the rear end, and the width change rate in the last stretching section is 20% or less. It provides a method of manufacturing an optical polyester protective film comprising the steps of stretching as possible, and (4) heat setting the stretched sheet.

According to an embodiment of the present invention, the amount of change in width in the first stretching section in the multi-stage stretching in step (3) may be more than twice the amount of change in width in the last stretching section.

In addition, the multi-stage stretching of step (3) is performed under a temperature condition of 140 ~ 170 ℃, the stretching temperature of the last stretching section is performed at a temperature exceeding 155 ℃, and the stretching temperature of the first stretching section is 15 ℃ compared to the last stretching section It can be carried out at a lower temperature.

In addition, the width change rate in the last stretching section in the multi-stage stretching of step (3) may be 15% or less.

In addition, the present invention provides an optical polyester protective film comprising polyethylene terephthalate, wherein the in-plane retardation measured at a wavelength of 632 nm is 650 nm or less, the optical axis is less than 40°, and the orientation angle is 13° or less.

According to an embodiment of the present invention, the deviation between the optical axis and the orientation angle at a predetermined point may be 45° or less, and more preferably 35° or less.

In addition, the thickness direction retardation in width may be 8000 nm or more.

In addition, the thickness direction retardation difference between two points symmetrical in the width direction with respect to the center of the width may be 55 nm or less.

In addition, the difference between the maximum value and the minimum value of the thickness direction retardation in width may be 70 nm or less.

Further, the width may be ±750 mm or more, more preferably ±1300 mm or more, and still more preferably ±1500 mm or more.

In addition, the in-plane retardation may be 570 nm or less.

Also, the orientation angle may be 10° or less.

In addition, the thickness direction retardation variation rate of Equation 3 below with respect to the thickness direction retardation measured at a total of five points spaced at equal intervals from the center of the width by 370 mm in both width directions may be 1% or less. In addition, more preferably, the thickness direction phase difference variation rate of Equation 3 below for the thickness direction retardation measured at a total of 9 points spaced at equal intervals with an interval of 370 mm from the width center in both width directions may be 1% or less. .

[Equation 3]

In addition, the present invention provides an optical laminate comprising an optical film and a polyester protective film according to the present invention disposed on at least one surface of the optical film.

In addition, the present invention provides an image display device including the optical laminate according to the present invention.

The polyester protective film according to the present invention is very suitable for the purpose of protecting the functional layer of the optical film as moisture resistance, heat resistance, and mechanical strength are guaranteed, and has the advantage of lowering the product cost due to the low unit price. In addition, unlike a conventional optical polyester protective film, the visibility of rainbow stains due to interference of transmitted light is significantly reduced, and when applied to various displays implemented by applying various types of light sources, the effect of the types of light sources is minimized. Excellent color reproducibility can be realized. Furthermore, as such excellent and uniform optical properties are realized within an increased width, it is very suitable for being employed in optical devices such as displays that have a large area.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art can easily implement them. The present invention may be embodied in many different forms and is not limited to the embodiments described herein.

The polyester protective film according to an embodiment of the present invention includes polyethylene terephthalate, and the in-plane retardation measured at a wavelength of 632 nm is 570 nm or less, the optical axis is less than 40°, and the orientation angle is 13° or less.

The polyester protective film includes polyethylene terephthalate, and the polyethylene terephthalate contains terephthalic acid as an acid component and ethylene glycol as a diol component. Polyesters copolymerized with polyvalent carboxylic acids other than terephthalic acid and/or other diol components other than ethylene glycol also fall within the category of polyethylene terephthalate according to the present invention.

In addition, the polyester protective film may further include a heterogeneous polyester component such as polyethylene naphthalate, polycarbonate, and polybutylene terephthalate in addition to polyethylene terephthalate.

The polyester protective film according to an embodiment of the present invention has an in-plane retardation of 570 nm or less measured at a wavelength of 632 nm, an optical axis of less than 40°, and an orientation angle of 13° or less, through which various different light sources are employed. Even in this case, it may be advantageous to prevent the recognition of rainbow spots due to distortion of transmitted light.

The in-plane retardation is calculated by Equation 1 below, and 'Nx' and 'Ny' mean the refractive index of the polyester film in the x-axis direction and the y-axis direction at a wavelength of 632 nm. For example, the x-axis direction may be MD, and the y-axis direction may be TD. In addition, d shows the thickness (unit: nm) of a polyester film.

The polyester protective film according to the present invention may have an in-plane retardation of 650 or less measured at a wavelength of 632 nm, preferably 570 nm or less, more preferably 150 to 570 nm, more preferably 150 to 550 nm. If the in-plane retardation exceeds 650 nm, it is difficult to maintain the degree of polarization, and rainbow spots may occur.

[Equation 1]

In-plane phase difference = (Nx - Ny)×d(nm)

In addition, the polyester protective film according to the present invention satisfies the in-plane retardation within the effective width of 650 nm or less, the orientation angle of the main chain is 13° or less, and the optical axis is less than 40°, thereby preventing the recognition of rainbow stains due to distortion can be advantageous

The orientation angle is defined as 0 degrees when it is parallel to the width direction of the polyester protective film, a clockwise inclination with respect to the film width direction is defined as (+), and a counterclockwise rotation is defined as (-). In addition, that the inclination of the orientation main axis with respect to the film width direction is the same direction is defined as the film width direction both ends of the width-set film showing the same code|symbol in the inclination measurement of the orientation main axis. In general, in a biaxially oriented polyester film, the inclination of the main axis of orientation is caused by a bowing phenomenon that occurs when a heat treatment process is performed in a stretching process. Since the bowing phenomenon is a phenomenon resulting from the formation of a catenary line (category curve) of the film in the stretching process to the heat treatment process, in a biaxially oriented polyester film, as it goes from the center in the width direction to the end of the film in the width direction, the orientation of the main axis The slope will usually show a tendency to increase. Moreover, in the two directions toward the width direction edge part from the width direction center of a biaxially-oriented polyester film, it has the orientation principal axis inclination of a reverse direction, respectively.

On the other hand, the bowing phenomenon affects the orientation angle of the main chain of the polyester resin, and the optical axis is affected by the mixed state of the main chain and the amorphous region of the polyester resin. have. The present invention is more advantageous to implement so that the optical axis does not exceed 40° by satisfying the orientation angle of the main chain of 13° or less, preferably 10° or less, and it is possible to prevent the phase difference of transmitted light from increasing.

In addition, since the optical film according to the present invention has an optical axis within a width of less than 40°, it is possible to prevent the recognition of rainbow spots due to distortion. In addition, when used together with a polarizing plate, light loss due to a difference in optical axis with the polarizing plate can be reduced. On the other hand, the optical axis may be measured as a film angle at the point in time when the black mode of the polarizing plate cross nicolo polarizing plate is realized.

In addition, according to an embodiment of the present invention, the difference between the optical axis and the orientation angle at a predetermined point can be at most 45° or less, more preferably 38° or less, more preferably 35° or less, even more preferably 30° or less. And, through this, it may be more suitable for achieving the object of the present invention, such as preventing the recognition of rainbow spots in various light sources and improving color reproducibility.

In addition, the width of the protective film means a region in which the above-described in-plane retardation is 650 nm or less, the optical axis is less than 40° and the orientation angle is 13° or less, and the width is a predetermined width in the direction of both ends based on the width center. The distance to the point may be, for example, more than ±750 mm, and as another example, ±1500 mm and ±2250 mm.

According to a preferred embodiment of the present invention, the thickness direction retardation measured at a wavelength of 632 nm of the polyester protective film may be 8000 nm or more, more preferably 8200 nm or more to 12000 nm. The thickness direction retardation is defined by Equation 2 below, and 'Nx', 'Ny', and 'Nz' are the x-axis direction, y-axis direction, and z-axis direction (thickness direction) of the polyester film at a wavelength of 632 nm. Refractive index (refractive index) means, d represents the thickness (unit: nm) of the polyester film.

[Equation 2]

Thickness direction retardation = {(Nx + Ny)/2 - Nz}×d(nm)

The retardation in the thickness direction affects the visibility of rainbow spots due to distortion, and when the retardation in the thickness direction is 8000 nm or more, visibility of rainbow spots due to distortion of transmitted light can be significantly reduced.

According to an embodiment of the present invention, the thickness direction retardation difference between two points symmetrical in the width direction with respect to the center of the width may be 55 nm or less. In addition, the difference between the maximum value and the minimum value of the retardation in the thickness direction within the width may be 70 nm or less, more preferably 50 nm or less, through which the visibility of rainbow spots can be more significantly reduced regardless of the position within the width, , it may be advantageous to further prevent the recognition of rainbow spots that increase when obliquely observed at an angle of 40° or more left and right. In addition, even when rainbow spots are not visually recognized by any light source, it is possible to prevent recognition of rainbow spots that may occur as the type of light source is changed. If the thickness direction retardation difference between two points symmetrical in the width direction with respect to the width center exceeds 55 nm, or the difference between the maximum value and the minimum value of the thickness direction retardation within the width exceeds 70 nm, the prevention effect of rainbow spots is reduced, and it may be difficult to achieve the object of the present invention, such as rainbow spots may be visually recognized depending on the location, and in particular, rainbow spots may be displayed even when observed only slightly obliquely from the front.

On the other hand, when the difference between the thickness direction retardation difference of 55 nm or less and the maximum and minimum value of the thickness direction retardation within the width of two points symmetrical in the width direction with respect to the width center simultaneously satisfies 70 nm, the rainbow within the width regardless of the position A synergistic effect on the anti-visibility effect of stains, the characteristics of transmitted light passing through the polyester protective film may be more uniform, and these advantages may contribute to uniform light leakage, uniform contrast ratio, uniform color tone, and the like.

In addition, the thickness direction phase difference variation rate of Equation 3 below with respect to the thickness direction retardation measured at a total of five points spaced at equal intervals from the center of the width by 370 mm in both width directions is 1% or less, more preferably 0.5 % or less, more preferably 0.3% or less, through which the optical properties of the transmitted light can be more uniform regardless of the position, and achieve the object of the present invention, such as a further increase in the anti-visibility effect of rainbow blot It may be more advantageous to: In addition, more preferably, the thickness direction phase difference variation rate of the following Equation 3 for the thickness direction retardation measured at a total of 9 points spaced at equal intervals with an interval of 370 mm from the width center in both width directions is 1% or less, more preferably It may be very advantageous to achieve the object of the present invention through satisfying 0.5% or less, more preferably 0.3% or less.

[Equation 3]

In addition, the polyester protective film may have a thickness of 30 to 100 μm, more preferably 50 to 75 μm. If the thickness exceeds 100 μm, an increase in in-plane retardation and a change in retardation may be large, and an optical axis may increase as the stress during stretching increases, which is undesirable for thinning. In addition, if the thickness is less than 30㎛, the mechanical strength may be reduced and it may be difficult to perform a function as a protective film, and there may be problems of wrinkle incorporation in the processing process using the protective film and breakage due to lack of mechanical strength.

The polyester protective film according to an embodiment of the present invention described above includes (1) melt-extruding a polyester resin to prepare an unstretched sheet, (2) 3.0 to 3.3 times the unstretched sheet in the longitudinal direction (MD) The first stretching step, (3) stretching the sheet in multiple stages in the width direction (TD) to a second stretching of 3.3 to 3.7 times in total, but stretching at least three stages so that the stretching ratio for each section becomes smaller toward the rear end, and the width change rate in the last stretching section It can be prepared including the steps of stretching so that this is 20% or less, and (4) heat setting the stretched sheet.

First, as step (1) according to the present invention, a step of melt-extruding a polyester resin to prepare an unstretched sheet is performed.

In step (1), a polyester resin containing polyethylene terephthalate may be melted to prepare a conventional film base material. In addition, the melting temperature during melt extrusion of the polyester resin may be appropriately changed in consideration of the type of polyester resin used, so the present invention is not particularly limited thereto.

The thickness of the unstretched sheet manufactured through step (1) may be 40 to 250 μm, and when it exceeds 250 μm, it may be difficult to control the phase difference, and it may be difficult to be employed in optical devices such as displays in a thinning trend.

Next, as the step (2) of the present invention, a first stretching step of stretching the unstretched sheet in the longitudinal direction (MD) is performed. In the first stretching, the stretching ratio may be 3.0 to 3.3. If the stretching ratio in the longitudinal direction is less than 3.0, it is difficult to control the thickness of the film, and the strength is low and there is a risk of being easily torn in the longitudinal direction. In addition, when the longitudinal stretch ratio exceeds 3.3, it may be difficult to implement the desired physical properties of the present invention, such as there is a risk that the orientation angle of the main chain is increased due to high residual shrinkage stress. In addition, the temperature during the first stretching may be carried out at 130 ~ 170 ℃, through this, it may be easy to manufacture a polyester protective film having the desired physical properties of the present invention.

Next, as the step (3) of the present invention, the sheet stretched in the longitudinal direction is stretched in multiple stages in the width direction (TD) to a total of 3.3 to 3.7 times the second stretch, but at least three stages are stretched so that the draw ratio for each section becomes smaller toward the rear end. perform the steps to Through this, the mechanical strength of the stretched film is guaranteed by achieving the total draw ratio in the second stretching, and it may be advantageous to minimize the bowing phenomenon. If the stretch ratio in the width direction is less than 3.3, it is difficult to control the thickness of the film, and there is a possibility that the film may be easily torn in the width direction. In addition, when the stretch ratio in the width direction exceeds 3.7, the residual shrinkage stress is high, so that thermal contraction is increased, and the deviation of the phase difference may be large, and it may be difficult to implement the physical properties desired by the present invention.

In addition, when stretching is performed twice or once without multi-stage stretching at least three times, it may be difficult to control the optical axis to a desired level, and it may be difficult to achieve uniformity of optical properties in the width direction. In addition, when the draw ratio does not decrease toward the rear end even in the case of multi-stage stretching, specifically, when the draw ratio increases than the previous section or when the same section exists, the optical axis is less than 40°, the orientation angle is 13° or less, and/or the in-plane retardation 650 It may not be easy to achieve physical properties of nanometers or less, and it may be difficult to achieve the object of the present invention, such as a large difference in optical properties and/or a very small width depending on the location.

In addition, the optical axis of less than 40 ° is achieved by stretching in multiple stages in the width direction, but stretching with a small stretching ratio toward the rear end, and stretching so that the width change rate is 20% or less, more preferably 15% or less in the last stretching section. can be advantageous In addition, it is possible to minimize the difference in optical properties at various points with different positions within the width, and ultimately, while realizing an optical film that expresses uniform properties regardless of location, rainbow blotches of transmitted light according to various types of light sources are eliminated. It may be more advantageous to prevent it. If the width change rate exceeds 20% in the last stretching section, it may be difficult to achieve an optical axis of less than 40°. In addition, even if there is a portion having an optical axis of less than 40° within the width, the width satisfying the optical axis condition based on the center of the width is very narrow, so it may be difficult to achieve a width exceeding ±375 mm in one example and ±750 mm in another example. have. Preferably, the width change rate in the last stretching section may be 15% or less, through which the thickness direction retardation deviation in the width direction can be realized to be smaller, and the maximum deviation between the optical axis and the orientation angle at a predetermined point is reduced, so that it can be used in various light sources. It may be possible to prevent rainbow stains, to improve color reproducibility, and to achieve the optical properties desired by the present invention in a wider width. However, in the last stretching section, the width change rate may be 7% or more, and if it is stretched to less than 7%, the optical axis rather increases, the difference between the optical axis and the orientation angle increases, and the thickness direction retardation uniformity may decrease.

On the other hand, the rate of change in width of the specific stretching section means the percentage of the change in width length in the specific stretching section with respect to the total change in width length through the second stretching, and is calculated through Equation 4 below.

[Equation 4]

Here, the total change in width length through the second stretching is an increase in the final width length through the second stretching compared to the width length before stretching, and is calculated as the difference between the final width length and the width length before stretching. In addition, the amount of change in width length in a specific stretching section is calculated as the difference between the width length at the end of the specific stretching section and the total width in the specific stretching section.

In addition, according to an embodiment of the present invention, during multi-stage stretching, the amount of change in width in the first stretching section may be more than twice the amount of change in width in the last stretching section, and through this, the uniformity of optical properties such as retardation in the thickness direction in the width direction is more guaranteed advantageous to In addition, since the maximum deviation between the optical axis and the orientation angle is reduced at a predetermined point, it is possible to prevent rainbow stains from various light sources, to improve color reproducibility, and to achieve the optical properties desired by the present invention in a wider range. can If the amount of change in width in the first stretching section is less than twice the amount of change in width in the last stretching section, it may be difficult to achieve the desired level of retardation in the thickness direction, and depending on the location and/or up and down, left and right angles of 40° or more (front It may not be easy to achieve the object of the present invention, such as a rainbow stain may be visually recognized when obliquely observed at 0°. In addition, it may be difficult to implement a protective film having a wide width in which optical properties are satisfied.

In addition, the temperature during the second stretching may be carried out at 140 ~ 170 ° C. Preferably, the temperature can be set differently depending on the stretching section during multi-stage stretching, and specifically, the stretching temperature of the last stretching section is at a temperature of more than 155 ° C. It can be carried out, and the stretching temperature of the first stretching section may be performed at a temperature lower than 15 ° C. compared to the stretching temperature of the last stretching section, and through this, it may be advantageous to achieve the desired physical properties of the present invention in an increased width. If the stretching temperature of the last stretching section is 155° C. or less, and/or if the stretching temperature in the first stretching section is higher than or equal to or lower than the stretching temperature in the last stretching section, it is sufficiently increased when performed at a low temperature of less than 10° C. It may be difficult to realize the specified width, and there is a possibility that the deviation of optical properties such as thickness direction retardation and in-plane retardation may be greatly realized for each location, or rainbow spots may be recognized due to transmitted light depending on the type of light source.

On the other hand, according to an embodiment of the present invention, step (3) described above can perform multi-stage stretching four times, and in the first section, which is the first drawing section, the draw ratio is 1.6 to 1.75, and the stretching temperature is 140 to 150 ℃, In the second section, the draw ratio is 1.45 ~ 1.6, the stretching temperature is 140 ~ 150℃, in the third section the stretching ratio is 1.2 ~ 1.35, the stretching temperature is 145 ~ 155℃, in the fourth section, the last stretching section, the stretching ratio is 1.05 ~ 1.15, The stretching temperature may be 155 ~ 170 ℃.

Next, as step (4) of the present invention, a step of heat-setting the stretched sheet is performed. The heat setting temperature may be performed at a temperature lower than a normal heat setting temperature to suppress the bowing phenomenon, thereby minimizing the stress difference between the stretched part and the heat part. The temperature at the time of heat setting may be, for example, 180 ~ 220 ℃, if it is less than 180 ℃, dimensional stability may be deteriorated due to poor heat setting, and if it exceeds 220 ℃, the bowing phenomenon becomes severe and the orientation angle and the optical axis increase can cause

According to an embodiment of the present invention, a primer layer may be further provided on one or both sides of the polyester protective film prepared by the above-described method. Although the protective film according to an embodiment of the present invention is designed to prevent visibility of rainbow stains by transmitted light, light reflected from the post-processed resin and interface causes reinforcing and destructive interference, respectively. A primer layer may be further provided to prevent .

As the primer layer, a known primer layer provided to prevent the recognition of rainbow stains on the optical film may be used without limitation. For example, the primer layer may include a binder resin, organic or inorganic particles, and a curing agent. The binder resin may include one or more of polyurethane-based resins and water-dispersed polyester resins as a subject. In addition, it is preferable to use the organic or inorganic particles having an average particle diameter of 10 to 500 nm in order to secure running properties. If the average particle diameter of the particles exceeds 500 nm, the haze may increase, and if the average particle diameter of the particles is less than 10 nm, the surface roughness is lowered, blocking occurs, or difficulty in film winding occurs, and the appearance may be deteriorated . In addition, the inorganic particles preferably have a refractive index of 1.5 or more as at least one of silica particles and a silica-organic composite. When the refractive index of the inorganic particles is less than 1.5, the overall refractive index of the primer layer is lowered, and rainbow spots may be visually recognized.

In addition, the primer layer may include at least one selected from an oxazoline-based curing agent, a carbodiimide-based curing agent, and a melamine-based curing agent. In particular, the oxazoline-based curing agent suppresses moisture permeating into the polyester protective film or reacts with moisture, thereby preventing blocking that may occur during double-sided coating. In addition, although the melamine-based curing agent reacts with the main agent, it is possible to prevent the blocking phenomenon that may occur during double-sided coating by improving the strength of the coating film through its own curing reaction between melamine.

In addition, the primer layer may further include additives such as anionic surfactants or antifoaming agents, and it is disclosed that various kinds of additives provided in conventional films or optical films other than surfactants may be included.

The polyester protective film according to an embodiment of the present invention described above may be provided on one or both sides of the optical film to implement an optical laminate. The optical film may be a known film in the optical field, and for example, a diffusion film, a reflective polarizing film, an absorption polarizing film, a retardation film, a compensation film, and the like.

In addition, the optical laminate in which the polyester protective film is integrated with the optical film may be mounted on various optical devices, and may be provided in, for example, an image display device. The image display apparatus may be a light-receiving type image display apparatus or a light emitting type image display apparatus, and in the present invention, a detailed description of the image display apparatus is omitted.

The present invention will be described in more detail through the following examples, but the following examples are not intended to limit the scope of the present invention, which should be construed to aid understanding of the present invention.

<Example 1>

A polyethylene terephthalate chip was melt-extruded to prepare an unstretched sheet on a casting roll. Thereafter, the unstretched sheet was first stretched in the longitudinal direction (MD) at a temperature of 150° C. and a draw ratio of 3.1, and then multi-staged four times in the width direction to perform a second stretching at a total draw ratio of 3.4. Specifically, in the second stretching, the first section is drawn at a temperature of 145 °C, a draw ratio of 1.7, the second section is at a temperature of 145 °C, a draw ratio of 1.5, a third section is a temperature of 150 °C, a draw ratio of 1.2, and the fourth section is a temperature of 160 °C, a draw ratio of 1.1. and then heat-setting at 210° C. to prepare a polyester protective film as shown in Table 1 below having a final width of 3370 mm and a thickness of 50 μm.

<Examples 2 to 3>

It was prepared in the same manner as in Example 1, except that the process conditions were changed as shown in Table 1 below to prepare a polyester protective film as shown in Table 1 below.

<Comparative Examples 1 to 2>

It was prepared in the same manner as in Example 1, except that the process conditions were changed as shown in Table 1 to prepare a polyester protective film as shown in Table 1 below.

<Experimental example>

The following physical properties were evaluated for Examples and Comparative Examples, and the results are shown in Table 1 below.

1. In-plane and thickness direction retardation measurement

The in-plane and thickness-direction retardation of a total of 17 points spaced at equal intervals from the center of the width of the prepared film in the direction of both ends were measured at intervals of 185 mm. Specifically, for the in-plane retardation, the biaxially stretched film is first placed between two polarizing plates, observed with cross nicol, and the angle with the film width direction is obtained in the vicinity where light does not leak, and the direction of the angle is Ny, A direction perpendicular thereto was defined as Nx, and the refractive index in each direction was measured. The refractive index was measured using an Abbe refractometer (ATAGO, NAR-3T), and the eyepiece has a polarization function, so that the refractive index can be measured separately for each direction. In addition, the thickness of the protective film was measured using a micrometer (VL-50aS, Mitsutoyo Corporation).

In addition, the result of measurements at five points in total by adding four points spaced at equal intervals with an interval of 370 mm from the center of width and the center of width to both ends, respectively, and 370 from the center of width and center of width to both ends in the direction of both ends. The thickness direction phase difference variation rate according to Equation 1 was calculated using the results measured at a total of 9 points by adding a total of 8 points spaced at an equal interval with an interval of mm.

In addition, the thickness direction retardation was calculated by substituting the measured refractive index and the thickness of the protective film into Equation 2, and the in-plane retardation was calculated by substituting Equation 3 into Equation 3.

On the other hand, in Example 1 as a measurement result, the maximum deviation of the retardation in the thickness direction of the center-based symmetric point was 45 nm, and the thickness-direction retardation of both sides based on the width center was similarly implemented.

In addition, as a result of the measurement, it was calculated that the difference between the maximum and minimum values of the thickness direction retardation in Example 1 was 45.2 nm, Example 2 was 67.5 nm, Example 3 was 79.9 nm, and Example 4 was 133.0 nm. It can be seen that the protective films of Examples 1 to 3 exhibited a small retardation difference in the thickness direction.

2. Measurement of optical axis and orientation angle

The optical axis and orientation angle of 17 points spaced at equal intervals were measured at intervals of 185 mm from the center of the width of the prepared film in the direction of both ends. Specifically, the optical axis was measured as the angle of the film at the time when the black mode of the polarizing plate cross nicolo polarizer was realized.

In addition, after the main chain orientation angle was biaxially stretched, the crystal direction of the main chain with respect to the film width direction was measured for the sample with an orientation angle measuring device (SST-4000, Shoji Nomura).

3. Evaluation of whether a rainbow stain is visible on an LCD TV

An LCD TV manufactured by Manufacturer A, which is commercially available, was prepared. After placing the polyester protective films according to Examples and Comparative Examples on the screen surface of the prepared LCD TV, the front side is 0°, the left end point is 90°, and the right end point is 90° from the viewing side of the liquid crystal panel. At the time of driving the light source, whether the rainbow stain was visually recognized at an angle of 60° up/down/left/right was evaluated according to the following criteria.

◎: Rainbow stains are not recognized at 60° up/down/left/right angles

○: A weak rainbow stain was recognized at any one of the 60° up/down/left/right angle points

△: A weak rainbow blotch was recognized at any two of the top/bottom/left/right 60° points

×: A weak rainbow blotch was recognized at any three or more of 60° up/down/left/right points

××: Strong rainbow blotch is recognized at at least one of 60° up/down/left/right points

4. Evaluation of whether a rainbow stain is recognized in OLED TV

We prepared an OLED TV produced by Manufacturer B, which is on the market. After placing the polyester protective film according to Example 1 on the screen surface of the prepared OLED TV, when the front side is 0°, the left end point is 90°, and the right end point is 90° from the viewer side, when the light source is driven It was evaluated whether the rainbow stain was visually recognized at an angle of 60° up/down/left/right.

As a result of evaluation, in the case of the protective film according to Example 1, rainbow stains were not observed at any point.

Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 quite MD draw ratio/temperature (℃) 3.1/
150 3.1/
150 3.1/
150 3.1/
150 3.2/
150 3.1/
150 3.1/
150 TD
(Stretch ratio/width change rate (%)
/Temperature (℃)) total draw ratio 3.37 3.37 3.35 3.31 3.4 3.32 3.5 1 section 1.7/
29.6/
145 1.7/
29.6/
145 1.6/
25.6/
145 1.8/
34.6/
145 3.4/
160 1.35/
15.1/
145 1.6/
24.1/
145 2nd section 1.5/35.9/
145 1.5/
35.9/
145 1.4/
27.2/
145 1.6/
46.7/
150 - 1.35/20.3/
145 1.4/
25.7/
145 Section 3 1.2/21.6/
150 1.2/
21.6/
150 1.3/
28.6/
150 1.15/
18.7/
160 - 1.35/27.5/150 1.3/
26.9/
150 4 sections 1.1/
12.9/
160 1.1/
12.9/
150 1.15/
18.6/
160 - - 1.35/
37.1/
160 1.2/
23.3/
160 Heat setting temperature (℃) 210 210 210 210 210 210 210 protective film Thickness (㎛) 50 50 50 50 50 50 50 Center of width Re(nm) 210 230 280 310 110 290 680 Center of width Rth(nm) 8400 8400 8400 8400 8166 8200 8200 Max Re(nm) 545 570 620 650 720 820 1300 5 points Rth change rate (%) 0.20 0.26 0.54 0.90 2.12 1.61 1.25 9 points Rth change rate (%) 0.23 0.54 0.91 1.45 2.50 1.89 1.60 Maximum optical axis (°) 36.7 37.5 38.8 39.7 52 59 43 Maximum orientation angle (°) 9.0 9.3 11.1 12.8 43.5 45.9 39.0 Maximum deviation of one point optical axis-orientation angle (°) 29.2 34.4 37.5 43.1 50.3 53.5 48.2 Effective width less than 40° of maximum optical axis (mm) ±1500 ±1300 ±750 ±750 ±185 ±370 ±370 Properties LCD TV ◎ ◎ ○ △ ×× × ×

As can be seen from Table 1, in the case of the protective film according to the comparative example, when it was attached to the surface of the LCD TV screen, more rainbow stains were displayed than the protective film according to the embodiment, indicating that it was not suitable as a protective film.

Although one embodiment of the present invention has been described above, the spirit of the present invention is not limited to the embodiments presented herein, and those skilled in the art who understand the spirit of the present invention can add components within the scope of the same spirit. , changes, deletions, additions, etc. may easily suggest other embodiments, but this will also fall within the scope of the present invention.

Claims (11)

It contains polyethylene terephthalate, and the in-plane retardation measured at a wavelength of 632 nm within a width of ±750 mm based on the width center is 650 nm or less, the optical axis is less than 40°, and an optical polyester protection with an orientation angle of 13° or less. film. According to claim 1,
An optical polyester protective film, characterized in that the deviation of the optical axis and the orientation angle at a predetermined point is 45° or less. According to claim 1,
An optical polyester protective film, characterized in that the retardation in the thickness direction within the effective width is 8000 nm or more. 4. The method of claim 3,
A polyester protective film for optical use, characterized in that the difference between the maximum value and the minimum value of the retardation in the thickness direction within the effective width is 70 nm or less. delete According to claim 1,
An optical polyester protective film, characterized in that the in-plane retardation is 570 nm or less. According to claim 1,
An optical polyester protective film, characterized in that the orientation angle is 10° or less. 3. The method of claim 2,
An optical polyester protective film with a deviation of 35° or less from the optical axis and the orientation angle at a predetermined point. 4. The method of claim 3,
Optical polyester, characterized in that the thickness direction retardation variation of the following Equation 3 for the thickness direction retardation measured at a total of 5 points spaced at equal intervals from the center of the width by 370 mm in both width directions is 1% or less protective film.
[Equation 3]

An optical laminate comprising an optical film and the polyester protective film according to any one of claims 1 to 4 and 6 to 9 disposed on at least one surface of the optical film. An image display device comprising the optical laminate according to claim 10.