Optical polyester protective film and preparation method thereof

Abstract

The present invention relates to a polyester protective film, and more particularly to an optical polyester protective film and a preparation method thereof.

Description

Technical Field

[0001] The present invention relates to a polyester protective film, and more particularly to an optical polyester protective film and a preparation method thereof. Background Art

[0002] Various displays for TVs, computers, mobile phones, etc. have been developed and commercialized, for example, from light-receiving liquid crystal display devices (LCD) to light-emitting display devices such as OLED displays that have been commercialized recently. These display devices include optical films with various functions, which are arranged on the optical path of light from the light source until it leaves the front side on the viewer side, and the amount of light transmission, the direction of light, and other optical characteristics are optimally adjusted by the optical film, thereby improving characteristics such as brightness, viewing angle, and brightness.

[0003] On the other hand, recently, display devices need to be able to withstand use in various environments, such as use in more severe environments. For example, display devices of mobile devices such as mobile phones or display devices of vehicles such as car navigation systems need to have durability for use in more severe environments. In addition, in large display devices such as televisions, the interior of the display device tends to be exposed to high temperature environments due to the increase in heat generation caused by large size and high brightness. Therefore, various optical films used in display devices also need to have less characteristic changes in severe environments such as high temperature and high humidity, that is, they need to have high durability.

[0004] On the other hand, the above-mentioned optical film will only be composed of a functional layer that performs the required function. However, in order to solve the problem of low moisture resistance, heat resistance and mechanical strength of the functional layer, many structures using a protective film bonded to the functional layer are adopted. For example, an absorbing polarizer is an optical film commonly used for LCD or OLED displays, wherein, in order to overcome low mechanical strength, etc., triacetyl cellulose (TAC) is generally widely used as a protective film for absorbing polarizers. However, when the thickness of the triacetyl cellulose film is thin, sufficient mechanical strength cannot be obtained, and the moisture permeability is high, so the polarizer is easily degraded. In addition, since the triacetyl cellulose film is expensive, a low-cost alternative material is needed.

[0005] Due to these demands, attempts to use polyester film as a protective film for optical films as an alternative to TAC film have been continued recently. Polyester film is cheaper than TAC film and has high mechanical strength.

[0006] However, polyester film has large birefringence characteristics, and has large birefringence characteristics in the plane and in the thickness direction during stretching. Therefore, when used as a protective film for optical films such as polarizers, especially when used as a protective film for optical films on the viewer side of liquid crystal cells, there is a problem of visible rainbow spots. In particular, with the recent high brightness and high color purity, such rainbow spots are more likely to be seen, so the use of polyester film as a protective film for optical films arranged on the viewing side of liquid crystal cells such as polarizers is limited. In addition, even if excellent characteristics are shown, such as no rainbow spots in the center of the width, there are still problems such as more rainbow spots occurring toward the end side in the width direction, or optical characteristics changing even if these rainbow spots do not occur, so it is difficult to present uniform physical properties throughout the width, and it is difficult to obtain a large width. Summary of the invention

[0007] Technical issues

[0008] An object of the present invention is to provide a polyester protective film which ensures moisture resistance, heat resistance, impact resistance, etc., has excellent and uniform optical properties over a wide width, significantly reduces the visibility of rainbow spots caused by interference of transmitted light even under various types of light sources, and achieves excellent color reproducibility, thereby being very suitable for use as a polyester protective film for optical film protection.

[0009] Solutions to the problem

[0010] In order to solve the above-mentioned problems, the present invention provides a method for preparing an optical polyester protective film, characterized in that it comprises: step (1), preparing an unstretched sheet by melt-extruding a polyester resin; step (2), performing a first stretching of the unstretched sheet in the length direction (MD) by 3.0 times to 3.3 times; step (3), stretching the sheet in multiple stages in the width direction (TD) to perform a second stretching by a total of 3.3 times to 3.7 times, and performing at least three stages of stretching, so that the stretching ratio of each stage becomes smaller towards the rear end, and the stretching is performed so that the width change rate in the last stretching stage is less than 20%; step (4), heat fixing the stretched sheet.

[0011] According to one embodiment of the present invention, in the multi-stage stretching of step (3), the width change of the initial stretching stage may be more than twice the width change of the final stretching stage.

[0012] Furthermore, the multi-stage stretching of step (3) can be carried out at a temperature of 140°C to 170°C, the final stretching stage can be carried out at a temperature greater than 155°C, and the initial stretching stage can be carried out at a temperature that is 15°C or more lower than the temperature of the final stretching stage.

[0013] Furthermore, in the multi-stage stretching of step (3), the width change rate of the final stretching stage may be 15% or less.

[0014] Furthermore, the present invention provides an optical polyester protective film comprising polyethylene terephthalate, having a thickness direction retardation of 8000 nm or more measured at a wavelength of 632 nm, and an optical axis of less than 40°.

[0015] According to an embodiment of the present invention, the thickness direction phase difference between two points symmetrical in the width direction with respect to the width center may be less than 55 nm.

[0016] Furthermore, the difference between the maximum value and the minimum value of the thickness direction phase difference may be 70 nm or less.

[0017] Furthermore, the width of the protective film may be ±750 mm or more based on the width center.

[0018] Furthermore, the in-plane phase difference can be 570 nm or less.

[0019] Furthermore, the thickness direction phase difference variation rate of the following mathematical formula 1 for the thickness direction phase difference measured at a total of five points spaced equidistantly at 370 mm from the width center to the two width directions may be 1% or less. Furthermore, more preferably, the thickness direction phase difference variation rate of the following mathematical formula 1 for the thickness direction phase difference measured at a total of nine points spaced equidistantly at 370 mm from the width center to the two width directions may be 1% or less.

[0020] [Mathematical formula 1]

[0021]

[0022] Furthermore, the present invention provides an optical laminate including: an optical film; and the polyester protective film according to the present invention disposed on at least one side of the optical film.

[0023] Furthermore, the present invention provides an image display device including the optical laminate according to the present invention.

[0024] Effects of the Invention

[0025] The polyester protective film according to the present invention ensures moisture resistance, heat resistance and mechanical strength, and is therefore very suitable for use in protecting the functional layer of an optical film, and has the advantage of being able to reduce product costs due to its low unit price. In addition, unlike conventional optical polyester protective films, the visibility of rainbow spots caused by interference with transmitted light is significantly reduced, and when applied to various displays realized by applying various types of light sources, the influence of the light source type is minimized, thereby achieving excellent color reproducibility. In addition, by achieving the above-mentioned excellent and uniform optical properties within a further increased width, it is very suitable for use in optical devices such as large-area displays. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The thickness direction phase difference curve of the polyester protective film according to one embodiment of the present invention at the locations in the width direction from the width center to both ends. DETAILED DESCRIPTION

[0027] The embodiments of the present invention are described in detail below so that a person having ordinary knowledge in the technical field to which the present invention belongs can easily implement the present invention. The present invention can be implemented in various forms and is not limited to the embodiments described here.

[0028] The polyester protective film prepared by the preparation method of one embodiment of the present invention comprises polyethylene terephthalate, has a thickness direction phase difference of 8000 nm or more measured at a wavelength of 632 nm, and has an optical axis of less than 40°.

[0029] The polyester resin includes polyethylene terephthalate, so it is conducive to achieving excellent heat resistance, moisture resistance, and impact resistance. At this time, the polyethylene terephthalate not only includes polyethylene terephthalate composed of terephthalic acid as an acidic component and ethylene glycol as a diol component, but also includes less than about 10 mol% of polycarboxylic acids other than terephthalic acid and / or other diol components other than ethylene glycol to copolymerize with respect to the total mole % of the acidic component and the diol component. The polyester resin according to the present invention also belongs to the scope of polyethylene terephthalate. In addition to polyethylene terephthalate, the polyester resin may also include heterogeneous polyester components such as polyethylene naphthalate, polycarbonate, and polybutylene terephthalate. In this case, the content of the heterogeneous polyester component relative to the total weight of the polyester resin may be, for example, less than 30% by weight, less than 20% by weight, less than 10% by weight, or less than 5% by weight.

[0030] The polyester protective film according to the present invention includes a thickness direction phase difference of 8000nm or more and an optical axis width of less than 40°, thereby, even when used in various light sources, it can be beneficial to prevent visible rainbow spots caused by distortion of transmitted light. When the conventional polyester protective film undergoes a biaxial stretching process and a heat setting process, a bowing phenomenon occurs due to residual stress, and the orientation angle of the polyester resin main chain that changes due to the bending phenomenon causes a change in the optical axis. In addition, during the manufacturing process, depending on the mixing state of the main chain and the amorphous region of the polyester resin, the optical axis may change along the width direction. At this time, the optical axis within the width of the optical film according to the present invention is less than 40°, so visible rainbow spots caused by distortion can be prevented. In addition, when used with a polarizing plate, light loss due to the difference in the optical axis of the polarizing plate can be reduced. On the other hand, the optical axis can be measured by the film angle at the time point when the black mode of the polarizing plate is achieved under the orthogonal Nicol of the polarizing plate.

[0031] Furthermore, the width of the protective film refers to the area that satisfies the above-mentioned thickness direction phase difference of more than 8000nm and the optical axis is less than 40°. The width can be the distance from the center of the width to the predetermined location in the two end directions. For example, it can be more than ±750mm. As another example, it can be more than ±1300mm. As another example, it can be less than ±2250mm, it can be less than ±1685mm, or it can be less than ±1500mm.

[0032] The thickness direction phase difference is defined by the following mathematical formula 2, where "Nx", "Ny" and "Nz" refer to the refractive index of the polyester protective film in the x-axis direction, y-axis direction and z-axis direction (thickness 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 represents the thickness of the polyester protective film (unit: nm).

[0033] [Mathematical formula 2]

[0034] Thickness direction phase difference = {(Nx+Ny) / 2-Nz}×d(nm)

[0035] The thickness direction phase difference affects the visibility of the rainbow spots caused by the distortion. The thickness direction phase difference measured at a wavelength of 632nm can be 8000nm or more, and more preferably, can be 8200nm to 12000nm. When the thickness direction phase difference is 8000nm or more, the visibility of the rainbow spots caused by the distortion of the transmitted light can be significantly reduced.

[0036] According to one embodiment of the present invention, the difference in thickness direction phase difference between two points symmetrical in the width direction with the width center as the reference may be less than 55nm. In addition, the difference between the maximum and minimum values ​​of the thickness direction phase difference within the width may be less than 70nm, more preferably, less than 50nm, thereby significantly reducing the visibility of rainbow spots regardless of the position within the width, and further preventing the increased visibility of rainbow spots when tilted at an angle of more than 40° up and down or left and right. Even when rainbow spots are not visible under a certain light source, the visibility of rainbow spots caused by changes in the light source type can be prevented. If the difference in thickness direction phase difference between two points symmetrical in the width direction with the width center as the reference is greater than 55nm or the difference between the maximum and minimum values ​​of the thickness direction phase difference within the width is greater than 70nm, the effect of preventing the visibility of rainbow spots is reduced, and rainbow spots may be seen depending on the position, especially when viewed from the front at several tilts, and it is difficult to achieve the purpose of the present invention as above.

[0037] On the other hand, when the conditions that the difference in thickness direction phase difference between two points symmetrical in the width direction with respect to the width center is less than 55nm and the difference between the maximum and minimum values ​​of the thickness direction phase difference within the width is 70nm are met at the same time, the visibility prevention effect of rainbow spots within the width is synergistic regardless of the position, and the transmission light characteristics through the polyester protective film can be more uniform. These advantages contribute to the uniformity of light leakage, uniformity of light-dark ratio, uniformity of color tone, etc.

[0038] Furthermore, the thickness direction phase difference variation rate of the following mathematical formula 1 for the thickness direction phase difference measured at a total of five points spaced equidistantly at 370 mm from the width center to the two width directions can be 1% or less, more preferably, 0.5% or less, and further preferably, 0.3% or less, thereby making the optical properties of the transmitted light more uniform regardless of the position, further increasing the effect of preventing the visibility of rainbow spots, etc., which is more conducive to achieving the purpose of the present invention. Furthermore, more preferably, the thickness direction phase difference variation rate of the following mathematical formula 1 for the thickness direction phase difference measured at a total of nine points spaced equidistantly at 370 mm from the width center to the two width directions can be 1% or less, more preferably, 0.5% or less, and further preferably, 0.3% or less, thereby greatly helping to achieve the purpose of the present invention.

[0039] [Mathematical formula 1]

[0040]

[0041] Furthermore, the in-plane phase difference within the width of the present invention may be 650 nm or less, more preferably 570 nm or less, and as another example, 550 nm or less. The in-plane phase difference is calculated by the following mathematical formula 3, wherein Nx, Ny, and d are defined as in mathematical formula 2. If the in-plane phase difference is greater than 570 nm, it is difficult to maintain the polarization degree, and rainbow spots may appear.

[0042] [Mathematical formula 3]

[0043] In-plane phase difference = (Nx-Ny) × d (nm)

[0044] Furthermore, the thickness of the polyester protective film may be 30 μm to 100 μm, more preferably 50 μm to 75 μm. If the thickness is greater than 100 μm, the increase in the in-plane phase difference and the phase difference variation will be large, and the stress will increase during stretching, resulting in an increase in the optical axis, so it is not preferred in terms of thinning. Furthermore, if the thickness is less than 30 μm, the mechanical strength is reduced, and it is difficult to perform the function of the protective film. In the processing steps using the protective film, the problem of breakage due to wrinkles and insufficient mechanical strength may occur.

[0045] The polyester protective film of one embodiment of the present invention is prepared by a method comprising the following steps: step (1), preparing an unstretched sheet by melt-extruding a polyester resin; step (2), performing a first stretching of the unstretched sheet in the length direction (MD) by 3.0 to 3.3 times; step (3), stretching the sheet in multiple stages in the width direction (TD) for a second stretching of a total of 3.3 to 3.7 times, and performing at least three stretching stages so that the stretching ratio of each stage becomes smaller toward the rear end, and the stretching is performed so that the width change rate in the last stretching stage is less than 20%; step (4), heat fixing the stretched sheet.

[0046] First, as step (1) according to the present invention, a step of preparing an unstretched sheet by melt-extruding a polyester resin is performed.

[0047] In the above step (1), the polyester resin including polyethylene terephthalate can be melted and prepared by a conventional film substrate preparation device. In addition, when the above polyester resin is melt-extruded, the melting temperature can be appropriately changed according to the type of polyester resin used, and therefore the present invention is not particularly limited thereto.

[0048] The thickness of the unstretched sheet prepared by the above step (1) can be 40 μm to 250 μm. When the thickness is greater than 250 μm, it is difficult to control the phase difference and it is difficult to be used in optical devices such as displays that tend to be thinner.

[0049] Secondly, as step (2) of the present invention, a step of first stretching the unstretched sheet in the length direction (MD) is performed. The stretching ratio in the above-mentioned first stretching can be 3.0 to 3.3. If the stretching ratio in the length direction is less than 3.0, it is difficult to control the film thickness, the strength is low, and it may be easy to tear in the length direction. In addition, when the stretching ratio in the length direction exceeds 3.3, it is difficult to achieve the physical properties required by the present invention due to the high residual shrinkage stress and the increased orientation angle of the main chain. In addition, the first stretching can be carried out at a temperature of 130°C to 170°C, thereby easily preparing a polyester protective film having the physical properties required by the present invention.

[0050] Secondly, as step (3) of the present invention, the sheet stretched in the length direction is stretched in multiple sections in the width direction (TD) to perform a second stretching of 3.3 to 3.7 times in total, and at least three stretchings are performed, so that the stretch ratio of each section is smaller toward the rear end. Thus, by achieving a total stretching ratio in the second stretching, the mechanical strength of the stretched film is ensured, and it may be helpful to minimize the bending phenomenon. If the stretching ratio in the width direction is less than 3.3, it is difficult to control the film thickness, and the film may be easily torn in the width direction. In addition, when the stretching ratio in the width direction exceeds 3.7, the residual shrinkage stress is high, so the thermal shrinkage rate increases, the deviation of the phase difference increases, etc., and it may be difficult to achieve the physical properties required by the present invention.

[0051] Furthermore, in the case where at least three multi-stage stretchings are not performed, but two multi-stage stretchings or one stretching is performed, it is 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. Furthermore, in the case of multi-stage stretching, if the stretching ratio does not decrease toward the rear end, specifically, if there is a stage where the stretching ratio is increased or the same as the front stage, it may not be easy to achieve physical properties such as an optical axis of less than 40° and a thickness direction phase difference of 8000 nm or more, and the difference in optical physical properties depending on the position is large and / or the width is realized to be very small, etc., and it may be difficult to achieve the purpose of the present invention.

[0052] Furthermore, the film is stretched in multiple stages in the width direction, and the stretching ratio is smaller toward the rear stage, and the film is stretched in a manner that the width change rate in the last stretching stage is less than 20%, so that the optical axis less than 40° can be achieved. Furthermore, the difference in optical physical properties at multiple locations with different positions in the width can be minimized, and finally, an optical film that exhibits uniform physical properties regardless of the position is achieved, and it is more conducive to preventing rainbow spots of transmitted light according to various types of light sources. When the width change rate of the last stretching stage exceeds 20%, it may be particularly difficult to achieve an optical axis less than 40°. In addition, even if there is a portion with an optical axis less than 40° in the width, the width that satisfies the optical axis condition based on the width center is very narrow, so it may be difficult to achieve, for example, a width exceeding ±375mm, and as another example, it may be difficult to achieve a width exceeding ±750mm. Preferably, the width change rate of the last stretching stage can be less than 15%, thereby facilitating the realization of a wider width, and the deviation of the thickness direction phase difference in the width direction will be realized to be smaller. However, in the final stretching stage, the width change rate may be greater than 7%. If the width change rate is less than 7% due to stretching, the optical axis will increase instead, and the thickness direction phase difference uniformity may decrease.

[0053] On the other hand, the width change rate of the specific stretching section refers to the percentage of the width change amount of the specific stretching section relative to the total width change amount by the second stretching, and is calculated by the following Mathematical Formula 4.

[0054] [Formula 4]

[0055]

[0056] Here, the total width change amount by the second stretching is the increase amount of the final width by the second stretching compared to the width before the stretching, and is calculated by the difference between the final width and the width before the stretching. In addition, the width change amount in a specific stretching section is calculated by the difference between the width at the end of the relevant specific stretching section and the width before the relevant specific stretching section.

[0057] Furthermore, according to one embodiment of the present invention, during multi-stage stretching, the width variation in the initial stretching section may be more than twice the width variation in the final stretching section, thereby facilitating the uniformity of optical physical properties such as thickness direction phase difference in the width direction. If the width variation in the initial stretching section is less than twice the width variation in the final stretching section, it may be difficult to achieve the desired level of thickness direction phase difference deviation, and rainbow spots may be seen depending on the position and / or when tilted at an angle of more than 40° (based on 0° on the front) in the upper and lower directions, left and right, etc., and it may be difficult to achieve the purpose of the present invention.

[0058] Furthermore, the second stretching can be performed at a temperature of 140°C to 170°C. Preferably, in the case of multi-stage stretching, different temperatures can be set according to the stretching stages. Specifically, the stretching of the last stretching stage can be performed at a temperature greater than 155°C, and the stretching temperature of the first stretching stage can be lower than the stretching temperature of the last stretching stage by more than 15°C, thereby facilitating the physical properties required by the present invention to be achieved at an increased width. If the stretching temperature of the last stretching stage is below 155°C, and / or the stretching temperature of the first stretching stage is higher than or equal to the stretching temperature of the last stretching stage, or although the stretching temperature of the first stretching stage is lower than the stretching temperature of the last stretching stage, the stretching temperature of the first stretching stage is lower than the stretching temperature of the last stretching stage by less than 15°C, it may be difficult to achieve a sufficiently increased width, and the deviation of optical physical properties such as the thickness direction phase difference and the in-plane phase difference may be large according to the position, or according to the type of light source, the visibility of rainbow spots caused by the transmitted light may be caused.

[0059] On the other hand, according to one embodiment of the present invention, in the above step (3), four multi-stage stretching can be performed, in which the stretching ratio in the first stage as the initial stretching stage can be 1.6 to 1.75, and the stretching temperature can be 140°C to 150°C, in the second stage the stretching ratio can be 1.45 to 1.6, and the stretching temperature can be 140°C to 150°C, in the third stage the stretching ratio can be 1.2 to 1.35, and the stretching temperature can be 145°C to 155°C, and in the fourth stage as the final stretching stage the stretching ratio can be 1.05 to 1.15, and the stretching temperature can be 155°C to 170°C.

[0060] Next, as step (4) of the present invention, a step of heat-fixing the stretched sheet is performed. The heat-fixing temperature may be a temperature lower than the usual heat-fixing temperature so as to suppress the bending phenomenon, thereby minimizing the stress difference between the stretched portion and the heat-fixing portion. The heat-fixing temperature may be, for example, 180°C to 220°C. If the heat-fixing temperature is lower than 180°C, the heat-fixing cannot be fully achieved, so the dimensional stability may be reduced. If the heat-fixing temperature exceeds 220°C, the bending phenomenon becomes serious, resulting in an increase in the orientation angle and the optical axis.

[0061] According to one 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 method. The physical properties of the protective film of one embodiment of the present invention are designed to prevent the visibility of rainbow spots due to transmitted light, but in order to prevent the visibility of rainbow spots that may occur due to enhanced interference and destructive interference caused by light reflected from the post-processing resin and the interface, respectively, a primer layer may be further provided.

[0062] As the primer layer, a known primer layer provided on an optical film to prevent the visibility of rainbow spots can be used without restriction. For example, the primer layer can be formed to include a binder resin, organic or inorganic particles and a curing agent. The binder resin can include one or more of a polyurethane resin and a water-dispersible polyester resin as a main agent. In addition, the organic or inorganic particles are used to ensure the running performance, and preferably use particles with an average particle size of 10nm to 500nm. If the average particle size of the particles is greater than 500nm, the haze may increase, and if the average particle size of the particles is less than 10nm, the surface roughness decreases, adhesion occurs, or difficulties occur in winding the film, resulting in a deteriorated appearance. In addition, the inorganic particles are at least one of silica particles and silica-organic composites, and the inorganic particles preferably have a refractive index of 1.5 or more. When the refractive index of the inorganic particles is less than 1.5, the overall refractive index of the primer layer decreases, and rainbow spots may be seen.

[0063] Furthermore, the primer layer may include one or more selected from oxazoline curing agents, carbodiimide curing agents and melamine curing agents as curing agents. In particular, oxazoline curing agents can inhibit moisture from penetrating into the polyester protective film or prevent adhesion that may occur during double-sided coating due to reaction with moisture. In addition, although the melamine curing agent reacts with the main agent, the coating strength is improved by the self-curing reaction between the melamines, thereby preventing adhesion that may occur during double-sided coating.

[0064] Also, the primer layer may further include additives such as anionic surfactants or defoamers, and may include various additives included in conventional films or films for optical purposes other than surfactants.

[0065] The polyester protective film of one embodiment of the present invention can be disposed on one or both sides of the optical film to realize an optical laminate. The optical film can be a known film in the optical field, for example, a diffusion film, a reflective polarizing film, an absorbing polarizing film, a retardation film, a compensation film, etc.

[0066] Furthermore, the optical laminate formed by integrating the polyester protective film and the optical film can be installed in various optical devices, for example, can be set in an image display device. The above-mentioned image display device can be a light-receiving image display device or a light-emitting image display device, and the description of the specific structure of the image display device will be omitted in the present invention.

[0067] The present invention will be described in more detail by the following examples. However, the following examples are not intended to limit the scope of the present invention but are intended to provide an understanding of the present invention.

[0068] <Example 1>

[0069] The polyethylene terephthalate slice was melt-extruded to prepare an unstretched sheet on a casting roll. Thereafter, the unstretched sheet was first stretched in the length direction (MD) at a temperature of 150°C and a stretch ratio of 3.1, and then stretched four times in the width direction, thereby performing a second stretch at a total stretch ratio of 3.4. Specifically, in the second stretching, the first section was stretched at a temperature of 145°C at a stretch ratio of 1.7, the second section was stretched at a temperature of 145°C at a stretch ratio of 1.5, the third section was stretched at a temperature of 150°C at a stretch ratio of 1.2, and the fourth section was stretched at a temperature of 160°C at a stretch ratio of 1.1, and then heat-fixed at a temperature of 210°C to prepare a polyester protective film having a final width of 3370 mm and a thickness of 50 μm as shown in Table 1 below.

[0070] <Examples 2 to 4>

[0071] Except for changing the process conditions as shown in Table 1 below, the same method as in Example 1 was carried out to prepare a polyester protective film as shown in Table 1 below.

[0072] <Comparative Examples 1 to 3>

[0073] Except for changing the process conditions as shown in Table 1 below, the same method as in Example 1 was carried out to prepare a polyester protective film as shown in Table 1 below.

[0074] <Experimental example>

[0075] The following physical properties of Examples and Comparative Examples were evaluated, and the results are shown in Table 1 below.

[0076] 1. Measurement of in-plane phase difference and thickness direction phase difference

[0077] The in-plane phase difference and thickness direction phase difference of a total of 17 points separated by equal intervals of 185mm from the width center of the prepared film to the two end directions were measured. Specifically, as for the in-plane phase difference, the biaxially stretched film is first placed between two polarizing plates and observed in an orthogonal Nicol state, and the angle between the width direction of the film is obtained near the light-tightness, and the direction of the angle is defined as Ny, and the direction perpendicular to Ny is defined as Nx, thereby determining the refractive index in each direction. The refractive index is measured using an Abbe refractometer (ATAGO, Japan, NAR-3T), and the eyepiece has a polarization function, so each direction is distinguished to measure the refractive index separately. In addition, a micrometer (VL-50aS, Mitutoyo, Japan) is used to measure the thickness of the protective film.

[0078] Furthermore, the thickness direction phase difference variation rate according to the above mathematical formula 1 is calculated using the results of measurements at a total of five points including the width center and four points equally spaced at 370 mm from the width center toward both ends and the results of measurements at a total of nine points including the width center and eight points equally spaced at 370 mm from the width center toward both ends.

[0079] Then, the measured refractive index and protective film thickness are substituted into the above-mentioned Mathematical Formula 2 to calculate the thickness direction phase difference, and are substituted into Mathematical Formula 3 to calculate the in-plane phase difference.

[0080] On the other hand, according to the measurement results, the maximum deviation of the thickness direction phase difference at the points symmetrical with respect to the center of Example 1 was 45 nm, that is, the thickness direction phase difference on both sides with respect to the width center was similar.

[0081] In addition, according to the measurement results, the difference between the maximum and minimum values ​​of the thickness direction phase difference in Example 1 was calculated to be 45.2 nm, the difference between the maximum and minimum values ​​of the thickness direction phase difference in Example 2 was calculated to be 67.5 nm, the difference between the maximum and minimum values ​​of the thickness direction phase difference in Example 3 was calculated to be 79.9 nm, and the difference between the maximum and minimum values ​​of the thickness direction phase difference in Example 4 was calculated to be 133.0 nm. It can be seen that the difference in the thickness direction phase difference of the protective films of Examples 1 to 3 is smaller than that of Example 4.

[0082] 2. Optical axis and orientation angle determination

[0083] The optical axis and orientation angle were measured at locations spaced 5 mm apart from the width center of the prepared film toward both ends. Specifically, the optical axis was measured by the film angle at the time point when the polarizing plate achieved a black mode under crossed Nicols.

[0084] In addition, as for the main chain orientation angle, after biaxial stretching, the main chain crystal direction of the sample with respect to the film width direction was measured using an orientation angle measuring device (SST-4000, Nomura Corporation).

[0085] 3. Evaluate whether rainbow spots are visible in LCD TVs

[0086] A commercially available LCD TV manufactured by manufacturer A was prepared. After the polyester protective films according to the embodiment and the comparative example were arranged on the prepared LCD TV screen surface in sizes of ±750 mm in width and ±1500 mm in width, respectively, with the viewer side of the liquid crystal panel as a reference, the front side was defined as 0°, the left end point was defined as 90°, and the right end point was defined as 90°, it was evaluated by the following criteria whether rainbow spots were visually recognized at an angle of 60° in the upper / lower / left / right when the light source was driven.

[0087] ◎: The rainbow spot cannot be identified at an angle of 60° up / down / left / right

[0088] ○: The rainbow spot can be faintly identified at any of the 60° up / down / left / right locations

[0089] △: Rainbow spots can be faintly identified at any two locations in the 60° angle up / down / left / right

[0090] ×: The rainbow spot can be faintly identified at any three or more locations in the 60° angle of up / down / left / right

[0091] ××: The rainbow spot can be clearly identified at any one of the 60° up / down / left / right angles.

[0092] 4. Evaluate whether rainbow spots are visible in OLED TVs

[0093] A commercially available OLED TV manufactured by manufacturer B was prepared. After the polyester protective film according to Example 1 was arranged on the prepared OLED TV screen surface, it was evaluated whether a rainbow spot was visually recognized at an angle of 60° in the upper / lower / left / right directions when the light source was driven, with the viewer side as a reference, the front side defined as 0°, the left end point defined as 90°, and the right end point defined as 90°.

[0094] According to the evaluation results, in the case of the protective film according to Example 1, rainbow spots were not recognized at any location.

[0095] Table 1

[0096]

[0097]

[0098] As can be seen from Table 1, in the case of the protective film according to the comparative example, when attached to the surface of an LCD TV screen, more rainbow spots were seen than in the protective film according to the embodiment, and therefore, the protective film is not suitable for use as a protective film.

[0099] Although an embodiment of the present invention has been described above, the concept of the present invention is not limited to the embodiment disclosed in this specification. Ordinary technicians in the technical field of the present invention who understand the concept of the present invention can easily propose other embodiments within the same scope of the concept by adding, changing, deleting, and appending structural elements, and these belong to the scope of the concept of the present invention.

Claims

1. A method for preparing an optical polyester protective film, characterized in that: include: Step (1), preparing an unstretched sheet by melt-extruding a polyester resin; Step (2), performing a first stretching of the unstretched sheet in the length direction by 3.0 to 3.3 times; Step (3), stretching the sheet in multiple stages in the width direction to perform a second stretching of 3.3 to 3.7 times in total, performing at least three stretching stages, making the stretching ratio of each stage smaller toward the rear end, and performing the stretching so that the width change rate in the last stretching stage is less than 20%; Step (4), heat fixing the stretched sheet.

2. The method for preparing an optical polyester protective film according to claim 1, characterized in that: In the multi-stage stretching of step (3), the width change in the initial stretching stage is more than twice the width change in the final stretching stage.

3. The method for preparing an optical polyester protective film according to claim 1, characterized in that: The multi-stage stretching in step (3) is carried out at a temperature of 140°C to 170°C, The final stretching stage is carried out at a temperature greater than 155°C. The stretching in the first stretching stage is carried out at a temperature lower than the temperature in the last stretching stage by at least 15°C.

4. The method for preparing an optical polyester protective film according to claim 1, characterized in that: In the multi-stage stretching of step (3), the width change rate in the final stretching stage is 15% or less.

5. An optical polyester protective film, characterized in that: The invention comprises polyethylene terephthalate, and in the width direction, based on the width center, within a range of at least ±750 mm, the thickness direction phase difference measured at a wavelength of 632 nm is calculated according to the following mathematical formula 2, is 8000 nm or more, the in-plane phase difference measured at a wavelength of 632 nm is calculated according to the following mathematical formula 3, is 650 nm or less, and the optical axis is less than 40° according to the following evaluation method: [Mathematical formula 2] Thickness direction phase difference = {(Nx+Ny) / 2-Nz}×d(nm) Wherein, "Nx", "Ny" and "Nz" refer to the refractive index of the polyester protective film in the x-axis direction, y-axis direction and z-axis direction at a wavelength of 632 nm, the x-axis direction is MD, the y-axis direction is TD, and d represents the thickness of the polyester protective film. [Evaluation method] The optical axis is measured by the angle of the polyester protective film at the time when the black mode of the polarizing plate is realized under the crossed Nicols of the polarizing plate. [Mathematical formula 3] In-plane phase difference = (Nx-Ny) × d (nm) Here, "Nx" and "Ny" refer to the refractive index in the x-axis direction and the y-axis direction of the polyester protective film, respectively, the x-axis direction is MD, the y-axis direction is TD, and d represents the thickness of the polyester protective film.

6. The optical polyester protective film according to claim 5, characterized in that: The thickness direction phase difference difference between two points symmetrical in the width direction with respect to the width center is 55 nm or less.

7. The optical polyester protective film according to claim 5, characterized in that: The difference between the maximum value and the minimum value of the thickness direction phase difference is 70 nm or less.

8. The optical polyester protective film according to claim 5, characterized in that: The in-plane retardation measured at a wavelength of 632 nm is 570 nm or less.

9. The optical polyester protective film according to claim 5, characterized in that: The thickness direction phase difference variation rate of the following mathematical formula 1 for the thickness direction phase difference measured at a total of five points spaced equidistantly at 370 mm from the width center in both width directions is 1% or less: [Mathematical formula 1] 10. The optical polyester protective film according to claim 9, characterized in that: The thickness direction phase difference variation rate of Mathematical Formula 1 for the thickness direction phase difference measured at a total of nine points equally spaced at 370 mm from the width center in both width directions is 1% or less.

11. An optical layered body, characterized in that: include: Optical films; and The polyester protective film according to any one of claims 5 to 10, which is disposed on at least one side of the optical film.

12. An image display device, characterized in that: The optical layered body according to claim 11 is included.

Images