Polyester film and its uses

Abstract

To provide a polyester film that is suitably used as a polarizer protective film by decreasing the number of whisker-like defects present on a slit end face of the film.SOLUTION: The polyester film has at most one whisker-like substance present per 100 μm film length, the whisker-like substance protruding to a length of 5 μm or more on an end face of the film slit along a mechanical flow direction in the production process of the film. A polyester film for a polarizer protective film is provided, which shows an in-plane retardation of 3000 nm or more and 30000 nm or less. A polarizing plate and a liquid crystal display device are disclosed in which the above film is used.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film that is slit along the machine direction during film production and has few whisker-like defects on the slit end surfaces, and more specifically to a polyester film for polarizer protective film that has few whisker-like defects on the slit end surfaces, and a polarizing plate and a liquid crystal display device using the same. [Background technology]

[0002] Polarizing plates used in liquid crystal displays (LCDs) typically consist of a polarizer made of iodine-dyed polyvinyl alcohol (PVA) or similar material sandwiched between two polarizer protective films, typically triacetyl cellulose (TAC) film. In recent years, thinner polarizers have been required to meet the demand for thinner LCDs and cost reductions. However, reducing the thickness of the TAC film used as the polarizer protective film to achieve this goal can result in insufficient mechanical strength and reduced moisture permeability. Furthermore, TAC film is very expensive, and there is a strong demand for cheaper alternative materials.

[0003] Although polyester films are more durable than TAC films, they have birefringence, which, unlike TAC films, causes optical distortion and reduces image quality when used as a polarizer protective film. Specifically, because birefringent polyester films have a certain optical anisotropy (retardation), when used as a polarizer protective film, rainbow-like color spots appear when observed from an oblique direction, resulting in reduced image quality. Therefore, in Patent Document 1, a measure to prevent color spots is taken by controlling the in-plane retardation of the polyester film within a specific range.

[0004] Furthermore, in the production of TAC films or polyester films in which the molecules are oriented in the stretching direction, the film has uneven thickness at both ends in the width direction, and these are cut off by a cutting device (slitter) after production (slitting process). However, the orientation of the molecular chains that make up the film can cause problems such as whiskers and fuzz to appear at the slit edges of the film, increasing the amount of foreign matter and causing film breakage during cutting. Therefore, Patent Document 2 proposes a method for solving these problems using a cutting technique.

[0005] On the other hand, during the production of polyester film, scrap film is generated. Attempts have been made to reuse or recycle scrap film, such as film edges cut and removed from products. However, when recycled polyester raw materials are used as the main component of films having coating layers such as highly adhesive resins or antistatic resins, there is a problem that the components of the coating layers deteriorate due to heat, resulting in significant coloration or cloudiness in the resulting film. Therefore, Patent Document 3 proposes a method for obtaining transparent, minimally colored polyester film containing self-recovered raw materials.

[0006] However, Patent Document 2 does not provide any knowledge on polyester films, and its applicability is questionable. Furthermore, Patent Document 3 has the drawback that, when considering application to optical applications, the addition of self-recovered raw materials poses a high hurdle of internal foreign matter generation, and in the coating layer compositions that have become more complex (diversified) in recent years, the inclusion of crosslinked components and the like hinders recovery and causes internal foreign matter. In addition, after removing the coating layer of the polyester film that is the self-recovered raw material by some method, only the base polyester film is left, and then this requires a complicated additional process of melt-extrusion and pelletizing to obtain recycled raw material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2011 / 162198 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-109262 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-039764 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the problems of the prior art. That is, an object of the present invention is to overcome the above-mentioned drawbacks, and in particular to provide a polyester film suitable for use as a polarizer protective film by reducing the number of whisker-like defects present on the slit end surface of the film.

[0009] The present inventors have conducted extensive research to achieve this object and have arrived at the present invention. That is, the present invention has the following configuration. 1. A 3-layer film having at least a surface layer made of polyester on both sides of an intermediate layer made of polyester. Layer A polyester film having a laminated structure, The thickness is 15 μm or more and 200 μm or less, The polyester is polyethylene terephthalate, the intrinsic viscosity of the polyester constituting the intermediate layer is lower than the intrinsic viscosity of the polyester constituting both surface layers, the intrinsic viscosity of both surface layers is 0.64 dL / g or more and 0.67 dL / g or less, and the intrinsic viscosity of the intermediate layer is 0.56 dL / g or more and 0.60 dL / g or less, and the number of whiskers protruding 5 μm or more at the edge of the film slit along the machine direction during film production is one or less per 100 μm of film length. 2. The entire polyester film 2. The polyester film according to 1 above, having an intrinsic viscosity of 0.55 dl / g or more and 0.65 dl / g or less. 3. The polyester film according to the above item 1 or 2, which has a haze value of 2.0% or less and a b value of -0.5 or more and 3.0 or less. 4.The polyester film according to any one of the first to fourth aspects of the present invention contains a self-recovering polymer generated in the process of producing the polyester film as part of the polyester constituting the polyester film. 3 1. The polyester film according to claim 1 , 5. The first to third plates each having a coating layer on at least one side thereof 4 1. The polyester film according to claim 1 , 6. The intermediate layer is made of the first to third polyester films containing a self-recovering polymer generated in the process of manufacturing the polyester film. 5 1. The polyester film according to claim 1 , 7. The first to third films are uniaxially stretched films. 6 1. The polyester film according to claim 1 , 8. The first to third optical fibers have an in-plane retardation of 3,000 nm or more and 30,000 nm or less. 7 2. The polyester film for a polarizer protective film according to claim 1, wherein the polyester film is a polyester film for a polarizer protective film. 9. On at least one surface of the polarizer, 8 A polarizing plate having the polyester film for a polarizer protective film according to claim 1 laminated thereon. 10. A liquid crystal display device having a backlight, a liquid crystal cell, and polarizing plates arranged on both sides of the liquid crystal cell, wherein the polarizing plate on at least one side is 9 A liquid crystal display device comprising the polarizing plate according to claim 1. [Effects of the Invention]

[0010] According to the present invention, a polyester film roll with less whiskers on the slit end surface can be provided, and the factors that cause defects can be reduced, making it more applicable to optical applications, particularly to polarizer protective films. BEST MODE FOR CARRYING OUT THE INVENTION

[0011] The present invention will be described in detail below.

[0012] (polyester film) The polyester film referred to in the present invention is a film made of a polyester resin, and is preferably a polyester film mainly composed of at least one selected from polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. It may also be a film made of a copolymer polyester in which a third component monomer is copolymerized with the above-mentioned polyester. Among these polyester films, polyethylene terephthalate film is most preferred in terms of the balance between physical properties and cost.

[0013] The polyester film may be a single layer or a multilayer. As long as the effects of the present invention are achieved, various additives may be incorporated into the polyester resin of each layer, as needed. Examples of additives include antioxidants, light stabilizers, antigelling agents, organic wetting agents, antistatic agents, UV absorbers, and surfactants.

[0014] (coating layer) The polyester film of the present invention preferably comprises a highly adhesive coating layer laminated on the above-mentioned polyester substrate film, which typically contains a binder resin, lubricant particles, a crosslinking agent, a surfactant, etc.

[0015] The lubricant particles in the coating layer may include (1) inorganic particles such as silica, kaolinite, talc, light calcium carbonate, heavy calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, zinc sulfate, zinc carbonate, titanium dioxide, satin white, aluminum silicate, diatomaceous earth, calcium silicate, aluminum hydroxide, hydrated halloysite, magnesium carbonate, and magnesium hydroxide; and (2) organic particles such as acrylic or methacrylic, vinyl chloride, vinyl acetate, nylon, styrene / acrylic, styrene / butadiene, polystyrene / acrylic, polystyrene / isoprene, methyl methacrylate / butyl methacrylate, melamine, polycarbonate, urea, epoxy, urethane, phenol, diallyl phthalate, and polyester. However, silica is particularly preferred for imparting adequate lubricity to the coating layer.

[0016] The average particle size of the lubricant particles is preferably 200 nm or more, more preferably 250 nm or more, even more preferably 300 nm or more, and particularly preferably 350 nm or more. When the average particle size of the lubricant particles is 200 nm or more, the particles are less likely to aggregate and lubricity can be ensured, which is preferable.

[0017] The average particle size of the lubricant particles is preferably 2000 nm or less, more preferably 1500 nm, even more preferably 1000 nm, and particularly preferably 700 nm. When the average particle size of the lubricant particles is 2000 nm or less, transparency is maintained and the particles do not fall off, which is preferable.

[0018] The lubricant particles may be surface-treated. Surface treatment methods include physical surface treatments such as plasma discharge treatment and corona discharge treatment, and chemical surface treatments using coupling agents, with the use of coupling agents being preferred. As coupling agents, organoalkoxymetal compounds (e.g., titanium coupling agents, silane coupling agents) are preferably used. When the lubricant particles, which are inorganic ultrafine particle fillers, are silica, treatment with a silane coupling agent is particularly effective. The surface treatment may be performed before preparing the coating solution, or the lubricant may be added as an additive during preparation of the coating solution and incorporated into the coating layer.

[0019] The binder resin constituting the coating layer is not particularly limited as long as it is a resin that provides easy adhesion, but specific examples of polymers include polyester resins, acrylic resins, urethane resins, polyvinyl resins (such as polyvinyl alcohol), polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxycellulose, starches, etc. Among these, polyester resins, acrylic resins, and urethane resins are preferably used from the viewpoint of particle retention and adhesion. Furthermore, polyester resins are optimal when compatibility with polyester films is taken into consideration. These binder resins may be used in combination.

[0020] The polyester resin may account for 100% by mass of the total solid components in the coating layer, but is preferably contained in an amount of 10% by mass to 90% by mass. It is more preferably contained in an amount of 20% by mass to 80% by mass. A polyester resin content of 90% or less is preferred because adhesion to a hard coat layer or the like under high temperature and high humidity conditions is maintained. Conversely, a content of 10% by mass or more is preferred because the presence of other urethane resins, etc., allows adhesion to a polyester film under normal temperature and high temperature and high humidity conditions to be maintained.

[0021] In the present invention, the coating layer may be formed by containing a crosslinking agent to form a crosslinked structure in the coating layer. By including a crosslinking agent, it becomes possible to further improve adhesion under high temperature and high humidity conditions. Specific crosslinking agents include urea-based, epoxy-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents. Among these, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents are preferred in terms of the stability of the coating liquid over time and the effect of improving adhesion under high temperature and high humidity treatment. Furthermore, a catalyst or the like can be used as needed to promote the crosslinking reaction.

[0022] The content of the crosslinking agent in the coating layer is preferably 5% by mass or more and 50% by mass or less of the total solid components. More preferably, it is 10% by mass or more and 40% by mass or less. When it is 5% by mass or more, the strength of the resin in the coating layer is maintained, and there is no risk of a decrease in adhesion under high temperature and high humidity, which is preferable. On the other hand, when it is 50% by mass or more, the flexibility of the resin in the coating layer is maintained, and adhesion is maintained at room temperature and under high temperature and high humidity, which is preferable.

[0023] The content of lubricant particles in the coating layer is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. When the content of lubricant particles in the coating layer is 0.1% by mass or more, it is preferable because an appropriate level of slippage is maintained.

[0024] The content of lubricant particles in the coating layer is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. When the content of lubricant particles in the coating layer is 20% by mass or less, the haze is kept low, which is preferable in terms of transparency.

[0025] The thickness of the coating layer is preferably 0.001 μm or more, more preferably 0.01 μm or more, even more preferably 0.02 μm or more, and particularly preferably 0.05 μm or more. When the thickness of the coating layer is 0.001 μm or more, the adhesion is good, which is preferable.

[0026] The thickness of the coating layer is preferably 2 μm or less, more preferably 1 μm or less, even more preferably 0.8 μm or less, and particularly preferably 0.5 μm or less. When the thickness of the coating layer is 2 μm or less, there is no risk of blocking occurring, which is preferable.

[0027] The coating layer may contain a surfactant to improve leveling during coating and to defoam the coating solution. The surfactant may be cationic, anionic, or nonionic, but silicone, acetylene glycol, or fluorine-based surfactants are preferred. These surfactants are preferably contained in the coating layer to an extent that does not impair adhesion or the effect of suppressing iridescence under fluorescent light.

[0028] In order to impart other functionality to the coating layer, various additives may be added to the extent that they do not impair the effect of suppressing iridescence under fluorescent light or the adhesion, such as fluorescent dyes, fluorescent brighteners, plasticizers, ultraviolet absorbers, pigment dispersants, antifoaming agents, defoaming agents, and preservatives.

[0029] As a coating method, either the so-called in-line coating method in which coating is carried out simultaneously with the formation of a polyester film, or the so-called off-line coating method in which coating is carried out using a separate coater after the polyester film has been formed, can be applied, but the in-line coating method is more efficient and more preferred.

[0030] Any known method can be used to apply the coating solution to the polyester film. Examples include reverse roll coating, gravure coating, kiss coating, die coating, roll brushing, spray coating, air knife coating, wire bar coating, pipe doctor coating, impregnation coating, and curtain coating. These methods can be used alone or in combination.

[0031] In the present invention, a method for providing a coating layer on a polyester film includes a method of applying a coating liquid containing a solvent, particles, and a resin to the polyester film and drying the applied liquid. Examples of the solvent include an organic solvent such as toluene, water, or a mixture of water and a water-soluble organic solvent. From the viewpoint of environmental conservation, water alone or a mixture of water and a water-soluble organic solvent is preferred.

[0032] The solid content of the coating solution varies depending on the type of binder resin and the type of solvent, but is preferably 2% by mass or more, more preferably 4% by mass or more, and is preferably 35% by mass or less, more preferably 15% by mass or less.

[0033] The drying temperature after coating also depends on the type of binder resin, the type of solvent, the presence or absence of a crosslinking agent, the solid content concentration, etc., but is preferably 80°C or higher and 250°C or lower.

[0034] The surface roughness (Ra) of the coating layer is related to the smoothness of the surface of the coating layer, and is preferably 0.01 nm or more, more preferably 0.1 nm or more, even more preferably 0.2 nm or more, and particularly preferably 0.5 nm or more. On the other hand, the upper limit of the surface roughness (Ra) of the coating layer is preferably 200 nm or less, more preferably 100 nm or less, even more preferably 80 nm or less, and particularly preferably 50 nm or less.

[0035] (Production of polyester film) The polyester film of the present invention can be produced by a general method for producing a polyester film, for example, by melting a polyester resin, extruding the extruded polyester into a sheet, and then stretching the resulting unoriented polyester sheet or film in the longitudinal direction at a temperature equal to or higher than the glass transition temperature by using a roll speed difference, if necessary, at a temperature equal to or higher than the glass transition temperature, and then stretching the resulting film in the transverse direction at a temperature equal to or higher than the glass transition temperature by using a tenter, or without stretching the film in the longitudinal direction, and then heat-treating the resulting film.

[0036] The polyester film of the present invention is believed to be applicable in a wide range of applications, but is suitably used as an optical film, particularly as a polarizer protective film.

[0037] The polyester film for protecting a polarizer may be either a uniaxially stretched film or a biaxially stretched film. However, when a biaxially stretched film is used as a polarizer protective film, depending on the in-plane retardation of the film, rainbow-like color spots may not be observed when observed from directly above the film surface, but rainbow-like color spots may be observed when observed from an oblique direction, so care must be taken.

[0038] This phenomenon occurs because biaxially stretched films are composed of index ellipsoids with different refractive indices in the machine, width, and thickness directions, and there exists a direction within the film where the retardation is zero (the index ellipsoid appears as a perfect circle) depending on the direction of light transmission. Therefore, when an LCD display screen is observed from a specific oblique direction, a point where the retardation is zero may appear, and rainbow-like color spots appear concentrically around that point. If the angle θ is defined as the angle from directly above the film surface (normal direction) to the point where the rainbow-like color spots are visible, this angle θ increases as the in-plane birefringence of the film increases, making the rainbow-like color spots less visible. Because the angle θ tends to be smaller in biaxially stretched films, uniaxially stretched films are preferred because they make the rainbow-like color spots less visible.

[0039] However, in a completely uniaxial (symmetrical) film, the mechanical strength in the direction perpendicular to the orientation direction may be significantly reduced. Therefore, in the present invention, it is also preferable that the film has biaxiality (biaxial symmetry) within a range in which rainbow-like color spots are not substantially produced, or within a range in which rainbow-like color spots are not produced within the viewing angle range required for the liquid crystal display screen.

[0040] The polyester film used for polarizer protective films preferably has an in-plane retardation of 3,000 to 30,000 nm. When the in-plane retardation is 3,000 nm or more, when used as a polarizer protective film, strong interference colors are not exhibited when observed from an oblique direction, and good visibility can be ensured, which is preferable. The lower limit of the in-plane retardation is preferably 4,500 nm, more preferably 5,000 nm, even more preferably 6,000 nm, particularly preferably 8,000 nm, and most preferably 10,000 nm.

[0041] On the other hand, the preferred upper limit of the in-plane retardation is 30,000 nm. Even if a polyester film having an in-plane retardation exceeding 30,000 nm is used, the effect of further improving visibility is substantially saturated. Furthermore, if the film has an in-plane retardation of 30,000 nm or less, the film thickness is not too large, and handling as an industrial material is favorable, which is preferable.

[0042] The in-plane retardation in the present invention can be determined by measuring the refractive index in two axial directions and the thickness, or by using a commercially available automatic birefringence measuring device such as KOBRA-21ADH (Oji Scientific Instruments Co., Ltd.). The wavelength for measuring the refractive index is 589 nm.

[0043] Specifically, the film-forming conditions for the polyester film of the present invention are as follows: the longitudinal stretching temperature and the transverse stretching temperature are preferably 80 to 130°C, and particularly preferably 90 to 120°C. The longitudinal stretching ratio is preferably 1.0 to 3.5 times, and particularly preferably 1.0 to 3.0 times. The transverse stretching ratio is preferably 2.5 to 6.0 times, and particularly preferably 3.0 to 5.5 times. In order to control the in-plane retardation, it is preferable to control the ratio between the longitudinal stretching ratio and the transverse stretching ratio. Increasing the difference between the longitudinal and transverse stretching ratios is preferable because it increases the retardation. Setting the stretching temperature low is also a preferable way to increase the retardation. In the subsequent heat treatment, the treatment temperature is preferably 100 to 250°C, and particularly preferably 180 to 245°C.

[0044] In order to suppress fluctuations in retardation, it is preferable that the thickness unevenness of the film is small. Since the stretching temperature and stretching ratio affect the thickness unevenness of the film, it is preferable to optimize the film formation conditions from the viewpoint of thickness unevenness as well.

[0045] The thickness unevenness of the film of the present invention is preferably 5.0% or less, more preferably 4.5% or less, even more preferably 4.0% or less, and particularly preferably 3.0% or less.

[0046] As described above, the in-plane retardation of a film can be controlled within a specific range by appropriately setting the stretching ratio, stretching temperature, and film thickness. For example, the greater the difference in stretching ratio between the longitudinal and transverse directions, the lower the stretching temperature, and the thicker the film, the easier it is to obtain a high in-plane retardation. Conversely, the smaller the difference in stretching ratio between the longitudinal and transverse directions, the higher the stretching temperature, and the thinner the film, the easier it is to obtain a low retardation. However, increasing the film thickness tends to increase the thickness direction phase difference. Therefore, it is desirable to appropriately set the film thickness within the range described below. In addition to controlling the in-plane retardation, it is preferable to set the final film-forming conditions taking into account the physical properties required for processing.

[0047] The thickness of the polyester film of the present invention is not particularly limited, but is preferably in the range of 15 to 300 μm, more preferably in the range of 20 to 200 μm, and even more preferably in the range of 30 to 150 μm. Even with a film having a thickness of less than 15 μm, it is possible, in principle, to obtain a retardation of 3,000 nm or more. However, in such a case, the anisotropy of the mechanical properties of the film becomes significant, making it prone to tearing and breaking, and thus less practical as an industrial material, which is not preferred. A particularly preferred lower limit of the thickness is 35 μm. On the other hand, it is preferable that the upper limit of the thickness of the polarizer protective film is 300 μm or less, since this prevents the polarizing plate from becoming too thick. From the viewpoint of practicality as a polarizer protective film, the upper limit of the thickness is preferably 200 μm. A particularly preferred upper limit of the thickness is 100 μm, which is approximately the same as that of a general TAC film. Even within the above thickness range, polyethylene terephthalate is suitable as the polyester constituting the film in order to control the in-plane retardation within the range of the present invention.

[0048] The polyester film of the present invention preferably has a light transmittance of 20% or less at a wavelength of 380 nm in order to prevent deterioration of optically functional dyes such as iodine dyes. The light transmittance at 380 nm is more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less. A light transmittance of 20% or less can prevent deterioration of the optically functional dye due to ultraviolet rays. The transmittance in the present invention is measured in a direction perpendicular to the plane of the film and can be measured using a spectrophotometer.

[0049] In order to achieve a transmittance of 20% or less at a wavelength of 380 nm for the polyester film of the present invention, it is desirable to appropriately adjust the type and concentration of the ultraviolet absorber contained in the polyester film and the film thickness. The ultraviolet absorber used in the present invention is a known substance. Examples of ultraviolet absorbers include organic and inorganic ultraviolet absorbers, with organic ultraviolet absorbers being preferred from the viewpoint of transparency. Examples of organic ultraviolet absorbers include, but are not limited to, benzotriazoles, benzophenones, cyclic iminoesters, and combinations thereof. However, from the viewpoint of durability, benzotriazoles and cyclic iminoesters are particularly preferred. When two or more ultraviolet absorbers are used in combination, ultraviolet rays of different wavelengths can be simultaneously absorbed, thereby further improving the ultraviolet absorption effect.

[0050] Examples of ultraviolet absorbers include benzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, and acrylonitrile-based ultraviolet absorbers. Examples of the hydroxyl-based ultraviolet absorbers include 2-[2'-hydroxy-5'-(methacryloyloxy) 2-[2'-hydroxy-5'-((dimethylphenyl)phenyl]-2H-benzotriazole methacryloyloxyethyl)phenyl]-2H-benzotriazole, 2-[2'-hydroxyethyl]phenyl hydroxy-5'-(methacryloyloxypropyl)phenyl]-2H-benzotriazo 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2,2',4,4' -Tetrahydroxybenzophenone, 2,4-di-tert-butyl-6-(5-chloro Benzotriazol-2-yl)phenol, 2-(2'-hydroxy-3'-tert- Butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(5-chloro( 2H)-Benzotriazol-2-yl)-4-methyl-6-(tert-butyl)phenyl 2,2'-methylenebis(4-(1,1,3,3-tetramethylbutyl)-6- (2H-benzotriazol-2-yl)phenol. Examples of terephthalate UV absorbers include 2,2'-(1,4-phenylene)bis(4H-3, 1-benzoxazinon-4-one), 2-methyl-3,1-benzoxazin-4-one 2-butyl-3,1-benzoxazin-4-one, 2-phenyl-3,1-benzoxazin-4-one oxazin-4-one, etc. However, the present invention is not limited to these.

[0051] The method for incorporating the ultraviolet absorber into the polyester film of the present invention can be a combination of known methods. For example, the ultraviolet absorber can be incorporated by a method in which a dried ultraviolet absorber is blended in advance with a polymer raw material using a kneading extruder to prepare a master batch, and then the predetermined master batch and the polymer raw material are mixed during film formation.

[0052] In this case, the concentration of the UV absorber in the masterbatch is preferably 5 to 30% by mass in order to disperse the UV absorber uniformly and to incorporate it economically. The conditions for preparing the masterbatch are preferably a kneading extruder, with the extrusion temperature being above the melting point of the polyester raw material and below 290°C for 1 to 15 minutes. At temperatures above 290°C, the weight loss of the UV absorber is significant, and the viscosity of the masterbatch also decreases significantly. At extrusion temperatures below 1 minute, it becomes difficult to uniformly mix the UV absorber. At this time, stabilizers, color adjusters, and antistatic agents may be added as needed.

[0053] In addition, in the present invention, it is preferable that the polyester film has a multilayer structure of at least three layers and that an ultraviolet absorber be added to the intermediate layer of the film. A polyester film with a three-layer or more structure containing an ultraviolet absorber in the intermediate layer can be specifically produced as follows: Polyester pellets alone are mixed for both surface layers, and a masterbatch containing an ultraviolet absorber and polyester pellets are mixed for the intermediate layer in a predetermined ratio, dried, and then fed into a known melt lamination extruder, extruded into a sheet through a slit die, and cooled and solidified on a casting roll to produce an unstretched film. That is, using two or more extruders and three or more manifolds or merging blocks (e.g., merging blocks with square merging sections), film layers constituting both surface layers and the intermediate layer are laminated, and a three-layer or more sheet is extruded from the die and cooled on a casting roll to produce an unstretched film. In the present invention, high-precision filtration is preferably performed during melt extrusion to remove foreign matter contained in the raw polyester that may cause optical defects. The filtering particle size (initial filtering efficiency 95%) of the filtering medium used for high-precision filtering of molten resin is preferably 15 μm or less. When the filtering particle size of the filtering medium is 15 μm or less, foreign matter of 20 μm or more can be sufficiently removed, which is preferable.

[0054] It is preferable that self-recovered polymer raw materials are used for the polyester film of the present invention.

[0055] Here, we will describe the method for producing self-recovery raw materials. The scrap film at the end of the film, which was slit in the machine direction and cut off from the product, was crushed and dried at a temperature of approximately 160°C, up to the glass transition temperature, to remove moisture. The dried crushed film was melted in an extruder and extruded from a die in the form of strands, which were then rapidly solidified with cold water. The solidified strands were cut into pellets using a strand cutter, and the surface was dried to prevent fusion, yielding self-recovery pellets.

[0056] The moisture content after drying is preferably 10 ppm or less. If it is 10 ppm or less, the hydrolysis of the polymer is unlikely to be accelerated, and there is no risk of a decrease in intrinsic viscosity, which is preferable.

[0057] The film at both ends of the master roll is of poor quality due to uneven thickness and the effects of clip marks made when the film is conveyed through the tenter, and is therefore usually cut off. However, when these portions are recovered and used as self-recovered raw material, care must be taken because, depending on the coating layer laminated on the cut portions, the polyester film produced from this self-recovered raw material may become contaminated with foreign matter, become discolored, or become cloudy.

[0058] In the present invention, the coating layer on both end portions of the master roll that is slit and cut off is different from the coating composition for laminating a coating layer having properties such as easy adhesion required for the product polyester film that is suitable for use in the polarizer protective film described above. Instead, it is a separate coating composition that is less likely to cause contamination, discoloration, or turbidity even if it is re-introduced as part of the raw polyester of the product polyester film, and is applied separately without overlapping with the coating layer laminated on the product polyester film. It is preferable to cut off the master roll up to the portions where the separate coating liquid is applied, located at both ends in the width direction, and use it as self-recovered raw material.

[0059] The coating liquid to be applied separately at both ends in the width direction of the film is not particularly limited as long as it has a composition that allows for the preparation of self-recoverable raw materials. However, in consideration of the stability and safety of the composition used, a coating liquid of a resin composition containing water as the main medium is preferred.

[0060] The aqueous coating solution may contain a small amount of organic solvent to improve the stability of the coating solution. Examples of such organic solvents include methyl ethyl ketone, acetone, ethyl acetate, tetrahydrofuran, dioxane, cyclohexanone, n-hexane, toluene, xylene, methanol, ethanol, 1-propanol, and 2-propanol. Multiple organic solvents may be contained.

[0061] The above-mentioned aqueous coating liquid may contain known additives, such as a heat stabilizer, an oxidation stabilizer, a weather stabilizer, an ultraviolet absorber, an organic lubricant, and organic or inorganic fine particles, as long as the effects of the present invention are not impaired.

[0062] Any known coating method may be used, such as reverse coating, gravure coating, microgravure coating, rod coating, die coating, roll coating, roll brushing, spray coating, air knife coating, curtain coating, or the like.

[0063] In the present invention, the separate coating layer on both ends of the film in the width direction is applied to at least one side of the polyester film, but it is preferable to apply the coating before the crystalline orientation of the polyester film is completed, and then stretch the polyester film in at least one direction and then complete the crystalline orientation of the polyester film, as this method can more significantly exhibit the effects of the present invention.

[0064] The amount of application is 1 m 2 The preferred thickness is 2 g or more and 15 g or less, more preferably 4 g or more and 13 g or less per unit area. The thickness of the final dried coating film is preferably 0.001 μm or more, more preferably 0.01 μm or more, even more preferably 0.02 μm or more, and particularly preferably 0.05 μm or more. When the coating layer has a thickness of 0.001 μm or more, sufficient slip properties can be ensured, the film roll slides well, and a film roll with few defects such as scratches can be obtained.

[0065] The thickness of the coating layer is preferably 2 μm or less, more preferably 1 μm or less, even more preferably 0.8 μm or less, and particularly preferably 0.5 μm or less. When the thickness of the coating layer is 2 μm or less, there is no risk of blocking occurring, which is preferable.

[0066] In the present invention, after the aqueous coating liquid is applied to the polyester substrate, a drying and stretching treatment is carried out. This drying is preferably carried out at a temperature of 80°C or higher and 250°C or lower, although this temperature varies depending on the type of binder resin, the type of solvent, the presence or absence of a crosslinking agent, the solids concentration, etc. The stretching treatment is carried out in accordance with the above-mentioned polyester film production method, and it is preferable to set the conditions accordingly.

[0067] In addition to the above, self-recovered raw materials can also be produced by peeling off the coating layer from edge rolls at the ends of slit rolls or substandard rolls that do not meet product standards.

[0068] There are no particular limitations on the method for peeling off the coating layer, as long as it does not cause problems such as foreign matter being mixed into the film after peeling. One method is to chemically and physically peel off only the coating layer, such as sandblasting, leaving only the resin layer, and this method may also be applied to the present invention.

[0069] The polyester film for polarizer protective film of the present invention preferably has a b-value, which indicates a yellowish-blue tint, of approximately zero even when self-recovered raw materials are used. When the b-value is 3.0 or less, the film does not appear yellowish, and when applied to a polarizer protective film, problems such as deterioration of image quality and discoloration do not occur, which is preferable. Furthermore, when the b-value is -0.5 or more, the film does not appear blue, and does not give a dark impression, which is preferable.

[0070] The same applies to the haze of the film, which is preferably 2.0% or less. If the haze is 2.0% or less, transparency is maintained and there is no risk of the display of a liquid crystal or the like being disturbed, which is preferable.

[0071] The intrinsic viscosity of the polyester film obtained by the present invention is preferably 0.55 dL / g or more, more preferably 0.56 dL / g or more, and even more preferably 0.57 dL / g or more. When the intrinsic viscosity is 0.55 dL / g or more, the mechanical strength of the film is maintained and there is no risk of problems with film formation stability or breakage resistance, which is preferable.

[0072] The intrinsic viscosity of the polyester film obtained by the present invention is preferably 0.65 dL / g or less, more preferably 0.64 dL / g or less, and even more preferably 0.63 dL / g or less. When the intrinsic viscosity is 0.65 dL / g or less, the film does not become too hard and cutting defects are unlikely to occur when both ends of the film in the width direction are cut off with a slitter, which is preferable because the cutting quality and cutting state are good.

[0073] Here, the intrinsic viscosity (dl / l) of the produced polyester film can be obtained by dissolving the polyester film in a specific solvent and measuring the solution using a viscosity tube.

[0074] The polyester film of the present invention preferably has no more than one whisker-like object (including cutting irregularities such as burrs and fluff) protruding 5 μm or more from the slit end face per 100 μm of film length.

[0075] In polyester film, the number of whisker-like features (cut defects) protruding 5 μm or more from the film edge is one or less per 100 μm of film length. This means that the film edge, slit in the machine direction by a slitter, is observed with an electron microscope or optical microscope, and whisker-like features (cut defects) protruding 5 μm or more from the edge per 100 μm of film length are counted, and the total is one or less.

[0076] If these whiskers (cutting defects) are less than 5 μm in size and protrude from the edge surface, they do not have a significant impact on the incorporation or adhesion of foreign matter during transportation or handling when applied to a polarizer protective film, and any foreign matter that does adhere is easy to remove, so they do not need to be a particular problem.

[0077] In order to obtain a polyester film of the present invention in which whisker-like features (cutting irregularities) protruding 5 μm or more from the slit edge are limited to one per 100 μm of film length, the slitting method is not particularly limited as long as it is a method that can obtain the film edge of the present invention, but preferred examples include a method that uses a laser beam as the slitting means (a method in which a beam is irradiated onto the film surface using a laser beam oscillator, and the film is instantly melted or evaporated by the laser irradiation energy and slit along the scanning path of the beam, and various oscillators such as a carbon dioxide laser, an argon laser, or a YAG laser are used for irradiating the laser beam), and a method of slitting using a blade (a Thomson blade, a rotary blade, etc.).

[0078] When using a cutting tool (Thomson cutting tool, rotary cutting tool, etc.) to slit the film, it is necessary to replace the cutting tool or update the cutting edge under certain conditions. For example, when slitting polyester film, if the cutting area is 200m 2 Not exceeding 160m, preferably 2 The polyester film of the present invention can be obtained by replacing the blade or updating the blade edge so that the cutting speed does not exceed 151 m / min. For example, when a 100 μm film is slit at a speed of 150 m / min for 7 consecutive days, the cutting speed is 151 m / min. 2 At this point, it is preferable to replace the blade or update the cutting edge.

[0079] The amount of self-recovering raw material added is not particularly limited as long as the above-mentioned film properties are satisfied, but the mass ratio of the self-recovering raw material to the total raw materials of the layer of the film to which it is added is preferably 3 mass% or more, more preferably 5 mass% or more, even more preferably 7 mass% or more, and particularly preferably 8 mass% or more. A mass% or more of 3 mass% or more is preferred because it makes it easy to adjust the intrinsic viscosity of the resulting film to 0.65 dL / g or less, the elastic modulus of the film is not too high, and whiskers protruding from the edge surfaces are reduced.

[0080] The amount of self-recovering raw material added is not particularly limited as long as the above-mentioned film properties are satisfied, but the mass ratio of the self-recovering raw material to the total raw materials of the layer of the film to which it is added is preferably 30 mass% or less, more preferably 28 mass% or less, even more preferably 26 mass% or less, and particularly preferably 25 mass% or less. If it is 30 mass% or less, the intrinsic viscosity of the film is likely to be 0.55 dL / g or more, and the mechanical strength of the film is maintained, which is preferable in terms of film formation stability, break resistance, etc.

[0081] The polyester film of the present invention has reduced whisker formation on the slit end surfaces. It has been found that the intrinsic viscosity of the resin used in film molding is related to the reduction in the number of whiskers, and by controlling the intrinsic viscosity of the resin within a certain range, whisker formation on the slit end surfaces can be favorably reduced.

[0082] In terms of the strength of the resulting polyester film, a certain level of intrinsic viscosity of the polyester constituting the film is desirable, and to achieve this, it is preferable to have a three-layer or more laminate structure having at least surface layers made of polyester on both sides of an intermediate layer made of polyester, with both surface layers being polyester layers with relatively high intrinsic viscosity and the intermediate layer being a polyester layer with relatively low intrinsic viscosity. In particular, it is preferable to configure the intermediate layer by incorporating a self-recovering polymer with a low intrinsic viscosity that is generated during the process of producing the polyester film.

[0083] The intrinsic viscosity of both surface layers of the polyester film having such a laminated structure of three or more layers is preferably 0.63 dL / g or more, more preferably 0.64 dL / g or more, and even more preferably 0.65 dL / g. When the intrinsic viscosity is 0.63 dL / g or more, the intrinsic viscosity of the entire film is not too low, which is preferable because problems such as breakage do not occur during film formation.

[0084] The intrinsic viscosity of both surface layers of such a polyester film having a laminate structure of three or more layers is preferably 0.67 dL / g or less, more preferably 0.66 dL / g or less, and even more preferably 0.65 dL / g. When the intrinsic viscosity is 0.67 dL / g or less, the resin does not have an excessively high intrinsic viscosity, and the pressure applied to filters during extrusion is not too high, which is preferable because problems such as poor discharge and uneven thickness can be avoided.

[0085] Furthermore, the intrinsic viscosity of the intermediate layer of such a polyester film having a laminate structure of three or more layers is preferably 0.56 dL / g or more, more preferably 0.57 dL / g or more, and even more preferably 0.58 dL / g or more. When the intrinsic viscosity is 0.56 dL / g or more, the intrinsic viscosity of the entire film is not too low, which is preferable because problems such as breakage do not occur during film formation.

[0086] The intrinsic viscosity of the intermediate layer of the polyester film obtained by the present invention is preferably 0.62 dL / g or less, more preferably 0.61 dL / g or less, and even more preferably 0.60 dL / g or less. When the intrinsic viscosity is 0.62 dL / g or less, the intrinsic viscosity of the resin is not too high, the pressure applied to the filter during extrusion is not high, and film formation can be performed without incurring discharge defects, making it easier to control the target thickness and reducing thickness unevenness.

[0087] Here, the intrinsic viscosity (dl / g) of each layer of a polyester film having a laminated structure of three or more layers can be calculated from the fluid viscosity and measurement temperature (°C) obtained from an online viscometer such as a capillary viscometer attached to the piping during each melt extrusion, and the intrinsic viscosity (dl / l) can be obtained using a pre-programmed conversion formula.

[0088] The intrinsic viscosity of the resin can control the intrinsic viscosity of the resulting polyester film and reduce the occurrence of whiskers, but it is also important in the layer structure of the polyester film. Taking a three-layer polyester structure, excluding any coating layers, as an example, the thickness composition ratio (surface layer:intermediate layer:surface layer) is preferably controlled between 4:92:4 and 13:74:13, and more preferably between 5:90:5 and 10:80:10. A thickness ratio of 4% or more for each of the two surface layers is preferable because it prevents an increase in film haze due to bleed-out of the UV absorber and other additives added to the intermediate layer.

[0089] When the ratio of each of the two surface layers is 13% or less, the intrinsic viscosity of the resulting polyester film does not become too high, and the effect of suppressing the generation of whiskers is clear, which is preferable.

[0090] Generally, a polarizing plate has a structure in which polarizer protective films are laminated on both sides of a polarizer, and at least one of the polarizer protective films is preferably the polyester film of the present invention. The polarizer is usually a film mainly made of polyvinyl alcohol (PVA) in which iodine compound molecules are adsorbed and aligned.

[0091] In the present invention, it is also preferable to use a polarizer protective film having various hard coat layers laminated on the surface thereof for the purpose of preventing reflection, suppressing glare, suppressing scratches, etc., in the polarizing plate, and the surface having such a hard coat layer is preferably laminated so as to be provided on the side that does not come into contact with the polarizer.

[0092] The liquid crystal display device of the present invention is a device in which polarizing plates are arranged on both sides of a liquid crystal cell, and images are displayed by a backlight arranged on one side of the cell, the light source of which is a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED). It is preferable that the polarizing plate of the present invention is used for at least one of the two polarizing plates. The liquid crystal display device may also have other components, such as a color filter, a lens film, a diffusion sheet, and an anti-reflection film, as appropriate.

[0093] The backlight configuration may be either an edge-light type in which the light guide plate and reflector are made of steel, or a direct-light type, but it is preferable to use a white LED as the backlight source for the liquid crystal display device. White LEDs are elements that emit white light by combining a phosphor-based LED that uses compound semiconductors to emit blue or ultraviolet light with a phosphor. Phosphors include yttrium-aluminum-garnet yellow phosphors and terbium-aluminum-garnet yellow phosphors, and white LEDs that combine a compound semiconductor blue LED with a yttrium-aluminum-garnet yellow phosphor have a continuous, wide emission spectrum and excellent luminous efficiency, making them promising for energy savings.

[0094] A method combining LEDs that emit red, green, and blue (three-color LED method) has also been put to practical use, but this method produces a narrow and discontinuous emission spectrum. Furthermore, fluorescent tubes such as CCFLs, which are widely used as conventional backlight light sources, also have a peak at a specific wavelength and a discontinuous emission spectrum, so caution is required in that the full effect of the present invention may not be obtained.

[0095] The arrangement of the polarizer protective film of the present invention in a liquid crystal display device is not particularly limited. In the case of a liquid crystal display device having a polarizer disposed on the incident light side (light source side), a liquid crystal cell, and a polarizer disposed on the exit light side (viewing side), the polarizer protective film is preferably disposed on the incident light side of the polarizer disposed on the incident light side, or on the exit light side of the polarizer disposed on the exit light side, and more preferably on the exit light side of the polarizer disposed on the exit light side. If the polarizer protective film of the present invention is disposed in a position other than the above, the polarization characteristics of the liquid crystal cell may be changed. On the other hand, it is preferable to use a film without in-plane retardation, such as a TAC film, an acrylic film, or a norbornene-based resin film, on the side on which the polarizer protective film of the present invention is not used. [Example]

[0096] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the spirit of the present invention, and all such modifications are included in the technical scope of the present invention. Here, Examples 2, 4, 8 and 10 below should be read as Reference Examples 2, 4, 8 and 10.

[0097] The evaluation methods used in the examples and the methods for evaluating various physical properties described in the text are shown below.

[0098] (1) Average particle size [Measurement method using a scanning electron microscope] The average particle size of particles used in coating layers and the like can be measured by the following method. Particles are photographed with a scanning electron microscope (SEM), and the maximum diameters (the distance between the two most distant points) of 300 to 500 particles are measured at a magnification such that the size of the smallest particle is 2 to 5 mm, and the average value is taken as the average particle size. The average particle size of particles present in the coating layer of the present invention can be measured by this measurement method.

[0099] (2) Intrinsic viscosity of resin In the present invention, the intrinsic viscosity of the resin of each layer is evaluated by measuring the fluid viscosity during melt molding. The evaluation was performed using an online viscometer VIS (manufactured by Gneuss) installed in the piping for the molten fluid, and the intrinsic viscosity (dl / l) was calculated using a pre-programmed conversion formula.

[0100] (3) Film thickness (d) The thickness d was determined in accordance with JIS K 7130-1999 "Method for measuring thickness of plastic films and sheets (Method A)."

[0101] (4) Refractive index (Nx, Ny, Nz) The refractive index in the MD direction (Nx), the refractive index in the TD direction (Ny), and the refractive index in the thickness direction (Nz) were determined in accordance with JIS K 7142-2014 "Measurement method for refractive index of plastic film (Method A)." The measurement wavelength was 589 nm.

[0102] (5) Birefringence (ΔNxy), in-plane retardation (Re) Retardation is the phase difference expressed as the product of the birefringence caused by the refractive index (Nx, Ny, Nz) in each axial direction of the film in the thickness direction (Z axis) of the film and two axial directions (x axis, y axis) that are perpendicular to this and also perpendicular to each other, and the thickness d of the film; here, it refers to in-plane retardation, which is the product of the birefringence Nxy caused by light incident on the film surface (xy plane) with MD as the x axis and TD as the y axis, and the thickness d, and each was calculated using the following formula. As is customary, the unit of in-plane retardation is nm. ΔNxy = |Nx-Ny| Re = Nxy / d

[0103] (6) Thickness retardation (Rth) The thickness direction retardation indicates the retardation caused by light incident in the thickness direction (z axis) and two axial directions (x axis, y axis) that are perpendicular to the thickness direction and perpendicular to each other with respect to the film surface, and here it was calculated using the following formula as the product of the average of the two birefringences in the xz plane and yz plane and the film thickness d. As usual, the unit is nm. Rth = (|Nx-Nz|+|Ny-Nz|) / 2×d

[0104] (7) Intrinsic viscosity of the entire film produced and the raw material pellets In accordance with JIS K 7367-2002 "Plastics - Determination of viscosity of diluted solutions using a capillary viscometer - Part 5: Thermoplastic polyester (TP) homopolymers and copolymers," the viscosity number obtained was measured under the following measurement conditions, and the value when the mass concentration c of the solution was set to 0 based on the relationship of the viscosity number to the mass concentration c of the solution was taken as the intrinsic viscosity (iV). Solvent: Phenol / 1,1,2,2-tetrachloroethane = 60 / 40 (wt%) Tube: Ubbelohde viscosity tube Temperature: 30±0.1℃

[0105] (8) Iridescence observation A polarizer was produced by attaching the films of the Examples and Comparative Examples, which were prepared by the method described below, to one side of a commercially available polarizer so that the absorption axis of the polarizer and the main alignment axis of the film (the higher of Nx and Ny) were perpendicular, and attaching a commercially available TAC film to the other side. This was then installed in a commercially available liquid crystal display device having a white LED as a backlight and a liquid crystal cell sandwiched between polarizing plates with two TAC films as polarizer protective films. The polarizing plate on the exit light side was removed, and the film of the Examples and Comparative Examples was installed so that it was on the exit light side. The liquid crystal display device was visually observed from the front and from an oblique angle, and the occurrence of iridescence was evaluated as follows. ○: No rainbow spots from any direction ×: When observed from an oblique direction, rainbow spots are clearly visible.

[0106] (9) Thickness variation rate in the width direction A continuous tape-like sample measuring 3 m in the width direction and 5 cm in the length direction was wound up, and the film thickness was measured using a continuous film thickness measuring device (manufactured by Anritsu Corporation) and recorded on a recorder. The maximum thickness (dmax), minimum thickness (dmin), and average thickness (d) were determined from the chart, and the thickness variation rate (%) was calculated using the following formula. The measurement was performed three times and the average value was calculated. If the length in the width direction was less than 3 m, the sample was joined together. The joined portions were deleted from the data. Thickness variation rate (%) = ((dmax-dmin) / d) × 100 The average value was calculated as the thickness variation rate (%) in the width direction, and was evaluated according to the following criteria. ○: Thickness variation rate in the width direction is 3% or less △: Thickness variation rate in the width direction is 5% or less ×: Thickness variation rate in the width direction is more than 5%

[0107] (10) Film transparency (haze) Measurement was carried out using a turbidity meter (manufactured by Nippon Denshoku, NDH2000) in accordance with JIS K 7136-2000 "Determination of haze for plastics - transparent materials".

[0108] (11) Color Tone (b-value) Measurements were made using a colorimeter (ZE2000, manufactured by Nippon Denshoku) in accordance with JIS K 7373-2006 "Plastics - Determination of yellowness and discoloration index."

[0109] (12) Surface roughness (Ra) In accordance with JIS B 0601-2001 "Geometric Product Specifications (GPS) - Surface Texture; Profile Method - Terms, Definitions, and Surface Texture Parameters," Ra was measured using a Surfcom (registered trademark) 304B (manufactured by Tokyo Seimitsu Co., Ltd.) under the following measurement conditions: cutoff 0.08 μm, stylus radius 2 μm, measurement length 0.8 mm, and measurement speed 0.03 mm / sec.

[0110] (13) Glass transition temperature Measurement was performed using a differential scanning calorimeter (Seiko Instruments, DSC6200) in accordance with JIS K 7121-2012 "Method for measuring transition temperature of plastics." A 10 mg resin sample was heated at a rate of 20°C / min over the temperature range of 25°C to 300°C, and the extrapolated glass transition onset temperature obtained from the DSC curve was taken as the glass transition temperature.

[0111] (14) Number average molecular weight 0.03 g of the resin was dissolved in 10 ml of tetrahydrofuran, and the number average molecular weight was measured using a GPC-LALLS low-angle light scattering photometer (LS-8000, manufactured by Tosoh Corporation) at a column temperature of 30°C, a flow rate of 1 ml / min, and a column (Shodex KF-802, 804, 806, manufactured by Showa Denko K.K.).

[0112] (15) Resin composition The resin was dissolved in deuterated chloroform and subjected to 1H-NMR analysis using a Varian Gemini 200 nuclear magnetic resonance analyzer (NMR), and the molar percentage of each component was determined from the integral ratio.

[0113] (16) Membrane formation stability (membrane permeability) The film was checked for breakage during the process from when the process temperature was stabilized until a film length of 5,000 m was formed. ○: Stable film formation possible (no breakage) △: Slightly unstable running (may break occasionally) ×: Frequent breakage occurs and stable film formation is not possible

[0114] (17) Measuring whiskers on the film edge The slit edge of a polyester film slit along the machine direction by a slitter was observed under an optical microscope or a scanning electron microscope at a magnification of 500 to 3000 times, and the number of whisker-like particles measuring 5 μm or more per 100 μm at different slit edge positions was counted from the photographed images. The counts at 10 positions were averaged to evaluate the number of whisker-like particles on the film edge.

[0115] (Production Example 1: Polyethylene terephthalate resin A) The esterification reactor was heated to 200°C, and 86.4 parts by mass of terephthalic acid and 64.4 parts by mass of ethylene glycol were added. While stirring, 0.017 parts by mass of antimony trioxide, 0.064 parts by mass of magnesium acetate tetrahydrate, and 0.16 parts by mass of triethylamine were added as catalysts. The temperature was then increased under pressure, and a pressurized esterification reaction was carried out at a gauge pressure of 0.34 MPa and 240°C. The esterification reactor was then returned to atmospheric pressure, and 0.014 parts by mass of phosphoric acid was added. The temperature was then increased to 260°C over 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. After 15 minutes, the mixture was dispersed using a high-pressure disperser. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor, where a polycondensation reaction was carried out at 280°C under reduced pressure. After the polycondensation reaction was completed based on the stirring torque, the strands were cooled in cooling water and solidified. The solidified strands were cut into pellets and further dried under reduced pressure to obtain polyethylene terephthalate resin A (hereinafter referred to as resin A) with an intrinsic viscosity of 0.65 dL / g.

[0116] (Production Example 2: Polyethylene terephthalate resin B) 10 parts by mass of dried ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazinon-4-one) and 90 parts by mass of particle-free polyethylene terephthalate resin A prepared in Production Example 1 were mixed, and the mixture was kneaded using an extruder to obtain ultraviolet absorber-containing polyethylene terephthalate resin B (hereinafter referred to as resin B) having an intrinsic viscosity of 0.58 dL / g.

[0117] (Production Example 3: Self-recovered polyethylene terephthalate resin C) The scrap film at the end of the film cut off from the product was crushed and dried at 150°C to remove moisture. The dried crushed film was melted in an extruder, extruded from a die in the form of strands, and rapidly solidified in cold water. The solidified strands were cut into pellets with a strand cutter, and the surface was dried to prevent fusion, yielding self-recovering polyethylene terephthalate resin C (hereinafter referred to as resin C) with an intrinsic viscosity of 0.50 dL / g.

[0118] (Production Example 4: Self-recovered polyethylene terephthalate resin D) The edge rolls and non-standard products generated during the slitting process were washed and peeled off outside. The resulting film with the coating layers peeled off from both surfaces was crushed and dried at 150°C to remove moisture. The dried crushed film was melted in an extruder, extruded into strands from a die, and rapidly solidified in cold water. The solidified strands were cut into pellets using a strand cutter, and the surface was dried to prevent fusion, yielding self-recovering polyethylene terephthalate resin D (hereinafter referred to as resin D) with an intrinsic viscosity of 0.60 dL / g.

[0119] (Production Example 5: Coating Solution X1: Preparation of Adhesion-Modifying Coating Solution) (Polymerization of polyester resin) A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 194.2 parts by weight of dimethyl terephthalate, 184.5 parts by weight of dimethyl isophthalate, 14.8 parts by weight of dimethyl-5-sodium sulfoisophthalate, 233.5 parts by weight of diethylene glycol, 136.6 parts by weight of ethylene glycol, and 0.2 parts by weight of tetra-n-butyl titanate. A transesterification reaction was carried out at a temperature of 160 to 220°C for 4 hours. The temperature was then raised to 255°C, and the reaction system was gradually reduced in pressure. The reaction was continued for 1 hour and 30 minutes under a reduced pressure of 30 Pa to obtain copolymer polyester resin (D-1). The resulting copolymer polyester resin was pale yellow and transparent. The reduced viscosity of the copolymer polyester resin was measured and found to be 0.70 dL / g. The glass transition temperature measured by DSC was 40°C.

[0120] (Preparation of Polyester Water Dispersion) A reactor equipped with a stirrer, thermometer, and reflux device was charged with 30 parts by mass of the polyester resin and 15 parts by mass of ethylene glycol n-butyl ether, and the mixture was heated to 110°C and stirred to dissolve the resin. After the resin was completely dissolved, 55 parts by mass of water was gradually added to the polyester solution while stirring. After the addition, the solution was cooled to room temperature while stirring to produce a milky white polyester water dispersion with a solids content of 30% by mass.

[0121] (Preparation of aqueous polyvinyl alcohol solution) 90 parts by mass of water was placed in a vessel equipped with a stirrer and a thermometer, and 10 parts by mass of polyvinyl alcohol resin (manufactured by Kuraray) with a degree of polymerization of 500 was gradually added while stirring. After the addition, the liquid was heated to 95°C while stirring to dissolve the resin. After dissolution, the liquid was cooled to room temperature while stirring to prepare an aqueous polyvinyl alcohol solution with a solids content of 10% by mass.

[0122] (Polymerization of Blocked Polyisocyanate Crosslinking Agent) A flask equipped with a stirrer, thermometer, and reflux condenser was charged with 100 parts by mass of a polyisocyanate compound having an isocyanurate structure (Duranate TPA, manufactured by Asahi Kasei Chemicals), 55 parts by mass of propylene glycol monomethyl ether acetate, and 30 parts by mass of polyethylene glycol monomethyl ether (average molecular weight 750), and the mixture was maintained at 70°C for 4 hours under a nitrogen atmosphere. The reaction solution temperature was then lowered to 50°C, and 47 parts by mass of methyl ethyl ketoxime was added dropwise. The infrared spectrum of the reaction solution was measured to confirm that the absorption of the isocyanate group had disappeared, yielding a blocked polyisocyanate aqueous dispersion with a solids content of 75% by mass.

[0123] The following coating materials were mixed to prepare coating solution X1. Water 40.75% by mass Isopropanol 25.00% by mass Polyester water dispersion 10.32% by mass Polyvinyl alcohol aqueous solution 20.82% by mass Blocked polyisocyanate crosslinking agent 0.50% by mass Particles 1.24% by mass (Silica sol with an average particle size of 100 nm, solid content of 40% by mass) Particles 0.74% by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) catalyst (organotin compound solid concentration 14% by mass) 0.48% by mass Surfactant 0.15% by mass (Silicone-based, solid content 10% by mass)

[0124] (Production Example 6: Coating Solution Y: Preparation of Water-Based Coating Solution) The following coating materials were mixed to prepare coating solution Y. Water 70.64% by mass Isopropanol 9.25% by mass Polyester water dispersion 15.00% by mass Particles 3.41% by mass (Silica sol with an average particle size of 100 nm, solid content of 20% by mass) Particles 1.70% by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass)

[0125] (Production Example 7: Coating Solution X2: Preparation of Adhesion-Modifying Coating Solution) The following coating materials were mixed to prepare coating solution X2. Water 54.89% by mass Isopropanol 15.00% by mass Polyester water dispersion 18.19% by mass Blocked polyisocyanate crosslinking agent 2.08% by mass Zinc oxide particles 9.37% by mass (Taki Chemical Ceramace S-8, solid content 8% by mass) Particles 0.17% by mass (Silica sol with an average particle size of 500 nm, solid content of 15% by mass) Surfactant 0.30% by mass (Silicone-based, solid content 10% by mass)

[0126] (Production Example 8: Coating Solution X3: Preparation of Adhesion-Modifying Coating Solution) (Polymerization of carbodiimide crosslinking agents) A flask equipped with a stirrer, thermometer, and reflux condenser was charged with 168 parts by weight of hexamethylene diisocyanate and 220 parts by weight of polyethylene glycol monomethyl ether (M400, average molecular weight 400). The mixture was stirred at 120°C for 1 hour. Then, 26 parts by weight of 4,4'-dicyclohexylmethane diisocyanate and 3.8 parts by weight of 3-methyl-1-phenyl-2-phosphorene-1-oxide (2% by weight based on the total isocyanate) were added as a carbodiimide catalyst. The mixture was stirred at 185°C for an additional 5 hours under a nitrogen stream. Infrared spectroscopy of the reaction solution confirmed the disappearance of absorption at wavelengths of 2200-2300 cm-1. The mixture was allowed to cool to 60°C, and 567 parts by weight of ion-exchanged water was added to obtain a carbodiimide crosslinker with a solids content of 40% by weight.

[0127] The following coating materials were mixed to prepare coating solution X3. Water 48.27% by mass Isopropanol 25.00% by mass Polyester water dispersion 20.09% by mass Carbodiimide crosslinking agent 2.86% by mass Zirconium oxide particles 1.88% by mass (Nissan Chemical ZR-40BL, solid content 40% by mass) Particles 1.60% by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) Surfactant 0.30% by mass (Silicone-based, solid content 10% by mass)

[0128] Example 1 As raw materials for the intermediate layer of the base film, 81 parts by weight of particle-free resin A, 9 parts by weight of resin C, and 10 parts by weight of resin B containing a UV absorber were dried under reduced pressure (1 Torr) at 135 ° C for 6 hours and then fed into extruder 2 (for intermediate layer II). Resin A was dried by conventional methods and fed into extruder 1 (for surface layers I and III), respectively, and melted at 285 ° C. These two layers of polymer were each filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), laminated in a two-type, three-layer merging block, extruded into a sheet from a die, and then wrapped around a casting drum at a surface temperature of 30 ° C using an electrostatic casting method, cooled, and solidified to produce an unstretched film. The output of each extruder was adjusted so that the thickness ratio of layers I, II, and III was 8:84:8. Furthermore, the intrinsic viscosity of the resins forming layers I and III was 0.65 dl / g, and the intrinsic viscosity of the resin forming layer II was 0.60 dl / g, as determined by the installed online viscometer.

[0129] Next, the unstretched film was coated on both sides by the reverse roll method so that the coating amount after drying was 0.1 g / m 2 The adhesiveness modifying coating liquid X1 was applied to the film in the width range that would remain usable as a product after slitting, excluding both ends in the film width direction, so that the film would be dried at 80°C for 20 seconds.

[0130] After slitting both ends of the unstretched film on which this coating layer was formed, the film was coated with a coating amount of 0.07 g / m2 by roll coating at a position 50 to 100 mm from the end, mainly in the area not coated with the adhesive property modifying coating liquid X1. 2 The water-based coating liquid Y was applied so as to avoid overlapping with the coating layer of the coating liquid X1 as much as possible.

[0131] The unstretched film, with its edges coated with the aqueous coating solution, was introduced into a tenter stretching machine. While the edges were held with clips, the film was introduced into a hot air zone at 125°C and stretched 4.0 times in the width direction. Next, while maintaining the stretched width, the film was treated at 225°C for 30 seconds and then further relaxed by 3% in the width direction, resulting in a uniaxially oriented PET film approximately 100 μm thick. The resulting film had an intrinsic viscosity of 0.61 dl / g, few whiskers on the slit edge, and good film permeability. Other film properties are listed in Table 1 below.

[0132] Example 2 A uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1, except that the resin composition ratio fed to Extruder 2 (for intermediate layer II) was 86 parts by mass of Resin A, 4 parts by mass of Resin C, and 10 parts by mass of Resin B. The intrinsic viscosity of the obtained film was 0.63 dL / g, the number of whiskers on the slit edge was small, and the film passability was good. Other film properties are shown in Table 1 below.

[0133] Example 3 A uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1, except that the resin composition ratio fed to Extruder 2 (for intermediate layer II) was 72 parts by mass of Resin A, 18 parts by mass of Resin C, and 10 parts by mass of Resin B. The intrinsic viscosity of the obtained film was 0.58 dL / g, the number of whiskers on the slit edge was small, and the film passability was good. Other film properties are shown in Table 1 below.

[0134] Example 4 A uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1, except that the resin composition ratio supplied to Extruder 2 (for intermediate layer II) was 90 parts by mass of Resin A and 10 parts by mass of Resin C. The intrinsic viscosity of the obtained film was 0.62 dL / g, the number of whiskers on the slit edge was small, and the film passability was good. Other film properties are shown in Table 1 below.

[0135] Example 5 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the final film thickness was 60 μm. The intrinsic viscosity of the obtained film was 0.59 dL / g, the number of whiskers on the slit edge was small, and the film permeability was good. Other film properties are shown in Table 1 below.

[0136] Example 6 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the extrusion rates of each extruder were adjusted so that the thickness ratio of Layer I to Layer II to Layer III was 5:90:5. The intrinsic viscosity of the obtained film was 0.57 dL / g, the number of beard-like particles on the slit edge was small, and the film permeability was good. Other film properties are shown in Table 1 below.

[0137] Example 7 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the extrusion rates of each extruder were adjusted so that the thickness ratio of Layer I, Layer II, and Layer III was 10:80:10. The intrinsic viscosity of the obtained film was 0.63 dL / g, the number of beard-like particles on the slit edge was small, and the film permeability was good. Other film properties are shown in Table 1 below.

[0138] Example 8 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the resin composition ratios fed to Extruder 2 (for intermediate layer II) were 76 parts by mass of Resin A, 14 parts by mass of Resin D, and 10 parts by mass of Resin B. The intrinsic viscosity of the obtained film was 0.62 dL / g, the number of whiskers on the slit edge was small, and the film passability was good. Other film properties are shown in Table 1 below.

[0139] Example 9 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the resin composition ratio supplied to Extruder 2 (for intermediate layer II) was 67 parts by mass of Resin A, 9 parts by mass of Resin C, 14 parts by mass of Resin D, and 10 parts by mass of Resin B. The intrinsic viscosity of the obtained film was 0.63 dL / g, the number of whiskers on the slit edge was small, and the film passability was good. Other film properties are shown in Table 1 below.

[0140] (Comparative Example 1) The resin composition ratio fed to Extruder 2 (for intermediate layer II) was 90 parts by mass of Resin A and 10 parts by mass of Resin B, and a uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1. During resin extrusion, the intrinsic viscosity of the resin forming Layer II was 0.64 dL / g, and the intrinsic viscosity of the resulting film was 0.67 dL / g, demonstrating good film permeability. However, there was a large number of whiskers on the slit end surface. Other film properties are shown in Table 2 below.

[0141] Example 10 The resin composition ratio fed to Extruder 2 (for intermediate layer II) was 63 parts by mass of Resin A, 27 parts by mass of Resin C, and 10 parts by mass of Resin B, and a uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1. During resin extrusion, the intrinsic viscosity of the resin forming Layer II was 0.55 dL / g, and the intrinsic viscosity of the resulting film was 0.54 dL / g. Although there were some dissatisfaction with film permeability and thickness fluctuation, there was little whisker-like material on the edge after slitting. Other film properties are shown in Table 2 below.

[0142] Example 11 The resin composition ratio fed to Extruder 2 (for intermediate layer II) was 63 parts by mass of Resin A, 27 parts by mass of Resin D, and 10 parts by mass of Resin B, and a uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1. The intrinsic viscosity of the resulting film was 0.61 dL / g, and although the thickness variation rate and yellowness were somewhat unsatisfactory, there was little whisker-like material on the edge after slitting. Other film properties are shown in Table 2 below.

[0143] Example 12 A uniaxially oriented PET film with a film thickness of approximately 100 μm was obtained in the same manner as in Example 1, except that the aqueous coating liquid was not applied to both ends of the unstretched film, the width of the coating of Adhesion Modifier Coating Liquid X1 was widened, and coating was continued up to the positions of both ends coated with Water-based Coating Liquid Y, and Water-based Coating Liquid Y was not applied. The intrinsic viscosity of the obtained film was 0.60 dL / g. Although there was some dissatisfaction with the film permeability, there was little whisker-like material on the end surfaces after slitting. Other film properties are shown in Table 2 below.

[0144] Example 13 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the extrusion rates of each extruder were adjusted so that the thickness ratio of Layer I, Layer II, and Layer III was 3:94:3. The intrinsic viscosity of the obtained film was 0.59 dl / g, and the film permeability was also good. Although the film was somewhat unsatisfactory in terms of haze and yellowness, there was little whisker-like material on the edge surface after slitting. Other film properties are shown in Table 2 below.

[0145] (Comparative Example 2) A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that the extrusion rates of each extruder were adjusted so that the thickness ratio of Layer I, Layer II, and Layer III was 14:72:14. The intrinsic viscosity of the obtained film was 0.63 dL / g, and the film permeability was also good. However, there were many beard-like particles on the slit end surface. Other film properties are shown in Table 2 below.

[0146] Example 14 The wire bar coating method was used to coat both sides of the unstretched film with a coating amount of 0.15 g / m after drying. 2 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that Adhesion Modifier Coating Solution X2 was applied instead of Adhesion Modifier Coating Solution X1 within the width range that would remain after slitting, excluding both ends in the film width direction. The intrinsic viscosity of the obtained film was 0.57 dL / g, the number of whiskers on the slit end surface was small, and the film permeability was good. Other film properties are shown in Table 2 below.

[0147] Example 15 By wire bar coating, the coating amount after drying was 0.10 g / m on one side of the unstretched film. 2 On the other side, the adhesiveness modifying coating solution X1 was replaced with the adhesiveness modifying coating solution X1 so that the coating amount after drying was 0.08 g / m 2 A uniaxially oriented PET film was obtained in the same manner as in Example 1, except that Adhesion Modifier Coating Solution X3 was applied over the entire width of the film, excluding both ends in the width direction, to obtain a film having a width that would remain the same as the final product after slitting. The intrinsic viscosity of the resulting film was 0.57 dL / g, the number of whiskers on the slit end surface was small, and the film permeability was good. Other film properties are shown in Table 2 below.

[0148] Tables 1 and 2 summarize various film properties of the polyester films of Examples 1 to 15 and Comparative Examples 1 and 2.

[0149] [Table 1]

[0150] [Table 2] [Industrial Applicability]

[0151] According to the present invention, a polyester film roll having fewer whiskers on the slit end surfaces can be provided, and the reduction of the factors that cause defects makes it more applicable to optical applications, particularly as a polarizer protective film. Furthermore, by using the polyester film of the present invention as a polarizer protective film, it is possible to provide a polarizing plate or a liquid crystal display device that can contribute to thinner and less expensive display devices without reducing visibility due to iridescence.

Claims

1. A polyester film for optical applications having a three-layer laminate structure including at least surface layers made of polyester on both sides of an intermediate layer made of polyester, the polyester film having a thickness of 15 μm or more and 200 μm or less; a thickness ratio of the surface layer:the intermediate layer:the surface layer is between 4:92:4 and 13:74:13; the polyester is polyethylene terephthalate, the intrinsic viscosity of the polyester constituting the intermediate layer is lower than the intrinsic viscosity of the polyesters constituting both surface layers, the intrinsic viscosities of both surface layers being 0.64 dL / g or more and 0.67 dL / g or less, and the intrinsic viscosity of the intermediate layer being 0.56 dL / g or more and 0.60 dL / g or less, and the number of whiskers protruding 5 μm or more at an edge of the film slit along the machine direction during film production is one or less per 100 μm of film length, a part of the polyester constituting the polyester film contains a self-recovered polymer generated in a process for producing the polyester film; the amount of the self-recovering polymer added is 3% by mass or more and 26% by mass or less, as a mass ratio of all raw materials of the layer of the film to which the self-recovering polymer is added, The film has a haze value of more than 0.4 and not more than 2.0%, a color tone b value of 0.7 or more and 3.0 or less, and is used for optical purposes. Polyester film.

2. 2. The polyester film according to claim 1, wherein the polyester film as a whole has an intrinsic viscosity of 0.55 dl / g or more and 0.65 dl / g or less, and is used for optical applications.

3. 3. The polyester film according to claim 1, which has a coating layer on at least one side thereof and is used for optical purposes.

4. 4. The polyester film according to claim 1, wherein the intermediate layer contains a self-recovering polymer generated in a process for producing the polyester film, and the polyester film is used for optical applications.

5. The polyester film according to any one of claims 1 to 4, which is a uniaxially stretched film and is used for optical applications.

6. 6. A polyester film for polarizer protective film, comprising the polyester film according to claim 1, which has an in-plane retardation of 3,000 nm or more and 30,000 nm or less.

7. A polarizing plate comprising a polarizer and the polyester film for polarizer protection film according to claim 6 laminated on at least one surface of the polarizer.

8. A liquid crystal display device comprising a backlight, a liquid crystal cell, and polarizing plates disposed on both sides of the liquid crystal cell, wherein the polarizing plate on at least one side is the polarizing plate according to claim 7.

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