JP6776612B2 - Laminated biaxially stretched polyester film - Google Patents

Description

The present invention relates to a laminated biaxially stretched polyester film preferably used for a so-called polarizer protective film, which is a film used for protecting a polarizer from ultraviolet rays and moisture from the outside by being laminated on a polarizer.

Thermoplastic resin films, especially biaxially stretched polyester films, have excellent mechanical properties, electrical properties, dimensional stability, transparency, chemical resistance, etc., and therefore are used in many magnetic recording materials, packaging materials, etc. It is widely used as a base film in applications. Particularly in recent years, there has been an increasing demand for various optical films such as polarizer protective films and transparent conductive films in the fields of flat panel displays, touch panels, and in-vehicle panel displays. Among them, in the field of polarizer protective film, replacement of the conventional TAC (triacetyl cellulose) film with a biaxially stretched polyester film is being actively studied for the purpose of cost reduction and thinning / miniaturization of the display. ..

A laminated biaxially stretched polyester film formed by alternately laminating a crystalline polyester resin and a thermoplastic resin different from the above can be used in a specific wavelength region according to the layer design by utilizing the difference in refractive index between the polyester resins. It is possible to reflect light rays. Here, by setting the reflected wavelength region to the ultraviolet region, it is expected to be used as a polarizer protective film that protects the liquid crystal from ultraviolet rays, but undulations occur in the spectral waveform depending on the lamination design, and polarization due to insufficient ultraviolet cut. There have been problems such as deterioration of the child and deterioration of visibility when the liquid crystal image display device is mounted due to the reduction of the amount of visible transmitted light.

Further, when a polyester film is used as a polarizer protective film in a liquid crystal image display device, a method of adding an ultraviolet absorber is used in order to prevent deterioration of the liquid crystal due to ultraviolet rays (Patent Document 1). However, when an ultraviolet absorber is used as an additive, a bleed-out phenomenon occurs near the mouthpiece or at the vacuum vent port during film forming, depending on the type and content of the ultraviolet absorber, and the film forming apparatus is contaminated or added. There has been a problem that the density is lowered and the quality of the film itself is impaired. In addition, when an ultraviolet absorber that can strongly absorb the long wavelength side of the ultraviolet region is excessively added, the light transmittance in the vicinity of the ultraviolet-visible light boundary decreases, causing coloration of the film, and the film is mounted on the liquid crystal image display device. There was a problem that the color tone deteriorated.

On the other hand, in addition to the ultraviolet ray blocking performance, the polarizer protective film is strongly required to have low retardation and low orientation angle so that the polarization state does not change before and after the film is transmitted. When the retardation is high, the direction of the polarized light transmitted from the backlight changes, so that when mounted on a liquid crystal image display device, the efficiency of transmitting the backlight light decreases and the brightness decreases. Further, in the case of a polyester film that undergoes orientation crystallization, the orientation angle of the molecules in the width direction may change depending on the film forming conditions of the film, which strongly affects the direction of the transmitted polarized light.

Japanese Unexamined Patent Publication No. 2013-210598

Therefore, the present invention eliminates the above-mentioned drawbacks, and is a thin, highly transparent laminated biaxially stretched polyester film that does not bleed out during film formation and does not show interference fringes due to ripples when mounted on a liquid crystal image display device. The purpose is to provide.

The present invention has the following configuration. That is,
A layer made of crystalline polyester A (layer A) and a layer made of a thermoplastic resin B different from the above (layer B) are alternately laminated in an amount of 50 or more, the outermost layer is the A layer, and the layer thickness of the outermost layer is The number of layers having a layer thickness of 5 μm or more or 50 nm or less and 50 nm or less is 10 or more consecutively counting from the second layer, and only the B layer of the A layer and the B layer contains an ultraviolet absorber. It is a laminated biaxially stretched polyester film, characterized in that the light transmittance T _{400 at} a wavelength of 400 nm is 80% or more.

The laminated biaxially stretched polyester film of the present invention does not bleed out various additives including an ultraviolet absorber during film formation, and is high without interference fringes even when mounted as a polarizer protective film on a display device such as a liquid crystal display. It has the effect of being able to display with dignity.

Hereinafter, the laminated biaxially stretched polyester film of the present invention will be described in detail.

In the laminated biaxially stretched polyester film of the present invention, 50 or more layers (A layer) made of crystalline polyester A and layers (B layer) made of a thermoplastic resin B different from the above are alternately laminated, and the outermost layer is A. It is a layer, and the number of layers having a layer thickness of 5 μm or more or 50 nm or less and a layer thickness of 50 nm or less is 10 or more consecutively counting from the second layer, and the A layer and the B layer Of these, a laminated biaxially stretched polyester film containing an ultraviolet absorber only in the B layer needs to have a light transmittance T _{400 at} a wavelength of 400 nm of 80% or more.

The polyester as described in the present invention is a condensed polymer obtained by polymerization of a monomer containing an aromatic dicarboxylic acid or an aliphatic dicarboxylic acid and a diol as main constituents. As a known method for producing polyester, a transesterification reaction (transesterification method) or a direct esterification reaction (direct polymerization method) is used. Here, examples of the aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 4,4'-. Examples thereof include diphenyldicarboxylic acid, 4,4'-diphenylether dicarboxylic acid, and 4,4'-diphenylsulfone dicarboxylic acid. Examples of the aliphatic dicarboxylic acid include adipic acid, suberic acid, sebacic acid, dimer acid, dodecandioic acid, 1,4-cyclohexanedicarboxylic acid and ester derivatives thereof. Of these, terephthalic acid and 2,6-naphthalenedicarboxylic acid, which exhibit a high refractive index, are preferably used. One of these dicarboxylic acid components may be used, or two or more of them may be used in combination.

Examples of the diol component include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, and 1,5-pentanediol. 1,6-Hexanediol, 1,2-Cyclohexanedimethanol, 1,3-Cyclohexanedimethanol, 1,4-Cyclohexanedimethanol, Diethylene glycol, Triethylene glycol, Polyalkylene glycol, 2,2-Bis (4-) Hydroxyethoxyphenyl) propane, isosorbate, spiroglycol and the like can be mentioned. Of these, ethylene glycol is preferably used. Only one type of these diol components may be used, or two or more types may be used in combination.

Further, in the present invention, the crystalline polyester A is, for example, polyethylene terephthalate and its copolymer, polyethylene naphthalate and its copolymer, polybutylene terephthalate and its copolymer, polybutylene naphthalate and its copolymer, and further. Can also use polyhexamethylene terephthalate and its copolymer, polyhexamethylene naphthalate and its copolymer, and the like. At this time, as the copolymerization component, it is preferable that one or more of the above-mentioned dicarboxylic acid component and diol component are copolymerized.

In the present invention, the thermoplastic resin B is not particularly limited, and chain polyolefins such as polyethylene, polypropylene, poly (4-methylpentene-1) and polyacetal, ring-opening metathesis polymerization of norbornenes, and addition polymerization. , Bioresolvable polymers such as alicyclic polyolefins, polylactic acid, polybutylsuccinate, which are addition copolymers with other olefins, polyamides such as nylon 6, nylon 11, nylon 12, nylon 66, aramid, poly Methyl methacrylate, polyvinyl chloride, vinylidene chloride, polyvinyl alcohol, polyvinyl butyral, vinyl ethylene acetate copolymer, polyacetal, polyglucolic acid, polystyrene, styrene copolymer polymethylmethacrylate, polycarbonate, polypropylene terephthalate, polyethylene terephthalate, polybutylene terephthalate, Polyethylene-2,6-naphthalate and other polyesters, polyether sulfone, polyether ether ketone, modified polyphenylene ether, polyphenylene sulfide, polyetherimide, polyimide, polyarylate, tetrafluoroethylene resin, trifluoride ethylene resin, 3 Polyethylene chloride resin, ethylene tetrafluoride-6 propylene fluoride copolymer, polyvinylidene fluoride and the like can be used. Among these, it is most preferable to select polyester as the thermoplastic resin B from the viewpoints of strength, heat resistance, transparency and versatility, and particularly from the viewpoint of adhesion and stackability with crystalline polyester A. .. Here, the thermoplastic resin B may be a copolymer or a mixture.

Alternating 50 or more layers means that the crystalline polyester A and the thermoplastic resin B are laminated in a regular arrangement in the thickness direction, for example, the rule of A (BA) n (n is a natural number). It refers to a state in which 50 or more layers of resin are laminated according to a specific arrangement. Specifically, the thermoplastic resin B refers to a resin that exhibits a melting point or a glass transition temperature different from that of the crystalline polyester A in differential scanning calorimetry (DSC). By alternately laminating resins having different thermal characteristics in this way, it is possible to highly control the orientation state of each layer when producing a laminated biaxially stretched polyester film. Further, in the case of a film in which 50 or more layers are alternately laminated, each resin is uniformly distributed as compared with a film in which several layers to a dozen or more layers are laminated, so that stable film forming property and mechanical properties can be obtained. It is expected. Further, as the number of layers increases, the growth of orientation in each layer tends to be suppressed, and by alternately stacking many layers, it becomes easier to control the retardation of the film described later. The number of layers is preferably 100 layers or more, and more preferably 200 layers or more. There is no upper limit to the number of layers, but as the number of layers increases, the manufacturing cost increases due to the increase in size of the manufacturing apparatus, and the handleability deteriorates due to the increase in film thickness. In particular, increasing the film thickness increases the absolute amount of the layer (A layer) made of crystalline polyester A, which increases the retardation described later. When used as a display material, it causes interference colors and rainbows. It is not preferable because it causes spots, and in reality, 1000 layers or less is suitable.

In the laminated biaxially stretched polyester film of the present invention, it is necessary that the outermost layer is the A layer and the thickness of the outermost layer is 50 nm or less or 5 μm or more. The outermost layer described here refers to a layer located at the outermost layer of the layer (A layer) made of crystalline polyester A, and is an adhesive layer used when bonding films to each other or a surface layer by a separate coating method. The coating layer made of the resin of the third component laminated on the above is not applicable. When the thickness of the outermost layer is 5 μm or more, it is possible to prevent the unevenness of the adhesive layer from being transferred to the laminated biaxially stretched polyester film, and further, suppression of flow marks, light transmittance and light reflectance spectra. The effect of suppressing ripples can be obtained. The thickness is preferably 7 μm or more. There is no upper limit to the thickness of the outermost layer, but it is realistic to set it to 10 μm or less in consideration of changes in other physical properties and optical performance due to an increase in the layer made of crystalline polyester A (layer A). On the other hand, a method of reducing the thickness of the outermost layer to 50 nm or less can also be used. By setting the thickness of the outermost layer to 50 nm or less, a strong effect of suppressing ripples in the light transmittance and the light reflectance spectrum can be obtained, and the layer thickness of the layer (A layer) made of crystalline polyester A located on the outermost layer can be increased. Since it becomes smaller as a whole, it is possible to contribute to the reduction of retardation. There is no lower limit to the thickness of the outermost layer, but if it is too thin, it is practically difficult to design the laminating device with high accuracy, and the influence of the layer (B layer) made of the thermoplastic resin B located in the second layer. It is also difficult to stretch the film during film formation. Therefore, it is preferable to design the outermost layer to have a thickness of 10 nm or more.

The ripple described here refers to a phenomenon in which the spectrum of the light transmittance and the light reflectance of the spectrum does not show a constant value and fluctuates in a wavy manner according to a change in wavelength, and is the outermost layer of the laminated biaxially stretched polyester film. It is caused by the interference between the light rays reflected in and the light rays reflected at the inner layer interface, and the interference between the light rays reflected at the inner layer interface. When this occurs in the visible light region, hue spots occur, leading to deterioration of visibility. In addition, when ripples occur in the ultraviolet region that is the target of the light transmittance cut of the present invention, the reflected light rays are partially lost, and stable light transmittance cannot be maintained. It is required to do. If the thickness of the outermost layer is designed to be 5 μm or more, the ripple can be eliminated as a long-period swell, and if it is designed to be 50 nm or less, the ripple can be changed to a short-period small swell and eliminated. It becomes.

The retardation is generally calculated from the product of the maximum value of the difference in refractive index in two orthogonal directions in the plane of the film and the thickness of the film, but it is easy in a laminated film such as the present invention. Since the refractive index of the film cannot be measured, the value of the retardation calculated by an indirect method is used as the retardation. Specifically, the value measured by the phase difference measuring device KOBRA series for measuring retardation by an optical method sold by Oji Measuring Instruments Co., Ltd. shall be used. In the polarizing element protective film used by bonding with a polarizer, when the retardation value is high, interference color and rainbow spots are generated when the film is mounted on a liquid crystal display, which causes a problem of deterioration in quality. Therefore, in the present invention, it is preferable to make the layer thickness of the layer (A layer) made of crystalline polyester A, which develops orientation by stretching or crystallization, as thin as possible in order to reduce retardation. From this point of view, as described above, controlling the thickness of the outermost layer to 50 nm or less is a preferable method from the viewpoints of both suppression of ripple and reduction of retardation and the viewpoint of thinning trend.

The laminated biaxially stretched polyester film of the present invention needs to have 10 or more consecutive layers having a layer thickness of 50 nm or less, counting from the second layer. The layer near the surface layer is a layer that is largely related to the realization of ripple-free light reflection as an optical layer, and the above-mentioned layer design is indispensable in any case where the outermost layer is 5 μm or more or 50 nm or less. .. Further, the total number of layers having a layer thickness of 50 nm or less is preferably 20 or more in succession, and more preferably 30 or more in succession. The range is changed by affecting the distribution of the thickness of the laminated layer.

The distribution of the laminated layer thickness includes a layer thickness distribution that increases or decreases from one side of the film to the opposite side, and a layer thickness distribution that increases or decreases after the layer thickness increases from one side of the film toward the center of the film. Alternatively, a layer thickness distribution that increases after the layer thickness decreases from one side of the film toward the center of the film is preferable. As a method of changing the layer thickness distribution, one that changes continuously such as linear, geometric progression, and difference sequence, and 10 to 50 layers have almost the same layer thickness, and the layer thickness changes stepwise. It is preferable to do.

When forming a laminated biaxially stretched polyester film of A (BA) n (n is a natural number) as in the present invention, a plurality of resins of crystalline polyester A and thermoplastic resin B are extruded by two or more extruders. A multi-manifold type feed block, a static mixer, or the like, which is a known laminating device, can be used by feeding out from different flow paths. In particular, in order to efficiently obtain the configuration of the present invention, it is preferable to use a feed block having fine slits in order to realize highly accurate lamination. When a laminate is formed using a slit type feed block, the thickness of each layer and the layer thickness distribution can be achieved by changing the length and width of the slit to incline the pressure loss. The length of the slit is the length of the comb tooth portion that forms a flow path for alternately flowing the layer made of crystalline polyester A (layer A) and the layer made of thermoplastic resin B (layer B) in the slit plate. That is. In order to reduce the thickness of the outermost layer to 50 nm or less, it can be achieved by making the width of the slit located at the end of the slit plate narrower than that of the slit forming the other thin film layer, or by adjusting the slit length longer. It becomes.

Further, in the laminated biaxially stretched polyester film of the present invention, it is essential that only the B layer of the A layer and the B layer contains an ultraviolet absorber. In the method of cutting ultraviolet rays by light reflection by alternately laminating layers A and B, light rays are generated depending on the size of the difference in refractive index between the layers A and B, which is generated by the combination of resins, stretching conditions, and heat treatment conditions. Since the reflectance changes, it is not easy to achieve complete UV protection. Therefore, it is necessary to include an ultraviolet absorber as a secondary method for achieving both light reflection due to lamination. When the method of containing the ultraviolet absorber and the light reflection by alternately laminating the A layer and the B layer are used together, the ultraviolet absorber is contained as compared with the method of realizing the light ray cut only by containing the ultraviolet absorber. Since the amount can be reduced, it has a great advantage especially from the viewpoint of suppressing bleed-out of the ultraviolet absorber.

The ultraviolet absorber may be added as an additive inside the resin, or may be copolymerized with the resin.
Most of the UV absorbers have a low molecular weight, and if they are not high molecular weight UV absorbers, problems such as volatilization in the air when melted and discharged as a sheet, precipitation on the surface of the film in a heat treatment process or reliability test, etc. Occurs. Therefore, by copolymerizing the resin, Ru can be kept reliably ultraviolet absorber in the layer. When copolymerizing an ultraviolet absorber with a resin, for example, when copolymerizing with a polyester-based resin, the hydroxy group contained in most of the ultraviolet absorbers is contained in the polyester resin by using a transesterification reaction or the like. This can be achieved by reacting with the carboxyl end.
The layer containing the ultraviolet absorber is only the B layer located in the inner layer of the laminated biaxially stretched polyester film . When an ultraviolet absorber is contained in the A layer including the outermost layer, if an ultraviolet absorber is used as an additive, the phenomenon of precipitation on the film surface (bleed-out phenomenon) as described above, and the sublimation / volatilization near the base. This is not preferable because the phenomenon is likely to occur, which contaminates the film forming machine and the precipitates have an adverse effect such as the occurrence of defects in the processing process. When the UV absorber is contained only in the B layer, the layer A composed of the outermost layer and the crystalline polyester A alternately located in the layer acts as a lid to prevent the precipitation of the UV absorber, so that the bleed-out phenomenon occurs. Is less likely to occur, which is preferable.

The concentration of the ultraviolet absorber in the entire laminated biaxially stretched polyester film is 2.5 wt% or less, preferably 1.5 wt% or less, and more preferably 1.0 wt% or less with respect to the total weight of the film. When the content concentration is higher than 2.5 wt%, the light transmittance is lowered and the white turbidity (haze value) of the film is increased, which causes a problem of deterioration of visibility when mounted on a liquid crystal image display device or the like. Not preferable.

In the laminated biaxially stretched polyester film of the present invention, it is necessary that the light transmittance T _{400 at} a wavelength of 400 nm is 80% or more. It is preferably 85% or more, more preferably 90% or more. The wavelength of 400 nm is located at the boundary between ultraviolet rays and visible rays, and if light rays on the longer wavelength side than the wavelength are reflected or absorbed, the film will be colored and the hue of the screen will change when mounted on an image display device. Not preferable.

As the ultraviolet absorber that can be used in the present invention, it is preferable to use a benzotriazole-based, benzophenone-based, benzoate-based, or triazine-based agent having a molecular weight of 300 g / mol or more. As the ultraviolet absorber, one of these may be selected, or two or more of them may be used in combination. There is a relationship between the molecular weight and the sublimation property of additives such as ultraviolet absorbers, and sublimation is unlikely to occur when an additive having a large molecular weight is used. The molecular weight is more preferably 400 g / mol or more, and further preferably 500 g / mol or more. Many UV absorbers with a high molecular weight have long-chain alkyl chains attached to the basic aromatic ring skeleton, which inhibit stacking between UV absorbers and crystallize in the resin to increase haze. It is desirable because it does not cause problems such as inviting.

The ultraviolet absorber that can be added is not particularly limited as the benzotriazole-based ultraviolet absorber, and is, for example, 2- (2'-hydroxy-5'-methylphenyl) benzotriazole, 2- (2'-hydroxy). -3', 5'-ditertiary butylphenyl) -5-chlorobenzotriazole, 2- (2'-hydroxy-3'-third butyl-5'-methylphenyl) -5-chlorobenzotriazole, 2- (2'-Hydroxy-5'-third octylphenyl) benzotriazole, 2- (2'-hydroxy-3', 5'-dicumylphenyl) benzotriazole, 2- (2'-hydroxy-3'-th Examples include 2- (2'-hydroxyphenyl) benzotriazoles such as tributyl-5'-carboxyphenyl) benzotriazole and 2,2'-methylenebis (4-thioctyl-6-benzotriazolyl) phenol. Be done.
The benzophenone-based ultraviolet absorber is not particularly limited, and for example, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-octoxybenzophenone, 5,5'-methylenebis (2-). 2-Hydroxybenzophenones such as hydroxy-4-methoxybenzophenone) can be mentioned.
The benzoate-based ultraviolet absorber is not particularly limited, and is, for example, phenylsalicylate, resorcinol monobenzoate, 2,4-ditertiary butylphenyl-3,5-ditertiary butyl-4-hydroxybenzoate, 2,4-. Examples thereof include di-tertiary amylphenyl-3,5-di-tertiary butyl-4-hydroxybenzoate and hexadecyl-3,5-di-tertiary butyl-4-hydroxybenzoate.

The triazine-based ultraviolet absorber is not particularly limited, but 2- (2-hydroxy-4-octoxyphenyl) -4,6-bis (2,4-dimethylphenyl) -s-triazine, 2- (2- (2-) Hydroxy-4-hexyloxyphenyl) -4,6-diphenyl-s-triazine, 2- (2-hydroxy-4-propoxy-5-methylphenyl) -4,6-bis (2,4-dimethylphenyl)- s-Triazine, 2- (2-hydroxy-4-hexyloxyphenyl) -4,6-dibiphenyl-s-triazine, 2,4-bis (2-hydroxy-4-octoxyphenyl) -6- (2) , 4-Dimethylphenyl) -s-triazine, 2,4,6-tris (2-hydroxy-4-octoxyphenyl) -s-triazine, 2- (4-isooctyloxycarbonylethoxyphenyl) -4,6 Examples thereof include triaryltriazines such as −diphenyl-s-triazine.

Other UV absorbers include, for example, phenylsalicylate, t-butylphenylsalicylate, p-octylphenylsalicylate, etc. in salicylic acid, and other natural products (eg, oryzanol, shea butter, bicarin). Etc.), biological systems (eg, keratinocytes, melanocytes, urocanin, etc.) and the like can also be used. A hindered amine compound can also be used in combination with these ultraviolet absorbers as a stabilizer. Inorganic UV absorbers are not preferable because they are incompatible with the base resin, leading to an increase in haze and deteriorating visibility when displaying an image.

In the laminated biaxially stretched polyester film of the present invention, it is preferable to use an ultraviolet absorber having a melting enthalpy of 50 J / g or less as the above-mentioned ultraviolet absorber. By using an ultraviolet absorber having a low melting enthalpy, it is possible to suppress sublimation and surface precipitation due to thermal energy associated with heating. Further, the triazine-based ultraviolet absorber is superior in that it is difficult to form a three-dimensional crystal structure as compared with other benzotriazole-based and benzophenone-based structures, and surface precipitation is difficult to increase haze. Therefore, it is more preferable to use a triazine-based ultraviolet absorber having a melting enthalpy of 50 J / g or less. In particular, a triazine skeleton having a long side chain can be more preferably used because it becomes difficult for molecules to form crystals. For the melting enthalpy of the ultraviolet absorber contained in the laminated biaxially stretched polyester film, the ultraviolet absorber is extracted from the laminated biaxially stretched polyester film by a method described later, and the extracted ultraviolet absorber is subjected to differential calorimetry (DSC). Required by doing. When it is difficult to extract the ultraviolet absorber from the laminated biaxially stretched polyester film by the method described later, the ultraviolet absorber before being added to the laminated biaxially stretched polyester film is determined by differential calorimetry (DSC).

It is more preferable that two or more kinds of ultraviolet absorbers are used in combination as the ultraviolet absorber contained in the laminated biaxially stretched polyester film of the present invention. The laminated biaxially stretched polyester film of the present invention is required to cut light rays having a wavelength of 380 nm or less with a minimum content of an ultraviolet absorber while suppressing sublimation and surface precipitation. Since each ultraviolet absorber has a different wavelength region in which light rays can be cut, it is preferable to appropriately combine ultraviolet absorbers having different wavelength regions in which light rays can be cut. In particular, when used in combination with a triazine-based ultraviolet absorber having a melting enthalpy of 50 J / g or less described in the previous section, it is more preferable because it exhibits an effect of suppressing sublimation of other ultraviolet absorbers exhibiting sublimation properties. Further, as the combined formulation, a formulation in which two or more types of ultraviolet absorbers are added may be used, addition and copolymerization may be used in combination, or two or more types may be copolymerized with the resin at the same time. A preferred embodiment is a method in which one kind of ultraviolet absorber is added to the crystalline polyester A by copolymerization, and one or more kinds of ultraviolet absorbers different from the above are added to the thermoplastic resin B. Since no ultraviolet absorber is added to the same resin layer at a high concentration, volatilization and surface precipitation can be effectively suppressed.

In the laminated biaxially stretched polyester film of the present invention, a fluorescent whitening agent may be contained in both the A layer, the B layer, or both the A layer and the B layer. The fluorescent whitening agent in the present invention refers to a molecule that absorbs ultraviolet rays in the wavelength range of 300 to 400 nm and emits visible light of purple to blue, and has not only the ability to absorb ultraviolet rays but also purple having a wavelength of 400 nm or more. It has the effect of increasing the light transmittance in the blue wavelength region. As the fluorescent whitening agent in the present invention, benzothiazole type, benzimidazole type, benzoxazole type and the like can be used, and the content concentration is 0.5 wt% or less, preferably 0.4 wt% or less. The additive layer is a layer made of a thermoplastic resin B located in the inner layer in the same manner as the method of adding an ultraviolet absorber described above in order to prevent bleeding out of molecules and precipitation phenomenon on the film surface during heat treatment. It is preferably contained in (B layer). Due to the characteristics of the fluorescent whitening agent, it can also be used as an ultraviolet absorber, but when the content of the fluorescent whitening agent is high, the white turbidity of the film becomes stronger due to the high emission intensity and the haze increases. In addition to this, there is a problem in visibility when mounted on an image display device. Therefore, it is preferable to use the fluorescent whitening agent in combination with the ultraviolet absorber only for the purpose of suppressing the hue development associated with the absorption of visible light by the ultraviolet absorber. When the ultraviolet absorber and the fluorescent whitening agent are used in combination, it is preferable that the total content concentration of the additive is 2.5 wt% or less with respect to the total weight of the film in addition to the above-mentioned content concentration condition of the fluorescent whitening agent. This is because, as described above, when the content concentration of the additive is high, the whitening of the film due to light diffusion becomes remarkable. Since the concentration of the ultraviolet absorber is changed according to the film thickness and the light absorption capacity of various additives, there is practically no lower limit of the concentration, but for example, when used as a polarizer protective film, a polarizer or a liquid crystal is used. It is required to contain a content concentration sufficient to prevent the molecules from being deteriorated by ultraviolet rays, and it is preferably contained in an amount of 0.5 wt% or more.
Polarizers used in image display devices have the function of transmitting light that has only a specific vibration direction, and polyvinyl alcohol (PVA) -based films dyed with iodine or dichroic dyes are the most common. It is widely used. This polarizer is made of an organic material, and in particular, deterioration occurs when it is irradiated with ultraviolet rays having a wavelength of 280 to 380 nm. Therefore, by cutting the ultraviolet rays in this region before reaching the polarizer, the polarizer It is possible to prevent deterioration or deterioration of liquid crystal molecules.

In response to the characteristics of the above-mentioned polarizer, the laminated biaxially stretched polyester film of the present invention preferably has a light transmittance T _{380 at} a wavelength of 380 nm of 20% or less, more preferably 10% or less, still more preferably 5%. It is as follows. By designing the laminated biaxial polyester film of the present invention, it is possible to sufficiently cut light rays in the ultraviolet wavelength region of 380 nm by achieving both light reflection due to the laminated structure and light absorption by various additives including an ultraviolet absorber. At a visible light wavelength of 400 nm, light can be sufficiently transmitted. Since the reflection wavelength region can be converted according to the design of the layer thickness, it is the most preferable system to have a reflection / absorption relationship that can complement each other in consideration of the absorption wavelength region of the ultraviolet absorber. ..

In the crystalline polyester A and the thermoplastic resin B of the present invention, various additives other than the above-mentioned ultraviolet absorber and fluorescent whitening agent, such as antioxidants, heat-resistant stabilizers, weather-resistant stabilizers, and organic-based easy substances, are contained. Lubricants, plasticizers, pigments, dyes, organic or inorganic fine particles, fillers, antioxidants, nucleating agents and the like may be contained to such an extent that they do not deteriorate the film properties that should be originally satisfied.

The laminated biaxially stretched polyester film of the present invention preferably has an average light reflectance in the wavelength range of 300 to 380 nm of 20% or more, more preferably 30% or more, still more preferably 50% or more. In laminated biaxially stretched polyester film, by designing the film thickness, interlayer distance and number of layers in a specific design, the effect of multiple interference reflection at the interface between air and film and the interface between film layers causes a specific wavelength. It is possible to reflect light rays in the range. Utilizing this effect, it is possible to specifically reflect only light rays having a wavelength of 300 to 380 nm in the ultraviolet region. Specifically, this can be achieved by setting the average layer thickness of each layer to 45 to 65 nm.

The laminated biaxially stretched polyester film in the present invention has a film width of 400 mm or more, an in-plane retardation Re (0 °) at the time of vertical incidence in that range of 150 nm or less, and an orientation angle of 15 ° or less. Is preferable. A more preferable value of retardation Re (0 °) is 100 nm or less, and more preferably 50 nm or less. The retardation value is preferably a value as close to 0 nm as possible, and when the value is larger than 150 nm, the light rays transmitted from the backlight are polarized, so that the brightness when mounted on an image display device is lowered. A point arises. When the retardation is reduced to 50 nm or less, the effect on polarization is negligible, so it is not necessary to consider the orientation of the crystal. However, when the retardation shows a value larger than about 100 nm, the orientation angle is large in the width direction. The change is not preferable because it strongly affects the direction of the transmitted polarized light. When control of the orientation angle is required, the orientation angle is more preferably 10 ° or less, still more preferably 5 ° or less.

The laminated biaxially stretched polyester film in the present invention preferably has a haze change amount (Δ haze) of 1.0 or less when treated for 200 hours under the condition of 85 ° C. and 85% RH. The haze as described in the present invention refers to the total haze of the film. This condition is a reliability test condition for judging long-term stability, which is particularly evaluated in the display field. More preferably, the Δ haze is 0.7 or less, still more preferably 0.5 or less, but since an increase in haze leads to a large deterioration in appearance, the amount of change in haze (Δ haze) after the test is set to 0. Closeness is the most suitable condition.

The crystalline polyester A in the present invention preferably contains polyethylene terephthalate and / or polybutylene terephthalate obtained by copolymerizing 5 to 40 mol% of polyethylene naphthalate as a main component. In order to exhibit high reflection performance in a thin film, it is necessary to adjust the optical thickness (refractive index x physical layer thickness) with respect to the wavelength to be reflected. When the film is thinned, the physical layer thickness becomes small, so that it is required to increase the refractive index, and in particular, to increase the difference in refractive index between the crystalline polyester A and the thermoplastic resin B. In order to increase the difference in refractive index, it is preferable to use as the crystalline polyester A a birefringent polymer whose birefringence increases as the molecular orientation increases, and the refractive index in the orientation direction is 1.65 due to stretching. It is preferable to use polyethylene terephthalate having a refractive index of about 1.69 or polybutylene terephthalate having a refractive index of about 1.64 to 1.68. Further, by copolymerizing polyethylene naphthalate having a refractive index of about 1.74 to 1.80 in the orientation direction after stretching, it is possible to further increase the refractive index after stretching. , Will be more preferable. The copolymerization amount of polyethylene naphthalate is preferably 10 to 40 mol%, more preferably 15 to 30 mol%. If the copolymerization amount is less than 5 mol%, the effect of the refractive index of polyethylene naphthalate cannot be obtained, while if the copolymerization amount is larger than 40 mol%, the resin itself impairs crystallinity, which is not preferable.

The laminated biaxially stretched polyester film in the present invention preferably has a film thickness of 5 μm or more and 15 μm or less. More preferably, it is 8 μm or more and 14 μm or less. When the film thickness is smaller than 5 μm, it may be difficult to realize the reflection performance by the layer design even when the crystalline polyester copolymerized with the polyethylene naphthalate is used. Further, since the thickness of the film is thin, there may be a problem in the handleability and the take-up property of the film after stretching. On the other hand, if it exceeds 15 μm, the layer thickness of the layer made of crystalline polyester A (layer A) becomes large, so that the value of retardation Re (0 °) becomes large and the visibility when mounted on the display deteriorates. Problems may arise.

The laminated biaxially stretched polyester film of the present invention can be preferably used for optical applications. In other words, applications that are required to prevent deterioration of the contents due to ultraviolet rays and protect the human body from ultraviolet rays, for example, functional film applications for the purpose of protecting the surface of displays, window covering applications for UV protection of trains and vehicles, and insect protection. It can be used in various fields such as exterior applications of buildings, packaging applications such as foods and medical devices, and agricultural applications. However, since it can exert its strength in that it can suppress the coloring due to polarized light by controlling the retardation to a low level, it is a performance that is preferably used in optical films around displays, especially in a polarizer protective film, which is a field in which the element is regarded as important. have.

Next, a preferable method for producing the laminated biaxially stretched polyester film of the present invention will be described below. Of course, the present invention is not construed as being limited to such an example.

Crystalline polyester A and thermoplastic resin B are prepared in the form of pellets or the like. If necessary, the pellets are dried in hot air or under vacuum, and then fed out from different flow paths using two or more extruders and fed into a multilayer stacking apparatus. As the multi-layer stacking device, a multi-manifold die, a feed block, a static mixer, or the like can be used, but in particular, in order to efficiently obtain the configuration of the present invention, it is preferable to use a feed block having fine slits. When such a feed block is used, the apparatus does not become extremely large, so that there are few foreign substances due to thermal deterioration, and even when the number of layers is extremely large, high-precision lamination is possible. In addition, the stacking accuracy in the width direction is also significantly improved as compared with the conventional technique. Further, in this device, since the thickness of each layer can be adjusted by the shape (length, width) of the slit as described above, it is possible to achieve an arbitrary layer thickness.

In the extruder, the resin that has been heated and melted above the melting point has a uniform extrusion amount of the resin by a gear pump or the like, and foreign substances and modified resin are removed through a filter and the like. These resins are formed into a desired shape by a die and then discharged.

The multi-layered sheet discharged from the die is extruded onto a cooling body such as a casting drum and cooled and solidified to obtain a casting film. At this time, it is preferable to use a wire-shaped, tape-shaped, needle-shaped, or knife-shaped electrode to bring the electrodes into close contact with a cooling body such as a casting drum by electrostatic force to quench and solidify. Further, a method in which air is blown out from a slit-shaped, spot-shaped, or planar device to be brought into close contact with a cooling body such as a casting drum to be rapidly cooled and solidified, or a method of being brought into close contact with a cooling body by a nip roll to be rapidly cooled and solidified is also preferable.

The casting film thus obtained is subsequently biaxially stretched in the longitudinal and width directions. The stretching may be biaxial stretching sequentially or biaxial stretching at the same time. Further, re-stretching may be performed in the longitudinal direction and / or the width direction.
The case of sequential biaxial stretching will be described first. Here, the stretching in the longitudinal direction means stretching to give the film a molecular orientation in the longitudinal direction, and is usually applied by the difference in peripheral speed of the rolls, and this stretching may be performed in one step. , Multiple roll pairs may be used in multiple stages. The stretching ratio varies depending on the type of resin, but is usually preferably 2 to 15 times, and when polyethylene terephthalate is used as one of the resins constituting the laminated polyester film, 2 to 7 times is particularly preferably used. .. The stretching temperature is preferably from the glass transition temperature to the glass transition temperature of the resin constituting the laminated uniaxially stretched polyester film + 100 ° C.

The laminated uniaxially stretched polyester film thus obtained is subjected to surface treatment such as corona treatment, frame treatment, and plasma treatment as necessary, and then has functions such as slipperiness, adhesiveness, and antistatic property. The provided water-based coating may be applied by in-line treatment.
Subsequently, the stretching in the width direction means stretching for giving the film orientation in the width direction, and usually, a tenter is used to carry the film while gripping both ends with clips to stretch the film in the width direction. The stretching ratio varies depending on the type of resin constituting the film, but is usually preferably 2 to 15 times, and particularly 2 to 7 times when polyethylene terephthalate is used as one of the resins constituting the laminated polyester film. It is preferably used. The stretching temperature is preferably glass transition temperature to glass transition temperature + 120 ° C. of the resin constituting the laminated biaxially stretched polyester film.

The laminated polyester film thus biaxially stretched is subjected to a heat treatment in a tenter above the stretching temperature and below the melting point, slowly cooled uniformly, cooled to room temperature, and wound up. Further, if necessary, in order to impart a low orientation angle and thermal dimensional stability of the film, a relaxation treatment or the like may be used in combination in the longitudinal direction and / or the width direction during heat treatment to slow cooling.

The case of simultaneous biaxial stretching will be described below. In the case of simultaneous biaxial stretching, the obtained cast film is subjected to surface treatment such as corona treatment, frame treatment, and plasma treatment as necessary, and then slipperiness, adhesiveness, antistatic property, etc. A functional water-based coating may be applied in-line.

Next, the cast film is guided to the simultaneous biaxial tenter, transported while gripping both ends of the film with clips, and stretched simultaneously and / or stepwise in the longitudinal direction and the width direction. Simultaneous biaxial stretching machines include a pantograph method, a screw method, a drive motor method, and a linear motor method. A drive motor system or a drive motor system in which the stretching ratio can be changed arbitrarily and relaxation processing can be performed at any location. The linear motor method is preferable. The stretching ratio varies depending on the type of resin, but usually, the area ratio is preferably 6 to 50 times, and when polyethylene terephthalate is used as one of the resins constituting the laminated biaxially stretched polyester film, the area ratio is set. 8 to 30 times is particularly preferably used. In particular, in the case of simultaneous biaxial stretching, it is preferable that the stretching ratios in the longitudinal direction and the stretching direction are the same and the stretching speeds are substantially the same in order to suppress the in-plane orientation difference. The stretching temperature is preferably glass transition temperature to glass transition temperature + 120 ° C. of the resin constituting the laminated biaxially stretched polyester film.

In order to impart flatness and dimensional stability to the biaxially stretched laminated polyester film in this way, it is preferable that the laminated polyester film is subsequently heat-treated in the tenter at the stretching temperature or higher and below the melting point. At the time of this heat treatment, in order to suppress the distribution of the main orientation axis in the width direction, it is preferable to perform the relaxation treatment in the longitudinal direction instantly immediately before and / or immediately after entering the heat treatment zone. After being heat-treated in this manner, it is uniformly slowly cooled, cooled to room temperature, and wound up. Further, if necessary, relaxation treatment may be performed in the longitudinal direction and / or the width direction during heat treatment and slow cooling. Immediately before and / or immediately after entering the heat treatment zone, relaxation treatment is performed in the longitudinal direction.

Next, a laminated sheet in which a hard coat layer containing a curable resin C as a main component is provided on the outermost surface of the laminated biaxially stretched polyester film according to the present invention will be described.
It is more preferable that the laminated biaxially stretched polyester film of the present invention is provided with a hard coat layer, which is a resin layer capable of adding functions such as scratch resistance and dimensional stability, on the outermost surface. In particular, in the case of an optical film for a display, it is required that the properties of the laminated biaxially stretched polyester film do not change in various reliability tests such as the treatment under the above-mentioned conditions of 85 ° C. and 85% RH. In the case of a laminated biaxially stretched polyester film oriented and crystallized by stretching, it is conceivable that the dimensions of the film will change due to heat shrinkage when the reliability test conditions are more severe. Since the thickness of the film increases due to heat shrinkage, there are problems such as improvement of the absorption performance of the ultraviolet absorber, shift of the reflected wavelength band to the long wavelength side, and undesired wavelength cut of visible light. Occurs. The hard coat layer may be directly coated on the laminated biaxially stretched polyester film, and as described in the above-mentioned manufacturing method, an in-line water-based coating layer capable of imparting functions such as slipperiness and adhesiveness is provided. May be coated.
Furthermore, by providing the hard coat layer on the outermost surface, a layer having a high density due to cross-linking is located on the outermost surface, and precipitation of oligomers and additives that could not be sufficiently cut by the laminated structure of the laminated biaxially stretched polyester film. Can be blocked even more strongly. Since the hard coat is later laminated on the laminated biaxially stretched polyester film by off-coating, the effect is particularly strongly exhibited in the reliability test.
The hard coat layer (C layer) containing the curable resin C as a main component may be provided on one side, but precipitation of oligomers and the like generally occurs from both sides of the film, and when laminated on only one side, it is on the laminated surface side. The shrinkage stress due to curing works strongly, and the laminated sheet itself may be significantly curled due to the laminated thickness of the hard coat layer. Therefore, the hard coat layer as a block layer such as an oligomer may be applied to both surfaces of the laminated biaxially stretched polyester film.
The easy-adhesion layer, which is applied to the laminated biaxially stretched polyester film in advance when laminating the hard coat layer, not only imparts functions such as easy slipperiness and easy adhesiveness, but also contains a curable resin C as a main component. When laminating the hard coat layer, it has the effect of improving the adhesion to the laminated biaxially stretched polyester film. When polyethylene terephthalate is used as the crystalline polyester constituting the A layer located on the outermost layer and acrylic resin is used as the curable resin C, the former has a refractive index of about 1.64 to 1.68, and the latter has a refractive index of 1. Since the difference in refractive index is as large as about 50, there is a concern that the interlayer adhesion may be deteriorated. Therefore, the refractive index of the easy-adhesion layer is preferably adjusted to a value of 1.53 to 1.60, more preferably 1.55 to 1.58.

The curable resin C constituting the hard coat layer is preferably highly transparent and durable. For example, an acrylic resin, a urethane resin, a fluorine resin, a silicon resin, a polycarbonate resin, or a vinyl chloride resin is used alone or mixed. Can be used. In particular, in terms of curability, flexibility, and productivity, the curable resin C is preferably made of an active energy ray-curable resin such as an acrylic resin typified by a polyacrylate resin. Further, when adding scratch resistance at the time of bending, which is required for a film applied to a portion where curved surface followability is required, the curable resin C is preferably made of a thermosetting urethane resin.

The active energy ray-curable resin used as a constituent component of the hard coat layer has, for example, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, as a monomer component constituting the active energy ray-curable resin. Dipentaerythritol tri (meth) acrylate, dipentaerythritol tetra (meth) acrylate, dipentaerythritol penta (meth) acrylate, dipentaerythritol hexa (meth) acrylate, trimethylpropantri (meth) acrylate, bis (methacryloylthio) Phenyl) sulfide, 2,4-dibromophenyl (meth) acrylate, 2,3,5-tribromophenyl (meth) acrylate, 2,2-bis (4- (meth) acryloyloxyphenyl) propane, 2,2- Bis (4- (meth) acryloyloxyethoxyphenyl) propane, 2,2-bis (4- (meth) acryloyloxydiethoxyphenyl) propane, 2,2-bis (4- (meth) acryloylpentaethoxyphenyl) propane , 2,2-Bis (4- (meth) acryloyloxyethoxy-3,5-dibromophenyl) propane, 2,2-bis (4- (meth) acryloyloxydiethoxy-3,5-dibromophenyl) propane, 2,2-bis (4- (meth) acryloyloxypentaethoxy-3,5-dibromophenyl) propane, 2,2-bis (4- (meth) acryloyloxyethoxy-3,5-dimethylphenyl) propane, 2 , 2-Bis (4- (meth) acryloyloxyethoxy-3-phenylphenyl) propane, bis (4- (meth) acryloyloxyphenyl) sulfone, bis (4- (meth) acryloyloxyethoxyphenyl) sulfone, bis ( 4- (Meta) acryloyloxypentaethoxyphenyl) sulfone, bis (4- (meth) acryloyloxyethoxy-3-phenylphenyl) sulfone, bis (4- (meth) acryloyloxyethoxy-3,5-dimethylphenyl) sulfone , Bis (4- (meth) acryloyloxyphenyl) sulfide, bis (4- (meth) acryloyloxyethoxyphenyl) sulfide, bis (4- (meth) acryloyloxypentaethoxyphenyl) sulfide, bis (4- (meth) Acryloyloxyethoxy-3-phenylphenyl) sulfide, bis (4- (meth) ) Use polyfunctional (meth) acrylic compounds such as acryloyloxyethoxy-3,5-dimethylphenyl) sulfide, di ((meth) acryloyloxyethoxy) phosphate, and tri ((meth) acryloyloxyethoxy) phosphate. These can be used alone or in combination of two or more.

In addition to these polyfunctional (meth) acrylic compounds, styrene, chlorostyrene, dichlorostyrene, bromostyrene, dibromostyrene, and divinyl are used to control the hardness, transparency, strength, refractive index, etc. of the active energy ray-curable resin. Styrene, vinyltoluene, 1-vinylnaphthalene, 2-vinylnaphthalene, N-vinylpyrrolidone, phenyl (meth) acrylate, benzyl (meth) acrylate, biphenyl (meth) acrylate, diallyl phthalate, dimetalyl phthalate, diallyl biphenylate, or A reaction product of (meth) acrylic acid and a metal such as barium, lead, antimony, titanium, tin, or zinc can be used. These may be used alone or in combination of two or more.

As a method for curing the active energy ray-curable resin, for example, a method of irradiating ultraviolet rays can be used. In this case, about 0.01 to 10 parts by weight of a photopolymerization initiator is added to the compound. It is desirable to add.

The active energy ray-curable resin used in the present invention contains isopropyl alcohol, ethyl acetate, methyl ethyl ketone, for the purpose of improving workability during coating and controlling the coating film thickness, as long as the effects of the present invention are not impaired. An organic solvent such as methyl isobutyl ketone or toluene can be blended.

In the present invention, the active energy ray means an electromagnetic wave that polymerizes an acrylic vinyl group such as ultraviolet rays, electron beams, and radiation (α rays, β rays, γ rays, etc.), and practically, ultraviolet rays are convenient. preferable. As the ultraviolet source, an ultraviolet fluorescent lamp, a low-pressure mercury lamp, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a xenon lamp, a carbon arc lamp, or the like can be used. Further, the electron beam method is advantageous in that the apparatus is expensive and requires operation under an inert gas, but it does not need to contain a photopolymerization initiator, a photosensitizer, or the like.

The thermosetting urethane resin used as a component of the hard coat layer (C layer) containing the curable resin C as a main component for adding scratch resistance is a polycaprolactone segment and a polysiloxane segment and / or a polydimethylsiloxane. A resin obtained by cross-linking a copolymer resin having a segment with a compound having an isocyanate group by a thermal reaction is preferable. By applying the thermosetting urethane resin, it is possible to strengthen the hard coat layer containing the curable resin C as the main component and at the same time promote the elastic recovery, and add appropriate scratch resistance to the optical film. It becomes possible.

The polycaprolactone segment constituting the heat-curable urethane resin has an effect of elastic recovery, and radically polymerizable polycaprolactone such as polycaprolactone diol, polycaprolactone triol, or lactone-modified hydroxyethyl acrylate can be used.

The polysiloxane and / or polydimethylsiloxane segment constituting the thermosetting urethane resin has the effect of improving the lubricity of the surface and reducing the frictional resistance by coordinating the surface of these components. As the resin having a polysiloxane segment, tetraalkoxysilane, methyltrialkoxysilane, dimethyldialkoxysilane, γ-glycidoxypropyltrialkoxysilane, γ-methacryloxypropyltrialkoxysilane and the like can be used. On the other hand, as a resin having a polydimethylsiloxane segment, various vinyl monomers such as methyl acrylate, isobutyl acrylate, methyl methacrylate, n-butyl methacrylate, styrene, α-methylstyrene, acrylonitrile, vinyl acetate, etc. are used in the polydimethylsiloxane segment. A copolymer obtained by copolymerizing vinyl chloride, vinyl fluoride, acrylamide, methacrylicamide, N, N-dimethylacrylamide, or the like can be preferably used.
The hard coat layer made of a thermosetting urethane resin is formed by linking and reacting resins and compounds at an arbitrary temperature to volatilize the solvent in the layer and at the same time thermally crosslinking the layers. In order to promote the thermal cross-linking reaction of the thermosetting urethane resin, the temperature in the heating step is preferably 150 ° C. or higher, more preferably 160 ° C. or higher. The heating temperature is preferably high, but the heat treatment is preferably performed at 170 ° C. or lower in consideration of the occurrence of shrinkage wrinkles due to heat shrinkage of the base material. The heating time is 1 minute or more, preferably 2 minutes or more, and the upper limit is not particularly set, but it is preferably 5 minutes or less from the viewpoint of dimensional stability and transparency of the laminated biaxially stretched polyester film. .. The laminated sheet that has been heat-treated at a high temperature for a short time in this way is subjected to an aging treatment at a temperature of 20 ° C. to 80 ° C. for 3 days or more, more preferably 7 days or more to increase the urethane bond and stretch the laminated sheet. It is preferable in terms of improving the degree.

The thickness of the hard coat layer should be appropriately adjusted according to the purpose of use, but is usually preferably 1 to 6 μm from the viewpoint of achieving both a thin film tendency in display applications and hard coat performance. It is preferably 1 to 3 μm, and more preferably 1 to 1.5 μm. When the thickness of the hard coat layer is thicker than 6 μm, the laminated biaxially stretched polyester film is inferior in mechanical strength when the hard coat layer is cured, and the laminated sheet is strongly curled, which is not preferable unless a considerably thick film is used.
The hard coat layer containing the curable resin C as a main component may contain various additives such as the above-mentioned various ultraviolet absorbers. By separately containing the laminated biaxially stretched polyester film and the hard coat layer, the content concentration of various additives distributed in each layer of the laminated biaxially stretched polyester film is reduced, so that the bleed-out phenomenon that occurs during resin extrusion can be prevented. It is preferable because it can be suppressed.
The concentration of the additive containing the ultraviolet absorber contained in the hard coat layer is preferably 5 wt% or less, more preferably 3 wt% or less, based on the entire curable resin composition constituting the hard coat layer. The content concentration should be appropriately adjusted in consideration of the performance of various additives and the thickness of the hard coat layer in order to achieve the desired cutting performance, but if it exceeds 5 wt%, the thickness of the hard coat layer Regardless of this, it is not preferable because the haze increases in the heat treatment process at the time of laminating the hard coat layer and in the reliability test.

(Characteristic measurement method and effect evaluation method)
The method for measuring the characteristics and the method for evaluating the effect in the present invention are as follows.

(1) The layer thickness, the number of layers, and the layer structure of the laminated structure film were determined by observing a transmission electron microscope (TEM) for a sample whose cross section was cut out using a microtome. That is, using a transmission electron microscope H-7100FA (manufactured by Hitachi, Ltd.), the cross section of the film was observed under the condition of an accelerating voltage of 75 kV, a cross-sectional photograph was taken, and the layer structure and the thickness of each layer were measured. In some cases, in order to obtain high contrast, a dyeing technique using RuO ₄ or OsO ₄ was used. Further, according to the thickness of the thinnest layer (thin film layer) among all the layers that can be captured in one image, if the thin film layer thickness is less than 50 nm, it is 100,000 times, and the thin film layer thickness is 50 nm or more and 500 nm. Observation was carried out at a magnification of 40,000 times when it was less than, and at a magnification of 10,000 times when it was 500 nm or more.

(2) Method for calculating layer thickness The TEM photographic image obtained in item (1) was captured using a CanonScanD123U with an image size of 720 dpi. Save the image in a personal computer as a bitmap file (BMP) or compressed image file (JPEG), and then use the image processing software Image-Pro Plus ver.4 (sold by Planetron Co., Ltd.) to create this file. Was opened and image analysis was performed. In the image analysis process, in the vertical sick profile mode, the relationship between the position in the thickness direction and the average brightness of the region sandwiched between the two lines in the width direction was read as numerical data. Using spreadsheet software (Excel2000), after adopting the data of position (nm) and brightness in sampling step 1, numerical processing of 3-point moving average was performed. Furthermore, the obtained data whose brightness changes periodically is differentiated, and the maximum value and the minimum value of the differential curve are read by the VBA (Visual Basic for Applications) program, and the adjacent intervals are set to one layer. The layer thickness was calculated as the layer thickness. This operation was performed for each photograph, and the layer thickness of all the layers was calculated.

(3) Reference Re (0 °)
A phase difference measuring device (KOBRA-21ADH) manufactured by Oji Measuring Instruments Co., Ltd. was used. A sample was cut out from the center of the film width direction at 5.0 cm in the width direction × 4.0 cm in the longitudinal direction, installed in the device so that the film width direction was an angle of 0 ° defined by this measuring device, and the incident angle was 0. The retardation at a wavelength of 590 nm at ° was measured.

(4) Orientation angle A phase difference measuring device (KOBRA-21ADH) manufactured by Oji Measuring Instruments Co., Ltd. was used. A sample is cut out from the center in the width direction of the film in a width direction of 5.0 cm × a longitudinal direction of 4.0 cm, installed in the device so that the film width direction has an angle of 0 ° defined by this measuring device, and the film width direction. The clockwise slope was + and the counterclockwise slope was-, and the absolute value was used as the measurement result.

(5) Light transmittance T ₃₈₀ , T ₄₀₀
A spectrophotometer U-4100 manufactured by Hitachi High-Technologies Corporation was used. When an integrating sphere is attached and the transmitted light measured using the reflected light of the aluminum oxide standard white plate (attached to the main body) is 100%, the relative light transmittance at 380 nm and 400 nm is measured, and the light transmittance at each wavelength is measured. It was a rate. As conditions, the slit was set to 2 nm (visible), the gain was set to 2, the scan speed was set to 600 nm / min, and the sampling pitch was set to 1 nm, and continuous measurement was performed.

(6) Average light reflectance A spectrophotometer U-4100 manufactured by Hitachi High-Technologies Corporation was used. When an integrating sphere was attached and the reflected light of the aluminum oxide standard white plate (attached to the main body) was 100%, the relative light reflectance in the 300 to 380 nm region was measured, and the average light reflectance in that range was obtained. .. As conditions, the slit was set to 2 nm (visible), the gain was set to 2, the scan speed was set to 600 nm / min, and the sampling pitch was set to 1 nm, and continuous measurement was performed.

(7) Haze measurement A haze meter (HGM-2DP) manufactured by Suga Test Instruments Co., Ltd. was used. A sample was cut out at a size of 10 cm × 10 cm from the central portion in the film width direction, and the total light transmittance and the haze value were measured by measuring according to the old JIS-K-7105. Three points were measured at equal intervals in the film width direction, and the average value was used as the measurement result.

(8) Melted enthalpy Laminated biaxially stretched polyester film was dissolved in 1,1,1,3,3,3-hexafluoro-2-isopropanol, and a low molecular weight substance was extracted by a gel permeation chromatography method at room temperature. Then, the powder obtained by drying was dissolved in chloroform, and the solution was recovered and dried to obtain an ultraviolet absorber powder. 5 mg of the obtained powder sample was sealed using an aluminum pan and a pan cover, and differential calorimetry (DSC) was performed using "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd. and data analysis "Disk Session SSC / 5200". ) Was performed. The temperature was raised from 25 ° C. to 300 ° C. at a heating rate of 20 ° C./min under a nitrogen atmosphere, and the melting enthalpy (J / g) at that time was read.

(9) 85 ° C. 85% RH Accelerated Moisture and Heat Resistance Test A constant temperature and humidity controller (LHL-114) manufactured by ESPEC CORPORATION was used. A sample was cut out in the same manner as the haze measurement, the haze value was measured with a haze meter (HGM-2DP) of Suga Test Instruments Co., Ltd., and then the sample was placed in a constant temperature and humidity chamber with the sample sandwiched between plain papers. After standing for 200 hours, the haze value was measured with the same haze meter, and the amount of change in the haze value was read as Δ haze.

(10) Hard coat layer (Examples 13 and 15)
30% of active energy ray-curable urethane acrylic resin (Shikou UV-1700B [refractive index: 1.50 to 1.51] manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) constituting the hard coat layer using a methyl ethyl ketone solvent. It was diluted to a concentration and uniformly applied on the outermost surface of the laminated biaxially stretched polyester film using a bar coater. Next, a condensing high-pressure mercury lamp (H04-L41 manufactured by Eye Graphics Co., Ltd.) having an irradiation intensity of 120 W / cm ² set at a height of 13 cm from the surface of the hard coat layer was used, and the integrated irradiation intensity was 180 mJ / cm. ^It was irradiated with ultraviolet rays so as to be No. ² and cured to obtain a laminated sheet in which a hard coat layer was laminated on a laminated biaxially stretched polyester film. An industrial UV checker (UVR-N1 manufactured by Nippon Battery Co., Ltd.) was used for measuring the integrated irradiation intensity of ultraviolet rays.

(11) Visibility On one surface of a polarizing plate having a degree of polarization of 99.9% prepared by adsorbing and orienting iodine in PVA, the size is 420 mm in the width direction and 310 mm in the longitudinal direction from the central portion in the width direction of the film. It was attached to the cut-out sample to make a test piece. The visibility was confirmed when the prepared test piece and the polarizing plate to which the film was not attached were superposed on the LED light source (A3-101 manufactured by Tritec) in the arrangement of the cross Nicol.

⊚: Interference fringes, interference colors, and rainbow spots are hardly seen.

◯: There are some interference fringes, interference colors, and rainbow spots, but there is no problem in practical use.

Δ: Interference fringes, interference colors, and rainbow spots are worrisome, so they can be used for displays, but we want to avoid them as much as possible.

X: Interference fringes, interference colors, and rainbow spots are clearly seen, so it is not suitable for display applications at all.

(12) Rainbow spot A white image was displayed on a liquid crystal display device of an LED light source in a dark room at 23 ° C., and a film cut out in a size of 420 mm in the width direction and 310 mm in the longitudinal direction was placed on the image display device. The presence or absence of iridescent coloring was confirmed by visual inspection while changing the elevation angle from 40 ° to 80 ° with respect to the direction perpendicular to the plane of the film.
⊚: Almost no change in hue can be seen with respect to the change in angle.
◯: The range in which the hue changes is a narrow range of elevation angle 40 ° to 50 °, and there is no problem in practical use. Δ: The range in which the hue changes is in the range of elevation angle 40 ° to 60 °, and use should be avoided as much as possible. Tai ×: Hue changes remarkably with angle change, which is not suitable for practical use.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, Examples 2, 3, and 9 below are used as reference examples.
(Example 1)
As the crystalline polyester A, polyethylene terephthalate (PET) having a melting point of 254 ° C. was used. Further, as the thermoplastic resin B, polyethylene terephthalate (PE / SPG15T / CHDC20) obtained by copolymerizing 20 mol% of cyclohexanedicarboxylic acid, which is an amorphous resin having no melting point, and 15 mol% of spiroglycol was used. A benzotriazole-based ultraviolet absorber having a molecular weight of 650 g / mol was added to the thermoplastic resin B so as to have a weight ratio of 2.0 wt% of the entire film, and kneaded. The prepared crystalline polyester A and thermoplastic resin B were put into two single-screw extruders, respectively, and the former was melted at 280 ° C. and the latter at 260 ° C. and kneaded. Next, after passing through five FSS type leaf disc filters, they are merged by a feed block having 255 slits while measuring with a gear pump, and 255 layers are alternately arranged in the thickness direction of a stacking ratio of 1.0. It was a laminated body. Here, the slit length was designed to be stepped, and the intervals were all constant. The obtained laminate had 128 layers made of crystalline polyester A (layer A) and 127 layers made of thermoplastic resin B (layer B), which were alternately laminated in the thickness direction. The thickness of the outermost layer was 40 nm. The laminate is supplied to a T-die, formed into a sheet, and then rapidly cooled and solidified on a casting drum whose surface temperature is maintained at 25 ° C. while applying an electrostatic application voltage of 8 kV with a wire, and the unstretched laminate is used. I got a cast film.

The obtained laminated cast film was heated in a roll group set at 100 ° C., and then stretched 3.3 times in the longitudinal direction of the film while being rapidly heated from both sides of the film by a radiation heater for a stretched section length of 100 mm. After that, it was cooled once. Subsequently, both sides of this laminated uniaxially stretched polyester film were subjected to corona discharge treatment in the air to set the wet tension of the base film to 55 mN / m, and the treated surfaces on both sides of the film (with a # 4 meta bar to form an easy-slip layer). A water-based coating film containing a vinyl acetate / acrylic resin containing 3 wt% of colloidal silica having a particle size of 100 nm is coated (hereinafter, "coating" means the above-mentioned contents), and is transparent and easy to slip. An adhesive layer was formed.

This laminated uniaxially stretched polyester film was guided to a tenter, preheated with hot air at 90 ° C., and then stretched 3.3 times in the film width direction at a temperature of 140 ° C. The stretching speed and temperature here were constant. The stretched film is directly heat-treated in a tenter with hot air at 190 ° C., followed by a 1% relaxation treatment in the width direction under the same temperature conditions, and then wound up to obtain a laminated biaxially stretched polyester film. Obtained. The obtained laminated biaxially stretched polyester film exhibited the physical properties as shown in Table 1. The light transmittances at a thickness of 15 μm, 380 nm and 400 nm were 9% and 84%, respectively, satisfying the target values, and no ripple in the transmission spectrum was observed. In addition, the basic physical properties of the film, including the in-plane orientation, also satisfied the requirements.

(Comparative Example 1)
In Example 1, a film was prepared in the same manner without adding an ultraviolet absorber. A colorless and transparent laminated biaxially stretched polyester film exhibiting the effect of ultraviolet reflection due to the laminated structure was obtained, but since no ultraviolet absorber was added, insufficient ultraviolet cut at a wavelength of 380 nm was remarkable, and a polarizer was obtained. It was a film that was not suitable for optical use for the purpose of protecting.

(Comparative Example 2)
A laminated biaxially stretched polyester film was obtained in the same manner as in Example 1 except that the number of layers having a layer thickness of 50 nm or less was only 5 consecutive layers counting from the second layer in Example 1. Since the obtained laminated biaxially stretched polyester film has a layer thickness of more than 50 nm in the vicinity of the surface layer, ripples due to light interference are generated, and loss of light transmittance is remarkably confirmed in the ultraviolet region. In particular, the transmission of ultraviolet rays causes deterioration of the polarizer and the liquid crystal molecules, so that the film obtained this time is not suitable for optical applications.

(Comparative Example 3)
In Example 1, two different types of resins were laminated with a feed block having 255 slits designed so that the thickness of the outermost layer was 80 nm, and a laminated body in which 255 layers were alternately laminated with a lamination ratio of 1.0. A laminated biaxially stretched polyester film was obtained in the same manner as in Example 1. In the obtained film, interference fringes due to optical interference reflection of the laminated structure were observed due to the generation of ripples due to the thick outermost layer.

(Example 2)
In Example 1, a laminated biaxially stretched polyester film was obtained in the same manner except that a benzotriazole-based ultraviolet absorber was added in an amount of 2 wt% only to the crystalline polyester A. Since the ultraviolet absorber was added to the layer made of crystalline polyester A including the outermost layer, volatilization of the ultraviolet absorber from the vicinity of the mouthpiece was confirmed during film formation. The basic performance of the laminated biaxially stretched polyester film was the same as that of Example 1, and it could be used for optical applications.

(Example 3)
In Example 1, a benzotriazole-based ultraviolet absorber was added in an amount of 1 wt% to a crystalline polyester A and 1 wt% to a thermoplastic resin B, and put into two different extruders to form a film. A laminated biaxially stretched polyester film was obtained in the same manner as in Example 1 except for the above. Unlike Example 2, the copolymerization of the ultraviolet absorber with the resin resulted in the suppression of the volatilization of the ultraviolet absorber from the vicinity of the base. The performance of the laminated biaxially stretched polyester film was equivalent to that of Example 1.

(Example 4)
In Example 1, the resin was laminated with a feed block having 491 slits to obtain a laminated body in which 491 layers were alternately laminated in the thickness direction with a lamination ratio of 1.3. Further, it was designed so that the layer thickness of the outermost layer was 5 μm. It was confirmed by transmission electron microscope observation that the obtained laminated biaxially stretched polyester film had 246 layers of crystalline polyester A layer and 245 layers of thermoplastic resin B layer, and were alternately laminated in the thickness direction. did. In addition, except that the content concentration of the benzotriazole-based ultraviolet absorber was set to 0.8 wt%, the formulation for adding the ultraviolet absorber and the stretching conditions of the film were carried out by the method described in Example 1. The obtained laminated biaxially stretched polyester film showed the physical properties shown in Table 1, and although the Re (0 °) value was a little high due to the thick presence of the crystalline polyester A layer which is the outermost layer, there was no ripple. The transmittance was also satisfactory. Further, since the outermost layer (layer A) made of crystalline polyester has a thickness of 5 μm, volatilization of the benzotriazole-based ultraviolet absorber added to the thermoplastic resin B was not confirmed, and the temperature was 85% at 85 ° C. The Δ haze in the reliability test of RH was particularly reduced as compared with Example 1.
(Example 5)
In Example 2, a film was obtained in the same manner as in Example 2 except that 0.7 wt% of a benzoxazine-based ultraviolet absorber having a molecular weight of 350 g / mol was added as an ultraviolet absorber. The melting enthalpy is as high as 117 J / g compared to the conventional benzotriazole-based UV absorbers, and although there is concern about sublimation during film formation, the outermost layer is as thick as 5 μm, so the UV absorbers volatilize. Was suppressed, and no contamination of the equipment during film formation was confirmed. The physical properties of the film also generally satisfied the targets except that the Re (0 °) value and the orientation angle were high, and could be used as an optical member.

(Example 6)
In Example 1, a laminated biaxially stretched polyester film was obtained by the same film-forming method as in Example 1 except that 1.8 wt% of a benzoxazine-based ultraviolet absorber having a molecular weight of 350 g / mol was added as an ultraviolet absorber. It was. The physical properties of the obtained laminated biaxially stretched polyester film are as shown in Table 1, but since the layer thickness of the outermost layer is as thin as 40 nm, the ultraviolet absorber sublimated when discharged from the base contaminates the film forming apparatus. However, the obtained laminated biaxially stretched polyester film satisfied the target values in terms of both physical properties and optical properties.

(Example 7)
In Example 1, the same as in Example 1 except that a triazine-based ultraviolet absorber having a melting enthalpy of 40 J / g and a molecular weight of 700 g / mol was added as an ultraviolet absorber so as to be 0.8 wt%. To prepare a laminated biaxially stretched polyester film. The triazine-based ultraviolet absorber did not volatilize from the base and did not precipitate on the surface even in the reliability test at 85 ° C. and 85% RH for 200 hours. Although the absorption wavelength extends to the long wavelength side of the ultraviolet region, the basic performance of the film satisfies the target, and it is the most suitable ultraviolet absorber used in the examples so far.

(Comparative Example 4)
In Example 7, a film was obtained in the same manner as in Example 7 except that 1.5 wt% of a triazine-based ultraviolet absorber was added. Ultraviolet rays Since this triazine-based ultraviolet absorber absorbs and absorbs ultraviolet absorbers on the long wavelength side, the transmittance in the visible light region having a wavelength of 400 nm was as low as 73%. The film itself had a yellow hue and could not be used for optical purposes.

(Example 8)
Except that the content concentration of the benzotriazole-based ultraviolet absorber in Example 1 was 1.1 wt%, and the triazine-based ultraviolet absorber used in Example 7 was added in combination with 0.4 wt% to the thermoplastic resin B. , A laminated biaxially stretched polyester film was obtained in the same manner as in Example 1. The combined use of the triazine-based ultraviolet absorber suppressed the volatilization from the mouthpiece confirmed in Example 1 and the increase in Δ haze in the reliability test.

(Example 9)
In Example 8, a laminated biaxially stretched polyester film was obtained by the same method except that the benzotriazole-based ultraviolet absorber was copolymerized with the crystalline polyester A and only the triazine-based ultraviolet absorber was added to the thermoplastic resin B. It was. As in Example 3, there was no film-forming contamination due to the effect of suppressing volatilization by copolymerization and the use of a triazine-based ultraviolet absorber that is difficult to volatilize and precipitate. The characteristics of the obtained film were equivalent to those of Example 8.

(Example 10)
In Example 8, a laminated biaxially stretched polyester film was obtained in the same manner except that 0.2 wt% of a benzotriazole-based fluorescent whitening agent was added to the thermoplastic resin B in addition to the ultraviolet absorber. Due to the effect of the fluorescent whitening agent, the light transmittance at a wavelength of 400 nm was improved and the clear feeling was improved. Other basic performances were equivalent to those of Example 8.

(Example 11)
In Example 10, a polyethylene terephthalate resin copolymerized with 5 mol% of isophthalic acid as the crystalline polyester A and a polyethylene terephthalate resin copolymerized with 25% isophthalic acid as the thermoplastic resin B were used. Furthermore, the discharge amount of each resin from the extruder was changed to set the stacking ratio to 0.7, and the concentrations of the benzotriazole-based ultraviolet absorber and the triazine-based ultraviolet absorber were 1.0 wt% and 0.3 wt%, respectively. A film was obtained in the same manner as in Example 10 except that the amount was reduced. By copolymerizing isophthalic acid to reduce the lamination ratio, not only the retardation Re (0 °) was reduced, but also the rainbow spots when viewed from an oblique direction were effectively reduced.

(Example 12)
In Example 8, the polyethylene terephthalate resin obtained by copolymerizing 5 mol% of spiroglycol (SPG) as the crystalline polyester and the polyethylene terephthalate resin obtained by copolymerizing 35 mol% of SPG as the thermoplastic resin B were used. In the same manner, a laminated biaxially stretched polyester film was obtained. The characteristics of the obtained film were almost the same as those of Example 8, but by adding SPG to both layers, the change in mechanical characteristics when the film was treated under the condition of 85 ° C. and 85% RH was strongly suppressed. The film was suitable for display applications.

(Example 13)
After preparing the laminated biaxially stretched polyester film of Example 8, a hard coat layer was laminated on one side of the film. By laminating the hard coat layer, the ripple due to the optical interference of the spectral spectrum was reduced, and the transparency of the film was increased. Further, since the hard coat having a strong crosslinkability was laminated, the Δ haze after the reliability test at 85 ° C. and 85% RH was the most suppressed result in the examples so far.

(Example 14)
In Example 8, a polyethylene terephthalate resin containing 20 mol% of polyethylene naphthalate (PEN) was used as the crystalline polyester A. Further, the same sequential biaxial stretching was performed except that the extrusion temperature of the crystalline polyester A was set to 240 ° C., the roll temperature for stretching in the longitudinal direction was set to 125 ° C., and the heat treatment temperature in the tenter was set to 180 ° C. A laminated biaxially stretched polyester film was obtained. Further, the extrusion conditions were adjusted so that the thickness of the laminated biaxially stretched polyester film was 8 μm and the lamination ratio was 0.7. Although it was a thin film of 8 μm, there was no problem in handleability. Further, since the polyethylene naphthalate resin originally absorbs light, the light transmittance at a wavelength of 380 nm is 10% even for a thin film, which satisfies the target value.

(Comparative Example 5)
Polyethylene naphthalate (PEN) resin was used as the crystalline polyester A and the thermoplastic resin B. The temperature of the two extruders was set to 290 ° C., and after stretching under the same conditions as in Example 14, a 5 μm single-film biaxially stretched polyester film was obtained. As the ultraviolet absorber, a combined formulation of a benzotriazole-based ultraviolet absorber and a triazine-based ultraviolet absorber was used as in Example 8. The absorption of PEN was strong, and the light transmittance at a wavelength of 400 nm was 73%, which did not satisfy the target value. In addition, the film itself is fragile, causing problems in windability and handleability. Furthermore, the retardation Re (0 °) also exceeded 200 nm, and rainbow spots were confirmed visually from an oblique direction, making it unsuitable for optical applications.

(Comparative Example 6)
A laminated biaxially stretched polyester film was obtained in the same manner as in Example 14 except that the thickness was designed to be 18 μm. Due to the absorption of polyethylene naphthalate derived from the resin, the light transmittance at a wavelength of 400 nm was 71%, which was lower than the target value. Further, the retardation Re (0 °) was as high as 362 nm, and rainbow spots were strongly confirmed by visual recognition from an oblique direction as in Comparative Example 5, which was not preferable for optical applications.

(Example 15)
In Example 14, a laminated biaxially stretched polyethylene terephthalate was obtained by the same method except that a polyethylene terephthalate resin to which 35 mol% of SPG was added was used as the thermoplastic resin B. Then, the hard coat layer was laminated 1 μm on both sides of the laminated biaxially stretched polyester film to obtain a laminated sheet. A triazine-based ultraviolet absorber having a molecular weight of 630 g / mol was added to the hard coat in an amount of 2 wt% to suppress deterioration of the polyethylene naphthalate component due to light absorption. By laminating the hard coat layer on both sides, the effect of volatilization, surface precipitation, and ripple suppression was exhibited, and the appearance was also good.

In the present invention, the layer thickness distribution near the surface layer is designed, and by using the reflection effect associated with the laminated structure and the addition of a small amount of an ultraviolet absorber in combination, visible light is transmitted without ripple and only ultraviolet rays are effectively shielded. Because it is a film with reduced retardation, it can be used as a protective film for thin film and highly transparent polarizing plates built into display devices such as liquid crystal displays, window films, release films for electronic materials, and agricultural / food / medical films. It can be preferably used.

Claims (13)

Of crystalline polyester A layer (A layer) and made of the different thermoplastic resin B layers of layer (B layer) laminated alternately 50 or more layers, the outermost layer is the layer A, the layer thickness of the outermost layer The number of layers having a layer thickness of 5 μm or more or 50 nm or less and a layer thickness of 50 nm or less is 10 or more consecutively counting from the second layer, and only the B layer of the A layer and the B layer contains an ultraviolet absorber. A laminated biaxially stretched polyester film, wherein the light transmittance T _{400 at} a wavelength of 400 nm is 80% or more.
The laminated biaxially stretched polyester film according to claim 1, which contains at least one type of benzotriazole-based, benzophenone-based, benzoate-based, or triazine-based UV absorber having a molecular weight of 300 g / mol or more as an ultraviolet absorber. The laminated biaxially stretched polyester film according to claim 1 or 2, which contains a triazine-based ultraviolet absorber having a melting enthalpy of 50 J / g or less as an ultraviolet absorber. The laminated biaxially stretched polyester film according to any one of claims 1 to 3, which contains two or more kinds of ultraviolet absorbers as an ultraviolet absorber. The laminated biaxially stretched polyester film according to any one of claims 1 to 4, wherein the A layer, the B layer, or both the A layer and the B layer contain a fluorescent whitening agent. The laminated biaxially stretched polyester film according to any one of claims 1 to 5, wherein the light transmittance T _{380 at a} wavelength of 380 nm is 20% or less. The laminated biaxially stretched polyester film according to any one of claims 1 to 6, wherein the average light reflectance at a wavelength of 300 to 380 nm is 20% or more. The lamination according to any one of claims 1 to 7, wherein the width of the laminated film is 400 mm or more, the retardation Re (0 °) is 150 nm or less, and the orientation angle is 15 ° or less. Biaxially stretched polyester film. The laminated biaxially stretched polyester film according to any one of claims 1 to 8, wherein the amount of change in haze (Δ haze) before and after the treatment for 200 hours under the condition of 85 ° C. and 85% RH is 1.0 or less. The laminated biaxially stretched polyester film according to any one of claims 1 to 9, wherein the crystalline polyester A is mainly composed of polyethylene terephthalate and / or polybutylene terephthalate obtained by copolymerizing 5 to 40 mol% of polyethylene naphthalate. The laminated biaxially stretched polyester film according to any one of claims 1 to 10, wherein the film thickness is 5 μm or more and 15 μm or less. The laminated biaxially stretched polyester film according to any one of claims 1 to 11, which is used for optical applications. A laminated sheet comprising a hard coat layer (C layer) containing a curable resin C as a main component on the outermost surface of the laminated biaxially stretched polyester film according to any one of claims 1 to 12.