JP5077142B2 - Biaxially oriented laminated polyester film - Google Patents

Description

  The present invention relates to a biaxially oriented laminated polyester film having good optical axis accuracy in the width direction, excellent light resistance, and suitable as a protective film used in a liquid crystal display device.

  High optical properties are required for films used for optical applications, particularly for constituent members of liquid crystal display devices. For example, a protective film for polarizing plates and retardation plates does not cause fluctuations in contrast and brightness, and does not cause interference colors in observation under crossed Nicols, which is performed in defect inspections such as scratches and foreign substances. Is required. In addition, when used as a protective film for a polarizer to be bonded to a polarizer, high optical properties are required so that no polarization deviation from the polarizer occurs.

  The protective film as described above is produced by providing a functional film such as a pressure-sensitive adhesive layer, an easy adhesion layer, and a release layer on a base film. Biaxially oriented polyester films are widely used as the base material for these protective films from the viewpoint of strength function and cost. A biaxially oriented polyester film having a structure in which linear polyester polymers are oriented is a birefringent body that optically exhibits birefringence. Therefore, the biaxially oriented polyester film has two optical axes perpendicular to the direction parallel to and perpendicular to the direction of molecular orientation. Therefore, when the optical axis of the base material of the protective film is laminated (laminated body) with the optical axis of the polarizer, polarizing plate, or retardation plate tilted, it is transmitted when placed under crossed Nicols. It exhibits light and interference colors, and becomes a factor that hinders visibility. Furthermore, if the direction of the optical axis of the base material of the protective film varies within the film plane, the contrast, brightness, interference color state fluctuates depending on the location of the laminate, and visibility is significantly reduced. Become. Moreover, when using together with a polarizer as a base material of a polarizer protective film, the weather resistance which can protect a polarizer from an ultraviolet-ray is required.

  Therefore, the biaxially oriented polyester film that is the base material of the protective film has excellent weather resistance and excellent optical axis accuracy, that is, the mechanical axis direction (longitudinal direction / width direction) of the film and the direction of the optical axis. Are matched with high accuracy, and the axial direction of the optical axis is required to be uniform in the film plane.

  However, the biaxially oriented polyester film produced by sequentially stretching in the machine direction-transverse direction tends to be distributed in the direction of the molecular orientation principal axis, that is, the optical axis in the film width direction, and is generated in the stretching and heat treatment processes. Due to the bowing phenomenon, the tilt angle of the optical axis with respect to the width direction increases as the film ends. Therefore, as a base material for a protective film for a polarizer, a polarizing plate or a retardation plate, only a product collected from the vicinity of the central portion in the film width direction having a relatively small inclination of the optical axis could be used. Therefore, it is a big problem in reducing the procurement cost of the base film, and consequently the supply cost of the protective film.

  Furthermore, along with the trend toward larger screens of liquid crystal displays in recent years, it is desired that the size of polarizers, polarizing plates, and retardation plates is increased, and the protective film is also widened. When the protective film becomes wider, for the reasons described above, it is necessary to collect the film from a wider area than the vicinity of the central portion so far, and the variation in the tilt angle of the optical axis in the film width direction within one protective film. (Fluctuation) also increases. That is, in a protective film used by being attached to a polarizer for a large-sized liquid crystal display device, a polarizing plate, or a retardation plate, excellent optical axis accuracy is required so that the optical axis is not inclined even in a wider region with respect to the width direction. It has become like this.

  In order to reduce the bowing phenomenon that causes the above problem, the following efforts have been made so far. For example, a uniaxially stretched film is transversely stretched with a tenter, cooled to below the glass transition temperature, and then the gripping of both ends of the film is once released and gripped again, and heat-fixed while raising the temperature in a temperature range of 120 to 240 ° C. Method (refer to Patent Document 1), immediately after transverse stretching, the film temperature is once lowered to a temperature not higher than the glass transition temperature to increase the rigidity, and the film on the heat treatment chamber side is prevented from being drawn into the stretching chamber (Patent Document 2). 3), a method in which only the narrow part near the center of the film is gripped by the nip roll while the both ends of the film after the transverse stretching is gripped by the gripping means of the tenter, and the center part is forcibly advanced (patent) In addition, after biaxially stretching the film, a method of heating so that the temperature at the end is higher than the center of the film (see Patent Documents 5 and 6). , A method of cooling at a temperature below the glass transition temperature (Tg) between stretching in the width direction and heat setting, and then re-stretching in the width direction at the maximum temperature of the heat setting temperature (see Patent Documents 7, 8, and 9) ), A method of providing either a triangular, trapezoidal, or arcuate cooling region in the heat setting region (see Patent Documents 10 and 11), a method of performing longitudinal relaxation simultaneously with lateral stretching (see Patent Document 12) It is.

JP-A-57-87331 Japanese Patent Laid-Open No. 3-130127 JP-A-3-216326 Japanese Patent Publication No. 63-24459 Japanese Patent Laid-Open No. 62-183327 JP-A-62-183328 JP 2001-328159 A JP 2002-172694 A JP 2002-361737 A Japanese Patent Laid-Open No. 2004-18588 JP 2004-18784 A JP 2004-358742 A

  However, any of these methods is an approach for suppressing the occurrence of bowing as much as possible in a biaxially stretched film having a molecular orientation main axis in the film width direction by sequentially stretching in the machine direction and the transverse direction. When the film is stretched sequentially in the machine direction-transverse direction, the straight line drawn in the width direction of the film before the tenter is put into an arcuate curve by bowing through stretching and heat setting. The geometric distortion (geometric bowing amount) of this curve is expressed by the maximum distance between the chord and arc of the curve. However, as shown in the examples of the present specification (Comparative Examples 2 and 3), the geometric bowing amount and the physical bowing amount represented by the inclination angle of the optical main axis are not necessarily proportional. However, it is necessary to avoid predicting the accuracy of the optical axis from the effect of improving the geometric bowing amount.

  Specifically, Patent Document 1 discloses a film in which a geometric bowing amount (maximum distance between a string and an arc) is halved compared to a conventionally known manufacturing method. However, the fact that Boeing is occurring remains unchanged. Further, there is no suggestion or disclosure of the improvement effect of the tilt angle of the optical axis.

  Patent Document 2 discloses a film in which the geometric bowing distortion is reduced to 2.58%, and Patent Document 3 discloses a film in which the geometric bowing distortion is reduced to 2.66%. ing. However, the bowing phenomenon still occurs in these proposals, and the tilt angle of the optical axis is not disclosed at all.

  Patent Document 4 discloses a film in which the geometric bowing amount is reduced to about one-tenth of the conventionally known method. However, no improvement effect is shown for the tilt angle of the optical axis.

  Patent Documents 5 and 6 disclose films having a trapezoidal geometric bow shape. However, there is no suggestion or disclosure of the improvement effect of the tilt angle of the optical axis.

  Patent Document 7 discloses a film having an orientation angle of 1.3 degrees or less with respect to the lateral direction of the film in a film having a width of 950 mm, excluding 200 mm at both ends. However, it is a conventionally known fact that the orientation angles are aligned in the lateral direction in the vicinity of the central portion in the film width direction, and no special effect is recognized in this proposal.

  Patent Document 7 discloses a film having an orientation angle of 7 degrees at a position 2 m from the center in the film width direction. In this proposal, since the full width of the film is not disclosed, it is unclear how uniform the orientation angle is obtained over the full width.

  Patent Document 8 discloses a film having an orientation angle of 4 degrees at a relative position 0.6 in the film width direction. However, as described above, it is a conventionally known fact that the orientation angles are aligned in the lateral direction at the central portion of the film.

  Patent Document 9 discloses a film having a maximum orientation angle of 4.6 degrees in an 80% region in the film width direction. However, the proposed film has five extreme values in the orientation angle distribution in the film width direction. Therefore, the problem that the orientation angle changes rapidly in the width direction of the film is inherent, and it is not an essential solution of the problem.

  Patent Documents 10 and 11 disclose films whose maximum orientation angles up to 80% in the film width direction are 5.1 degrees and 4 degrees, respectively. However, these methods have not yet fundamentally solved the problem of optical distortion.

  In Patent Documents 7, 9, 10, 11, and 12, in order to suppress the geometric bowing amount of the biaxially oriented polyester film, the stretching ratio in the longitudinal direction is set to be smaller than the stretching ratio in the width direction. Has been. This is intended to reduce the distortion due to bowing that occurs in the film by reducing the draw ratio in the longitudinal direction. Although the geometric bowing amount is suppressed by this, the problem of the optical axis accuracy has not been essentially solved as described above.

  Furthermore, the Example of patent document 7 is disclosing the biaxially stretched film which has a molecular orientation main axis | shaft in the longitudinal direction of a film by extending | stretching sequentially in a horizontal direction-a vertical direction. However, in Patent Document 7, for the purpose of reducing bowing, the draw ratio in the longitudinal direction is set to be equal to or less than the draw ratio in the width direction as described above. In this way, if the orientation main axis is simply in the longitudinal direction, the optical axis direction is easily affected by bowing, and further, the optical axis of the film is strongly influenced by the wave of molecular orientation in the longitudinal direction (variation). As shown in the examples of the present specification (Comparative Example 1), the optical axis direction in the film surface varies and the direction is not uniform.

  In other words, all of the methods in these patent documents are approaches to suppress the geometric bowing amount of the film as much as possible, and the optical axis accuracy of the protective film for a polarizer, a polarizing plate, or a retardation plate is improved. The problem has not been solved essentially.

The object of the present invention is excellent in weather resistance, and the mechanical axis direction (longitudinal direction / width direction) of the film coincides with the direction of the optical axis with high accuracy, and the direction of the optical axis is uniform in the film plane. It is to provide a biaxially oriented laminated polyester film suitable as a substrate for a protective film for a polarizer, a polarizing plate or a retardation plate.
Another object of the present invention is to provide a method for stably producing a biaxially oriented laminated polyester film having the above characteristics at a low cost.
And it aims at contributing to the image quality improvement of a large-screen liquid crystal display, and its spread by them.

  As a result of intensive studies, the present inventor has found that the above problems can be solved by highly orienting the molecular orientation of the base film in the longitudinal direction. That is, the protective substrate film for a polarizer, a polarizing plate or a retardation plate of the present invention, and a production method thereof have the following configurations.

The first invention is a biaxially oriented laminated polyester film characterized by satisfying the following requirements (1) to (3).
(1) It has a molecular orientation main axis in the film longitudinal direction. (2) The difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is 0.03 or more and 0.10 or less. (3) Wavelength The second aspect of the invention, in which the light transmittance at 380 nm is 20% or less, is that the maximum inclination angle of the optical principal axis with respect to the mechanical axis direction of the biaxially oriented laminated polyester film is 5 degrees or less, and at 30 cm intervals in the film width direction. The biaxially oriented laminated polyester film is characterized in that the measured change in the tilt angle of the optical principal axis is 5 degrees or less.
A third invention is a protective film for a polarizer, a polarizing plate or a retardation plate, characterized in that the biaxially oriented laminated polyester film is a base material.
4th invention is the said biaxially oriented laminated polyester film manufacturing method, Comprising: After the said biaxially oriented laminated polyester film extends | stretches an unstretched film to the width direction, it extends | stretches to a longitudinal direction, Then, it heat-processes. It is a manufacturing method of the biaxially oriented laminated polyester film characterized by being manufactured.
5th invention is the said biaxially oriented laminated polyester film manufacturing method, Comprising: The said biaxially oriented laminated polyester film is further extended | stretched in a longitudinal direction after carrying out simultaneous biaxial stretching of the unstretched film to the width direction and a longitudinal direction. It is a manufacturing method of the biaxially oriented lamination | stacking polyester film characterized by performing by performing heat processing and then performing heat processing.

Since the biaxially oriented laminated polyester film of the present invention is highly molecularly oriented in the longitudinal direction and the difference between the refractive index in the longitudinal direction and the refractive index in the width direction is controlled within a specific range, the direction of the optical axis Is not affected by the geometric bowing, and the fluctuation range in the optical axis direction with respect to the mechanical axis of the film is small. For this reason, the optical axis and the mechanical axis coincide with each other with high accuracy.
Moreover, according to the manufacturing method of this invention, said characteristic can be acquired stably irrespective of the taking position of the width direction of a biaxially-oriented laminated polyester film. The biaxially oriented laminated polyester film is excellent in weather resistance and can protect functional dyes such as a polarizer.

  In the present invention, the polyester is a crystalline polyester that can be obtained by polycondensation of an aromatic dicarboxylic acid or its ester and a diol. The aromatic dicarboxylic acid typically includes terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, and the diol includes ethylene glycol, diethylene glycol, tetramethylene glycol, neopentyl glycol, and the like. It is done.

  The above polyester is obtained by direct polycondensation of aromatic dicarboxylic acid and glycol, or a method of polycondensation after transesterification of aromatic dicarboxylic acid dialkyl ester and glycol, or diglycol ester of aromatic dicarboxylic acid Can also be obtained by a polycondensation method.

  Specific examples of such polyester include polyethylene terephthalate, polyethylene-2,6-naphthalate, polytetramethylene terephthalate, polytetramethylene-2,6-naphthalate, and the like. The polyester may be a copolyester obtained by copolymerizing various components in addition to the homopolyester. Further, it may be a polymer blend of polyester and another copolymer. Examples of other copolymers that can be blended include polyolefins and other polyesters. A blend of a homopolymer and a polyalkylene copolymer polymer, in particular a polyetherester copolymer or a blend of different polyalkylene glycol copolymer polymers is preferred. Among these, in particular, polyethylene terephthalate (PET) is most preferably used from the comprehensive performance such as transparency, mechanical properties, surface smoothness, solvent resistance, scratch resistance, moisture permeability, cost and the like with few impurities.

  The intrinsic viscosity of the polyester is preferably in the range of 0.45 to 0.70. If the intrinsic viscosity is lower than 0.45, the film is easily torn, and if it is higher than 0.70, the increase in filtration pressure is increased and high-precision filtration becomes difficult.

  The biaxially oriented polyester film of the present invention preferably has a haze of 4.0% or less, more preferably 3.5% or less, and even more preferably 3.0% or less. When the haze is 4.0% or less, high brightness is obtained when used as a protective film for a polarizer, which is preferable. Moreover, as a minimum of the said haze, it is preferable that it is 0.6% or less, It is more preferable that it is 1.0% or less, It is further more preferable that it is 1.5% or less. If the haze is less than 0.6%, the slipperiness required for handling may be reduced. In addition, when the transparency is extremely high, a defect at a level that does not cause a problem in defect inspection may be seen, so that it may not be preferable as a substrate for a protective film for a polarizing plate or a retardation plate.

  The biaxially oriented polyester film of the present invention needs to have a transmittance of 20% or less at a wavelength of 380 nm. The transmittance at a wavelength of 380 nm is preferably 15% or less, and more preferably 5% or less. If the transmittance is 20% or less, alteration of the optical functional dye such as a polarizer by ultraviolet rays can be suppressed. In order to reduce the transmittance of the biaxially oriented polyester film at a wavelength of 380 nm to 20% or less, for example, the concentration of the following ultraviolet absorber and the thickness of the base film are adjusted as appropriate. In addition, the transmittance | permeability in this invention is measured by the perpendicular | vertical method with respect to the plane of an adhesive property modified base film optical laminated film, and is measured using a spectrophotometer (for example, Hitachi U-3500 type). be able to.

  The biaxially oriented laminated polyester film of the present invention preferably has a multilayer structure of three or more layers. When the layer having the ultraviolet absorber is the A layer and the other surfaces are the B layer and the C layer, the layer structure in the film thickness direction is B / A / B, B / A / C, B / A / C / B. Alternatively, a configuration such as B / A / C / A / B is conceivable. Each of the A to C layers may have the same or different configuration of the polyester resin, but preferably has a B / A / B configuration (two types and three layers). In any case, it is necessary to provide a layer containing an ultraviolet absorber as the intermediate layer. By containing an ultraviolet absorber in the intermediate layer, bleeding out of the additive can be suitably prevented, and deterioration of adhesion due to bleeding out of the additive can be suppressed.

  In the two-layer / three-layer configuration of the A layer and the B layer, when the configuration is B / A / B, the layer configuration is such that the ratio of the thickness of the A layer to the entire layer is in the range of 0.02 to 0.20. Preferably, the layer structure is in the range of 0.05 to 0.15. When the lower limit is lower than 0.02, the ultraviolet absorbent may bleed out. Moreover, when an upper limit exceeds 0.20, since a weather resistance may fall, it is unpreferable.

  The ultraviolet absorber used in the present invention is a known substance. Examples of the ultraviolet absorber include an organic ultraviolet absorber and an inorganic ultraviolet absorber, and an organic ultraviolet absorber is preferable from the viewpoint of transparency. Examples of the organic ultraviolet absorber include benzotoazole, benzophenone, cyclic imino ester, and combinations thereof, but are not particularly limited as long as the absorbance is within the range defined by the present invention. However, from the viewpoint of durability, benzotoazole and cyclic imino ester are particularly preferable. When two or more kinds of ultraviolet absorbers are used in combination, ultraviolet rays having different wavelengths can be absorbed simultaneously, so that the ultraviolet absorption effect can be further improved.

Examples of the benzophenone ultraviolet absorber, benzotriazole ultraviolet absorber, and acrylonitrile ultraviolet absorber include 2- [2′-hydroxy-5 ′-(methacryloyloxymethyl) phenyl] -2H-benzotriazole, 2- [2 ′. -Hydroxy-5 '-(methacryloyloxyethyl) phenyl] -2H-benzotriazole, 2- [2'-hydroxy-5'-(methacryloyloxypropyl) phenyl] -2H-benzotriazole, 2,2'-dihydroxy- 4,4′-dimethoxybenzophenone, 2,2 ′, 4,4′-tetrahydroxybenzophenone, 2,4-di-tert-butyl-6- (5-chlorobenzotriazol-2-yl) phenol, 2- ( 2'-hydroxy-3'-tert-butyl-5'-methylphenyl)- 5-chlorobenzotriazole, 2- (5-chloro (2H) -benzotriazol-2-yl) -4-methyl-6- (tert-butyl) phenol, 2,2'-methylenebis (4- (1,1 , 3,3-tetramethylbutyl) -6- (2H-benzotriazol-2-yl) phenol, etc. Examples of cyclic imino ester UV absorbers include 2,2 ′-(1,4-phenylene). Bis (4H-3,1-benzoxazinon-4-one), 2-methyl-3,1-benzoxazin-4-one, 2-butyl-3,1-benzoxazin-4-one, 2-phenyl -3,1-benzoxazin-4-one, etc. However, it is not particularly limited thereto.

  In order to impart handling properties (slidability) to the film, it is preferable to add particles as a lubricant to the film. Typical examples of inorganic particles added to the polyester resin include silica, colloidal silica, alumina, aluminum sol, kaolin, talc, mica, calcium carbonate, calcium phosphate, and the like. Moreover, acrylic particles, styrene particles, olefin particles, imide particles or the like can be used as the organic particles.

  The inorganic particles or organic particles preferably have an average particle size of 0.01 μm or more and 10 μm or less, more preferably 0.05 μm or more and 8 μm or less, and most preferably 0.1 μm or more and 3 μm or less. It is better to use a diameter.

  When the average particle size of the particles is smaller than 0.01 μm, it is necessary to increase the amount of addition in order to achieve easy slipping, and it is difficult to control the haze value within a necessary range. In addition, when the average particle size of the particles is larger than 10 μm, it is not preferable because the added particles drop off significantly during the film forming process and contaminate the process.

  In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method. Take a picture of the particles with a scanning electron microscope (SEM), at a magnification such that the size of one smallest particle is 2-5 mm, the maximum diameter of 300-500 particles (between the two most distant points) The average value is taken as the average particle diameter.

  Moreover, when high transparency is required, it is also a preferable aspect that substantially no particles are contained inside the film. Here, “substantially no particles” means, for example, in the case of inorganic particles, when inorganic elements are quantified by fluorescent X-ray analysis, they are 50 ppm or less, preferably 10 ppm or less, most preferably the detection limit or less. Means content. This means that even if particles are not actively added to the base film, contaminants derived from foreign substances and raw material resin or dirt adhering to the line or equipment in the film manufacturing process will be peeled off and mixed into the film. It is because there is a case to do.

  Further, in the biaxially oriented polyester film of the present invention, various additives such as an antioxidant, a heat stabilizer, a weather stabilizer, an ultraviolet absorber, and an organic lubricant as long as the effects of the present invention are not impaired. , Pigments, dyes, fillers, antistatic agents, and the like may be used in combination.

  As a result of intensive studies on the relationship between optical axis accuracy and bowing, the present inventors surprisingly found that the biaxially stretched film highly molecularly oriented in the longitudinal direction of the film had its position in the width direction, It has been found that the optical axis is highly coincident with the mechanical axis (longitudinal direction, width direction) of the film regardless of the degree of geometric bowing. That is, the biaxially oriented polyester film used as the base material for the protective film for a polarizer, a polarizing plate or a retardation plate of the present invention has (1) a molecular orientation principal axis in the film longitudinal direction, (2 It is important that the difference between the refractive index (nx) in the longitudinal direction of the film and the refractive index (ny) in the width direction of the film is 0.03 or more and 0.10 or less.

The reason why the biaxially oriented polyester film in the present invention exhibits the above characteristics is considered as follows.
As described above, since the biaxially oriented polyester film produced by sequentially stretching in the longitudinal direction-lateral direction is stretched in the lateral direction (film width direction) in the subsequent stage, the molecular orientation as a whole is oriented in the width direction. And has a molecular orientation main axis in the film width direction. Therefore, in such a biaxially oriented polyester, the direction of the optical principal axis is distributed in the width direction. Therefore, the bowing caused by transverse stretching and being promoted by heat fixation causes distortion of physical properties in the width direction, so that the optical principal axis distributed in the width direction is easily affected by the bowing, and becomes the end in the film width direction. The distortion of the optical main axis increases.

  On the other hand, since the biaxially oriented polyester film of the present invention is highly molecularly oriented in the longitudinal direction of the film, it is less susceptible to bowing that causes distortion in the width direction. However, although it is difficult to be affected by Boeing, simply aligning the alignment main axis in the longitudinal direction is strongly affected by the wave (variation) of the molecular orientation in the longitudinal direction. It will not be uniform. Therefore, by further controlling the refractive index difference between the refractive index (nx) in the film longitudinal direction and the refractive index (ny) in the film width direction within the above range, the wave of molecular orientation in the longitudinal direction is suppressed, and optical The direction of the axis was made uniform in the film plane. Therefore, the optical axis of the biaxially oriented polyester film of the present invention is highly consistent with the mechanical axis direction (longitudinal direction and width direction) of the film regardless of the position in the width direction and the degree of geometric bowing. To do. In addition, the manufacturing method of such a film is mentioned later.

  When the difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is less than 0.03, the inclination of the optical axis of the biaxially oriented polyester film becomes large due to the influence of geometric bowing. It is not preferable. Further, when the refractive index difference is less than 0.03, the wave of molecular orientation in the longitudinal direction (variation) becomes large, and the variation of the tilt angle of the optical principal axis with respect to the mechanical axis of the film becomes large. The difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is preferably 0.04 or more, and more preferably 0.05 or more.

  On the other hand, the difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction needs to be 0.10 or less, more preferably 0.09 or less, and further 0.08 or less. Is preferred. If the difference between the refractive index in the longitudinal direction of the film and the refractive index in the width direction of the film is greater than 0.10, the retardation value becomes too large, whitish under polarized light, and foreign matter and other inspections are difficult. Further, the film tends to tear in the orientation direction of the fill, and the mechanical strength decreases.

  The biaxially oriented polyester film having the above characteristics has the following preferable characteristics. That is, the maximum inclination angle of the optical main axis with respect to the machine axis direction (longitudinal direction, width direction) is within 5 degrees. Moreover, the fluctuation | variation of the inclination-angle of the optical principal axis measured at 30 cm intervals in the film width direction is less than 5 degree | times.

  The maximum inclination angle is more preferably within 4 degrees, further preferably within 3 degrees, and most preferably within 2 degrees. Here, the machine axis direction means two directions orthogonal to the longitudinal direction (machine direction) of the film and the width direction. In the case of a biaxially oriented polyester, the optical axis has two optical axes, a direction perpendicular to the direction of molecular orientation and a direction perpendicular to the direction of molecular orientation. In the present invention, the optical axis is in the longitudinal direction with the molecular orientation main axis, and has another optical axis in the width direction. When the maximum inclination angle of these optical axes with respect to the mechanical axis exceeds 5 degrees, transmitted light and interference color are exhibited during defect inspection under crossed Nicols, which hinders inspection. Further, the variation of the tilt angle of the optical main axis measured at an arbitrary 30 cm interval in the film width direction is more preferably within 4 degrees, further preferably within 3 degrees, and most preferably within 2 degrees. Further, if the fluctuation of the tilt angle of the optical main axis exceeds 5 degrees per 30 cm of the film width, an interference color is partially generated or the difference between light and dark becomes large, which also hinders inspection.

The biaxially oriented polyester film preferably has a thermal shrinkage rate at 150 ° C. for 30 minutes of 3.0% or less in both the longitudinal direction and the width direction. When the heat shrinkage rate exceeds 3.0%, the film is greatly shrunk by heating during post- processing, and flatness deterioration, wrinkles, curls, etc. are generated, which is not preferable.

  Moreover, the thickness of the film of the present invention is not particularly limited and is arbitrary, but is preferably 9 to 300 μm, and most preferably 12 to 50 μm. If the thickness exceeds 300 μm, there is a problem in cost, retardation is increased, and visibility in crossed Nicol tends to be lowered. On the other hand, when the thickness is less than 9 μm, the mechanical properties are lowered, and the function as a protective film for a polarizer, a polarizing plate or a retardation plate cannot be achieved.

  The biaxially oriented polyester film having the above characteristics can be obtained by the following means, but any biaxially oriented polyester film having the characteristics of the present invention is included in the scope of the present invention even if it is obtained by a method other than the following method. It is.

(1) Longitudinal stretching in the latter stage of stretching In the present invention, in order to orient the orientation main axis of the biaxially oriented polyester film in the longitudinal direction, it is preferable to perform longitudinal stretching in the latter stage of the stretching step. The molecular orientation of the biaxially stretched film tends to be strongly influenced by the stretching direction to be performed later, and the orientation main axis of the film can be easily oriented in the longitudinal direction by the above embodiment.

(2) Stretch ratio in the longitudinal direction As described above, in the conventional technical idea of suppressing geometric bowing, it has been desirable to reduce the stretch ratio in the longitudinal direction. However, in the present invention, it is desirable that the biaxially oriented polyester film has a high longitudinal draw ratio in order to suppress fluctuations in the molecular orientation in the longitudinal direction. By increasing the draw ratio in the longitudinal direction, the orientation main axis of the film is highly oriented in the longitudinal direction, and the optical axis direction becomes uniform in the film plane. In order to produce a biaxially oriented polyester film having the above characteristics, the draw ratio in the longitudinal direction is preferably 3.0 to 6.0 times, particularly 4.0. -5.0 times is more preferable. Moreover, the draw ratio in the width direction is preferably 2.0 to 5.0 times, and more preferably 2.5 to 4.0 times. As for the draw ratio in the longitudinal direction and the width direction, it is desirable that the draw ratio in the longitudinal direction is 1.0 or more, preferably 1.5 or more larger than the draw ratio in the width direction.

(3) Heat setting Boeing is caused by transverse stretching, and strain is increased and fixed by subsequent heat setting. Therefore, from the viewpoint of suppressing bowing, it is ideally preferable not to perform heat setting or to lower the heat setting temperature. However, in the present invention, since there is no influence of bowing, heat setting at a continuous high temperature is possible. This makes it possible to obtain a biaxially oriented polyester film having high thermal dimensional stability.

  Hereinafter, as an example, a specific production method when polyethylene terephthalate (PET) is used will be described in detail.

  As a method of blending a film with a UV absorber, a known method can be used in combination.For example, a master batch is prepared by blending a dried UV absorber and a polyester raw material in advance using a kneading extruder. In addition, it can mix | blend by the method etc. which mix this predetermined masterbatch and a polyester raw material at the time of film forming of a base film.

  At this time, the concentration of the UV absorber in the master batch is preferably 5 to 30% by weight in order to uniformly disperse the UV absorber and economically blend it. As a condition for producing the master batch, it is preferable to use a kneading extruder and to extrude it at a temperature not lower than the melting point of the polyester raw material and not higher than 290 ° C for 1 to 15 minutes. Above 290 ° C, the weight loss of the UV absorber is large, and the viscosity of the master batch is greatly reduced. When the extrusion temperature is 1 minute or less, uniform mixing of the UV absorber becomes difficult. At this time, if necessary, a stabilizer, a color tone adjusting agent, and an antistatic agent may be added.

  Specifically, a base film having a three-layer structure containing an ultraviolet absorber in the intermediate layer can be produced as follows. PET pellets alone for the outer layer, and master batches containing UV absorbers for the intermediate layer and PET pellets are mixed at a predetermined ratio, dried and then fed to a known melt laminating extruder, Extruded into a sheet form from a die and cooled and solidified on a casting roll to make an unstretched film. That is, using two or more extruders, a three-layer manifold or a merging block (for example, a merging block having a square merging portion), a film layer constituting both outer layers and a film layer constituting an intermediate layer are laminated, An unstretched film is formed by extruding a three-layer sheet from the die and cooling with a casting roll.

  In the present invention, it is preferable to perform high-precision filtration during melt extrusion in order to remove foreign substances contained in the raw material polyester that cause optical defects. The filter particle size (initial filtration efficiency 95%) of the filter medium used for high-precision filtration of the molten resin is preferably 15 μm or less. When the filter particle size of the filter medium exceeds 15 μm, removal of foreign matters of 20 μm or more tends to be insufficient.

  Next, the film stretching process will be described. The biaxially oriented polyester film used in the present invention is characterized by being highly oriented in the longitudinal direction. Therefore, as a stretching method, an unstretched film is stretched in the width direction using a tenter, the film is heated by a roll group, and further stretched in the longitudinal direction using an infrared heater, etc., and then guided to the tenter again and heat-set. It is preferable to employ any one of a method for performing stretching, that is, a sequential biaxial stretching method in which stretching is performed in the order of TD-MD, or a simultaneous biaxial stretching method in which stretching in the longitudinal direction and the width direction is performed in the same tenter.

First, sequential biaxial stretching in which stretching is performed in the order of the above-described TD-MD will be described.
First, the unstretched sheet is stretched in the transverse direction by gradually widening the width of the tenter rail with a tenter-type stretching machine to obtain a uniaxially oriented polyester film. The temperature of the preheating zone is preferably not less than the glass transition temperature of the resin and not more than 130 ° C. The preheating zone is preferably composed of one or more zones defined by a predetermined preheating temperature. After preheating, the film is subsequently stretched, and the stretching ratio is preferably 2.0 to 5.0 times, particularly preferably 2.5 to 4.0 times. The temperature of the stretching zone is in the range of the glass transition point Tg to (Tg + 50) ° C. of the polyester. When the stretching temperature is lower than Tg, it cannot be uniformly stretched and causes unevenness in thickness. Moreover, when the stretching temperature is higher than (Tg + 50) ° C., the uniformity of the thickness in the width direction is deteriorated, which is not preferable. After stretching, the film is gradually cooled by using one or two or more zones, and after the temperature is lowered to (Tg−20) ° C. or lower, the clip is removed and guided to the next step.

  Next, it is stretched in the longitudinal direction by a roll type stretching machine comprising a group of rolls heated to a temperature equal to or higher than the glass transition point Tg of the polyester. The draw ratio is 3.0 to 6.0 times in the longitudinal direction, particularly 4.0. -5.0 times is more preferable. When the stretching is performed, auxiliary heating is performed using an infrared heater or the like, and the film stretching temperature is set in the range of (Tg + 10) to (Tg + 50) ° C. When the temperature is lower than (Tg + 10) ° C., breakage tends to occur. When the temperature is higher than (Tg + 50), the longitudinal orientation is lowered, which is not preferable for achieving the object of the present invention. After stretching, the film is cooled to Tg or less by a cooling roll and then guided to the next step.

  Subsequently, the film is again guided to the tenter and heat-set, and the temperature of the heat-set zone is preferably in the range of (Tg + 130) to (Tm-10) ° C, more preferably (Tg + 140) to (Tm-20) ° C. Range. (Tm represents the melting point of the polyester.) When the temperature of the heat setting zone is lower than (Tg + 130) ° C., the heat shrinkage rate becomes high, which may cause wrinkling and curling during processing. When the temperature of the heat setting zone is higher than (Tm-100) ° C., crystallization progresses, and it tends to tear in the stretching direction in the trimer process and the slit process, which is not preferable. Moreover, the relaxation process can be performed in the longitudinal direction and the width direction as necessary. The amount of relaxation varies depending on the conditions, but is about 1 to 10%. Moreover, about the process after heat setting, it cuts and processes the film edge currently hold | gripped with the TD clip, the process of performing a corona process, the process of performing a relaxation process between rolls, etc. with a product winder It is wound up.

Next, simultaneous biaxial stretching will be described.
Both ends of the obtained unnew-stretched polyester film are held by clips and guided to the preheating zone. The temperature of the preheating zone is preferably not less than the glass transition temperature of the resin and not more than 130 ° C. The preheating zone is preferably composed of one or more zones defined by a predetermined preheating temperature.

  After preheating, the film is stretched simultaneously in the longitudinal direction and the width direction, and the stretching ratio is 3.0 to 6.0 times in the longitudinal direction, particularly 4.0. -5.0 times is more preferable. Further, the draw ratio in the width direction is more preferably 2.0 to 5.0 times and 2.5 to 4.0 times. As for the draw ratio in the longitudinal direction and the width direction, it is desirable that the draw ratio in the longitudinal direction is 1.0 or more, preferably 1.5 or more larger than the draw ratio in the width direction.

  The stretching end point is preferably set so that the stretching in the longitudinal direction and the width direction ends simultaneously or the setting where the stretching in the longitudinal direction ends later. In the latter, it is preferable that the stretching in the longitudinal direction performs 15% or more of the entire stretching ratio in the longitudinal direction after the stretching in the width direction is completed.

  The temperature of the stretching zone is in the range of Tg to (Tg + 50) ° C. When the stretching temperature is lower than Tg, it cannot be uniformly stretched and causes unevenness in thickness. Moreover, when the stretching temperature is higher than (Tg + 50) ° C., the uniformity of the thickness in the width direction is deteriorated, which is not preferable.

  After the stretching of the film, the film is continuously heat-set, but before that, it is preferable to provide an intermediate zone between the stretching zone and the heat-setting zone of the film. By providing the intermediate zone, heat exchange between the stretching zone and the heat setting zone can be suppressed, and the temperature accuracy of each zone can be improved.

  The intermediate zone may be unheated, or may be a hot air circulation system in which the temperature is controlled in the range of Tg-50 ° C to Tg + 60 ° C, more preferably in the range of Tg-40 ° C to Tg + 30 ° C. . When the intermediate zone of the hot air circulation system is employed, if the temperature of the circulating air is Tg−50 ° C. or less, a large amount of heat is required for heating the film in the heat fixing zone, which is not preferable. Conversely, Tg + 60 ° C. or higher is not preferable because the optical axis accuracy of the biaxially stretched film may deteriorate. The time for the film to pass through the intermediate zone is preferably 2 to 20 seconds, more preferably 4 to 15 seconds. If it is 2 seconds or less, the film temperature is not sufficiently lowered, and if it is 20 seconds or more, the zone length necessary for cooling becomes long, and the equipment becomes large. Further, instead of providing the intermediate zone, stretching and heat setting may be performed by separate tenters, and the film temperature may be adjusted within the above range.

  The temperature of the heat setting zone is preferably in the range of (Tg + 130) to (Tm−10) ° C., more preferably in the range of (Tg + 140) to (Tm−20) ° C. When the temperature of the heat setting zone is lower than (Tg + 130) ° C., the heat shrinkage rate becomes high, which causes wrinkles and curls during processing. When the temperature of the heat setting zone is higher than (Tm−10) ° C., crystallization proceeds, and it tends to tear in the stretching direction in the trimer process and the slit process, resulting in a decrease in yield.

  Moreover, the relaxation process can be performed in the longitudinal direction and the width direction as necessary. The amount of relaxation varies depending on the conditions, but is preferably about 1 to 10%.

  Moreover, about the process after heat setting, it cuts and processes the film edge currently hold | gripped with the TD clip, the process of performing a corona process, the process of performing a relaxation process between rolls, etc. with a product winder It is wound up.

  The biaxially oriented polyester film produced by the above method has adhesiveness, water resistance, and chemical resistance with layers formed on the film such as an adhesive layer, a release layer, an antistatic layer, and a protective layer. In order to improve the above, surface treatment, that is, corona discharge treatment (in air, nitrogen, carbon dioxide, etc.) or easy adhesion treatment may be performed by a known method. Various methods known in the art can be used for the easy adhesion treatment, and a method of applying various easy adhesives before stretching or after uniaxial stretching is suitably employed.

  Next, the description will be further described with reference to examples. However, the present invention is not limited to the following examples without departing from the gist thereof. In addition, the evaluation method of the physical property in a following example and a comparative example is as follows.

(1) Refractive index In accordance with JIS K 7142-1996 5.1 (Method A), the refractive index (nx) in the longitudinal direction of the film and the refractive index (ny) in the width direction were measured with an Abbe refractometer using sodium D line as a light source. , (Nx−ny) to obtain the refractive index difference.

(2) Direction of molecular orientation main axis In the refractive index measurement, when nx> ny, the longitudinal direction was determined, and when nx ≦ ny, the width direction was determined.

(3) Molecular chain principal axis orientation angle (θ), optical principal axis tilt angle (ξ)
About the distance of a film width direction, an edge is set to 0% in the film width | variety of the tenter exit (Examples 1-4 and 2nd tenter exit in the comparative example 1) in preparation of the biaxially oriented polyethylene terephthalate film mentioned later. The edge is 100%. For an area corresponding to 10% to an area corresponding to 90% of the film width (corresponding to the entire width of the release film described below), n 100 mm square film samples were continuously formed at a pitch of 100 mm in the width direction. Cut out. The square film sample was cut at a right angle with respect to either the longitudinal or width axis. For each film sample, using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd., the orientation angle (θi) of the molecular chain principal axis relative to the film longitudinal direction, and the machine axis direction (longitudinal direction) defined by the following formula , Or the width direction), the tilt angle (ξi) of the optical principal axis was measured. Note that n is an integer obtained by multiplying the total width of the film by 0.8 and rounding up the value after dividing by 10 mm. I represents a sample number, i = 1 to n.
When | θ | ≦ 45 degrees ξ = | θ |
When | θ |> 45 degrees ξ = | 90 degrees− | θ ||
Of the inclination angles of the optical principal axis measured from the film sample, the maximum value was defined as the maximum inclination angle (ξmax) of the optical principal axis.

  In the measurement of the orientation angle, the orientation angles (θi, θi + 1, θi + 2, θi + 3) of four consecutive film samples were compared, and the difference (Δθi) between the maximum value and the minimum value was obtained. Among Δθ1 to Δθn-3, the maximum value was defined as the change in the tilt angle of the optical principal axis (Δθmax).

(4) Thermal shrinkage at 150 ° C. [HS150]
In accordance with JIS C 2318-1997 5.3.4 (dimensional change), the dimensional change rate (%) in the longitudinal direction (MD) and the width direction (TD) was measured.

(5) Haze The haze was measured according to JIS K 7105 “Testing method for optical properties of plastic” haze (cloudiness value). NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd. was used for the measuring instrument.

(6) Light transmittance at a wavelength of 380 nm Using a spectrophotometer (manufactured by Hitachi, U-3500 type), the light transmittance in a wavelength region of 300 to 500 nm is measured using an air layer as a standard, and the light transmittance at a wavelength of 380 nm. Asked.

(7) Visibility under polarized light A polarizing plate is orthogonally crossed and installed in a crossed Nicol state on a surface light source backlight (transmitted light BOX A3-2 manufactured by SFC). The film cut out from the film roll is sandwiched between the polarizing plates so that the slit end of the film and the polarizing axis of the polarizing plate are vertical or horizontal, and the visibility is observed.
A: Almost no interference color occurs. Even when viewed obliquely, interference colors are hardly generated. There are no partially colored spots.
◯: Some interference color occurs, but the colored spots are within the allowable range.
Δ: Interference color occurs, and some colored spots occur.
X: The interference color is intense and there is a partial colored spot.

Example 1
10 parts by weight of a dried UV absorber (2,2 ′-(1,4-phenylene) bis (4H-3,1-benzoxazinon-4-one), PET resin pellets containing no particles (with intrinsic viscosity 0.62 dl / g) 90 parts by weight were mixed, and an ultraviolet absorbent-containing masterbatch (A) was prepared using a kneading extruder, and the extrusion temperature at this time was 285 ° C.

  As a raw material for the base film intermediate layer, 90 parts by weight of PET resin pellets having an intrinsic viscosity of 0.62 dl / g not containing particles and 10 parts of a UV-absorbing master batch (A) were vacuum-dried at 135 ° C. for 6 hours (1 Torr ) And then into an extruder 2 (for the intermediate layer B layer), polyethylene terephthalate pellets containing 0.06% by mass of silica particles having an average particle diameter of 2.5 μm (inherent viscosity is 0.62 dl / g) Were fed to extruders 1 (for outer layer A layer) and 3 (for outer layer C layer), respectively, and dissolved at 285 ° C. These two polymers are each filtered through a filter material of stainless steel sintered body (nominal filtration accuracy 10 μm particle 95% cut), laminated in a three-layer confluence block, extruded into a sheet form from the die, and then electrostatically applied. It was wound around a casting drum having a surface temperature of 30 ° C. using a casting method, and solidified by cooling to produce an unstretched film. At this time, the discharge amount of each extruder was adjusted so that the thickness ratio of the A layer, the B layer, and the C layer was 5: 90: 5.

This unstretched film was guided to a tenter stretching machine (first tenter), preheated at 95 ° C. for about 6 seconds in a preheating zone, and then stretched at a stretching ratio of 3.2 times in the width direction in a stretching zone at 100 ° C. Thereafter, the film was cooled to 50 ° C., and then the clip was released. Then, it led to the roll group heated at 100 degreeC, and it adjusted so that extending | stretching temperature might be 115 degreeC using an infrared heater, and performed 4.5 time extending | stretching to the longitudinal direction. Then, after cooling a film to 40 degreeC with a cooling roll, it guide | induced to the tenter (2nd tenter) again, heat-fixed at 220 degreeC, and after cooling to 50 degreeC, the holding | grip of the clip was open | released. Subsequently, both ends corresponding to 15% of the entire film width of the second tenter outlet width were cut and wound into a roll shape to obtain a biaxially oriented polyethylene terephthalate film having a width of about 1500 mm and a thickness of 38 μm.
The properties of the obtained film are shown in Table 1.

(Example 2)
A biaxially oriented polyethylene terephthalate film having a thickness of 38 μm was obtained in the same manner as in Example 1, except that the draw ratio in the longitudinal direction was 4.7 times.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 1.
As in Example 1, the visibility of the obtained film under polarized light was good over the entire width of the film.

(Comparative Example 1)
A biaxially oriented polyethylene terephthalate film having a thickness of 38 μm was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was 3.7 times and the stretching ratio in the longitudinal direction was 3.5 times.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 1.
The visibility of the obtained film under polarized light emitted an interference color and was x.

(Example 3)
83 parts by weight of PET resin pellets having an intrinsic viscosity of 0.62 dl / g without containing particles and 17 parts of an ultraviolet absorber-containing masterbatch (A) as a raw material for the intermediate layer, the thickness of the A layer, the B layer, and the C layer A biaxially oriented polyethylene terephthalate film having a thickness of 38 μm was obtained in the same manner as in Example 1 except that the ratio was kept at 5: 90: 5 and the thickness of the adhesive modified polyester film was 25 μm.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 1.
As in Example 1, the visibility of the obtained film under polarized light was good over the entire width of the film.

(Comparative Example 2)
The unstretched film obtained by the same method as in Example 1 is led to a group of rolls heated to 75 ° C., adjusted using an infrared heater so that the stretching temperature becomes 95 ° C., and 3.3 times in the longitudinal direction. Stretched. Next, the film was led to a tenter, stretched 3.9 times in the width direction at 120 ° C., subsequently heat-set at 220 ° C., cooled the film, then released from the clip, and 15% of the total film width at the tenter outlet width. Both ends corresponding to the above were cut and then rolled up to obtain a biaxially oriented polyethylene terephthalate film having a thickness of 38 μm.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 1.
Although the visibility of the obtained film under polarized light showed good characteristics at the center of the film, it exhibited a remarkable interference color at the end of the film and could not withstand use.

  In the production of the biaxially oriented polyethylene terephthalate film of this comparative example, the geometric bowing amount was measured by the following method. Before introducing the tenter, a straight line extending in the longitudinal direction was drawn with magic ink over the entire width in the film width direction. When such a straight line was observed after heat setting, it became a curved curve that was bowed in a concave shape with respect to the film traveling direction by bowing. The maximum distance between the bowed curve and the string was measured, the maximum distance was divided by the total film width distance, and the geometric bowing amount was calculated as a percentage, which was approximately 15%.

(Comparative Example 3)
The unstretched film obtained by the same method as in Example 1 is led to a group of rolls heated to 75 ° C., adjusted using an infrared heater so that the stretching temperature becomes 95 ° C., and 3.3 times in the longitudinal direction. Stretched. Next, it was led to a tenter, stretched 2.6 times in the width direction at 120 ° C, and further stretched 1.5 times in the width direction at 220 ° C. And both ends corresponding to 15% of the total film width of the tenter exit width were cut and wound into a roll to obtain a biaxially oriented polyethylene terephthalate film having a thickness of 38 μm.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 1.
As in Comparative Example 2, the visibility of the obtained film under polarized light showed good characteristics at the center of the film, but exhibited a remarkable interference color at the end of the film and was unusable.

  In the production of this comparative example, the geometric bowing amount was measured in the same manner as in comparative example 2. As a result, the bowing amount was about 5%, and a marked improvement was observed in comparison with the bowing amount of Comparative Example 2. However, as shown in Table 1, the improvement effect on the maximum tilt angle of the optical main axis was very slight, and no improvement in distortion as much as the geometric bowing amount was improved.

Example 4
After producing an unstretched film in the same manner as in Example 1, the film was guided to a tenter-type simultaneous biaxial stretching machine, and both ends of the film were gripped with clips, and the lengthwise direction and the widthwise direction at 95 ° C. were multiplied by 3.2 times. Simultaneous biaxial stretching. Subsequently, the film was stretched 1.4 times in the longitudinal direction while raising the temperature to 120 ° C., then heat-fixed at 220 ° C., cooled to 50 ° C., and then the clip was opened. Subsequently, both ends corresponding to 15% of the total film width of the tenter outlet width were cut and wound into a roll shape to obtain a biaxially oriented polyethylene terephthalate film having a thickness of 38 μm.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 2.
The visibility of the obtained film under polarized light was ◎.

(Example 5)
The unstretched film prepared by the same method as in Example 1 is guided to a tenter type simultaneous biaxial stretching machine, and both ends of the film are gripped by clips and simultaneously two times at 90 ° C. in the longitudinal direction and in the width direction. Axial stretched. Subsequently, the film was stretched 1.67 times in the longitudinal direction while raising the temperature to 115 ° C., heat-fixed at 220 ° C., cooled to 50 ° C., and then the clip was opened. Subsequently, both ends corresponding to 15% of the total film width of the tenter outlet width were cut and wound into a roll shape to obtain a biaxially oriented polyethylene terephthalate film having a thickness of 38 μm.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 2.
The visibility of the obtained film under polarized light was even better than that of Example 5 and was extremely good.

(Comparative Example 4)
The unstretched film prepared by the same method as in Example 5 was guided to a tenter type simultaneous biaxial stretching machine, and both ends of the film were gripped by clips, and simultaneously two times at 95 ° C. in the longitudinal direction and the width direction. Axial stretched. Subsequently, the film was stretched 1.17 times in the longitudinal direction while raising the temperature to 120 ° C., then heat-fixed at 220 ° C., cooled to 50 ° C., and then the clip was opened. Subsequently, both ends corresponding to 15% of the total film width of the tenter outlet width were cut, and then wound into a roll to obtain a biaxially oriented polyethylene terephthalate film.
The properties of the obtained biaxially oriented polyethylene terephthalate film are shown in Table 2.
The visibility of the obtained film under polarized light exhibited a remarkable interference color at the end of the film and was not practical.
In addition, the refractive index difference (difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction) of the film obtained in this comparative example is 0.02, and the refractive index difference in Example 5 (0. 04), but the maximum tilt angle of the optical main axis was remarkably deteriorated.

  The obtained results are shown in Tables 1 and 2. In each of the examples and comparative examples, the heat shrinkage rate was greater in the longitudinal direction than in the width direction.

  The biaxially oriented polyester film of the present invention can be suitably used as a substrate for a protective film for a polarizer, a polarizing plate or a retardation plate. In particular, it is suitable as a protective film for polarizers, polarizing plates, and retardation plates, which are constituent members of large liquid crystal display devices.

Claims (4)

A biaxially oriented laminated polyester film characterized by satisfying the following requirements (1) to (4).
(1) It has a molecular orientation main axis in the film longitudinal direction. (2) The difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is 0.03 or more and 0.10 or less. (3) Wavelength (4) Optical principal axis measured at 30 cm intervals in the film width direction with a maximum inclination angle of the optical principal axis with respect to the machine axis direction of the biaxially oriented laminated polyester film having a light transmittance of 380 nm of 20% or less The fluctuation of the tilt angle is 5 degrees or less A protective film for a polarizer, a polarizing plate, or a retardation plate, comprising the biaxially oriented laminated polyester film according to claim 1 as a base material. A biaxially oriented laminated polyester film satisfying the following requirements (1) to (4), wherein the biaxially oriented laminated polyester film is stretched in the longitudinal direction after stretching the unstretched film in the width direction, and then heat treated. A biaxially oriented laminated polyester film obtained by performing the step .
(1) It has a molecular orientation main axis in the film longitudinal direction. (2) The difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is 0.03 or more and 0.10 or less. (3) Wavelength (4) Optical principal axis measured at 30 cm intervals in the film width direction with a maximum inclination angle of the optical principal axis with respect to the machine axis direction of the biaxially oriented laminated polyester film having a light transmittance of 380 nm of 20% or less The fluctuation of the tilt angle is 5 degrees or less A biaxially oriented laminated polyester film satisfying the following requirements (1) to (4), wherein the biaxially oriented laminated polyester film is further biaxially stretched in the width direction and the longitudinal direction, and further in the longitudinal direction. A biaxially oriented laminated polyester film obtained by stretching the film , followed by heat treatment .
(1) It has a molecular orientation main axis in the film longitudinal direction. (2) The difference between the refractive index in the film longitudinal direction and the refractive index in the film width direction is 0.03 or more and 0.10 or less. (3) Wavelength (4) Optical principal axis measured at 30 cm intervals in the film width direction with a maximum inclination angle of the optical principal axis with respect to the machine axis direction of the biaxially oriented laminated polyester film having a light transmittance of 380 nm of 20% or less The fluctuation of the tilt angle is 5 degrees or less