JP6264408B2 - Polarizer - Google Patents

Description

  The present invention relates to a biaxially oriented polyethylene terephthalate film for protecting a polarizer. Specifically, biaxially oriented polyethylene for protecting a polarizer that has excellent thermal dimensional stability, has reduced iridescent spots even at a wide viewing angle when used as a polarizer protective film, and has good visibility. It relates to a terephthalate film.

  A polarizing plate used in a liquid crystal display (LCD) has a configuration in which a polarizer composed of a polyvinyl alcohol resin and a dichroic dye such as iodine is sandwiched between two polarizer protective films. A triacetyl cellulose film (TAC film) has been preferably used as the protective film. However, in recent years, although there has been a demand for thinning the polarizing plate, there has been a problem that when the TAC film, which is a protective film, is thinned, moisture permeability deteriorates and mechanical durability decreases. In addition, when the polarizing plate is produced, heat treatment may be performed in bonding or adhesive processing, and dimensional stability may be required particularly for large screen applications.

  Therefore, it has been proposed to use a polyester film instead of a TAC film for the purpose of achieving thinning while maintaining the durability of the protective film (Patent Documents 1 to 4). In addition, it is disclosed that a polyester film having a high retardation is used for the purpose of providing sufficient mechanical strength (Patent Document 5).

JP 2002-116320 A JP 2004-219620 A JP 2004-205773 A JP 2009-42653 A JP 2008-3541 A

  The protective film for the polarizing plate is required to have a small optical distortion. Therefore, in the method for producing a biaxially stretched polyester film disclosed in Patent Documents 1 to 5, in order to reduce the inclination of the orientation main axis, stretching in approximately one direction is performed, or introduction of a copolymer component is introduced. It is illustrated. However, with such a film, the strength of the film is weak, the film becomes easy to tear, and the thickness unevenness is deteriorated, so that it cannot be practically used.

  Further, a wide-screen viewing angle is being demanded for a large-screen monitor intended for living use. In such a wide viewing-angle application, when a film having a high retardation as in Patent Document 5 is used, an oblique direction is used. From the observation, there was a problem that rainbow-like spots occurred.

  The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide a polyester film having reduced rainbow-like spots even at a wide viewing angle as a polarizer protective film while having excellent dimensional stability and high transparency of a biaxially oriented polyethylene terephthalate film. It is to provide.

That is, the present invention is as follows.
(1) The heat shrinkage rate at 150 ° C. is 6.0% or less, the retardation obtained by the following formula (1) is 500 nm or less, and the variation in retardation in the film width direction is 250 nm / m or less, And the plane orientation coefficient (DELTA) P of a film is 0.160-0.175, The biaxially-oriented polyethylene terephthalate film for polarizer protection.
(Retardation) = | n1-n2 | × (film thickness) (1)
(Where n1 is the refractive index in the orientation principal axis direction in the film plane, and n2 is the refractive index in the direction perpendicular to the orientation principal axis in the film plane)
(2) The biaxially oriented polyethylene terephthalate film for protecting a polarizer, wherein the light transmittance at a wavelength of 380 nm is 20% or less.
(3) The biaxially oriented polyethylene terephthalate film for protecting a polarizer, wherein a film haze change amount ΔHz (ΔHz = haze after heating−haze before heating) when heat-treated at 90 ° C. for 200 hours is less than 5.0.
(4) The above-mentioned biaxially oriented polyethylene terephthalate film for protecting a polarizer, having an easy-adhesion layer on at least one surface.

  The biaxially oriented polyethylene terephthalate film for protecting a polarizer of the present invention has high dimensional stability, and when used as a polarizer protective film, rainbow-like spots are reduced even at a wide viewing angle over a wide range, and visibility is improved. Is good. Therefore, it is suitable for a polarizer protective film, particularly for LCDs for large monitors that require a wide viewing angle.

(polyethylene terephthalate)
Hereinafter, the present invention will be described in detail.
The film of the present invention is made of polyethylene terephthalate. Here, polyethylene terephthalate contains ethylene glycol and terephthalic acid as main components. By using poly (ethylene terephthalate), excellent mechanical strength and transparency as a protective film can be achieved. Conventionally, in order to suppress retardation in a polyester film, a predetermined amount of a copolymerization component has been used, but dimensional stability at high temperatures may not be obtained. On the other hand, since the film of the present invention is made of polyethylene terephthalate, high dimensional stability can be obtained even in high-temperature processing such as post-processing, so that a film with little retardation fluctuation can be obtained. In addition, the occurrence of optical distortion due to bowing in heat setting can be reduced as compared with the case of having a copolymer component.

  As a polymerization method of such polyethylene terephthalate (hereinafter simply referred to as PET), a direct polymerization method in which terephthalic acid and ethylene glycol and, if necessary, other dicarboxylic acid component and diol component are directly reacted, and dimethyl terephthalate are used. Any production method such as a transesterification method in which an ester (including a methyl ester of another dicarboxylic acid as necessary) and ethylene glycol (including another diol component as necessary) are transesterified can be used. .

  The intrinsic viscosity of polyethylene terephthalate is preferably in the range of 0.45 to 0.70. When the intrinsic viscosity is lower than 0.45, the effect of improving the tear resistance is lowered. When the intrinsic viscosity is higher than 0.70, the increase in the filtration pressure is increased and high-precision filtration becomes difficult.

(Oligomer)
In the present invention, in order to achieve good visibility in the display, when heat processing is performed in a post-processing step, or when used as a display member for a long time at a high temperature, when heat-treated at 90 ° C. for 200 hours The film haze change amount ΔHz (ΔHz = haze after heating−haze before heating) is preferably less than 5.0%. The upper limit of the film haze change amount ΔHz is more preferably 4.0% or less, and particularly preferably 3.0% or less. This increase in ΔHz due to heat treatment is a phenomenon that occurs due to the precipitation of oligomers on the film surface. When it exceeds 5.0%, the oligomers crystallize on the surface when used in the above applications, and the transparency of the film. May be reduced.

  In order to make the oligomer precipitation amount on the film surface by heat treatment within the above range, a laminated polyester film composed of at least three layers by coextrusion is used, and a polyester having a low oligomer content is used as the outermost layer constituting the surface layer. it can. Alternatively, by suppressing the oligomer precipitation on the film surface by providing a coating layer that has the effect of preventing oligomer precipitation in-line / off-line, the amount of oligomer precipitation on the film surface after heat treatment is suppressed, and ΔHz is in the above range. It can be.

  The film of the present invention can be formed into a laminated structure using a co-extrusion method. In this case, the outermost layer thickness is preferably 3 μm or more, more preferably 5 μm or more, and only one side thickness. It is 25% or less of the total thickness, more preferably 20% or less, and particularly preferably 10% or less. If the thickness is less than 3 μm, oligomers contained in the inner layer may precipitate on the film surface due to heat history during processing, etc., and there may be contamination of the production line and an increase in the amount of foreign matter on the film surface. If the thickness is greater than 25% of the total thickness, the amount of particles blended in the outermost layer may increase, and transparency may be impaired.

  The oligomer includes cyclic oligomers mainly composed of cyclic trimers, linear oligomers and linear oligomers mainly composed of linear trimers, terephthalic acid, terephthalic acid monoglycol ester, etc. The oligomer of the present invention mainly consists of a cyclic trimer. A method for forming such a polyester film layer having a small amount of oligomer is not particularly limited, but is once polymerized as disclosed in JP-A-48-101462, JP-A-49-32973, and the like. By further polymerizing the chip in a solid phase, low molecular weight substances such as oligomers are reduced in the chip state, and a film is formed using these raw materials, or low molecular weight substances such as oligomers in the chip using a solvent. A method of forming a film by removing the film and a method of extracting and removing low molecular weight substances such as oligomers from a biaxially stretched heat-set film using a solvent are suitable. In particular, in the method of adding the former solid-phase polymerization operation, if the temperature in the extrusion process to the film is high and the time is long, low molecular weight substances such as oligomers with reduced angles increase due to thermal equilibrium. Therefore, it is preferable to extrude at as low a temperature as possible and in a short time.

(UV absorber)
When suppressing deterioration of optical functional pigments such as iodine pigments, it is desirable that the film of the present invention has a light transmittance of 20% or less at a wavelength of 380 nm. The light transmittance at 380 nm is more preferably 15% or less, further preferably 10% or less, and particularly preferably 5% or less. If the light transmittance is 20% or less, the optical functional dye can be prevented from being deteriorated by ultraviolet rays. In addition, the transmittance | permeability in this invention is measured by the perpendicular | vertical method with respect to the plane of a film, and can be measured using a spectrophotometer (for example, Hitachi U-3500 type).

  In order to reduce the transmittance of the film of the present invention at a wavelength of 380 nm to 20% or less, it is desirable to appropriately adjust the concentration of the ultraviolet absorber and the thickness of the film. 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′-me Tilphenyl) -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, Examples thereof include 2-phenyl-3,1-benzoxazin-4-one, but are not particularly limited thereto.

  Further, in addition to the ultraviolet absorber, various additives other than the catalyst can be contained within a range not impeding the effects of the present invention. Examples of additives include inorganic particles, heat resistant polymer particles, alkali metal compounds, alkaline earth metal compounds, phosphorus compounds, antistatic agents, light proofing agents, flame retardants, thermal stabilizers, antioxidants, and antigelling agents. And surfactants

  As a method of blending the ultraviolet absorber, known methods can be used in combination. For example, a master batch is prepared by blending a dried ultraviolet absorber and a polyethylene terephthalate raw material in advance using a kneading extruder. In addition, the film can be blended by a method of mixing the predetermined master batch and the polyethylene terephthalate raw material during film formation.

  At this time, the concentration of the UV absorber in the master batch is preferably 5 to 30% by mass 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. If the extrusion time 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.

  A film having a three-layer structure containing an ultraviolet absorber in the intermediate layer can be specifically produced as follows. Polyethylene terephthalate (PET) pellets alone for the outer layer, master batches containing UV absorbers for the intermediate layer and PET pellets are mixed in a prescribed ratio, dried, and then fed to a known melt laminating extruder. Then, the sheet is extruded from a slit-shaped die and cooled and solidified on a casting roll to form 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.

(particle)
In order to impart easy slipping to the film used in the present invention, particles can be contained. In order to obtain high transparency, it is also a preferred embodiment that the film has a multilayer structure and that the particles are contained only in the surface layer. In this case, it is desirable that the intermediate layer does not substantially add particles. However, when the recovered raw material is used for the intermediate layer, the intermediate layer has a particle concentration of 300 ppm or less in order to make the transparency in a favorable range. More preferably, it can be adjusted to 200 ppm or less, and more preferably 150 ppm or less.

  Typical examples of the inorganic particles to be added 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.

  In addition, it is preferable to use particles having an average particle diameter 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 good to use. 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 maintain the slipperiness, and it is difficult to control the haze value and the film surface roughness within the necessary ranges. 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.

  The content of the particles in the film is preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less. When the content of the particles is less than 0.01% by mass, the slipperiness is inferior, and scratches are generated due to friction with a roll or the like in the process, which is not preferable. Moreover, when the content of the particles is more than 5% by mass, it is difficult to control the haze value and the film surface roughness within the necessary ranges.

  Further, when the film of the present invention requires a high degree of transparency, it is also possible to take a method in which particles that cause a decrease in transparency are not substantially contained in the film.

  The above-mentioned “substantially contain no particles” means, for example, in the case of inorganic particles, a content of 50 ppm or less, preferably 10 ppm or less, most preferably less than the detection limit when inorganic elements are quantified by fluorescent X-ray analysis. Means quantity. This means that even if particles are not actively added to the film, contaminants derived from foreign substances, raw resin, or dirt adhering to the lines and equipment in the film manufacturing process may be peeled off and mixed into the film. Because there is.

  Even if particles for the purpose of imparting slipperiness to the film are not added, for example, if the easy-adhesion layer laminated in-line is provided with slipperiness by containing fine particles with a uniform particle size, good winding Therefore, it is not necessary to add particles to the base film.

  Examples of particles to be included in such an easy-adhesion layer include calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, and other inorganic particles, Examples thereof include organic particles such as crosslinked polymer particles and calcium oxalate.

(Easily adhesive layer)
The polyethylene terephthalate film of the present invention may be provided with an easy-adhesion layer in order to improve adhesion with polyvinyl alcohol. The easy adhesion layer preferably has an easy adhesion layer mainly composed of at least one of polyester resin, polyurethane resin or polyacrylic resin. Here, the “main component” refers to a component that is 50% by mass or more of the solid components constituting the easy-adhesion layer. The coating solution used for forming the easy-adhesion layer is preferably an aqueous coating solution containing at least one of a water-soluble or water-dispersible copolymerized polyester resin, an acrylic resin, and a polyurethane resin. Examples of these coating solutions include water-soluble or water-dispersible co-polymers disclosed in Japanese Patent No. 3567927, Japanese Patent No. 3589232, Japanese Patent No. 3589233, Japanese Patent No. 3900191, and Japanese Patent No. 4150982. Examples thereof include a polymerized polyester resin solution, an acrylic resin solution, and a polyurethane resin solution.

Furthermore, at least one of the above-described polyester resin, polyurethane resin or polyacrylic resin, and the easy-adhesion layer mainly composed of polyvinyl alcohol resin are good for polyvinyl alcohol films widely used for polarizer materials. Adhesive. Here, the “principal component” means containing 50% or more of all resin components contained in the easy-adhesion layer.

  The mixing ratio of the polyvinyl alcohol resin is preferably 10 to 50%, more preferably 20 to 40%, in mass ratio with respect to the resin composed of at least one of polyester resin, polyurethane resin or polyacrylic resin. When the polyvinyl alcohol resin is reduced, the adhesiveness with the polyvinyl alcohol film is lowered, which is not preferable. When the polyvinyl alcohol resin is increased, the adhesiveness with the polyethylene terephthalate film of the substrate is lowered, which is not preferable.

  As the method for applying the easy-adhesion layer, an in-line coating method in which coating is performed during film formation or an off-line coating method in which coating is performed after film formation can be applied, but in-line coating method is preferable from the viewpoint of productivity. . Specific examples of the method for providing the resin coating layer include a reverse roll coating method, a gravure coating method, a kiss coating method, a roll flash method, a spray coating method, a die coating method, an air knife coating method, a Mayer bar coating method, and a pipe doctor. Method, impregnation / coating method, curtain coating method and the like, and these methods may be used alone or in combination. Any solvent can be used as the solvent used to form the easy-adhesion layer as long as it does not dissolve or swell the film, but it is environmentally friendly to use water or an aqueous mixed solvent of water and an organic solvent such as alcohol. Is preferable. Furthermore, the composition constituting the easy-adhesion layer can be used in combination with the above-mentioned particles and optional components such as an antistatic agent.

The easy-adhesion layer can be obtained by applying the coating solution on one or both sides of a longitudinal uniaxially stretched film, drying at 100 to 150 ° C., and further stretching in the transverse direction. The final coating amount of the easy adhesion layer is preferably controlled to 0.05 to 0.20 g / m ² . If the coating amount is less than 0.05 g / m ² , the adhesion with polyvinyl alcohol may be insufficient. On the other hand, when the coating amount exceeds 0.20 g / m ² , blocking resistance may be lowered.

(Film characteristics)
It is important that the heat shrinkage rate at 150 ° C. for 30 minutes in the width direction and the longitudinal direction of the film is 6.0% or less, preferably 5.8% or less, more preferably 5.5% or less. When the heat shrinkage ratio exceeds 6.0%, the film shrinks greatly when heat processing is performed in a post-processing step or when it is used for a long time as a member such as a display application. This is not preferable because distortion of optical characteristics occurs, flatness deteriorates, wrinkles, curls, and the like occur.

The plane orientation degree ΔP of the film of the present invention is 0.160 to 0.175. The plane orientation degree ΔP indicates the orientation strength of the entire film surface. By making the above-mentioned plane orientation degree ΔP of the film in such a range, it is possible to achieve excellent mechanical strength in terms of post-workability. In the case where there is a difference in orientation between the front and back surfaces within the above range, the waist strength (strength) as a film that can resist the curing shrinkage of the curable resin can be maintained. Here, the plane orientation degree ΔP is obtained by the following equation.
ΔP = (nx + ny) / 2−nz
Here, nx, ny, and nz represent the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction, respectively.

  When the plane orientation degree ΔP is 0.160 or more, the film strength in the post-processing can be suitably maintained, which is suitable for automating the bonding to the polarizing film. Further, if the degree of plane orientation ΔP is 0.175 or less, the optical influence due to birefringence is reduced, which is suitable for setting the retardation within the range described later. The upper limit of the degree of plane orientation ΔP of the present invention is preferably 0.173, and more preferably 0.172. The lower limit of the degree of plane orientation ΔP of the present invention is preferably 0.161, more preferably 0.162, and still more preferably 0.163.

  As a result of intensive observation of rainbow spots observed from an oblique direction on a wide viewing angle monitor, the present inventor can eliminate the above rainbow spots very well over the entire width of the film by setting the retardation of the film to 500 nm or less. And found the present invention. That is, the retardation of the film of the present invention is 500 nm or less, preferably less than 500 nm, more preferably 480 nm or less, still more preferably 460 nm or less, and still more preferably 400 nm or less.

  It is not well understood why the rainbow spots generated when the film is laminated on the polarizing film are suppressed by setting the retardation within the above range. However, the above rainbow spots are not visible from the front direction and can be seen only at an angle of 45 ° or less, and moreover, it becomes easier to see when the alignment main axis of polyethylene terephthalate and the absorption axis of the polarizing film are perpendicular. Therefore, it is speculated that the rainbow spots are caused by the optical path difference in the oblique direction due to the influence of birefringence while the elliptically polarized light beam that has passed through the polarizing film passes through the polyethylene terephthalate. The Here, it is considered that by setting the retardation of the film within the above range, the optical path difference is eliminated and the generation of rainbow spots is suppressed.

Here, the retardation is obtained by the following method.
(Retardation) = | n1-n2 | × (film thickness)
N1 is the refractive index in the orientation main axis direction in the film plane, and n2 is the refractive index in the direction perpendicular to the orientation main axis in the film plane.
Although smaller retardation is preferable, it is considered that the lower limit is about 10 nm from the viewpoint of the biaxial stretching method.

  The film of the present invention has a retardation variation of 250 nm / m or less in the film width direction. Thereby, even if it is the film extract | collected from which position of a film roll, the retardation in a plane is stable in the wide film corresponding to enlargement of a screen, and generation | occurrence | production of a color spot can be suppressed. The variation in retardation in the film width direction is preferably 200 nm / m or less, more preferably 180 nm / m or less.

  The length in the film width direction of the present invention is not particularly limited depending on the specification, but is preferably 1.0 m or more, more preferably 1.5 m or more, further preferably 2.0 m or more, and even more preferably 3.0 m. That's it. When the film width direction is equal to or more than the above lower limit, it can suitably correspond to a polarizing plate for large screen applications. In order to cope with an increase in screen size, the length in the film width direction is preferably large. However, in order to suppress the occurrence of color spots due to fluctuations in retardation, the length is preferably 6.0 m or less.

  In order to control the thermal dimensional stability, ΔP and retardation of the film within the above ranges while maintaining a predetermined range of heat shrinkage ratio, (1) film thickness, (2) film stretching ratio, (3) multistage It is desirable to suitably adjust the transverse stretching in (4) heat setting conditions, which will be described later.

(1) Film thickness In order to suppress retardation, the film is preferably thin. The thickness of the film of the present invention is preferably 70 μm or less, and more preferably 60 μm or less. On the other hand, in order to maintain the mechanical strength as the protective film, the thickness of the film is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 20 μm or more.

  Moreover, in order to suppress the fluctuation | variation of a retardation, it is desirable for the thickness variation of a film to be small. That is, the thickness unevenness of the film of the present invention is preferably 5.0% or less, more preferably 4.5% or less, still more preferably 4.0% or less, and 3.0% It is particularly preferred that

(2) Film stretch ratio The stretch ratio during film stretching is preferably 2 to 6 times, more preferably 3 to 5 times, and even more preferably 3.5 to 4.5 times in both the longitudinal direction and the width direction. When the film stretching ratio is less than the above lower limit, it is below the lower limit of ΔP, not only the mechanical strength is lowered, but also the thickness unevenness of the film is likely to occur, and the fluctuation of the retardation is likely to increase. When the film stretching ratio exceeds the above upper limit, the upper limit of ΔP is exceeded, and not only the influence of birefringence becomes strong, but also breakage during film formation tends to occur.

  In order to control the retardation within the above range, ideally, it is preferable to balance the aspect ratio of the film stretch ratio. That is, the upper limit of the ratio of (stretching ratio in the longitudinal direction) / (stretching ratio in the width direction) is preferably 1.5 or less, and more preferably 1.2 or less. The lower limit of the ratio (stretching ratio in the longitudinal direction) / (stretching ratio in the width direction) is preferably 0.8 or more, more preferably 0.9 or more, and even more preferably 1.0 or more. When adopting sequential stretching, the orientation of the film tends to be strongly influenced by the stretching direction in the last stage. Therefore, in order to balance the alignment in the longitudinal direction and the width direction and to suitably suppress the retardation, it is preferable to make the stretching ratio in the longitudinal direction larger than the stretching ratio in the width direction as described above.

(3) Transverse stretching in multiple stages When the biaxially stretched polyethylene terephthalate film in the present invention is obtained by longitudinal-transverse sequential biaxial stretching, it is desirable to perform stretching in the width direction in at least two stages. In particular, it is preferable to perform stretching in the width direction by dividing into two or more sections having different stretching temperatures. In this case, the subsequent stretching temperature is preferably 5 ° C. or more, more preferably 10 ° C. or more higher than the preceding stretching temperature. The distortion of the optical properties caused by bowing in the width direction generated in the preceding stage can be alleviated in the subsequent stretching process at a higher temperature, and the fluctuation of the retardation value in the width direction can be suppressed. Moreover, when performing transverse stretching in two stages, it is preferable that the stretching ratio in the subsequent stage is smaller than the stretching ratio in the preceding stage. Deterioration of the uneven thickness of the film can be suppressed by lowering the subsequent draw ratio. Specifically, when the transverse stretching is performed in two stages, the first stage stretching is performed in the range of 120 ° C. to 200 ° C. by 1.5 to 4.0 times, and then further performed at 150 ° C. to 230 ° C. It is preferable to redraw at a magnification of 01 to 2.0 times. Thereafter, two-stage stretching is performed in the range of 120 ° C. to 230 ° C. (TD in the first direction in the width direction is TD1, and the second stage is TD2), and one or more steps are performed in the range of 150 ° C. to 230 ° C. (n-stage stretching). TDn) will be described as an example.

  When performing two-stage stretching in the above range, the stretching temperature of TD1 is desirably 120 ° C. to 200 ° C., and preferably 130 ° C. to 150 ° C. When it is 120 ° C. or lower, the film is broken, and when it is 200 ° C. or higher, the distortion of the film properties in the width direction is increased. The draw ratio is preferably 1.5 to 4.0 times. Furthermore, it is preferable that the extending | stretching temperature of TD2 is 150 to 230 degreeC, More preferably, it is 180 to 220 degreeC. The draw ratio is preferably 1.01 to 2.0 times. In this way, by stretching 3.0 to 4.5 times as a whole in the range of 120 ° C. to 230 ° C., the retardation value in the width direction while maintaining the plane orientation capable of maintaining the mechanical strength. Fluctuations can be reduced.

(4) Heat setting conditions In order to improve the thermal dimensional stability of the film, it is preferable to perform heat setting at a high temperature. Specifically, the upper limit of the heat setting temperature is preferably higher than 200 ° C, more preferably 210 ° C or higher. However, when the heat setting temperature is high, the influence of optical distortion due to bowing tends to occur, and the fluctuation of retardation may be increased. Therefore, the upper limit of the heat setting temperature is preferably 230 ° C. or less.

  The film of the present invention can be obtained by alone or in combination with the above condition control, but is not limited only to the above production conditions. Needless to say, the present invention employs a polyethylene terephthalate film having specific optical characteristics, so that no rainbow spots occur in a wide polarizing plate, and visibility and dimensional stability are improved. It is important to find out.

(Film production method)
Next, although the detail of the manufacturing method of the polyethylene terephthalate film of this invention is demonstrated, naturally it is not limited to this.
After sufficiently vacuum-drying PET pellets substantially free of particles for the purpose of imparting slipperiness, they are supplied to an extruder, melted and extruded into a sheet at 270 to 290 ° C., cooled and solidified, and unstretched A PET sheet is formed. At this time, high-precision filtration is performed in order to remove foreign substances contained in the resin at an arbitrary place where the molten resin is maintained at 270 to 290 ° C. The filter medium used for high-precision filtration of molten resin is not particularly limited, but the filter medium of the sintered stainless steel is excellent in removal performance of aggregates and high-melting-point organic substances mainly composed of Si, Ti, Sb, Ge, Cu. It is. Furthermore, the filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 15 μm or less.

  In the extrusion method, PET is melted and extruded from an extrusion die, and cooled and solidified with a cooling roll to obtain an unstretched sheet. If necessary, lamination may be performed using 2 or 3 extruders, 2 or 3 layers of multi-nihold, or a feed block. In order to improve the sheet flatness, it is preferable to use an electrostatic application adhesion method or a liquid application adhesion method in order to improve the adhesion between the sheet and the rotary cooling drum.

  The unstretched film obtained as described above is stretched in the longitudinal direction within the range of the above-mentioned stretch ratio with a roll heated to 80 to 120 ° C. to obtain a uniaxially oriented PET film. In order to reduce the distortion of the orientation main axis due to the so-called bowing phenomenon, a method in which the stretching temperature is set high so that the stretching ratio in the longitudinal direction is reduced within a range where there is no problem of unevenness in thickness may be employed.

  Although it does not specifically limit as a method of forming the easily bonding layer of this invention in the said uniaxially oriented PET film surface which is a base film, The coating method is used preferably. For example, as a coating method, air doctor coating method, knife coating method, rod coating method, forward rotation roll coating method, reverse roll coating method, gravure coating method, kiss coating method, bead coating method, slit orifice coating method, cast coating method, Examples include spray coating.

  In addition, the film is a surface activation treatment such as a corona discharge treatment, a glow discharge treatment, a flame treatment, an ultraviolet irradiation treatment, an electron beam irradiation treatment, and an ozone treatment, as long as the object of the present invention is not impaired. May be applied.

  A coating liquid is apply | coated to the single side | surface or both surfaces of a uniaxially oriented PET film by the method as described above. Next, both ends of the film are gripped by clips, guided to a hot air zone heated to 80 to 180 ° C., and stretched in the width direction after drying in the width direction after drying. Subsequently, it is guided to the heat treatment zone within the above-mentioned heat setting temperature range, and heat treatment is performed to complete the crystal orientation. In this heat treatment step, a relaxation treatment of 1 to 12% may be performed in the width direction or the longitudinal direction as necessary.

  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) Haze and haze variation (ΔHz)
The obtained biaxially oriented polyester film was cut into a 50 mm square, and 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.
After the measurement, the film was set in an oven heated to 90 ° C., and the film was taken out after 200 hours. The haze of the heated film was measured by the same method as above to obtain a haze after heating. The haze difference before and after heating according to the following formula was defined as the amount of change in haze (ΔHz).
ΔHz = (haze after heating−haze before heating)

(2) Longitudinal and widthwise thermal shrinkage at 150 ° C. In accordance with JIS C 2318-1997 5.3.4 (dimensional change), the dimensional change rate (%) in the longitudinal direction and the width direction was measured.

(3) Plane orientation coefficient (ΔP)
In accordance with JIS K 7142-1996 5.1 (Method A), the refractive index in the film longitudinal direction (nx), the refractive index in the width direction (ny), and the refractive index in the thickness direction (nz) using an Abbe refractometer with the sodium D line as the light source. ) And the plane orientation coefficient (ΔP) was calculated by the following formula.
ΔP = (nx + ny) / 2−nz
Here, nx, ny, and nz represent the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction, respectively.

(4) Retardation of film Measurement of thickness Take a tape-like sample (5 cm in the horizontal direction × 1 m in the vertical direction) continuous in the longitudinal direction, and measure the thickness at 20 points at a 1 cm pitch using a Seiko EM electronic micrometer, Millitron 1240. It calculated | required as the average value.
B-1. Film Sampling Film samples (10 cm × 10 cm) were taken at 10 cm intervals in the film width direction.
B-2. Refractive index measurement (measurement of n1, n2)
About each film sample, the orientation angle | corner ((theta)) of a molecular chain orientation main axis | shaft is calculated | required using the MOA-6004 type | mold molecular orientation meter by Oji Scientific Instruments. The refractive index measured in the orientation main axis direction was n1, and the refractive index measured in the direction perpendicular thereto was n2, and the retardation of the film was determined as follows. The refractive index was measured according to JIS K 7142-1996 5.1 (Method A) using an Abbe refractometer using sodium D line as a light source.
(Retardation) = | n1-n2 | × film thickness (nm)
B-3. Retardation The average value of the retardation obtained from each film sample was determined and used as the retardation of the film. Also, the difference between the maximum and minimum retardation values obtained from each film sample was calculated, and the difference between the film samples that obtained the maximum and minimum values was divided by the distance (m). Fluctuated.

(5) 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.

(6) Iridescent observation The polyester film of the present invention is attached to one side of a polarizing film made of PVA and iodine so that the absorption axis of the polarizing film and the main axis of orientation of the film are perpendicular, and a TAC film is attached to the opposite surface. A polarizing plate was prepared. The obtained polarizing plate was installed in the backlight so that the polyester film was the outermost surface. The polarizing plate was observed from an angle of 45 degrees oblique to the orientation axis direction of the film, and evaluated as follows. The direction of the orientation main axis of the film was measured using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments.
◎: No rainbow spots.
○: Very thin rainbow spots can be observed.
X: Iridescent spots can be clearly observed.

(7) Thickness unevenness of film A tape-like sample (length 1 m) continuous in the longitudinal direction was collected, and a thickness of 100 points was measured at a 1 cm pitch using a Seiko EM electronic micrometer, Millitron 1240. To do. From the measured values, the maximum value (dmax), the minimum value (dmin), and the average value (d) of the thickness were obtained, and the thickness unevenness (%) was calculated by the following formula. In addition, the measurement was performed 3 times and the average value was calculated | required.
Thickness unevenness (%) = ((dmax−dmin) / d) × 100

(8) PVA easy adhesion The following polyvinyl alcohol solution was applied to the surface of the easy adhesion layer of the biaxially oriented polyethylene terephthalate film provided with the easy adhesion layer obtained in Examples 1, 9, and 10 with a No. 10 wire bar. And dried at 110 ° C. for 1 minute. As the coating material, a material to which a red dye was added so as to facilitate the determination was used. The prepared evaluation target film was attached to a glass plate having a thickness of 5 mm to which a double-sided tape was attached, and the opposite surface of the evaluation target laminated film on which the polyvinyl alcohol resin layer was formed was attached to the double-sided tape. Next, 100 grid-like cuts that penetrated through the polyvinyl alcohol resin layer and reached the base film were made using a cutter guide with a gap interval of 2 mm. Then, an adhesive tape (Nichiban Co., Ltd. cello tape (registered trademark) CT-24; 24 mm width) was attached to the grid-shaped cut surface. The air remaining on the interface at the time of pasting was pushed with an eraser to bring it into close contact, and then the adhesive tape was peeled off vigorously and vertically. At this time, the number of squares peeled off was counted. In addition, what is partially peeled within one square is also included in the number of peeled. The results are shown in Table 4.
○: The number of remaining cells after the one-time peel test is 90 or more Δ: The number of remaining cells after the one-time peel test is 30 or more and less than 90 ×: The number of remaining cells after the one-time peel test is less than 30

(Polyvinyl alcohol solution)
5 g of polyvinyl alcohol (manufactured by Kuraray, PVA117) was gradually added to 95 g of ion-exchanged water, and the mixture was heated to 70 to 90 ° C. and stirred to prepare a 5 mass% PVA-A aqueous solution. After the polyvinyl alcohol was dissolved, about 0.2 g of a water-based red dye (Leva Fix Brill Red E2B, manufactured by Bayer) was added little by little so as to facilitate peeling observation.

(Production Example 1-Polyester A)
When the temperature of the esterification reactor was raised to 200 ° C., 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged and 0.017 parts by mass of antimony trioxide as a catalyst while stirring. 0.064 parts by mass of magnesium acetate tetrahydrate and 0.16 parts by mass of triethylamine were charged. Next, the pressure was raised and the pressure esterification reaction was carried out under the conditions of gauge pressure 0.34 MPa and 240 ° C., then the esterification reaction can was returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. Furthermore, it heated up to 260 degreeC over 15 minutes, and 0.012 mass part of trimethyl phosphate was added. Next, after 15 minutes, dispersion treatment was performed with a high-pressure disperser, and 0.1 mass% of sodium tripolyphosphate aqueous solution was contained as sodium atoms with respect to the silica particles. 0.2 parts by mass of an ethylene glycol slurry of silica particles having an average particle diameter of 2.5 μm, which was filtered through a metal filter having an opening of 5 μm, was added as a particle content. After 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction can and subjected to a polycondensation reaction at 280 ° C. under reduced pressure.
After completion of the polycondensation reaction, it is filtered through a NASRON filter with a 95% cut diameter of 5 μm, extruded into a strand from a nozzle, and cooled and solidified using cooling water that has been filtered (pore diameter: 1 μm or less) in advance. And cut into pellets. The obtained polyethylene terephthalate resin (A) had an intrinsic viscosity of 0.62 dl / g, an oligomer content of 0.96% by mass, and contained substantially no inert particles and internally precipitated particles. (Hereafter, abbreviated as PET (A).)

(Production Example 2-Polyester B)
On the other hand, in the production of PET (A), a polyethylene terephthalate resin (B) having an intrinsic viscosity of 0.62 dl / g and containing no silica particles was obtained. (Hereafter, abbreviated as PET (B).)

(Production Example 3-Polyester C)
The PET (B) obtained by the above method was pre-crystallized in advance at 160 ° C. and then solid-phase polymerized in a nitrogen atmosphere at a temperature of 220 ° C., with an intrinsic viscosity of 0.63 dl / g and an oligomer content of 0.1. 27 mass% polyethylene terephthalate resin (C) was obtained. (Hereafter, abbreviated as PET (C).)

(Production Example 4-Polyester D)
10 parts by weight of a dried UV absorber (2,2 ′-(1,4-phenylene) bis (4H-3,1-benzoxazinon-4-one), PET (B) containing no particles (inherent viscosity Was 0.62 dl / g) and 90 parts by mass were mixed, and a polyethylene terephthalate resin (D) containing an ultraviolet absorber was obtained using a kneading extruder (hereinafter abbreviated as PET (D)).

(Production Example 5-Polyester E)
10 mass of dried ultraviolet absorber (2- (5-chloro (2H) -benzotriazol-2-yl) -4-methyl-6- (tert-butyl) phenol (TINUVIN 326, manufactured by Ciba Specialty Chemicals) And 90 parts by mass of PET (B) (inherent viscosity is 0.62 dl / g) containing no particles, were mixed, and a polyethylene terephthalate resin (E) containing an ultraviolet absorber was obtained using a kneading extruder (hereinafter referred to as “polyethylene terephthalate resin”). And abbreviated as PET (E).)

Example 1
After drying 100 parts by mass of PET (B) resin pellets having an intrinsic viscosity of 0.62 dl / g and containing no particles as a raw material for the base film intermediate layer at 135 ° C. for 6 hours under reduced pressure (1 Torr), the extruder 2 (intermediate layer) II layer), PET (A) and PET (B) are mixed and adjusted so that the content of silica particles is 0.10% by mass, dried by a conventional method, and the extruder 1 (outer layer I layer and For outer layer III) and dissolved at 285 ° C. After filtering these two kinds of polymers with a filter medium made of a sintered stainless steel (nominal filtration accuracy of 10 μm particles 95% cut), laminating them in a two-kind / three-layer confluence block, and extruding them into a sheet form from a die, The film was wound around a casting drum having a surface temperature of 30 ° C. using an electrostatic application casting method, and then cooled and solidified to produce an unstretched film. At this time, the discharge amount of each extruder was adjusted so that the ratio of the thickness of the I layer, the II layer, and the III layer was 5: 90: 5.

  This unstretched film was heated to 100 ° C. with a heated roll group and an infrared heater, and then stretched 3.6 times in the longitudinal direction with a roll group having a difference in peripheral speed to obtain a uniaxially oriented PET film.

  Next, a coating liquid having a solid content concentration of 14.6% by mass was prepared using a water-based polyurethane resin with a solvent having a solvent mass ratio of water / isopropanol = 70/30. The coating solution was microfiltered with a felt type polypropylene filter medium having a filtration particle size (initial filtration efficiency 95%) of 25 μm, applied to one side of a uniaxially oriented PET film by a reverse roll method, and then dried at 80 ° C. for 20 seconds.

The uniaxially stretched film on which the coating layer was formed was guided to a tenter stretching machine, and the film was guided to a hot air zone at a temperature of 140 ° C. while being gripped by a clip, and stretched 3.6 times in the width direction. Next, it was led to a hot air zone having a temperature of 210 ° C. while maintaining the width stretched in the width direction, and the hot air zone was stretched 1.2 times in the width direction. Further, after heat treatment in a hot air zone at a temperature of 210 ° C. for about 5 seconds, a 3% relaxation treatment was performed in the width direction to obtain a polyethylene terephthalate film having an easy adhesion layer coating amount of 0.2 g / m ² and a film thickness of about 38 μm. Obtained. Three rolls having a width of 1 m were taken out from the full width roll. (The center position was matched, and the samples were collected continuously in the width direction, and R, C, and L were taken from the right side.)

(Example 2)
Except that the unstretched film was stretched 3.8 times in the longitudinal direction, a film was formed in the same manner as in Example 1 to obtain a polyethylene terephthalate film having a thickness of about 38 μm.

(Example 3)
Except that the unstretched film was stretched 3.4 times in the longitudinal direction, then stretched 2.7 times in the width direction in a hot air zone at a temperature of 140 ° C, and further stretched 1.3 times in a hot air zone at 210 ° C. 1 to obtain a polyethylene terephthalate film having a thickness of about 38 μm.

Example 4
Except that the unstretched film was stretched 3.6 times in the longitudinal direction, then stretched 2.5 times in the width direction in a hot air zone at a temperature of 140 ° C, and further stretched 1.7 times in a hot air zone at 210 ° C. 1 to obtain a polyethylene terephthalate film having a thickness of about 38 μm.

(Example 5)
An unstretched film is stretched 3.6 times in the longitudinal direction, then stretched 3.3 times in the width direction in a hot air zone at a temperature of 140 ° C, and further stretched 1.3 times in a hot air zone at 225 ° C, and hot air at 225 ° C A polyethylene terephthalate film having a thickness of about 38 μm was obtained by forming a film in the same manner as in Example 1 except that heat setting was performed in the zone.

(Example 6)
An unstretched film is stretched 3.6 times in the longitudinal direction, then stretched 2.7 times in the width direction in a hot air zone at a temperature of 140 ° C, and further stretched 1.6 times in a hot air zone at 210 ° C, and hot air at 225 ° C A polyethylene terephthalate film having a thickness of about 38 μm was obtained in the same manner as in Example 5 except that the film was heat-set in the zone.

(Example 7)
In Example 1, the intermediate layer II was formed by the same method except that 90 parts by mass of PET (B) and 10 parts by mass of PET (D) were mixed to obtain a polyethylene terephthalate film having a thickness of about 38 μm. .
(Example 8)
In Example 1, the intermediate layer II was formed by the same method except that 90 parts by mass of PET (B) and 10 parts by mass of PET (E) were mixed to obtain a polyethylene terephthalate film having a thickness of about 38 μm. .

(Comparative Example 1)
The same as Example 1 except that the unstretched film was stretched 3.4 times in the longitudinal direction and 4.3 times in the width direction at 140 ° C., heat-set at 240 ° C., and then relaxed 3% in the width direction. The film was formed by this method to obtain a polyethylene terephthalate film having a thickness of 38 μm.

(Comparative Example 2)
A polyethylene terephthalate film having a thickness of about 48 μm was obtained in the same manner as in Comparative Example 1 except that the casting speed was changed.

(Comparative Example 3)
A polyethylene terephthalate film having a thickness of about 25 μm was obtained in the same manner as in Comparative Example 1 except that the casting speed was changed.

(Comparative Example 4)
Except that the heat setting was 240 ° C., a film was formed in the same manner as in Example 1 to obtain a polyethylene terephthalate film having a thickness of about 38 μm.

(Comparative Example 5)
Except that the unstretched film was stretched 2.8 times in the longitudinal direction, a film was formed in the same manner as in Example 1 to obtain a polyethylene terephthalate film having a thickness of 38 μm.

(Comparative Example 6)
A film was formed in the same manner as in Example 1 except that heat setting was set at 190 ° C. to obtain a polyethylene terephthalate film having a thickness of about 38 μm.

(Comparative Example 7)
A polyethylene terephthalate film having a thickness of about 38 μm was formed by the same method as in Example 1 except that the unstretched film was stretched 2.4 times in the longitudinal direction, 3.2 times in the width direction, and heat setting was 220 ° C. Got.

Example 9
Except that the aqueous polyurethane resin of the coating layer was changed to a mixed resin of 60 parts by mass of aqueous polyurethane and 40 parts by mass of polyvinyl alcohol, a film was formed in the same manner as in Example 1 to obtain a polyethylene terephthalate film having a film thickness of about 38 μm. .

(Example 10)
A polyethylene terephthalate film having a film thickness of about 38 μm was obtained by forming a film in the same manner as in Example 1 except that the aqueous polyurethane resin of the coating layer was changed to a mixed resin of 60 parts by mass of aqueous polyester and 40 parts by mass of polyvinyl alcohol. .

  Table 1 shows the film stretching conditions for each example. The obtained evaluation results are shown in Tables 2 and 3. When the polyethylene terephthalate film of the present invention is used as a polarizer protective film, it can suppress the occurrence of rainbow spots and maintain good visibility even when it is collected from any position in the film width direction. . Table 4 shows the PVA easy-adhesion evaluation results.

  The biaxially oriented polyethylene terephthalate film of the present invention has dimensional stability and transparency, and can reduce iridescent spots even at a wide viewing angle. Therefore, it is suitably used as a polarizer protective film.

Claims (3)

The heat shrinkage at 150 ° C. is 6.0% or less,
The retardation calculated by the following formula (1) is 500 nm or less,
The fluctuation of retardation in the film width direction is 250 nm / m or less,
And a biaxially oriented polyethylene terephthalate film for protecting a polarizer having a plane orientation coefficient ΔP of 0.160 to 0.175 of the film,
A liquid crystal display having a polarizing plate attached to one side of a polarizing film made of polyvinyl alcohol and iodine.
(Retardation) = | n1-n2 | × (film thickness) (1)
(Where n1 is the refractive index in the orientation principal axis direction in the film plane, and n2 is the refractive index in the direction perpendicular to the orientation principal axis in the film plane) The polarizing plate has a biaxially oriented polyethylene terephthalate film for protecting a polarizer on one side of a polarizing film made of polyvinyl alcohol and iodine, and the absorption axis of the polarizing film is perpendicular to the main axis of orientation of the biaxially oriented polyethylene terephthalate film for protecting the polarizer. The liquid crystal display according to claim 1, wherein the liquid crystal display is attached so as to become. The liquid crystal display according to claim 1 or 2, wherein the polarizing plate is obtained by attaching a triacetyl cellulose film to the other side of the polarizing film.