WO2013080949A1

WO2013080949A1 - Liquid crystal display device, polarizing plate, and polarizer protective film

Info

Publication number: WO2013080949A1
Application number: PCT/JP2012/080562
Authority: WO
Inventors: 村田　浩一; 佐々木　靖
Original assignee: 東洋紡株式会社
Priority date: 2011-11-29
Filing date: 2012-11-27
Publication date: 2013-06-06
Also published as: JP6179548B2; JP5804079B2; KR20140097413A; CN103959148A; KR101737679B1; TW201329569A; JP2015166875A; CN103959148B; JP2017215596A; JP2019174822A; JPWO2013080949A1; TWI515485B; JP2017201417A; JP6761381B2

Abstract

The purpose of the present invention is to further suppress the occurrence of spectral irregularities in a liquid crystal display device that uses a light emitting diode as a light source, and uses an oriented polyester film having a given retardation as a polarizer protective film.　This liquid crystal display device includes a backlight light source, two polarizing plates, and a liquid crystal cell disposed between the two polarizing plates. The backlight light source is a white light source having a continuous emission spectrum. The two polarizing plates are each formed from a polarizer and protective films on both sides of the polarizer. At least one of the protective films on either side is an oriented polyester film having a retardation of 3000 to 30000 nm. The polarizing axis of the polarizer and the main orientation axis of the oriented polyester film, which is a protective film of the polarizer, are substantially parallel.

Description

Liquid crystal display device, polarizing plate and polarizer protective film

The present invention relates to a liquid crystal display device. Specifically, the present invention relates to a liquid crystal display device in which generation of rainbow spots is improved.

A polarizing plate used in a liquid crystal display device (LCD) is usually configured by sandwiching a polarizer obtained by dyeing iodine in polyvinyl alcohol (PVA) or the like between two polarizer protective films. In general, a triacetyl cellulose (TAC) film is used. In recent years, with the thinning of LCDs, there has been a demand for thinner polarizing plates. However, if the thickness of the TAC film used as a protective film is reduced for this purpose, sufficient mechanical strength cannot be obtained, moisture permeability increases, and the polarizer tends to deteriorate. Further, TAC films are very expensive, and there is a strong demand for inexpensive alternative materials.

Therefore, it has been proposed to use a polyester film instead of the TAC film so that the polarizing plate can be made thin so that high durability can be maintained even if the thickness is small as a polarizer protective film (Patent Documents 1 to 3). ).

JP 2002-116320 A JP 2004-219620 A JP 2004-205773 A

The polyester film is superior to the TAC film in durability, but unlike the TAC film, it has birefringence. Therefore, when it is used as a polarizer protective film, there is a problem that the image quality is deteriorated due to optical distortion. That is, since the polyester film having birefringence has a predetermined optical anisotropy (retardation), when used as a polarizer protective film, a rainbow-like color spot is generated when observed from an oblique direction, and the image quality is deteriorated. . Therefore, Patent Documents 1 to 3 take measures to reduce retardation by using a copolyester as the polyester. However, even in that case, the iridescent color spots could not be completely eliminated.

As a means for solving the above problems, the present inventors have found that a white light source having a continuous emission spectrum is used as a backlight light source, and an oriented polyester film having a certain retardation is used as a polarizer protective film. It was. However, the inventors have made further studies on the liquid crystal display device having such a configuration, and even in such an improved liquid crystal display device, polyester as a polarizer protective film is provided on both of the pair of polarizing plates. When the film is used, it has been rediscovered that, when observed from an oblique direction, rainbow spots may occur depending on the angle, and the problem of rainbow spots has not been completely solved.

That is, when a liquid crystal display device is industrially produced using a polarizing plate using a polyester film as a polarizer protective film, the direction of the polarization axis of the polarizer and the orientation main axis of the polyester film are usually perpendicular to each other. Be placed. This is because the polyvinyl alcohol film, which is a polarizer, is produced by uniaxial stretching, and the polyester film, which is a protective film, is usually produced by transverse stretching after longitudinal stretching. This is because the film orientation main axis direction is the horizontal direction, and when these long objects are bonded together to produce a polarizing plate, the polyester film orientation main axis and the polarizer polarization axis are usually perpendicular. In this case, by using an oriented polyester film having a specific retardation as a polyester film, and using a white light source having a continuous emission spectrum as a backlight light source, rainbow spots were greatly improved, but were observed from an oblique direction. Occasionally, it was rediscovered that, depending on the angle, thin rainbow spots were observed.

As a result of studying the above problem day and night, the present inventor made the polarization axis of the polarizer and the orientation main axis of the oriented polyester film (polarizer protective film) substantially parallel with respect to the two polarizing plates arranged on both sides of the liquid crystal. By doing so, it has been found that rainbow spots caused by the angle of viewing the liquid crystal display device are greatly reduced. The present invention has been completed as a result of further research and improvement based on such knowledge.

The representative present invention is as follows.
Item 1.
A liquid crystal display device having a backlight light source, two polarizing plates, and a liquid crystal cell disposed between the two polarizing plates,
The backlight source is a white light source having a continuous emission spectrum;
Each of the two polarizing plates comprises a polarizer and protective films on both sides thereof,
At least one of the protective films on both sides is an oriented polyester film having a retardation of 3000 to 30000 nm,
The polarizing axis of the polarizer and the orientation main axis of the oriented polyester film as the protective film are substantially parallel.
Liquid crystal display device.
Item 2.
Item 2. The liquid crystal display device according to item 1, wherein the ratio (Re / Rth) of retardation of the oriented polyester film to thickness direction retardation is 0.2 or more and 1.2 or less.
Item 3.
Item 3. The liquid crystal display device according to item 1 or 2, wherein the white light source having the continuous emission spectrum is a white light emitting diode.
Item 4.
The polyester film consists of three or more layers,
Contains a UV absorber in a layer other than the outermost layer,
The light transmittance at 380 nm is 20% or less,
Item 4. The liquid crystal display device according to any one of Items 1 to 3.

The liquid crystal display device, polarizing plate, and polarizer protective film of the present invention can obtain a spectrum of transmitted light that approximates the light source at any observation angle, and significantly suppress the occurrence of rainbow-like color spots. It is possible to ensure good visibility. Moreover, in suitable one Embodiment, the polarizer protective film of this invention is equipped with the mechanical strength suitable for thickness reduction.

Generally, a liquid crystal display device has a rear module, a liquid crystal cell, and a front module in order from the backlight light source side to the image display side (viewing side). The rear module and the front module are generally composed of a transparent substrate, a transparent conductive film formed on the liquid crystal cell side surface, and a polarizing plate disposed on the opposite side. Here, the polarizing plate is disposed on the backlight source side in the rear module, and is disposed on the image display side (viewing side) in the front module.

The liquid crystal display device of the present invention includes at least a backlight light source and a liquid crystal cell disposed between two polarizing plates as constituent members. Moreover, you may have suitably other structures other than these, for example, a color filter, a lens film, a diffusion sheet, an antireflection film etc. suitably.

The configuration of the backlight may be an edge light method using a light guide plate or a reflection plate as a constituent member, or may be a direct type, but in the present invention, it is a continuous light source for a liquid crystal display device. It is preferable to use a white light source having a broad emission spectrum. Here, the continuous and broad emission spectrum means an emission spectrum in which there is no wavelength at which the light intensity becomes zero in the wavelength region of at least 450 nm to 650 nm, preferably in the visible light region. Examples of such a white light source having a continuous and broad emission spectrum include a white light emitting diode (white LED). White LEDs include phosphors, that is, elements that emit white light by combining a phosphor that emits blue light or ultraviolet light using a compound semiconductor, and organic light-emitting diodes (Organic light-emitting diodes: OLEDs). Etc. are included. Examples of the phosphor include yttrium / aluminum / garnet yellow phosphor and terbium / aluminum / garnet yellow phosphor. In particular, white light-emitting diodes, which are composed of light-emitting elements that combine blue light-emitting diodes using compound semiconductors with yttrium, aluminum, and garnet-based yellow phosphors, have a continuous and broad emission spectrum and are also efficient in light emission. Since it is excellent, it is suitable as the backlight light source of the present invention. In addition, since the white LED with low power consumption can be widely used by the method of the present invention, it is possible to achieve an energy saving effect.

Fluorescent tubes such as cold cathode tubes and hot cathode tubes that have been widely used as backlight light sources in the past have only a discontinuous emission spectrum whose emission spectrum has a peak at a specific wavelength. Since it is difficult to obtain the effect of the period, it is not preferable.

The polarizing plate has a configuration in which both sides of a polarizer in which iodine is dyed on PVA or the like is sandwiched between two polarizer protective films, but the present invention is at least one of the polarizer protective films constituting the polarizing plate. A polyester film having a specific range of retardation is used.

The mechanism for suppressing the occurrence of rainbow-like color spots according to the above aspect is considered as follows.

When an oriented polyester film having birefringence is disposed on one side of the polarizer, the linearly polarized light emitted from the polarizer is disturbed when passing through the polyester film. The transmitted light shows an interference color peculiar to retardation which is a product of birefringence and thickness of the oriented polyester film. Therefore, if a discontinuous emission spectrum such as a cold cathode tube or a hot cathode tube is used as the light source, the transmitted light intensity varies depending on the wavelength, and a rainbow-like color spot is generated (see: 15th Micro Optical Conference Proceedings, No. 1). 30-31).

In contrast, white light emitting diodes usually have a continuous and broad emission spectrum in a wavelength region of at least 450 nm to 650 nm, preferably in the visible light region. Therefore, when attention is paid to the envelope shape of the interference color spectrum by the transmitted light transmitted through the birefringent body, it is possible to obtain a spectrum similar to the emission spectrum of the light source by controlling the retardation of the oriented polyester film. Thus, the emission spectrum of the light source and the envelope shape of the interference color spectrum by the transmitted light that has passed through the birefringent body are similar to each other, so that rainbow-like color spots do not occur and the visibility is remarkable. It is thought to improve.

As described above, by using a white light emitting diode having a wide emission spectrum as a light source, it becomes possible to approximate the envelope shape of the spectrum of transmitted light to the emission spectrum of the light source with only a relatively simple configuration.

In order to achieve the above effects, the oriented polyester film used for the polarizer protective film preferably has a retardation of 3000 to 30000 nm. When the retardation is less than 3000 nm, when used as a polarizer protective film, it exhibits a strong interference color when observed from an oblique direction, so the envelope shape is different from the emission spectrum of the light source, and good visibility cannot be ensured. . The preferred lower limit of retardation is 4500 nm, the next preferred lower limit is 5000 nm, the more preferred lower limit is 6000 nm, the still more preferred lower limit is 8000 nm, and the still more preferred lower limit is 10,000 nm.

On the other hand, the upper limit of retardation is 30000 nm. Even if an oriented polyester film having a retardation higher than that is used, not only a further improvement in visibility can be obtained, but also the thickness of the film is considerably increased, and the handling property as an industrial material is reduced. It is not preferable.

The retardation of the present invention can be obtained by measuring the refractive index and thickness in the biaxial direction, or by using a commercially available automatic birefringence measuring device such as KOBRA-21ADH (Oji Scientific Instruments). it can. In this specification, retardation means in-plane retardation.

In the present invention, at least one of the protective films provided on both sides of the polarizer is a polarizer protective film having the specific retardation. The polarizer protective film having the specific retardation is used for polarizing plates on both the incident light side (light source side) and the outgoing light side (viewing side). In the polarizing plate arranged on the incident light side, the polarizer protective film having the above specific retardation may be disposed on the incident light side from the polarizer, or on the liquid crystal cell side, on both sides. However, it is preferably arranged at least on the incident light side. For the polarizing plate disposed on the outgoing light side, the polarizer protective film having the specific retardation described above is disposed on both sides regardless of whether it is disposed on the liquid crystal side starting from the polarizer, or disposed on the outgoing light side. Although it may be arranged, it is preferably arranged at least on the outgoing light side. From the viewpoint of ensuring good polarization characteristics, the polarizer protective film on the incident light side of the polarizing plate arranged on the incident light side, and the polarizer protective film on the outgoing light side of the polarizing plate arranged on the outgoing light side It is preferable to use a polarizer protective film having the above specific retardation.

The polarizing plate of the present invention has a structure in which both sides of a known polarizer such as a film in which iodine is dyed on polyvinyl alcohol (PVA) or the like is sandwiched between two polarizer protective films, and at least any polarized light The child protective film is a polarizing plate protective film having the specific retardation. As the other polarizer protective film, it is preferable to use a film having no birefringence such as a TAC film, an acrylic film, and a norbornene-based film.

When oriented polyester films are used as the protective films on both sides of the polarizer, the orientation main axes of both oriented polyester films are preferably substantially parallel to each other.

In the liquid crystal display device of the present invention, the polarization axis of the polarizer and the orientation main axis of the oriented polyester film (polarizer protective film) are substantially parallel. Here, the term “substantially parallel” means that the angle formed by the polarization axis of the polarizer and the orientation main axis of the polarizer protective film is −15 ° to 15 °, preferably −10 ° to 10 °, more preferably −5 ° to It means 5 °, more preferably −3 ° to 3 °, still more preferably −2 ° to 2 °, and even more preferably −1 ° to 1 °. In a preferred embodiment, substantially parallel is substantially parallel. Here, “substantially parallel” means that the polarization axis and the alignment main axis are parallel to such an extent that a deviation inevitably generated when the polarizer and the protective film are bonded to each other is allowed. Although the mechanism has not yet been elucidated, the occurrence of rainbow spots on the liquid crystal display screen is suppressed by the fact that the polarization axes of the polarizers of the two polarizing plates and the orientation main axis of the oriented polyester film are substantially parallel. be able to. The direction of the orientation main axis can be determined by measuring with a molecular orientation meter (for example, MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments).

The polarizing plate in which the polarizer and the polarizer protective film satisfy the above relationship can be obtained, for example, by the following procedure. That is, the polarizer and the oriented polyester film can be cut to an appropriate size and bonded so that the polarizing axis of the polarizer and the orientation main axis of the oriented polyester film are substantially parallel. In addition, the polarization axis of the polarizer can be obtained by continuously laminating a long film of a polarizer film made of polyvinyl alcohol that has been stretched uniaxially and a long film of an oriented polyester film that has been stretched substantially uniaxially. It is also possible to produce a polarizing plate in which the main orientation axes of the oriented polyester film are substantially parallel.

The polarizing plate used in the present invention may be provided with various functional layers, that is, a hard coat layer, an antiglare layer, an antireflection layer, and the like on the oriented polyester surface for the purpose of preventing reflection, suppressing glare, and suppressing scratches. This is a preferred mode. When providing various functional layers, the oriented polyester film preferably has an easy adhesion layer on the surface thereof. At that time, from the viewpoint of suppressing interference due to reflected light, it is preferable to adjust the refractive index of the easy-adhesion layer so that it is close to the geometric mean of the refractive index of the functional layer and the refractive index of the oriented polyester film. The refractive index of the easy-adhesion layer can be adjusted by a known method. For example, the refractive index of the easy-adhesion layer can be easily adjusted by containing a binder resin with titanium, germanium, or other metal species.

The oriented polyester used in the present invention may be polyethylene terephthalate or polyethylene naphthalate, but may contain other copolymerization components. These resins are excellent in transparency and excellent in thermal and mechanical properties, and the retardation can be easily controlled by stretching. In particular, polyethylene terephthalate is the most suitable material because it has a large intrinsic birefringence and a large retardation can be obtained relatively easily even if the film is thin.

Also, for the purpose of suppressing deterioration of optical functional dyes such as iodine dyes, the protective film of the present invention desirably 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. The transmittance in the present invention is measured by a method perpendicular to the plane of the film, and can be measured using a spectrophotometer (for example, Hitachi U-3500 type).

In order to reduce the transmittance at a wavelength of 380 nm of the protective film of the present invention to 20% or less, it is achieved by adding an ultraviolet absorber in the film or by applying a coating solution containing the ultraviolet absorber on the film surface. It is desirable to appropriately adjust the type, concentration, and thickness of the ultraviolet absorber. 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 benzotriazole, 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 -3,4-benzoxazin-4-one, etc., but is not particularly limited thereto.

In addition to the ultraviolet absorber, it is also preferable to include various additives 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. Moreover, in order to show high transparency, it is also preferable that a polyester film does not contain a particle | grain substantially. “Substantially free of particles” means, for example, in the case of inorganic particles, when the inorganic element is quantified by fluorescent X-ray analysis, the content is 50 ppm or less, preferably 10 ppm or less, particularly preferably the detection limit or less. Means quantity.

Furthermore, the oriented polyester film of the present invention can be subjected to corona treatment, coating treatment, flame treatment or the like in order to improve the adhesion to the polarizer.

In the present invention, in order to improve the adhesion to the polarizer, at least one surface of the film of the present invention has an easy-adhesion layer mainly composed of at least one of a polyester resin, a polyurethane resin or a polyacrylic resin. Is preferred. 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 of the present invention is preferably an aqueous coating solution containing at least one of water-soluble or water-dispersible copolymerized polyester resin, acrylic resin, and 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.

The easy-adhesion layer can be obtained by applying the coating solution on one or both sides of a uniaxially stretched film in the longitudinal direction, drying at 100 to 150 ° C., and further stretching in the transverse direction. The final coating amount of the easy adhesion layer after drying 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 the resulting polarizer may be insufficient. On the other hand, when the coating amount exceeds 0.20 g / m ² , blocking resistance may be lowered. When providing an easily bonding layer on both surfaces of a polyester film, the application quantity of an easily bonding layer on both surfaces may be the same or different, and can be independently set within the above range.

It is preferable to add particles to the easy-adhesion layer in order to impart slipperiness. It is preferable to use particles having an average particle size of 2 μm or less. When the average particle diameter of the particles exceeds 2 μm, the particles easily fall off from the coating layer. As particles to be included in the easy adhesion layer, for example, titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, Examples include inorganic particles such as calcium fluoride, and organic polymer particles such as styrene, acrylic, melamine, benzoguanamine, and silicone. These may be added alone to the easy-adhesion layer, or may be added in combination of two or more.

Further, as a method for applying the coating solution, a known method can be used. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. can be mentioned. Or it can carry out in combination.

The average particle size of the above particles is measured by the following method. Take a picture of the particles with a scanning electron microscope (SEM) and take the largest diameter of 300-500 particles (between the two most distant points) at a magnification such that the size of one smallest particle is 2-5 mm. Distance) is measured, and the average value is taken as the average particle diameter.

The oriented polyester film, which is the protective film of the present invention, can be manufactured according to a general polyester film manufacturing method. For example, the polyester resin is melted and the non-oriented polyester extruded and formed into a sheet shape is stretched in the longitudinal direction by utilizing the speed difference of the roll at a temperature equal to or higher than the glass transition temperature, and then stretched in the transverse direction by a tenter. The method of performing heat processing is mentioned.

The oriented polyester film of the present invention may be a uniaxially stretched film or a biaxially stretched film, but when the biaxially stretched film is used as a polarizer protective film, it is observed from directly above the film surface. Although rainbow-like color spots are not seen, rainbow-like color spots may be observed when observed from an oblique direction.

The cause of this phenomenon is that the biaxially stretched film is composed of refractive index ellipsoids having different refractive indexes in the running direction, the width direction, and the thickness direction, and the retardation becomes zero due to the light transmission direction inside the film ( This is because there is a direction in which the refractive index ellipsoid appears to be a perfect circle. Therefore, when the liquid crystal display screen is observed from a specific oblique direction, a point where the retardation becomes zero may be generated, and a rainbow-like color spot is generated concentrically around that point. When the angle from the position directly above the film surface (normal direction) to the position where the rainbow-like color spots are visible is θ, the angle θ increases as the birefringence in the film increases, and the rainbow-like color increases. Spots are difficult to see. The biaxially stretched film tends to reduce the angle θ, and therefore the uniaxially stretched film is more preferable because rainbow-like color spots are less visible.

However, a perfect uniaxial (uniaxial symmetry) film is not preferable because the mechanical strength in the direction orthogonal to the orientation direction is significantly reduced. The present invention has biaxiality (biaxial symmetry) in a range that does not substantially cause rainbow-like color spots or a range that does not cause rainbow-like color spots in a viewing angle range required for a liquid crystal display screen. It is preferable.

The present inventors specify the ratio of the retardation of the protective film (in-plane retardation) and the retardation in the thickness direction (Rth) as a means to suppress the occurrence of rainbow spots while maintaining the mechanical strength of the protective film. It was found that control was performed so as to be within the range. Thickness direction retardation means an average of retardation obtained by multiplying two birefringences ΔNxz and ΔNyz by film thickness d when the film is viewed from the cross section in the thickness direction. The smaller the difference between the in-plane retardation and the thickness direction retardation, the more isotropic the birefringence action due to the observation angle, and the smaller the change in retardation due to the observation angle. Therefore, it is considered that rainbow-like color spots due to the observation angle are less likely to occur.

The ratio of the retardation of the polyester film of the present invention to the retardation in the thickness direction (Re / Rth) is preferably 0.200 or more, more preferably 0.500 or more, and further preferably 0.600 or more. As the ratio of the retardation to the retardation in the thickness direction (Re / Rth) is larger, the birefringence action is more isotropic, and the occurrence of iridescent color spots due to the observation angle is less likely to occur. In a complete uniaxial (uniaxial symmetry) film, the ratio of the retardation to the retardation in the thickness direction (Re / Rth) is 2.0. However, as described above, the mechanical strength in the direction orthogonal to the orientation direction is significantly lowered as the film approaches a complete uniaxial (uniaxial symmetry) film.

On the other hand, the ratio of the retardation of the polyester film of the present invention to the retardation in the thickness direction (Re / Rth) is preferably 1.2 or less, more preferably 1.0 or less. In order to completely suppress the occurrence of rainbow-like color spots due to the observation angle, the ratio of the retardation to the thickness direction retardation (Re / Rth) does not have to be 2.0, and 1.2 or less is sufficient. is there. Even if the ratio is 1.0 or less, it is possible to satisfy the viewing angle characteristics (180 degrees left and right, 120 degrees up and down) required for the liquid crystal display device.

The film forming conditions of the polyester film of the present invention will be specifically described. The longitudinal stretching temperature and the transverse stretching temperature are preferably 80 to 130 ° C, particularly preferably 90 to 120 ° C. The longitudinal draw ratio is preferably 1.0 to 3.5 times, particularly preferably 1.0 to 3.0 times. The transverse draw ratio is preferably 2.5 to 6.0 times, and particularly preferably 3.0 to 5.5 times. In order to control the retardation within the above range, it is preferable to control the ratio of the longitudinal draw ratio and the transverse draw ratio. If the difference between the vertical and horizontal draw ratios is too small, it is difficult to increase the retardation, which is not preferable. Also, setting the stretching temperature low is a preferable measure for increasing the retardation. In the subsequent heat treatment, the treatment temperature is preferably from 100 to 250 ° C., particularly preferably from 180 to 245 ° C.

In order to suppress the fluctuation of retardation, it is preferable that the thickness unevenness of the film is small. Since the stretching temperature and the stretching ratio greatly affect the thickness variation of the film, it is necessary to optimize the film forming conditions from the viewpoint of the thickness variation. In particular, when the longitudinal draw ratio is lowered to increase the retardation, the value of the longitudinal thickness unevenness may be increased. Since there is a region where the vertical thickness unevenness becomes extremely high within a specific range of the draw ratio, it is desirable to set the film forming conditions outside this range.

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% or less. It is particularly preferred. The thickness unevenness of the film can be measured by any means. For example, a tape-like sample (length 3 m) continuous in the film flow direction is collected, and an electronic micrometer (Millitron) manufactured by Seiko EM Co., Ltd. 1240) or the like is used to measure the thickness at 100 points at a pitch of 1 cm, and the maximum thickness (dmax), minimum value (dmin), and average value (d) are determined. %) Can be calculated.
Thickness unevenness (%) = ((dmax−dmin) / d) × 100

As described above, the retardation of the film can be controlled within a specific range by appropriately setting the stretching ratio, the stretching temperature, and the thickness of the film. For example, it becomes easier to obtain a higher retardation as the stretching ratio between the longitudinal stretching and the lateral stretching is higher, the stretching temperature is lower, and the film is thicker. On the contrary, it becomes easier to obtain a lower retardation as the stretching ratio between the longitudinal stretching and the lateral stretching is lower, the stretching temperature is higher, and the film thickness is thinner. Moreover, the higher the stretching temperature and the lower the total stretching ratio, the easier it is to obtain a film having a lower ratio of retardation to thickness direction (Re / Rth). Conversely, the lower the stretching temperature and the higher the total stretching ratio, the easier it is to obtain a film with a higher ratio of retardation to thickness direction retardation (Re / Rth). The final film forming conditions must be set in consideration of the physical properties necessary for processing in addition to the retardation control.

The thickness of the oriented polyester film of the present invention is arbitrary, but it is preferably in the range of 15 to 300 μm, more preferably in the range of 15 to 200 μm. In principle, it is possible to obtain a retardation of 3000 nm or more even with a film having a thickness of less than 15 μm. However, in that case, the anisotropy of the mechanical properties of the film becomes remarkable, and it becomes easy to cause tearing, tearing, etc., and the practicality as an industrial material is remarkably lowered. A particularly preferable lower limit of the thickness is 25 μm. On the other hand, if the upper limit of the thickness of the polarizer protective film exceeds 300 μm, the thickness of the polarizing plate becomes too thick, which is not preferable. From the viewpoint of practicality as a polarizer protective film, the upper limit of the thickness is preferably 200 μm. A particularly preferable upper limit of the thickness is 100 μm, which is about the same as a general TAC film. Polyethylene terephthalate is preferable as the polyester used as the film substrate in order to control the retardation within the range of the present invention even in the above thickness range.

In addition, as a method of blending the ultraviolet absorber with the polyester film in the present invention, a known method can be used in combination. For example, a preliminarily kneaded extruder is used to blend the dried ultraviolet absorber and the polymer raw material. A master batch can be prepared and blended by, for example, a method of mixing the predetermined master batch and a polymer raw material during film formation. The addition weight of the ultraviolet absorber added to the film is preferably 0.3 to 1.5%, more preferably 0.4 to 1.0%.

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 mix it economically. As a condition for producing the master batch, it is preferable to use a kneading extruder and to extrude 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 residence time is 1 minute or less, uniform mixing of the ultraviolet absorber becomes difficult. At this time, if necessary, a stabilizer, a color tone adjusting agent, and an antistatic agent may be added.

In the present invention, it is preferable that the film has a multilayer structure of at least three layers, and an ultraviolet absorber is added to the intermediate layer of the film. A film having a three-layer structure containing an ultraviolet absorber in the intermediate layer can be specifically produced as follows. Polyester pellets alone for the outer layer, master batches containing UV absorbers for the intermediate layer and polyester pellets are mixed at a predetermined ratio, dried, and then supplied to a known melt laminating extruder, which is slit-shaped. 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.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a scope that can meet the gist of the present invention. These are all included in the technical scope of the present invention. In addition, the evaluation method of the physical property in the following examples is as follows.

(1) Film orientation principal axis The orientation principal axis direction of the film was determined using a molecular orientation meter (manufactured by Oji Scientific Instruments, MOA-6004 type molecular orientation meter).

(2) Retardation (Re)
Retardation is a parameter defined by the product (ΔNxy × d) of the biaxial refractive index anisotropy (ΔNxy = | Nx−Ny |) on the film and the film thickness d (nm). Yes, it is a scale showing optical isotropy and anisotropy. The biaxial refractive index anisotropy (ΔNxy) was determined by the following method. Using a molecular orientation meter (MOA-6004 type molecular orientation meter, manufactured by Oji Scientific Instruments Co., Ltd.), the orientation principal axis direction of the film is obtained and 4 cm × 2 cm so that the orientation principal axis direction is parallel to the long side of the measurement sample. A rectangle was cut out and used as a measurement sample. For this sample, the biaxial refractive index (Nx, Ny) perpendicular to each other and the refractive index (Nz) in the thickness direction were determined by an Abbe refractometer (Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm). The absolute value of the refractive index difference (| Nx−Ny |) of the axis was defined as the anisotropy (ΔNxy) of the refractive index. The thickness d (nm) of the film was measured using an electric micrometer (manufactured by Fine Reef, Millitron 1245D), and the unit was converted to nm. Retardation (Re) was determined from the product (ΔNxy × d) of refractive index anisotropy (ΔNxy) and film thickness d (nm).

(3) Thickness direction retardation (Rth)
Thickness direction retardation is obtained by multiplying two birefringences ΔNxz (= | Nx−Nz |) and ΔNyz (= | Ny−Nz |) by film thickness d when viewed from the cross section in the film thickness direction. It is a parameter which shows the average of retardation obtained. Thickness direction retardation (Rth) is calculated by calculating Nx, Ny, Nz and film thickness d (nm) by the same method as the measurement of retardation, and calculating an average value of (ΔNxz × d) and (ΔNyz × d). )

(4) 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 of each film is measured using an air layer as a standard. The transmittance was determined.

(5) Observation of rainbow spots Any one of polyester films 1 to 10 prepared by the method described later is attached to one side of a polarizer composed of PVA and iodine so that the polarization axis of the polarizer and the orientation axis of the film are perpendicular or parallel. Then, a TAC film (manufactured by FUJIFILM Corporation, thickness 80 μm) was attached to the opposite surface to prepare a polarizing plate. The obtained polarizing plate was placed on both sides of the liquid crystal so that each polarizing plate was in a crossed Nicols condition to produce a liquid crystal display device. At this time, each polarizing plate was arrange | positioned so that the said polyester film might become the other side (distal) with respect to a liquid crystal. As a light source of the liquid crystal display device, a white LED composed of a light emitting element in which a blue light emitting diode and a yttrium / aluminum / garnet yellow phosphor were combined was used as a light source (Nichia Chemical, NSPW500CS). Visual observation was performed from the front and oblique directions of the polarizing plate of such a liquid crystal display device, and the presence or absence of rainbow spots was determined as follows.

A: No iridescence from any direction.
B: When observed from an oblique direction, a thin rainbow can be observed depending on the angle.
C: Iris can be observed when observed from an oblique direction.
D: Iris can be observed when observed from the front and diagonal directions.

(6) Tear Strength The tear strength of each film was measured according to JIS P-8116 using an Elmendorf tear tester manufactured by Toyo Seiki Seisakusho. The tearing direction was performed so as to be parallel to the orientation main axis direction of the film, and was determined as follows.
○: Tear strength is 50 mN or more ×: Tear strength is less than 50 mN

(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. Subsequently, the pressure was raised and the esterification reaction was performed under the conditions of a gauge pressure of 0.34 MPa and 240 ° C., and then the esterification reaction vessel 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. Then, after 15 minutes, dispersion treatment was performed with a high-pressure disperser, and after 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction can and subjected to 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 and contained substantially no inert particles and internally precipitated particles. (Hereafter, abbreviated as PET (A).)

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

(Production Example 3-Adjustment of Adhesive Modification Coating Solution)
A transesterification reaction and a polycondensation reaction were carried out by a conventional method, and as a dicarboxylic acid component (based on the total dicarboxylic acid component) 46 mol% terephthalic acid, 46 mol% isophthalic acid and 8 mol% sodium 5-sulfonatoisophthalate, A water-dispersible sulfonic acid metal base-containing copolymer polyester resin having a composition of 50 mol% ethylene glycol and 50 mol% neopentyl glycol as a glycol component (based on the entire glycol component) was prepared. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, 0.06 parts by mass of a nonionic surfactant were mixed and then heated and stirred. After adding 5 parts by mass of a water-dispersible sulfonic acid metal base-containing copolymer polyester resin and continuing to stir until the resin is no longer agglomerated, the resin water dispersion is cooled to room temperature to obtain a solid content concentration of 5.0% by mass. A uniform water-dispersible copolymerized polyester resin liquid was obtained. Furthermore, after dispersing 3 parts by mass of aggregated silica particles (Silicia 310, manufactured by Fuji Silysia Co., Ltd.) in 50 parts by mass of water, 99.46 parts by mass of the water-dispersible copolyester resin solution was mixed with 99.46 parts by mass of Silicia 310. 0.54 parts by mass of the aqueous dispersion was added, and 20 parts by mass of water was added with stirring to obtain an adhesive modified coating solution.

(Polarizer protective film 1)
After drying 90 parts by mass of PET (A) resin pellets containing no particles as a raw material for the base film intermediate layer and 10 parts by mass of PET (B) resin pellets containing an ultraviolet absorber at 135 ° C. for 6 hours under reduced pressure (1 Torr) , And supplied to the extruder 2 (for the intermediate layer II layer). Also, the PET (A) was dried by an ordinary method and supplied to the extruder 1 (for the outer layer I layer and the 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 thickness ratio of the I layer, the II layer, and the III layer was 10:80:10.

Next, after applying the adhesive property-modifying coating solution on the both sides of this unstretched PET film by a reverse roll method so that the coating amount after drying was 0.08 g / m ² , the coating was dried at 80 ° C. for 20 seconds. .

The unstretched film on which this coating layer was formed was guided to a tenter stretching machine, guided to a hot air zone at a temperature of 125 ° C. while being gripped by a clip, and stretched 4.0 times in the width direction. Next, while maintaining the width stretched in the width direction, the film was treated at a temperature of 225 ° C. for 30 seconds and further subjected to a relaxation treatment of 3% in the width direction to obtain a uniaxially oriented PET film having a film thickness of about 50 μm.

(Polarizer protective film 2)
By changing the thickness of the unstretched film, a uniaxially oriented PET film was obtained in the same manner as the polarizer protective film 1 except that the thickness was about 100 μm.

(Polarizer protective film 3)
The unstretched film on which the coating layer produced by the same method as that of the polarizer protective film 1 is formed is heated to 105 ° C. using a heated roll group and an infrared heater, and then a roll group having a difference in peripheral speed. After stretching 1.5 times in the running direction, the film was stretched 4.0 times in the width direction in the same manner as the polarizer protective film 1 to obtain a biaxially oriented PET film having a film thickness of about 50 μm.

(Polarizer protective film 4)
In the same manner as for the polarizer protective film 3, the film was stretched 2.0 times in the running direction and 4.0 times in the width direction to obtain a biaxially oriented PET film having a film thickness of about 50 μm.

(Polarizer protective film 5)
A uniaxially oriented PET film having a film thickness of 50 μm was obtained in the same manner as in the polarizer protective film 1 without using a PET resin (B) containing an ultraviolet absorber in the intermediate layer.

(Polarizer protective film 6)
In the same manner as the polarizer protective film 3, the film was stretched 4.0 times in the running direction and 1.0 times in the width direction to obtain a uniaxially oriented PET film having a film thickness of about 100 μm.

(Polarizer protective film 7)
In the same manner as for the polarizer protective film 1, the film was stretched 1.0 times in the running direction and 3.5 times in the width direction to obtain a uniaxially oriented PET film having a film thickness of about 75 μm.

(Polarizer protective film 8)
A uniaxially oriented PET film having a thickness of about 275 μm was obtained by changing the thickness of the unstretched film using the same method as for the polarizer protective film 1.

(Polarizer protective film 9)
In the same manner as for the polarizer protective film 3, the film was stretched 3.6 times in the running direction and 4.0 times in the width direction to obtain a biaxially oriented PET film having a film thickness of about 38 μm.

(Polarizer protective film 10)
A uniaxially oriented PET film having a thickness of about 10 μm was obtained by changing the thickness of the unstretched film using the same method as for the polarizer protective film 1.

(Polarizer protective film 11)
A uniaxially oriented PET film having a film thickness of 50 μm was obtained in the same manner as in the polarizer protective film 5 except that a single layer was used. Note that rainbow spots were observed using an OLED instead of a white LED composed of a light emitting element combining a blue light emitting diode and a yttrium / aluminum / garnet yellow phosphor as a light source of a liquid crystal display device.

Table 1 below shows the results of rainbow spot observation and tear strength measurement of the liquid crystal display devices produced using the polarizer protective films 1 to 11 as described above.

In Table 1, “relationship between orientation main axis and polarization axis” means the relationship between the polarization axis of the polarizer of the polarizing plate in the liquid crystal display device and the orientation main axis of the oriented polyester film used as the protective film. In Table 1, the polarizer protective film No. 1-1 shows the case where the polarizer protective film 1 is used as the polarizer protective film and the cold cathode tube is used as the light source. Polarizer protective film No. 1-2 is a polarizer protective film No. 1-2. A case where 1 is bonded to the polarizer so that the angle formed by the alignment main axis and the polarization axis of the polarizer is 8 ° (substantially parallel) is shown. Polarizer protective film No. 1-3 is a polarizer protective film No. 1-3. A case where 1 is bonded to the polarizer so that the angle formed by the alignment main axis and the polarization axis of the polarizer is 4 ° (substantially parallel) is shown.

As shown in Table 1, by using the polarizer protective films 1 to 8 or 11 and arranging the angle formed by the polarization axis of the polarizer of the polarizing plate and the orientation main axis of the oriented polyester film substantially in parallel, When viewing the screen of the liquid crystal display device from this angle, no rainbow spots were observed. On the other hand, even when the same polarizer protective films 1 to 8 and 11 are used, when the principal axis of orientation of the polyester film constituting the polarizing plate and the polarization axis of the polarizer are perpendicular to each other, the angle at which the screen of the liquid crystal display device is viewed Depending on the case, rainbow spots may occur. Further, when the polarizer protective film 9 or 10 was used or when a cold cathode tube was used as the light source, clear rainbow spots were observed when the screen of the liquid crystal display device was observed obliquely.

Claims

A liquid crystal display device having a backlight light source, two polarizing plates, and a liquid crystal cell disposed between the two polarizing plates,
The backlight source is a white light source having a continuous emission spectrum;
Each of the two polarizing plates comprises a polarizer and protective films on both sides thereof,
At least one of the protective films on both sides is an oriented polyester film having a retardation of 3000 to 30000 nm,
The polarizing axis of the polarizer and the orientation main axis of the oriented polyester film as the protective film are substantially parallel.
Liquid crystal display device.
The liquid crystal display device according to claim 1, wherein a ratio of the retardation of the oriented polyester film to the retardation in the thickness direction (Re / Rth) is 0.2 or more and 1.2 or less.
3. The liquid crystal display device according to claim 1, wherein the white light source having the continuous emission spectrum is a white light emitting diode.
The polyester film consists of three or more layers,
Contains a UV absorber in a layer other than the outermost layer,
The light transmittance at 380 nm is 20% or less,
The liquid crystal display device according to any one of claims 1 to 3.