TWI515485B - Liquid crystal display device, polarizing plate and polarizer protective film - Google Patents

Description

Liquid crystal display device, polarizing plate and polarizer protective film

The present invention relates to a liquid crystal display device. More specifically, it relates to a liquid crystal display device with improved generation of rainbow spots.

A polarizing plate used for a liquid crystal display device (LCD) is usually composed of two polarizing mirror protective films sandwiching a polarizing lens coated with iodine on polyvinyl alcohol (PVA) or the like, usually using triacetyl cellulose ( The TAC) film acts as a polarizer protective film. In recent years, with the thinning of LCDs, there has been a demand for thinning of polarizing plates. However, if the thickness of the TAC film used as the protective film is made thin, sufficient mechanical strength cannot be obtained, and the moisture permeability becomes high, and the polarizer is easily deteriorated. Also, TAC films are very expensive and are strongly pursuing inexpensive alternative materials.

Therefore, the thickness of the polarizing plate is reduced, and even if the thickness of the polarizer protective film is reduced, it is proposed to use a polyester film instead of the TAC film in order to maintain high durability (Patent Documents 1 to 3).

Prior technical literature Patent literature

Patent Document 1 Japanese Patent Laid-Open Publication No. 2002-116320

Patent Document 2 Japanese Patent Laid-Open Publication No. 2004-219620

Patent Document 3 Japanese Patent Laid-Open Publication No. 2004-205773

Compared with TAC film, polyester film has excellent durability, but because it has different birefringence from TAC film, when it is used as a polarizer When the film is protected, there is a problem that the image quality is lowered due to optical distortion. In other words, the polyester film having birefringence has a specified optical anisotropy (hysteresis value). When used as a polarizer protective film, if it is observed obliquely, a rainbow-like stain will occur, and the image quality will be observed. reduce. Therefore, in Patent Documents 1 to 3, measures for reducing the hysteresis value by using a copolyester as a polyester are carried out. However, even in this case, rainbow-like stains cannot be completely eliminated.

The present inventors have found that a white light source having a continuous luminescence spectrum is used as a backlight source, and an alignment polyester film having a certain hysteresis value is further used as a polarizer protective film as a means for solving the above problems. However, the inventors of the present invention have repeatedly conducted intensive studies on the liquid crystal display device having such a configuration, and found that even in such a modified liquid crystal display device, a polyester film is used as a polarizing plate for both of them as polarized light. In the case of a mirror protective film, when viewed from an oblique direction, there is a situation in which a rainbow spot is generated due to a different angle, and the problem of the rainbow spot is not completely solved.

That is, when a polarizing plate using a polyester film as a polarizing film protective film is used, when the liquid crystal display device is industrially produced, the directions of the polarizing axes of the polarizing mirror and the alignment main axis of the polyester film are generally arranged perpendicular to each other. This is because the polyvinyl alcohol film of the polarizer is manufactured by longitudinal uniaxial stretching, and the polyester film of the protective film is usually formed by extending in the longitudinal direction and then extending laterally, and the direction of the alignment direction of the polyester film is transverse, if it is laminated. When such a long strip is used to manufacture a polarizing plate, the alignment main axis of the polyester film and the polarizing axis of the polarizing mirror are usually perpendicular. It has been found that by using an alignment polyester film having a specific hysteresis value as a polyester film, a white light source having a continuous luminescence spectrum is used as the backlight. Although the source can greatly improve the rainbow spot, when viewed from an oblique direction, a faint rainbow spot is observed depending on the angle.

As a result of the discussion of the above-mentioned problems, the present inventors have found that the two polarizing plates disposed on both sides of the liquid crystal are slightly aligned by the alignment axis of the polarizing mirror and the alignment polyester film (polarizing film protective film). Parallel, the rainbow spots generated by the angle of viewing the liquid crystal display device are greatly reduced. The present invention is based on this knowledge and is an invention that is continuously researched and improved.

Representative inventions are as follows.

Item 1. A liquid crystal display device comprising a backlight source, two polarizing plates, and a liquid crystal display device disposed between the two polarizing plates, wherein the backlight source is a white light source having a continuous emission spectrum. The two polarizing plates include respective polarizing mirrors and protective films on both sides thereof, and at least one of the protective films on both sides has an alignment polyester film having a hysteresis value of 3,000 to 30,000 nm, and the polarizing axis of the polarizing mirror and the protective film thereof The alignment main axis of the alignment polyester film is slightly parallel.

The liquid crystal display device according to Item 1, wherein the ratio (Re/Rth) of the retardation value to the thickness direction hysteresis value of the alignment polyester film is 0.2 or more and 1.2 or less.

The liquid crystal display device according to any one of items 1 to 2, wherein the white light source having a continuous light emission spectrum is a white light emitting diode.

The liquid crystal display device according to any one of claims 1 to 3, wherein the polyester film contains three or more layers, and an ultraviolet absorber is contained in a layer other than the outermost layer, and a light transmittance of 380 nm is 20%. the following.

According to the liquid crystal display device, the polarizing plate and the polarizer protective film of the present invention, even at any observation angle, the spectrum of the transmitted light can obtain a spectrum close to the light source, thereby ensuring good visibility of deliberate suppression of rainbow spot generation. . Further, in a preferred embodiment, the polarizer protective film of the present invention has mechanical strength suitable for thinning.

[Formation for carrying out the invention]

In general, the liquid crystal display device has a rear module, a liquid crystal cell, and a front module in the order from the backlight source side toward the side of the display image (visual identification 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 display image side (visual identification side) in the front module.

The liquid crystal display device of the present invention includes at least a backlight source and a liquid crystal cell disposed between the two polarizing plates as a constituent member. Further, other configurations than those described above may be appropriately employed, such as a color filter, a lens film, a diffusion sheet, an antireflection film, and the like.

The configuration of the backlight may be an edge light method in which a light guide plate or a reflection plate is used as a constituent member, or may be a direct type. However, in the present invention, it is preferable to use a continuous wide-angle luminescence spectrum. The white light source serves as a backlight source of the liquid crystal display device. The term "continuous broad-spectrum luminescence spectrum" as used herein means a luminescence in a wavelength region of at least 450 nm to 650 nm, preferably in the region of visible light, in the absence of a wavelength of zero light intensity. spectrum. As a white light source having such a continuous wide-spectrum luminescence spectrum, for example, a white light-emitting diode (white LED) can be cited. The white LED includes a phosphor type, that is, an element that emits white by combining a light-emitting diode or a phosphor that emits blue light or ultraviolet light using a compound semiconductor, and an organic light-emitting diode (Organic light-emitting) Diode: OLED). In terms of phosphors, there are yellow phosphors of 钇, aluminium, and garnet, or yellow phosphors of enamel, garnet, and garnet. Among them, a white light-emitting diode comprising a light-emitting element in which a blue light-emitting diode using a compound semiconductor and a yellow light-emitting body of a yttrium-aluminum-garnet-based yellow phosphor is used, since not only has a continuous wide-spectrum luminescence spectrum, but also The light-emitting efficiency is also excellent, so it is suitable as the backlight source of the present invention. Moreover, according to the method of the present invention, since the white LED having low power consumption can be widely used, the effect of energy saving can be exhibited.

Since the luminescence spectrum of a fluorescent tube such as a cold cathode tube or a hot cathode tube which has been widely used as a backlight source has only a discontinuous luminescence spectrum having a peak at a specific wavelength, it is difficult to obtain the desired effect of the present invention, and thus it is not preferable. .

The polarizing plate has a configuration in which two polarizing mirror protective films are sandwiched between two sides of a PVA or the like which are dyed with iodine, but the present invention is characterized in that a polyester film having a specific range of hysteresis value is used as a constituent polarizing film. At least one of the polarizer protective film of the plate.

The mechanism for suppressing the occurrence of rainbow streaks by the above-described aspects is conceived as follows.

When an aligning polyester film having birefringence is disposed on one side of the polarizer, linear polarized light emitted by the polarizer may be emitted when passing through the polyester film. Scattered. The transmitted light shows a characteristic interference color in terms of the hysteresis value of the product of the birefringence and the thickness of the polyester film. Therefore, if a discontinuous luminescence spectrum such as a cold cathode tube or a hot cathode tube is used as a light source, different penetration light intensities are displayed depending on the wavelength, and a rainbow-like color spot is generated (reference: 15th micro-optical conference (Microoptics) Conference), drafts, 30-31).

On the other hand, in the white light-emitting diode, a continuous broad-spectrum light-emitting spectrum is usually obtained in a wavelength region of at least 450 nm to 650 nm, preferably in a visible light region. Therefore, if attention is paid to the envelope curve shape of the interference color spectrum caused by the penetrating light passing through the birefringent body, the spectrum similar to the light emission spectrum of the light source can be obtained by controlling the hysteresis value of the polyester film. As described above, it is considered that since the envelope curve shape of the interference color spectrum caused by the luminescence spectrum of the light source and the penetrating light penetrating the birefringent body is similar, the rainbow-like color patch does not occur and the vision is remarkably improved. Identification.

As described above, by using a white light-emitting diode having a wide-amplitude luminescence spectrum for a light source, the shape of the envelope curve of the spectrum of the transmitted light can be approximated to the luminescence spectrum of the light source with only a relatively simple configuration.

In order to exert the above effects, the alignment polyester film used for the polarizer protective film preferably has a hysteresis value of 3,000 to 30,000 nm. When the hysteresis value is less than 3000 nm, when it is used as a polarizer protective film, since it exhibits a strong interference color when viewed obliquely, the shape of the envelope curve is different from that of the light source, and good visibility cannot be ensured. Preferably, the lower limit value of the hysteresis value is 4500 nm, the second preferred lower limit value is 5000 nm, the more preferred lower limit value is 6000 nm, and the still further lower limit value is 8000 nm, and further preferably a lower limit value is further preferred. 10000nm.

On the other hand, the upper limit of the hysteresis value is 30000 nm. Even if an alignment polyester film having a hysteresis value exceeding the hysteresis value is used, not only is the effect of improving further visibility not substantially obtained, but also the thickness of the film is considerably thick, and the workability as an industrial material is lowered, which is not preferable. .

Further, the hysteresis value of the present invention can be determined by measuring the refractive index and the thickness in the biaxial direction, and can also be obtained by using a commercially available automatic birefringence measuring device of KOBRA-21ADH (Oji Scientific Instruments Co., Ltd.). In the present specification, the hysteresis value means the in-plane hysteresis value.

The present invention is characterized in that at least one of the protective films provided on both sides of the polarizer has a polarizer protective film having the above specific hysteresis value. The polarizer protective film having the specific hysteresis value is used for a polarizing plate on both the incident light side (light source side) and the outgoing light side (visual identification side). In the polarizing plate disposed on the incident light side, the polarizer protective film having the specific hysteresis value may be disposed on the incident light side as the starting point of the polarizing mirror, or may be disposed on the liquid crystal cell side, and may be disposed on both sides. Preferably, it is disposed at least on the incident light side. In the polarizing plate disposed on the light-emitting side, the polarizer protective film having a specific hysteresis value may be disposed on the liquid crystal side as the starting point of the polarizing mirror, or may be disposed on the outgoing light side, and may be disposed on both sides. To be disposed at least on the outgoing light side. From the viewpoint of ensuring good polarization characteristics, it is preferable to use a polarizer protective film having the above specific hysteresis value for a polarizer protective film disposed on the incident light side of the polarizing plate on the incident light side and a polarizing plate disposed on the outgoing light side. A polarizer protective film on the exiting light side.

The polarizing plate of the present invention is characterized in that it has a structure in which two polarizing mirrors are sandwiched between two polarizing mirror protective films, such as a film coated with iodine on polyvinyl alcohol (PVA) or the like, and at least one of which is polarized. Mirror protective film is A polarizing plate protective film having the above specific hysteresis value. It is preferable to use a film having no birefringence such as a TAC film or an acrylic film or a norbornene film for another polarizer protective film.

When the alignment polyester film is used as the protective film on both sides of the polarizer, it is preferred that the alignment main axes of the alignment polyester films on both sides are slightly parallel to each other.

In the liquid crystal display device of the present invention, the polarization axis of the polarizer is slightly parallel to the alignment main axis of the alignment polyester film (polarizer protective film). The term "slightly parallel" means that the angle between the polarizing axis of the polarizer and the alignment main axis of the polarizer protective film is -15° to 15°, preferably -10° to 10°, more preferably -5°. 5°, further preferably -3° to 3°, further preferably -2° to 2°, and more preferably -1° to 1°. In a preferred embodiment, the parallelism is substantially parallel. Here, the term "substantially parallel" means that the polarizing axis and the alignment main axis are parallel to the extent that the offset is inevitably generated when the polarizer and the protective film are bonded. Although the mechanism has not been clarified, the polarization axes of the polarizers of the two polarizing plates are slightly parallel to the alignment main axis of the alignment polyester film, so that generation of rainbow spots on the liquid crystal display screen can be suppressed. The direction of the alignment spindle can be determined by measurement using a molecular alignment meter (for example, a molecular alignment meter manufactured by Oji Scientific Co., Ltd., MOA-6004).

The polarizer and the polarizer protective film satisfy the polarizing plate as described above, and can be obtained, for example, by the following procedure. That is, the polarizer and the alignment polyester film can be cut to an appropriate size to fit the polarization axis of the polarizer and the alignment main axis of the alignment polyester film in a slightly parallel manner. Further, by continuously bonding a long strip of a polarizing film comprising a longitudinally uniaxially extending polyvinyl alcohol to a long strip of an oriented polyester film extending substantially longitudinally and uniaxially, A polarizing plate in which the polarizing axis of the polarizer and the main alignment axis of the alignment polyester film are slightly parallel can be manufactured.

In the polarizing plate used in the present invention, various functional layers, that is, a hard coat layer, an antiglare layer, an antireflection layer, etc., are provided on the surface of the alignment polyester for the purpose of preventing reflection or suppressing glare, suppressing damage, and the like. The preferred aspect. When various functional layers are provided, the alignment polyester film preferably has an easy-adhesion layer on its surface. At this time, from the viewpoint of suppressing interference due to reflected light, it is preferable to adjust the refractive index of the easy-adhesion layer to the vicinity of the multiplication and average of the refractive index of the functional layer and the refractive index of the alignment polyester film. The refractive index of the easy-adhesion layer can be adjusted by a known method, for example, by making the adhesive resin contain titanium or bismuth, other metal species.

The alignment polyester used in the present invention may be polyethylene terephthalate or polyethylene naphthalate, and may contain other copolymerization components. These resins are excellent in transparency and excellent in heat and mechanical properties, and the hysteresis value can be easily controlled by extension processing. In particular, polyethylene terephthalate is the most suitable material because of its large intrinsic birefringence, and even if the thickness of the film is thin, it is relatively easy to obtain a high hysteresis value.

In addition, for the purpose of suppressing deterioration of the optical functional dye such as iodine dye, the light transmittance of the protective film of the present invention at a wavelength of 380 nm is preferably 20% or less. The light transmittance at 380 nm is more preferably 15% or less, further preferably 10% or less, and particularly preferably 5% or less. When the light transmittance is 20% or less, deterioration of the optical functional dye due to ultraviolet rays can be suppressed. Further, the transmittance in the present invention is measured by a method in which the plane of the film is perpendicular, and can be measured using a spectrophotometer (for example, Hitachi U-3500 type).

In order to make the transmittance of the protective film of the present invention at a wavelength of 380 nm 20% or less, it is possible to add an ultraviolet absorber to the film or apply a coating liquid containing the ultraviolet absorber to the surface of the film, and it is preferable to appropriately adjust the ultraviolet rays. The type and concentration of the absorbent and the thickness of the film. The ultraviolet absorber used in the present invention is a well-known substance. The ultraviolet absorber is an organic ultraviolet absorber and an inorganic ultraviolet absorber, and an organic ultraviolet absorber is preferred from the viewpoint of transparency. Examples of the organic ultraviolet absorber include a benzotriazole-based, a diphenylketone-based, a cyclic imido ester-based, and the like, and a combination thereof is not particularly limited as long as it is within the range of the absorbance specified in the present invention. . However, from the viewpoint of durability, a benzotriazole-based or cyclic imido ester-based system is particularly preferred. When two or more types of ultraviolet absorbers are used in combination, since ultraviolet rays of respective wavelengths can be simultaneously absorbed, the ultraviolet absorption effect can be further improved.

Examples of the diphenylketone-based ultraviolet absorber, the benzotriazole-based ultraviolet absorber, and the acrylonitrile-based ultraviolet absorber include 2-[2'-hydroxy-5'-(methacryloxyl group A). Phenyl]-2H-benzotriazole, 2-[2'-hydroxy-5'-(methacryloxyethyl)phenyl]-2H-benzotriazole, 2-[2'-hydroxy-5'-(methacryloxypropyl)phenyl]-2H-benzotriazole,2,2'-dihydroxy-4,4'-dimethoxydiphenyl ketone, 2 , 2',4,4'-tetrahydroxydiphenyl ketone, 2,4-di-t-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-(t-butyl)phenol, 2,2'-methylenebis(4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole- 2-yl)phenol, etc. Examples of the cyclic imido ester-based ultraviolet absorber include 2,2'-(1,4-phenylene)bis(4H-3,1-benzo). Keto-4-one), 2-methyl-3,1-benzo 4-ketone, 2-butyl-3,1-benzo 4-ketone, 2-phenyl-3,1-benzo 4-ketone and the like. However, it is not particularly limited to this.

Further, in addition to the ultraviolet absorber, it is preferable to contain various additives within a range that does not impair the effects of the present invention. Examples of the additive include inorganic particles, heat resistant polymer particles, alkali metal compounds, alkaline earth metal compounds, phosphorus compounds, antistatic agents, light stabilizers, flame retardants, heat stabilizers, antioxidants, and antigels. Chemical agent, surfactant, and the like. Further, in order to exhibit high transparency, it is preferred that the polyester film contains substantially no particles. The term "substantially free of particles" means, for example, inorganic particles. When the inorganic element is quantified by fluorescent X-ray analysis, the weight is 50 ppm or less, preferably 10 ppm or less, and particularly preferably the detection limit. The following content.

Further, in order to improve the adhesion to the polarizer, the alignment polyester film of the present invention may be subjected to corona treatment, coating treatment, flame treatment or the like.

In the present invention, in order to improve the adhesion to the polarizer, it is preferred that at least one of the polyester resin, the polyurethane resin or the polyacrylic resin is mainly composed of at least one side of the film of the present invention. Easy to adhere to the layer. Here, the "main component" means a component of 50% by mass or more of the solid component constituting the easy-adhesion layer. The coating liquid used for the formation of the easy-adhesion layer of the present invention is preferably an aqueous coating liquid containing at least one of a water-soluble or water-dispersible copolymerized polyester resin, an acrylic resin, and a polyurethane resin. Examples of the coating liquids include, for example, Japanese Patent No. 3567927 and Japanese Patent No. 3589232. Water-soluble or water-dispersible copolyester resin solution, acrylic resin solution, polyurethane resin disclosed in Japanese Patent No. 3589233, Japanese Patent No. 3900191, Japanese Patent No. 4150982, and the like. Solution, etc.

The easy-adhesion layer is obtained by coating the coating liquid on one or both sides of the longitudinal uniaxially stretched film, drying at 100 to 150 ° C, and extending in the lateral direction. The coating amount of the final easy-adhesion layer after drying is preferably managed at 0.05 to 0.20 g/m2. When the coating amount is less than 0.05 g/m 2 , the adhesion to the obtained polarizer may be insufficient. On the other hand, when the coating amount exceeds 0.20 g/m 2 , the anti-blocking property may be lowered. When the easy-adhesion layer is provided on both sides of the polyester film, the coating amounts of the easy-adhesion layers on both sides may be the same or different, and may be independently set within the above range.

In order to impart slipperiness to the easy-adhesion layer, it is preferred to add particles. It is preferred to use particles having an average particle diameter of 2 μm or less of fine particles. When the average particle diameter of the particles exceeds 2 μm, the particles are likely to fall off from the coating layer. Examples of the particles contained in the easy-adhesion layer include titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, vermiculite, alumina, talc, kaolin, clay, calcium phosphate, mica, lithium bentonite, and zirconia. Inorganic particles such as tungsten oxide, lithium fluoride or calcium fluoride, or organic polymer particles such as styrene, acrylic, melamine, benzoguanamine or polyoxyn. These may be added separately to the easy-adhesion layer, or two or more types may be combined and added.

Further, as a method of applying the coating liquid, a well-known method can be used. For example, a reverse roll coating method, a gravure roll coating method, a roll coating method, a roll brush method, a spray coating method, an air knife coating method, a wire rod coating method, a tube scraping method, etc. may be mentioned, and these methods may be used alone or in combination. get on.

Further, the measurement of the average particle diameter of the above particles was carried out by the following method. A photograph of the particles was taken by a scanning electron microscope (SEM), and the maximum diameter of 300 to 500 particles (distance between the farthest points) was measured with a minimum particle size of 2 to 5 mm. The value is taken as the average particle diameter.

The alignment polyester film of the protective film of the present invention can be produced by following the method of producing a general polyester film. For example, an unaligned polyester obtained by melting a polyester resin and extruding it into a sheet shape may be used, and at a temperature equal to or higher than the glass transition temperature, the speed difference in the roll is extended in the longitudinal direction, and then the tenter is used. The method of laterally extending and applying heat treatment.

The alignment polyester film of the present invention may be a uniaxially stretched film, and may be a biaxially stretched film. However, when a biaxially stretched film is used as a polarizing film protective film, even if it is observed from directly above the film surface, no rainbow is observed. It is a kind of stain, but it must be noticed because it observes a rainbow-like stain when it is observed from an oblique direction.

The reason for this phenomenon is that the biaxially stretched film contains refractive index ellipsoids having different refractive indices in the traveling direction, the width direction, and the thickness direction, and the hysteresis value becomes different as the direction of light penetration inside the film is different. The direction of zero (refractive index ellipsoid looks like a true circle). Therefore, if the liquid crystal display screen is viewed from a specific direction in the oblique direction, there is a case where the hysteresis value becomes zero, and the rainbow-like color spots are concentrically formed around the point. Further, if the angle from the directly above (normal direction) of the film surface to the position at which the rainbow-colored stain is observed is θ, the larger the birefringence in the film surface, the larger the angle θ becomes. Rainbow-like stains will become more difficult to see. In the case of biaxially stretched films, due to the angle The degree θ tends to be small, and it is preferable that the uniaxially stretched film becomes difficult to see a rainbow-like color patch.

However, in the case of a completely uniaxial (uniaxial symmetry) film, the mechanical strength in the direction orthogonal to the alignment direction is remarkably lowered, which is not preferable. The present invention preferably has biaxiality (biaxial symmetry) in a range in which rainbow-like color spots do not substantially occur or in a range in which a rainbow-colored color patch does not occur in a viewing angle range required for a liquid crystal display screen.

The inventors of the present invention have found that the mechanical strength of the protective film is maintained while suppressing the rainbow spot such that the ratio of the hysteresis value (in-plane hysteresis value) of the protective film to the hysteresis value (Rth) in the thickness direction converges within a specific range. The means of occurrence. The phase difference in the thickness direction means an average of the phase differences obtained by multiplying the two birefringences ΔNxz and ΔNyz of the film by the film thickness d from the thickness direction cross section. Since the difference between the in-plane hysteresis value and the thickness direction hysteresis value is smaller, the effect of birefringence caused by the different observation angles will increase the isotropic direction more and more, so the change of the hysteresis value due to the difference in the observation angle is smaller. Therefore, it is considered that rainbow-colored spots caused by different viewing angles may become difficult to produce.

The ratio (Re/Rth) of the hysteresis value to the thickness direction retardation value of the polyester film of the present invention is preferably 0.200 or more, more preferably 0.500 or more, still more preferably 0.600 or more. The greater the contrast between the hysteresis value and the thickness direction hysteresis value (Re/Rth), the more the birefringence will increase the isotropic direction, and the generation of rainbow-like stains caused by the different observation angles will become more difficult. Further, in the case of the completely uniaxial (uniaxial symmetry) film, the ratio of the hysteresis value to the thickness direction hysteresis value (Re/Rth) was 2.0. However, as described above, with an approximately completely uniaxial (uniaxial symmetry) film, the mechanical strength in a direction orthogonal to the alignment direction is remarkably lowered.

On the other hand, the ratio (Re/Rth) of the hysteresis value to the thickness direction hysteresis value of the polyester film of the present invention is preferably 1.2 or less, more preferably 1.0 or less. In order to completely suppress the generation of the rainbow-like color unevenness caused by the difference in the observation angle, the ratio of the retardation value to the thickness direction phase difference (Re/Rth) need not be 2.0, and it is sufficient to be 1.2 or less. Moreover, even if the ratio is 1.0 or less, the viewing angle characteristics required by the liquid crystal display device (about 180 degrees left and right and 120 degrees above and below) can be sufficiently satisfied.

When the film forming conditions of the polyester film of the present invention are specifically described, the longitudinal stretching temperature and the lateral stretching temperature are preferably 80 to 130 ° C, particularly preferably 90 to 120 ° C. The longitudinal stretching ratio is preferably 1.0 to 3.5 times, and particularly preferably 1.0 to 3.0 times. Further, the lateral stretching ratio is preferably 2.5 to 6.0 times, and particularly preferably 3.0 to 5.5 times. In order to control the hysteresis value within the above range, it is more preferable to control the ratio of the longitudinal stretching ratio to the lateral stretching ratio. If the difference between the vertical and horizontal stretching ratios is too small, it becomes difficult to increase the hysteresis value. Further, in terms of increasing the hysteresis value, setting a low extension temperature is also a preferable correspondence. 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 variation of the hysteresis value, it is preferable that the thickness unevenness of the film is small. Since the stretching temperature and the stretching ratio have a large influence on the thickness unevenness of the film, it is necessary to optimize the film forming conditions from the viewpoint of thickness unevenness. In particular, if the longitudinal stretching ratio is lowered in order to increase the hysteresis value, the value of the longitudinal thickness unevenness may become high. Since there is a region in which the longitudinal thickness unevenness becomes extremely high in a certain range of the stretching ratio, it is preferable to set the film forming conditions outside the 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 particularly preferably 3.0% or less. The thickness unevenness of the film can be measured by any means, for example, a continuous strip sample (length 3 m) is taken along the flow direction of the film, and a measuring machine such as an SEIKO‧EM (electronic) micrometer (Millitron 1240) is used. The thickness of 100 points was measured at intervals of 1 cm, and the maximum value (dmax), the minimum value (dmin), and the average value (d) of the thickness were determined, and the thickness unevenness (%) was calculated by the following formula.

Uneven thickness (%) = ((dmax-dmin) / d) × 100

As described above, in order to control the hysteresis value of the film to a specific range, it can be carried out by appropriately setting the stretching ratio or the stretching temperature and the thickness of the film. For example, the higher the stretching ratio of the longitudinal extension and the lateral extension, the lower the extension temperature, and the thicker the film, the easier it is to obtain a high hysteresis value. On the contrary, the lower the difference between the stretching ratio of the longitudinal extension and the lateral extension, the higher the extension temperature, and the thinner the thickness of the film, the easier it is to obtain a low hysteresis value. Further, the higher the elongation temperature and the lower the total stretching ratio, the easier it is to obtain a film having a low ratio of hysteresis value to retardation value (Re/Rth). On the contrary, the lower the extension temperature is, the higher the total stretching ratio is, the easier it is to obtain a film having a higher ratio of hysteresis value to the retardation value in the thickness direction (Re/Rth). Further, in addition to the control of the hysteresis value, the final film forming conditions must be set in consideration of physical properties required for processing.

The alignment polyester film of the present invention has an arbitrary thickness, but preferably has a range of 15 to 300 μm, more preferably 15 to 200 μm. Even in the case of a film having a thickness of less than 15 μm, a hysteresis value of 3000 nm or more can be obtained in principle. However, in this case, the directionality of the mechanical properties of the film becomes remarkable, and cracks, breakage, and the like are likely to occur, as industrial materials. Practicality will be significantly reduced. A particularly preferred lower limit of 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 may become too thick. The upper limit of the thickness is preferably 200 μm from the viewpoint of practicality as a polarizer protective film. The upper limit of the particularly preferable thickness is 100 μm which is equivalent to that of a general TAC film. In the above thickness range, in order to control the hysteresis value within the scope of the present invention, polyethylene terephthalate which is used as a film substrate is also suitable.

Further, as for the method of blending the ultraviolet absorber in the polyester film of the present invention, a well-known method may be used in combination, for example, by blending a dried ultraviolet absorber by using a kneading extruder in advance. A masterbatch is prepared from a polymer raw material, and a predetermined method of mixing the master batch with a polymer raw material at the time of film formation is blended. The addition weight of the ultraviolet absorber added to the film is preferably from 0.3 to 1.5%, more preferably from 0.4 to 1.0%.

In order to uniformly disperse the ultraviolet absorber and economically blend, it is preferable to set the concentration of the ultraviolet absorber of the master batch to a concentration of 5 to 30% by mass. The conditions for producing the master batch are preferably from 1 to 15 minutes at a temperature at which the extrusion temperature is 290 ° C or more above the melting point of the polyester material, using a kneading extruder. At 290 ° C or higher, the amount of reduction of the ultraviolet absorber becomes large, and the viscosity of the master batch decreases. When the residence time is 1 minute or less, uniform mixing of the ultraviolet absorber becomes difficult. At this time, a stabilizer, a color tone adjuster, and an antistatic agent may be added as needed.

Further, in the present invention, it is preferred that the film has a multilayer structure of at least three or more 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, specifically Make it as follows. The granules of the individual polyesters for the outer layer, the masterbatch containing the ultraviolet absorbing agent for the intermediate layer, and the granules of the polyester are mixed and dried in a predetermined ratio, and then supplied to a known extruder for melt lamination. The slit-shaped die was extruded into a sheet shape, and cooled and solidified on a casting roll to produce an unstretched film. That is, two or more extruders, three-layer collecting tubes, or confluent sections (for example, confluent sections having rectangular confluences) are used, and a thin film layer constituting the two outer layers and a thin film layer constituting the intermediate layer are laminated, and the metal nozzle is used. Three sheets of the sheet were extruded and cooled on a casting roll to produce an unstretched film. Further, in the present invention, in order to remove foreign matter contained in the polyester of the raw material which causes optical defects, it is preferred to perform high-precision filtration at the time of melt extrusion. 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, the removal of foreign matter of 20 μm or more is likely to be insufficient.

[Examples]

In the following, the present invention will be more specifically described by the following examples, but the present invention is not limited by the following examples, and may be appropriately modified and implemented within the scope of the present invention. The technical scope of the present invention. Further, the evaluation methods of the physical properties in the following examples are as follows.

(1) Film alignment spindle

The direction of the alignment main axis of the film was obtained by using a molecular alignment meter (manufactured by Oji Scientific Instruments Co., Ltd., MOA-6004 molecular alignment meter).

(2) Hysteresis value (Re)

The so-called hysteresis value is defined by the product of the anisotropy (ΔNxy=|Nx-Ny|) of the orthogonal biaxial refractive index on the film and the film thickness d (nm) (ΔNxy×d). The parameters are also the scales showing the isotropy and anisotropy of optics. The anisotropy (ΔNxy) of the biaxial refractive index is obtained by the following method. Using a molecular alignment meter (manufactured by Oji Scientific Instruments Co., Ltd., MOA-6004 molecular alignment meter), the direction of the alignment main axis of the film was obtained, and the direction of the alignment main axis was cut in parallel with the long side of the measurement sample to cut 4 cm × 2 cm. A rectangle is used as a sample for measurement. With respect to this sample, an orthogonal biaxial refractive index (Nx, Ny) and a refractive index (Nz) in the thickness direction were obtained by an Abbe refractometer (NAR-4T, manufactured by ATAGO Co., Ltd., measuring wavelength: 589 nm). The absolute value (|Nx-Ny|) of the above-described biaxial refractive index difference is taken as the anisotropy (ΔNxy) of the refractive index. The thickness d (nm) of the film was measured using an electric micrometer (Millitron 1245D, manufactured by FEINPRUF Co., Ltd.), and the unit was converted into nm. The hysteresis value (Re) is obtained from the product of the anisotropy of the refractive index (ΔNxy) and the thickness d (nm) of the film (ΔNxy×d).

(3) Thickness direction hysteresis value (Rth)

The hysteresis value in the thickness direction is obtained by multiplying the two birefringences ΔNxz (|Nx-Nz|) and ΔNyz (|Ny-Nz|) by the film thickness d when viewed from the cross section of the film thickness direction. The average parameter of the hysteresis value. Nx, Ny, Nz and film thickness d (nm) were obtained by the same method as the measurement of hysteresis value, and the average value of (ΔNxz × d) and (ΔNyz × d) was calculated to obtain the thickness direction hysteresis value. (Rth).

(4) Light transmittance at a wavelength of 380 nm

Using a spectrophotometer (manufactured by Hitachi, Ltd., model U-3500), the light transmittance of each film at a wavelength of 300 to 500 nm was measured using an atmosphere as a standard, and the light transmittance at a wavelength of 380 nm was determined.

(5) Rainbow spot observation

On one side of the polarizer including PVA and iodine, one of the polyester films 1 to 10 produced by the method described later is adhered to the alignment axis of the polarizer perpendicularly or parallel to the alignment axis of the film. A TAC film (manufactured by FUJI FILM Co., Ltd., thickness: 80 μm) was adhered to the reverse surface to prepare a polarizing plate. The obtained polarizing plate was sandwiched between liquid crystals, and each of the polarizing plates was placed under the condition of crossed Nicols, and one piece was placed on each side to fabricate a liquid crystal display device. At this time, each of the polarizing plates is disposed such that the polyester film and the liquid crystal are on the opposite side (distal position). In the light source of the liquid crystal display device, a white LED including a light-emitting element in which a blue light-emitting diode and a yttrium aluminum garnet-based yellow phosphor are combined is used for a light source (Nichia Chemical Co., Ltd., NSPW500CS). From the front side and the oblique direction of the polarizing plate of the liquid crystal display device, the presence or absence of the rainbow spot was visually observed, and the determination was made as follows.

A: No rainbow spots occur in either direction.

B: When viewed from an oblique direction, a faint rainbow spot can be observed depending on the angle.

C: A rainbow spot can be observed when viewed obliquely.

D: Rainbow spots can be observed when viewed from the front and obliquely.

(6) tear strength

The tear strength of each film was measured using an Elmendorf tear tester manufactured by Toyo Seiki Seisakusho Co., Ltd., in accordance with JIS P-8116. The tearing direction is performed in parallel with the direction of the alignment axis of the film, and is determined as follows.

○: tear strength is 50mN or more

×: tear strength is less than 50mN

(Manufacturing Example 1 - Polyester A)

The temperature of the esterification reactor was raised, and 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged at a time point of reaching 200 ° C, and 0.017 parts by mass of antimony trioxide and 0.064 parts by mass were charged while stirring. Magnesium acetate tetrahydrate and 0.16 parts by mass of triethylamine were used as a catalyst. Subsequently, the temperature was raised under pressure, and the pressure esterification reaction was carried out under the conditions of a gauge pressure of 0.34 MPa and 240 ° C. Thereafter, the esterification reaction tank was returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. Further, the temperature was raised to 260 ° C over 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. Then, after 15 minutes, the dispersion treatment was carried out in a high-pressure disperser, and after 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction tank, and a polycondensation reaction was carried out at 280 ° C under reduced pressure.

After completion of the polycondensation reaction, the filter was treated with a 95%-blocking filter of NASLON filter having a diameter of 5 μm, extruded into a strand shape from a nozzle, and cooled by using a cooling water previously subjected to filtration treatment (pore diameter: 1 μm or less). Cured 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 internal precipitated particles (hereinafter abbreviated as PET (A)).

(Manufacturing Example 2 - Polyester B)

Mix 10 parts by mass of dried UV absorber (2,2'-(1,4-phenylene) bis(4H-3,1-benzo) Keto-4-ketone), 90 parts by mass of PET-free (A) (inherent viscosity: 0.62 dl/g), using a kneading extruder to obtain polyethylene terephthalate containing a UV absorber Resin (B) (hereinafter abbreviated as PET (B)).

(Production Example 3 - Preparation of Adhesive Modification Coating Liquid)

The transesterification reaction and the polycondensation reaction were carried out by a usual method to prepare 46 mol% of terephthalic acid, 46 mol% of isophthalic acid, and 8 mol of dibasic acid component (for the entire dibasic acid component). Water-dispersible sulfonic acid metal salt of sodium 5-sulfonate isophthalate, 50 mol% ethylene glycol and 50 mol% neopentyl glycol as a diol component (for the entire diol component) Base copolyester resin. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, and 0.06 parts by mass of a nonionic surfactant were mixed, and then heated and stirred. When 77 ° C was reached, 5 was added. The above-mentioned copolymerized polyester resin containing a water-dispersible sulfonic acid metal salt group is further stirred until the resin agglomerates disappear, and the aqueous resin dispersion liquid is cooled to normal temperature to obtain uniform water dispersibility of a solid content concentration of 5.0% by mass. Copolymerized polyester resin solution. Further, after dispersing 3 parts by mass of aggregated ceria particles (Sylysia 310 manufactured by FUJI SILYSIA Co., Ltd.) in 50 parts by mass of water, 0.54 parts by mass of the above water-dispersible copolymerized polyester resin liquid is added to 0.54 part by weight. A mass part of the aqueous dispersion of Sylysia 310 was added with 20 parts by mass of water while stirring to prepare an adhesive modified coating liquid.

(Polarizer protective film 1)

90 parts by mass of the particle-free PET (A) resin pellet and 10 parts by mass of the PET (B) resin pellet containing the ultraviolet absorber were used as a raw material for the base film intermediate layer, and dried under reduced pressure at 135 ° C ( After 1 hr), it is supplied to the extruder 2 (for the intermediate layer II layer), and the PET (A) is dried by a usual method, and each is supplied to the extruder 1 (for the outer layer I and the outer layer III). Melt at 285 °C. Each of the two types of polymers was filtered with a filter material of a stainless steel sintered body (nominal filtration accuracy of 10 μm particles 95%), and laminated by two types of three-layer joining sections, and extruded into a sheet shape from a metal nozzle, and then used. An electrostatic casting method was applied, and the film was wound on a casting drum having a back surface temperature of 30 ° C to be 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 10:80:10.

Subsequently, the above-mentioned adhesive modified coating liquid was applied to both surfaces of the unstretched PET film by a reverse roll method so that the coating amount after drying became 0.08 g/m 2 , and then dried at 80 ° C for 20 seconds.

The unstretched film formed with the coating layer was guided to a tenter stretching machine, and while holding the end portion of the film with a jig, it was guided to a hot air region at a temperature of 125 ° C and extended 4.0 times in the width direction. Next, the stretched width was maintained in the width direction, treated at a temperature of 225 ° C for 30 seconds, and then 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)

A uniaxially oriented PET film was produced in the same manner as the polarizer protective film 1 except that the thickness of the unstretched film was changed to a thickness of about 100 μm.

(Polarizer protective film 3)

The unstretched film formed by the same method as that of the polarizer protective film 1 was heated to 105 ° C using a heated roll group and an infrared heater, and then a roll group having a peripheral speed difference was used. After the traveling direction was extended by 1.5 times, the biaxially oriented PET film having a film thickness of about 50 μm was obtained by extending 4.0 times in the width direction in the same manner as the polarizer protective film 1.

(Polarizer protective film 4)

In the same manner as the polarizer protective film 3, a biaxially oriented PET film having a film thickness of about 50 μm was obtained by extending 2.0 times in the traveling direction and 4.0 times in the width direction.

(Polarizer protective film 5)

In the same manner as the polarizer protective film 1, the PET resin (B) containing the ultraviolet absorber was not used in the intermediate layer, and a uniaxially oriented PET film having a film thickness of 50 μm was obtained.

(Polarizer protective film 6)

In the same manner as the polarizer protective film 3, a uniaxially oriented PET film having a film thickness of about 100 μm was obtained by extending 4.0 times in the traveling direction and 1.0 times in the width direction.

(Polarizer protective film 7)

In the same manner as the polarizer protective film 1, a uniaxially oriented PET film having a film thickness of about 75 μm was obtained by extending 1.0 times in the traveling direction and 3.5 times in the width direction.

(Polarizer protective film 8)

A uniaxially oriented PET film having a thickness of about 275 μm was produced by changing the thickness of the unstretched film in the same manner as in the polarizer protective film 1.

(Polarizer protective film 9)

In the same manner as the polarizer protective film 3, the biaxially oriented PET film having a film thickness of about 38 μm was obtained by extending 3.6 times in the traveling direction and 4.0 times in the width direction.

(Polarizer protective film 10)

A uniaxially oriented PET film having a thickness of about 10 μm was produced by changing the thickness of the unstretched film in the same manner as in the polarizer protective film 1.

(Polarizer protective film 11)

A uniaxially oriented PET film having a film thickness of 50 μm was produced in the same manner as the polarizer protective film 5 except that a single layer was formed. Further, a white LED including a light-emitting element in which a blue light-emitting diode and a yttrium aluminum garnet-based yellow phosphor were combined was used, and an OLED was used for a light source of a liquid crystal display device to perform rainbow spot observation.

The results of rainbow spot observation and measurement of tear strength for the liquid crystal display device produced using the polarizer protective films 1 to 11 as described above are shown in Table 1 below.

The relationship between the "alignment main axis and the polarization axis" in Table 1 means the relationship between the polarization axis of the polarizer of the polarizing plate in the liquid crystal display device and the alignment main axis of the alignment polyester film used as the protective film. In Table 1, the polarizer protective film No. 1-1 shows a case where the polarizer protective film 1 is used as a polarizer protective film, and a cold cathode tube is used as a light source. The polarizer protective film No. 1-2 is a case where the polarizer protective film No. 1 is bonded to the polarizer so that the angle between the alignment axis and the polarizing axis of the polarizer is 8° (slightly parallel). The polarizer protective film No. 1-3 is a case where the polarizer protective film No. 1 is bonded to the polarizer such that the alignment angle between the main axis and the polarizing axis of the polarizer is 4° (slightly parallel).

As shown in Table 1, by using the polarizer protective film 1~8 or 11 and arranging the angle between the polarizing axis of the polarizer of the polarizing plate and the alignment main axis of the alignment polyester film, the liquid crystal display is viewed from any angle. No rainbow spots were observed on the screen of the device. On the other hand, even if the same polarizer protective films 1 to 8 and 11 are used, when the alignment main axis of the polyester film constituting the polarizing plate and the polarizing axis of the polarizing mirror are perpendicular to each other, the angle of the screen of the viewing liquid crystal display device may be different. And the situation of generating rainbow spots. Further, in the case of using the polarizer protective film 9 or 10 or the case of using a cold cathode tube as a light source, a clear rainbow spot is observed when the screen of the liquid crystal display device is viewed obliquely.

Claims (7)

A liquid crystal display device comprising a backlight source, two polarizing plates, and a liquid crystal display device disposed between the two polarizing plates, wherein the backlight source is a white light source having a continuous luminescence spectrum, and the two The polarizing plate has a polarizer and a polarizer protective film on at least one side thereof, and the polarizer protective film comprises an alignment polyester film having a hysteresis value of 3000 to 30000 nm, and the hysteresis value and the thickness direction hysteresis value of the alignment polyester film The ratio (Re/Rth) is 0.2 or more and 2.0 or less, and the polarizing axis of the polarizer is slightly parallel to the alignment main axis of the alignment polyester film of the protective film. A liquid crystal display device comprising a backlight source, two polarizing plates, and a liquid crystal display device disposed between the two polarizing plates, the backlight source is a white light emitting diode, and the two polarizing plates are Each having a polarizer and a polarizer protective film on at least one side thereof, the polarizer protective film comprising an alignment polyester film having a hysteresis value of 3,000 to 30,000 nm, and a ratio of a hysteresis value to a thickness direction hysteresis value of the alignment polyester film ( The Re/Rth) is 0.2 or more and 2.0 or less, and the polarizing axis of the polarizer is slightly parallel to the alignment main axis of the alignment polyester film of the protective film. The liquid crystal display device of claim 2, wherein the white light emitting diode is a white light emitting diode of a phosphor type. The liquid crystal display device of claim 2, wherein the white light emitting diode comprises a light emitting element in which a blue light emitting diode and a yttrium aluminum garnet yellow phosphor are combined. Liquid crystal display device according to any one of claims 1 to 4 The ratio of the hysteresis value of the alignment polyester film to the retardation value in the thickness direction (Re/Rth) is 0.2 or more and 1.2 or less. The liquid crystal display device according to any one of claims 1 to 4, wherein the polyester film comprises three or more layers, and an ultraviolet absorber is contained in a layer other than the outermost layer, and a light transmittance of 380 nm is 20%. the following. The liquid crystal display device according to any one of claims 1 to 4, wherein the two polarizing plates respectively comprise a polarizing mirror and a protective film on both sides thereof, and at least one of the protective films on both sides has a 3000 to 30000 nm Hysteresis value of the alignment polyester film.