KR20060097433A - A broadband reflection typed brightness enhancement polarizer and a liquid crystal display having the same - Google Patents

Abstract

The present invention is a broadband reflective high brightness polarizing plate in which a plurality of cholesteric liquid crystal films formed to have different selective reflection wavelength ranges by varying a mixing ratio of a curable nematic liquid crystal compound and a curable chiral compound, wherein the cholesteric As the curable nematic liquid crystal compound and the curable chiral compound contained in the liquid crystal films, a curable nematic liquid crystal compound represented by the following general formula (1) and a curable chiral compound represented by the following general formula (2) are used. Since the broadband reflective high brightness polarizing plate of the present invention has a wide range of selective reflection wavelengths of the cholesteric liquid crystal films constituting the polarizing plate, even if only a plurality of liquid crystal films are stacked, the visible light region is covered. Therefore, the polarizing plate of the present invention is economical because the manufacturing process is shortened, and the thickness is also relatively thin, thereby maximizing the luminance improvement characteristic when used in the liquid crystal display device.

Description

Broadband reflective type high brightness polarizing plate and liquid crystal display having the same {A BROADBAND REFLECTION TYPED BRIGHTNESS ENHANCEMENT POLARIZER AND A LIQUID CRYSTAL DISPLAY HAVING THE SAME}

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to

1 is a cross-sectional view schematically showing the structure of a broadband reflective high brightness polarizer of the present invention;

2 is an optical conceptual diagram showing the viewing angle deviation of the cholesteric liquid crystal film,

3 is a conceptual diagram showing the phase difference characteristics according to the angle of the cholesteric liquid crystal film,

4 is a conceptual diagram showing a phase difference characteristic required according to an angle for compensation;

5 is a conceptual diagram illustrating a phase difference characteristic according to an angle after compensation;

6 is a selective reflection area spectrum of a polarizing plate according to Example 1 of the present invention,

7 is a selective reflection region spectrum of a polarizing plate according to a comparative example,

8 is a graph showing the viewing angle characteristics of the polarizing plate according to the second embodiment of the present invention,

9 is a graph showing the viewing angle characteristics of the polarizing plate according to the first embodiment of the present invention.

The present invention relates to a broadband reflective high brightness polarizer in which a plurality of cholesteric liquid crystal films formed to have different selective reflection wavelength ranges are stacked.

Display panels are widely used in display devices such as electronic calculators, electronic clocks, car navigation systems, office automation devices, mobile phones, laptop computers, and information communication terminals. For example, liquid crystal displays have transparent electrodes and alignment layers formed therein. The liquid crystal is injected between the opposing upper panel and the lower panel. Since the liquid crystal display device currently used uses linear polarization, the absorption type polarizing film is arrange | positioned before and behind a panel, and is used.

Absorption-type polarizing film is usually produced by adsorbing iodine or dichroic dye on a polyvinyl alcohol film and stretching it in a certain direction. The polarizing film thus prepared has weak mechanical strength in the direction of the transmission axis and shrinks due to heat or moisture. There is a disadvantage that the polarization function is significantly reduced. Therefore, it is used by the structure which adhere | attached with the adhesive agent between support bodies, such as a cellulose acetate film. As described above, the absorption type polarizing film using the polyvinyl alcohol film absorbs light vibrating in one direction and passes only the light vibrating to the other to make linearly polarized light. Therefore, the efficiency of the polarizing film cannot theoretically exceed 50%. This is a major cause of lowering the efficiency and brightness of the LCD.

On the other hand, if the reflection type polarizing plate manufactured using the cholesteric liquid crystal is further used between the liquid crystal panel having the absorption type polarizing film and the reflecting plate can improve the disadvantages of the absorption type polarizing film described above. The cholesteric liquid crystal has a selective reflection characteristic in which the twisted direction of the helical structure of the liquid crystal coincides with the circular polarization direction, and the wavelength reflects only circular polarized light such as the spiral pitch of the liquid crystal. By using this selective reflection characteristic, it is possible to manufacture a polarizing plate which converts unpolarized light of a certain wavelength band into a specific circularly polarized light. That is, when non-polarized light in which left circularly polarized light and right circularly polarized light are mixed in half is incident on a cholesteric liquid crystal film having a leftward or preferential spiral structure, circularly polarized light such as a spiral direction is reflected and circularly polarized light in the opposite direction is Is transmitted. At this time, the transmitted circularly polarized light changes to linearly polarized light while passing through the 1 / 4λ retardation film. On the other hand, the reflected circularly polarized light is changed by the polarization direction upon re-reflection in the reflecting plate, thereby transmitting the liquid crystal film. Therefore, when additionally using a polarizing plate made of a cholesteric liquid crystal film, the theoretically there is no loss of light, it is possible to always make the luminance dramatically.

However, since the backlight used in the liquid crystal display generates light in the visible light band (400 to 700 nm), which is mainly a color display area, the selective reflection area of the cholesteric liquid crystal film must cover all visible light areas. If not, light transmitted through the polarizing plate in a non-polarized state is present, which degrades the image quality of the liquid crystal display. Since only one cholesteric liquid crystal film does not cover all of these visible light bands, a plurality of cholesteric liquid crystal films formed to have different selective reflection wavelength regions as disclosed in Korean Patent Laid-Open Publication No. 1999-65280 The polarizing plate should be manufactured by laminating them. However, since the cholesteric liquid crystal compounds commonly used have a narrow width of the selective reflection wavelength region, at least four cholesteric liquid crystal films should be stacked to cover all visible light bands. As a result, the manufacturing process of the polarizing plate is complicated and the economical efficiency is lowered, and the thickness of the manufactured polarizing plate is increased, thereby degrading the luminance characteristics.

The technical problem to be solved by the present invention is to solve the above problems, the width of the selective reflection wavelength region of the cholesteric liquid crystal films constituting the polarizing plate can cover all the visible light region even if only a few liquid crystal film laminated, Accordingly, an object of the present invention is to provide a broadband reflective high brightness polarizing plate which can be manufactured thinly and economically, and a liquid crystal display device having the same.

In order to achieve the above technical problem, the broadband reflective high brightness polarizing plate of the present invention has a plurality of cholesteric liquid crystal films formed to have different selective reflection wavelength regions by varying the mixing ratio of the curable nematic liquid crystal compound and the curable chiral compound. A stacked broadband reflective high brightness polarizing plate, wherein the curable nematic liquid crystal compound and the curable chiral compound contained in the cholesteric liquid crystal films are respectively curable nematic liquid crystal compounds represented by the following Chemical Formula 1 and curable represented by the following Chemical Formula 2. Chiral compounds are used.

<Formula 1>

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Any one selected from the group consisting of (m is an integer from 1 to 12 and n is an integer from 1 to 3), M is a mesogen group

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<Formula 2>

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로 이루어진 군으로부터 선택된 어느 하나이고, C는 카이칼 그룹으로서

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로 이루어진 군으로부터 선택된 어느 하나이다.In Formula 2, S is a spacer

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Any one selected from the group consisting of C is a caikal group

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Any one selected from the group consisting of.

In the broadband reflective high brightness polarizing plate of the present invention, it is preferable that three cholesteric liquid crystal films having a center wavelength of a selective reflection wavelength range of 450 to 480 nm, 530 to 550 nm, and 590 to 610 nm are laminated.

In the broadband reflective high brightness polarizer of the present invention, it is preferable to further laminate a 1 / 4λ retardation film for changing circularly polarized light to linearly polarized light on one surface of the broadband reflective high brightness polarizer. It is more preferable to use one larger than the refractive index of the direction.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention. Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

1 is a cross-sectional view schematically showing the structure of a broadband reflective high brightness polarizer of the present invention. Referring to FIG. 1, the broadband reflective high luminance polarizer 10 of the present invention has a structure in which cholesteric liquid crystal films 1a, 1b, and 1c having different selective reflection wavelength regions are stacked. The cholesteric liquid crystal films 1a, 1b, and 1c are made of a cholesteric liquid crystal formed by mixing a curable nematic liquid crystal compound represented by the following Chemical Formula 1 and a curable chiral compound represented by the following Chemical Formula 2.

<Formula 1>

,

및

로 이루어진 군으로부터 선택된 어느 하나이고(m은 1~12의 정수이고 n은 1~3의 정수임), M은 메소겐 그룹으로서In Formula 1, S is a spacer

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Any one selected from the group consisting of (m is an integer from 1 to 12 and n is an integer from 1 to 3), M is a mesogen group

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<Formula 2>

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로 이루어진 군으로부터 선택된 어느 하나이고, C는 카이칼 그룹으로서

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로 이루어진 군으로부터 선택된 어느 하나이다.In Formula 2, S is a spacer

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Any one selected from the group consisting of (m is an integer from 1 to 12 and n is an integer from 1 to 3), M is a mesogen group

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Any one selected from the group consisting of C is a caikal group

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Any one selected from the group consisting of.

The width of the selective reflection region of the cholesteric liquid crystal film depends on the nematic liquid crystal compound. When the cholesteric liquid crystal film is manufactured using the curable nematic liquid crystal compound of Formula 1, the cholesteric liquid crystal having a large width of the selective reflection region is formed. Films can be produced. Accordingly, even if only a small number of cholesteric liquid crystal films are stacked, a high brightness polarizer covering all visible light regions is formed, and thus a high brightness polarizer having a small thickness and economical efficiency can be provided. On the other hand, since the curable chiral compound of Formula 2 is excellent in compatibility with the curable nematic liquid crystal compound of Formula 1, it contributes to the improvement of the polarization characteristics of the cholesteric liquid crystal film.

In the broadband reflective high brightness polarizing plate of the present invention, the selective reflection wavelength region of the cholesteric liquid crystal films 1a, 1b, and 1c is determined according to the mixing ratio of the curable nematic liquid crystal compound and the curable chiral compound. When the mixing ratio of the cholesteric liquid crystal films constituting the high luminance polarizing plate of the present invention is adjusted so that the center wavelength of the selective reflection wavelength region is 450 to 480 nm, 530 to 550 nm and 590 to 610 nm, respectively, three cholesteric Even if only the liquid crystal films are stacked, a polarizing plate having high brightness may be manufactured by covering all visible light regions efficiently.

On the other hand, on one surface of the broadband reflective high-brightness polarizing plate of the present invention, it is preferable to further laminate a 1/4 lambda retardation film for changing circularly polarized light into linearly polarized light. In particular, the 1 / 4λ retardation film is more preferably used that the refractive index of the thickness direction is larger than the refractive index of the planar direction, look at the reason as follows.

The cholesteric liquid crystal has a characteristic that the color changes depending on the viewing angle. When the film of the cholesteric liquid crystal is applied to the backlight of the liquid crystal display device, there is a problem that the brightness is lowered and the color of the screen is changed depending on the viewing angle. To solve this problem, it is necessary to use a film that can compensate for the viewing angle characteristics. As shown in FIG. 2, when the light passes through the retardation film, the light traveling in a square does not completely change into linearly polarized light but has an elliptical polarization close to the linearly polarized light. Therefore, while passing through the linear polarizing film, a component such as a transmission axis of the polarizing film transmits, but the other components are absorbed, and thus the amount of light transmitted is reduced. This is why the luminance decreases as the angle increases. The reason why the color of the transmitted light changes according to the incident angle is the same as the reason that the luminance decreases. The color of light we see is determined by how many three primary colors exist: red, green, and blue. In the approach described above for the reason of the decrease in luminance, since the birefringence of light of different colors is different in the cholesteric liquid crystal film and the retardation film, the transfer angle of each light passing through the linear polarizing film is finally Depending on the color of the light will be different. In order to solve this problem according to the angle, it is necessary to use a compensation film opposite to the birefringence of the cholesteric liquid crystal film. The cholesteric liquid crystal film is isotropic on the film plane, but since the refractive index is negative C-plate smaller than the value on the film plane in the thickness direction of the film (see FIG. 3), the compensation film is positive C as shown in FIG. 4. It must be a plate. That is, when a film having isotropic on the film plane and having a larger refractive index in the thickness direction of the film is used, phase retardation caused by birefringence of the cholesteric liquid crystal film can be restored. When the phase delays of the cholesteric liquid crystal film and the compensation film are combined in FIGS. 3 and 4, as shown in FIG. 5, the phase delay value according to the angle is almost eliminated. In other words, the case where the light proceeds perpendicularly to the film or the case where the light proceeds in a square does not change the polarization state of the light because the phase delay disappears.

In the present invention, in order to impart such a property, a retardation film having a refractive index larger in the thickness direction than a refractive index in the planar direction is used. Methods for producing such retardation films are disclosed in Japanese Patent Nos. 2612196, 2994013, 2818983, 3309452, 3168850, and the like.

Looking at the manufacturing method of the broadband reflective high brightness polarizing plate of the present invention as an example.

First, the curable nematic liquid crystal compound of Formula 1 and the curable chiral compound of Formula 2 are dissolved in an organic solvent with a photoinitiator at a predetermined ratio, roll coated onto a substrate, and then passed through a dryer to orient the liquid crystal material while drying the solvent. To prepare a liquid crystal film through UV irradiation. In the same manner, the liquid crystal films having different selective reflection wavelength ranges were prepared by varying the mixing ratio of the curable nematic liquid crystal compound of Chemical Formula 1 and the curable chiral compound of Chemical Formula 2, and then bonded to the liquid crystal films prepared using an adhesive. The polarizing plate is manufactured by laminating | stacking. On the surface of the polarizing plate thus prepared, a retardation film is also laminated by using an adhesive.

Hereinafter, examples will be described in detail to help understand the present invention. However, embodiments according to the present invention can be modified in many different forms, the scope of the invention should not be construed as limited to the following examples. Embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

Example 1

First, three cholesteric liquid crystal films having different selective reflection wavelength ranges were prepared. The curable nematic liquid crystal compound (LC1057 from BASF) and the curable chiral compound (LC756 from BASF) were dissolved in cyclopentanone at a concentration of 30% by weight. The content of the curable chiral compound added in the preparation of each cholesteric liquid crystal film was 7.7% by weight, 6.2% by weight and 5.5% by weight relative to the content of the curable nematic liquid crystal compound, respectively, and 5% by weight of the photoinitiator (IG184, Ciba). -Geigy) and 0.2% by weight of leveling agent (BYK361, BYK) were added as additives. Each of the prepared solutions was roll coated on a polyethylene terephthalate film coated with a horizontal alignment layer, and then the solvent was dried in the dryer to orient the liquid crystal, and then UV light irradiation was used to prepare cholesteric liquid crystal films, respectively. The drying conditions were 85 ° C., and UV light irradiation used a 300 W (center wavelength 360 nm) lamp. The center wavelengths of the selective reflection regions of the respective cholesteric liquid crystal films obtained were 470 nm, 535 nm, and 600 nm, respectively.

The prepared cholesteric liquid crystal films were pressure-bonded using an adhesive in the order of the central wavelength from the short wavelength to the long wavelength. The adhesive was in the form of a solution, as in the case of applying the liquid crystal material, after the thin film coating by a roll coating method, the solvent was dried and pressed in a dryer. After pressing, it was aged for a certain time and completely bonded. The thickness of the stacked liquid crystal films was about 13 microns.

1 / 4λ retardation film (thickness 50 microns, retardation 110nm, Nx: 1.5817, Ny: 1.5795, Nz: 1.5778) coated with a pressure sensitive adhesive (PSA) on the short wavelength surface of the laminated liquid crystal film prepared by the above method After pressing at room temperature to prepare a final high brightness polarizing plate.

Example 2

A high-brightness polarizing plate was manufactured in the same manner as in Example 1, except that a 1 / 4λ retardation film having a retardation of 115 nm, Nx: 1.5806, Ny: 1.5783, and Nz: 1.5841 was used.

Comparative Example 1

A polarizing plate was manufactured in the same manner as in Example 1 except that the curable nematic liquid crystal compound and the curable chiral compound were each used with Merck Co., Ltd. RMS 02 and Merck Co., Ltd. RMS 03.

6 and 7 show selective reflection region spectra of the polarizing plates according to Example 1 and Comparative Example 1 prepared by the above-described method. Referring to FIG. 6, the polarizing plate in which the cholesteric liquid crystal films formed of the curable nematic liquid crystal compound and the curable chiral compound according to the present invention is laminated has a wide range of selective reflection region, so that only the three liquid crystal films are stacked. You can see that they cover all of them. However, as shown in FIG. 7, in the polarizing plate of Comparative Example 1 in which a cholesteric liquid crystal film formed of a conventional nematic liquid crystal compound and a chiral compound is laminated, there is a visible light band not covered to achieve high luminance characteristics. It can be seen that there is a problem. In fact, when the polarization plates of Example 1 and Comparative Example 1 were mounted on the panel of the liquid crystal display and the luminance was measured, when the luminance when the reflective polarizer was not used is 100, the polarization plate of Example 1 is mounted. The luminance at was 135 and the luminance at the time of mounting the polarizing plate of Comparative Example 1 was 125.

8 and 9 are graphs showing viewing angle characteristics of the polarizing plates of Example 1 and Example 2, respectively. It can be seen that the polarization plate of FIG. 2 in which the refractive index in the thickness direction is larger than the refractive index in the planar direction is laminated (FIG. 8) has less color coordinate variation depending on the angle than the polarizing plate of FIG. .

As described above, the broadband reflective high brightness polarizing plate of the present invention has a wide range of selective reflection wavelength range of the cholesteric liquid crystal films constituting the polarizing plate. Therefore, even if only three liquid crystal films are stacked, all visible light regions are covered. Accordingly, the polarizing plate of the present invention is economical because the manufacturing process is shortened, and the thickness is also relatively thin, thereby maximizing luminance improvement characteristics when used in the liquid crystal display device. In addition, when the 1 / 4λ retardation film having a refractive index in the thickness direction greater than the refractive index in the planar direction is laminated on the broadband reflective high-brightness polarizing plate of the present invention, color coordinate variation according to the angle is reduced, thereby providing excellent viewing angle characteristics.

Claims (6)

A broadband reflective high brightness polarizing plate in which a plurality of cholesteric liquid crystal films formed by varying the mixing ratio of the curable nematic liquid crystal compound and the curable chiral compound to have different selective reflection wavelength ranges, The curable nematic liquid crystal compound and the curable chiral compound contained in the cholesteric liquid crystal films are respectively a curable nematic liquid crystal compound represented by Formula 1 and a curable chiral compound represented by Formula 2 below Reflective high brightness polarizer; <Formula 1>

In Formula 1, S is a spacer

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And

Any one selected from the group consisting of (m is an integer from 1 to 12 and n is an integer from 1 to 3), M is a mesogen group

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in It is any one selected from the group consisting of. <Formula 2>

In Formula 2, S is a spacer

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Any one selected from the group consisting of (m is an integer from 1 to 12 and n is an integer from 1 to 3), M is a mesogen group ,

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Any one selected from the group consisting of C is a caikal group

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Any one selected from the group consisting of. 2. The broadband reflective high luminance polarizer of claim 1, wherein the cholesteric liquid crystal films having the central wavelengths of the selective reflection wavelength range of 450 to 480 nm, 530 to 550 nm, and 590 to 610 nm, respectively, are laminated. Broadband reflective high brightness polarizer. The broadband reflective high luminance polarizer of claim 2, wherein the cholesteric liquid crystal films having three center wavelengths of 470 nm, 535 nm, and 600 nm, respectively, in the selective reflection wavelength range are laminated. Polarizer. The broadband reflective high brightness polarizing plate according to claim 1 or 2, further comprising a 1/4? Phase difference film for changing circularly polarized light into linearly polarized light on one surface of the broadband reflective high brightness polarizing plate. 5. The broadband reflective high brightness polarizing plate according to claim 4, wherein the 1 / 4λ retardation film has a refractive index in the thickness direction greater than that in the planar direction. A liquid crystal display device comprising the broadband reflective high brightness polarizing plate of any one of claims 1 to 5.

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