KR20030013942A - A reflective polarizer using narrow-band cholesteric liquid crystal and its preparation method - Google Patents

Abstract

PURPOSE: A reflection type polarization plate using cholesteric liquid crystal of narrow selective-reflection band width and a method for fabricating the same are provided to improve luminance and fidelity of picture. CONSTITUTION: A reflection type polarization plate using cholesteric liquid crystal of narrow selective-reflection band width includes a cholesteric liquid crystal layer(10) formed by stacking cholesteric liquid crystal films in the sequence of pitch from short one to long one for covering peak portions of a light source of an LCD backlight, a compensating film(20) stacked on the cholesteric liquid crystal films for reducing the change in the luminance and color in every corner, a λ/4 phase difference film(30) stacked on the compensating film for converting circular polarization light to rectilinear polarization light, and a linear polarization plate(40) stacked on the λ/4 phase difference film with a transmission axis located at 45° position with respect to the phase difference film for covering a part of visible light areas.

Description

Reflective polarizer using cholesteric liquid crystal with narrow selective reflection band and its manufacturing method {A REFLECTIVE POLARIZER USING NARROW-BAND CHOLESTERIC LIQUID CRYSTAL AND ITS PREPARATION METHOD}

According to the present invention, when the reflective polarizer is manufactured using a cholesteric liquid crystal having a narrow selective reflection band and covering only part of the visible light, the luminance increase and color at the front of the LCD backlight unit are satisfied. A reflective polarizing plate and a method of manufacturing the same.

Until now, reflective polarizers using cholesteric liquid crystals have been designed such that the selective reflection spectrum of cholesteric liquid crystals covers the entire visible light region.

US Patent US5737044 describes a method that can cover the entire area of visible light by having a structure in which the pitch of the cholesteric liquid crystal is sequentially changed from a short pitch to a long pitch in one layer of the film. It also describes the difference in viewing angle and color when the short pitch plane of the cholesteric liquid crystal film is toward the light source and when the long pitch plane is toward the light source.

In another US patent US6016177, a cholesteric liquid crystal film having a different pitch from each other in a short pitch to a long pitch using an adhesive or a heat-adhesive method, rather than a sequential change in the pitch of the liquid crystal in one layer as in US5737044. A method of laminating films to cover the entire area of visible light is described.

These conventional methods all use a liquid crystal material having a sequentially varying pitch or a method of stacking multiple layers of different single pitch liquid crystal films so that the cholesteric liquid crystal film has a reflection spectrum covering the entire visible light region. Used. Particularly, when the lamination method was used, more layers had to be laminated to cover the entire area when the selective reflection band width was narrow, resulting in a long manufacturing process and a poor yield.

The present invention uses a method of stacking different single-pitch liquid crystal films, but relates to a method capable of realizing the characteristics of the reflective polarizer while covering only a part of the entire region without visible light.

That is, an object of the present invention is to provide a reflective polarizer that satisfies the luminance increase and color at the front of the LCD backlight unit even though only a portion of the visible light region is covered using a cholesteric liquid crystal film having a narrow selective reflection band. will be.

In addition, it is an object of the present invention to find the center wavelength of the cholesteric liquid crystal film that satisfies the luminance increase and color at the front of the LCD backlight unit even when covering only a part of the visible light region.

1 is a structure of a reflective polarizer using a cholesteric liquid crystal according to the present invention.

2 is a structure of a cholesteric liquid crystal film layer according to an embodiment of the present invention.

3 shows a reflection spectrum of a cholesteric liquid crystal having a narrow selective reflection band covering only a peak portion of an LCD backlight light source.

4 shows a reflection spectrum of the cholesteric liquid crystal covering the entire region of visible light.

<Description of the symbols for the main parts of the drawings>

10: cholesteric liquid crystal film layer 11: blue CLC film

12: Green CLC Film 13: Red CLC Film

20: compensation film 30: lambda / 4 phase difference film

40: linear polarizing plate

In order to achieve the above object, in the present invention,

A cholesteric liquid crystal film layer in which a cholesteric liquid crystal film having a narrow selective reflection band is sequentially stacked from a short pitch to a long pitch so as to cover only a peak portion of the LCD backlight light source; A compensation film laminated on the cholesteric liquid crystal film layer to reduce brightness and color change in a square; A λ / 4 retardation film laminated on the compensation film and converting circularly polarized light into linearly polarized light; A reflection using narrow cholesteric liquid crystal having a narrow selective reflection band, comprising a linear polarizing plate laminated on the λ / 4 retardation film such that the transmission axis is 45 ° with the retardation film. A polarizing plate and a manufacturing method thereof are provided. One embodiment of a reflective polarizer according to the present invention is shown in FIG. 1.

The cholesteric liquid crystal used in the present invention uses a cholesteric liquid crystal having a narrow selective reflection band so as to cover only a peak portion of the LCD backlight light source as shown in FIG. 3. Preferably, the cholesteric liquid crystal uses a selective reflection bandwidth of 30-70 nm. In particular, when the selective reflection center wavelength is 550 nm, a cholesteric liquid crystal having a bandwidth of about 50 nm is used. When using a cholesteric liquid crystal having a band width of 50 nm as described above, in order to cover the entire visible light region (400-750 nm), seven layers having different pitches should be laminated. The cholesteric liquid crystal film of the cover only a portion of the visible light region.

The pitch of the cholesteric liquid crystal is controlled by the ratio of the nematic liquid crystal and the chiral liquid crystal. The larger the amount of the chiral liquid crystal, the shorter the pitch is the cholesteric liquid crystal. The cholesteric liquid crystal film is sequentially laminated from a short pitch to a long pitch. In a preferred embodiment of the present invention, as shown in FIG. 2, a blue film 11 and a green film 12 are shown. ) And a red film 13 are sequentially stacked.

At this time, preferably, the selective reflection center wavelength of the blue film 11 is 460-480 nm, the selective reflection center wavelength of the green film 12 is 550-570 nm, and the red film The selective reflection center wavelength of the (red film) 13 is 600-620 nm.

In the present invention, the "lamination" of the cholesteric liquid crystal film or other films described below refers to lamination using an adhesive or a pressure sensitive adhesive (PSA) according to a conventional method. do. Unless specifically limited in this specification, "lamination" is all interpreted with the same meaning.

When light does not pass through the cholesteric liquid crystal film layer 10 at any angle but passes vertically, the polarization state of the light is changed due to retardation caused by the refractive anisotropy of the cholesteric liquid crystal film. Therefore, the light passing through exhibits different characteristics from the light passing vertically through the film surface. The compensation film 20 is used to correct this phenomenon.

The compensation film 20 is a negative c-plate having a refractive index distribution of the cholesteric liquid crystal film larger than the value in the direction perpendicular to the film surface in the film surface direction and having the same refractive index value in the surface direction. (plate), so we do the opposite. That is, the compensation film uses a positive c-plate having a uniform refractive index in the film surface direction and smaller than the value in the direction perpendicular to the film surface, and the phase delay value and size of the cholesteric liquid crystal film Adjust the thickness so that they are the same and in opposite directions.

In a preferred embodiment of the present invention, the compensation film uses a nematic liquid crystal is vertically aligned, and the film thickness is 2-4 ㎛.

Since the cholesteric liquid crystal film separates light into left circularly polarized light and right circularly polarized light, the light passing through the cholesteric liquid crystal film and the compensation film is circularly polarized. Therefore, a quarter-wave phase difference is used to change the circularly polarized light into linearly polarized light. Use film.

In the exemplary embodiment of the present invention, a stretched film made of polycarbonate (PC) material having a phase delay value of 115 nm is used as the lambda / 4 phase difference film to satisfy the luminance increase rate of the transmitted light and the front color. In another embodiment, a liquid crystal film in which nematic liquid crystals are horizontally aligned as the λ / 4 retardation film.

In the case of a quarter-wave retardation film, the refractive index of two directions orthogonal to each other in the surface direction can be obtained. The direction in which the refractive index is small is called the fast axis, and the direction in which the refractive index is large is called the slow axis. When circularly polarized light is incident on a quarter-wave retardation film, light may be linearly polarized at 45 ° with respect to a fast axis depending on the direction of circular polarization, or may be light polarized at −45 °. (When light with a left polarized light passes through a quarter-wave retardation film, light linearly polarized at -45 ° with respect to the fast axis).

Therefore, when laminating the linear polarizing plate on the quarter-wave retardation film, the alignment should be aligned so that the transmission axis of the linear polarizing plate and the polarization of the light transmitted through the quarter-wave retardation film coincide.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the scope of the present invention is not limited by the following examples, and those skilled in the art to which the present invention pertains should be within the equivalent scope of the technical concept of the present invention and the claims to be described below. Of course, various modifications and variations are possible.

Example 1

(1) FIG. 3 shows the reflection spectrum when laminating cholesteric liquid crystal films having selective reflection wavelengths of 470, 560 and 610 nm. A cholesteric liquid crystal material having selective reflection wavelengths of 470, 560, and 610 nm exhibiting such a reflection spectrum was coated on a polyester base film using a Gravure coating method, and the solvent was dried, followed by UV light. The liquid crystal material was cured to obtain a film state. The thickness of the prepared cholesteric liquid crystal film was about 3 μm.

(2) Among the cholesteric liquid crystal films prepared in the above (1), the selective reflection wavelengths of 470 nm and 560 nm are laminated with an acrylic adhesive, and the PET base film toward the 560 nm cholesteric liquid crystal film. After delamination, the 610 nm liquid crystal film was further bonded in the same manner, and the PET base film on the 610 nm side was peeled off.

(3) coating a vertical alignment layer on the PET base film, coating the nematic liquid crystal on the gravure coating method, vertically aligning the liquid crystal, drying the solvent, and irradiating UV light to maintain the alignment state of the nematic liquid crystal. The compensation film was prepared in a film state. The birefringence index of the nematic liquid crystal was Δn = 0.13, and the film thickness was 2.5 μm. The prepared compensation film was bonded to the 610 nm surface of the cholesteric liquid crystal film layer obtained in (2), and then the PET base film on the compensation film side was peeled off.

(4) Compensation film surface of the film layer obtained in (3) above using a pressure sensitive adhesive (PSA) for a 1/4 wavelength retardation film having a phase delay value of 115 nm and a thickness of about 70 μm of a polycarbonate (PC) material. And cemented.

(5) A commercially available linear polarizing plate (for example, a linear flat plate of Nitto-Denko, a high permeable linear flat plate of LG Chem) is aligned and bonded so that the 1/4 wavelength retardation film obtained in the above (4) coincides with the transmission axis. To prepare a reflective CLC polarizing plate of the present invention.

The prepared reflective CLC polarizer was placed on a commercially available LCD backlight unit (for example, a unit used for LG 13.3 inches of LCD of LG-philips) and compared with a linear polarizer. When comparing the reflective polarizer of the present invention prepared as described above based on the portion where only the linear polarizer was placed, the luminance increase rate was 134%, and the color was 1.34, when the color of the linear polarizer was only 1; Green is 1.35 and red is 1.19.

Example 2

Using a 2.3 μm thick compensation film; The nematic liquid crystal having a phase delay value of about 120 nm and a film thickness of about 1.2 μm is used in the same manner as in Example 1 except that a liquid crystal film having a horizontal orientation is used as a quarter-wave retardation film. A reflective CLC polarizing plate was prepared. The prepared reflective CLC polarizing plate was placed on the LCD backlight unit in the same manner as in Example 1 and compared with the portion on which only the linear polarizing plate was placed.

As a result, the luminance increase rate at the front was 136%, and the color was 1.16 for blue, 1.37 for green, and 1.15 for red when the area with only the linear polarizer was 1.

Comparative example

4 shows a reflection spectrum of the cholesteric liquid crystal covering the entire visible light because the bandwidth of the selective reflection spectrum is wide. Except for using the cholesteric liquid crystal layer having such a reflection spectrum, a reflective polarizer was manufactured in the same manner as in Example 1, and then the linear polarizer was placed on the LCD backlight unit in the same manner as in Example 1. Comparison was made with the placed part.

As a result, the luminance increase rate in front was 134%, and the color showed 1.34 for blue, 1.35 for green, and 1.34 for red when the area with only linear polarizer was 1. Compared with Example 1, the front luminance increase rate was the same, and in terms of color, the value of the reflective polarizer when covering the entire visible light region was uniformly increased compared with the case where only the linear polarizer was included. In addition, the color of Example 1 had more green components than the color of Comparative Example 1.

According to the present invention, according to the present invention, a cholesteric liquid crystal having a narrow bandwidth of the selective reflection spectrum is used to cover the entire visible light region even if only a part of the visible light region is not covered. It is possible to produce a reflective polarizing plate which hardly differs from the above. In addition, when the bandwidth is narrow, even if the reflection spectrum does not cover the entire visible light area, the reflective polarizer is sufficient to increase the brightness of the LCD, so that many layers of cholesteric liquid crystal film are laminated to cover the entire visible light area. There is no need to shorten the manufacturing process, thereby increasing the manufacturing yield.

Claims (14)

A cholesteric liquid crystal film layer in which a cholesteric liquid crystal film having a narrow selective reflection band is sequentially stacked from a short pitch to a long pitch so as to cover only a peak portion of the LCD backlight light source; A compensation film laminated on the cholesteric liquid crystal film layer to reduce brightness and color change in a square; A λ / 4 retardation film laminated on the compensation film and converting circularly polarized light into linearly polarized light; A reflection using narrow cholesteric liquid crystal having a narrow selective reflection band, comprising a linear polarizing plate laminated on the λ / 4 retardation film such that the transmission axis is 45 ° with the retardation film. Type polarizer. The reflective polarizer of claim 1, wherein the cholesteric liquid crystal film has a selective reflection bandwidth of 30-70 nm. The reflective polarizing plate of claim 1 or 2, wherein the cholesteric liquid crystal film layer is laminated in the order of a blue film, a green film, and a red film. The method of claim 3, wherein the selective reflection center wavelength of the blue film is 460-480 nm, the selective reflection center wavelength of the green film is 550-570 nm, the selection of the red film The reflective polarizing plate is characterized in that the 600-620 nm wavelength. The method of claim 1 or 2, wherein the compensation film is a nematic liquid crystal is a vertically aligned using a reflective polarizing plate, characterized in that the film thickness is 2-4 ㎛. The reflective polarizing plate according to claim 1 or 2, wherein the λ / 4 retardation film is a polycarbonate film. The polarizing plate according to claim 1 or 2, wherein the λ / 4 retardation film is a liquid crystal film having a nematic liquid crystal oriented horizontally. Forming a cholesteric liquid crystal film layer by sequentially laminating a cholesteric liquid crystal film having a narrow selection reflection band from a short pitch to a long pitch so as to cover only a peak portion of the LCD backlight light source; Stacking a compensation film on the cholesteric liquid crystal film layer to reduce luminance and color change in a square; Stacking a [lambda] / 4 retardation film for converting circularly polarized light into linearly polarized light on the compensation film; A reflective type using a narrow cholesteric liquid crystal having a narrow selective reflection band, wherein a linear polarizing plate is laminated on the λ / 4 retardation film so that the transmission axis is 45 ° with the retardation film to cover only a part of the visible light region. Method of manufacturing a polarizing plate. The method according to claim 8, wherein the cholesteric liquid crystal film has a selective reflection bandwidth of 30-70 nm. The method according to claim 8 or 9, wherein the cholesteric liquid crystal film layer is laminated in the order of a blue film, a green film and a red film. The method of claim 10, wherein the selective reflection center wavelength of the blue film is 460-480 nm, the selective reflection center wavelength of the green film is 550-570 nm, the selection of the red film Reflective center wavelength is 600-620 nm manufacturing method. The method of claim 8 or 9, wherein the compensation film is a nematic liquid crystal is used vertically oriented and the film thickness is 2-4 ㎛ manufacturing method characterized in that. The method according to claim 8 or 9, wherein the lambda / 4 phase difference film is a polycarbonate film. The method of claim 8 or 9, wherein the lambda / 4 phase difference film is a manufacturing method, characterized in that the nematic liquid crystal film is horizontally aligned.