KR20100013159A - Optical sheet having reflective polarizer with curl free, back light unit having the optical sheet and liquid crystal display device having the back light unit - Google Patents

Abstract

PURPOSE: An optical sheet, a backlight unit and a LCD device thereof are provided to reduce the inferiority of a backlight and maintain the high definition of the LCD by preventing a curl. CONSTITUTION: A reflective type polarizing plate(21) is formed into an anisotropic material with two refractive index of an extraordinary reflective index and an ordinary reflective index. A phase difference film(22) arranged in the one side of the reflective type polarizing plate changes a circularly polarized light to a linearly polarized light. A compensation film(23) is arranged on the phase difference film compensates the ordinary refractive index and the extraordinary refractive index of the reflective type polarizing plate. A diffusion film(24) uniformly diffuses light emitted from the backlight unit.

Description

An optical sheet having a reflective polarizer with curl free, a back light unit having the optical sheet and liquid crystal display device having the back light unit}

The present invention relates to an optical sheet having a reflective polarizing plate, and more particularly, to an optical sheet having a reflective polarizing plate with curl prevention, a backlight unit employing the same, and a liquid crystal display device having the backlight unit.

Recently, as a flat panel display device used in a notebook computer, a television, or a mobile phone that requires thinning, miniaturization, and low power consumption, a plasma display panel (PDP), a field emission display device (field) An emission display device (FED), a thin film transistor liquid crystal display device (TFT-LCD), and the like have been developed, and among them, a liquid crystal display device having excellent color reproducibility and thinness is widely used.

Since the liquid crystal display of the flat panel display is not a self-emission type unlike the PDP and the FED, in order to display an image, light must be irradiated using a backlight unit that is a separate light source from the back side.

The liquid crystal display device has a structure in which a liquid crystal is injected between an upper panel and a lower panel in which a transparent electrode and an alignment layer are formed inside and disposed to face each other. The liquid crystal display uses linear polarization, and an absorption type polarizing film is disposed above and below the panel for this purpose.

The absorption type polarizing film absorbs light oscillating in one direction and passes only the light oscillating in the other direction to make linearly polarized light. Therefore, the efficiency of the polarizing film does not exceed 50% in theory. This is a major cause of lowering the efficiency and brightness of the liquid crystal display.

In order to solve the above problems, a reflective polarizing plate is disposed between the liquid crystal display panel and the reflecting plate including the absorption type polarizing film. The reflective polarizer includes a stack of cholesteric liquid crystal layers, a stack of anisotropic materials having various thicknesses, and an isotropic material layer, and complementarily in an anisotropic or isotropic matrix, isotropic or isotropic islands. Several types are being developed, including the arrangement of.

In the case of the reflective polarizing plate as described above, a method of forming a composite optical sheet by bonding with various other optical films is known. In particular, a reflective polarizing plate using cholesteric liquid crystal is used to convert circularly polarized light into linearly polarized light. A method of imparting new optical properties and durability by bonding a? / 4 retardation film and adhering a diffusing film, a protective film and the like to a reflective polarizing plate is known.

As shown in FIG. 1, a cross-sectional view of the optical sheet 10 in which the lambda / 4 phase difference film 12 and the diffusion film 13 are adhered to the reflective polarizing plate 11 is shown.

However, in the optical sheet 10 having such a laminated structure, a plurality of films laminated by the change of the surrounding environment such as temperature and / or humidity change have different thermal contraction rates, thereby causing curl to occur in the optical sheet. This causes a problem in the backlight unit, thereby degrading the image quality of the liquid crystal display.

FIG. 2 is a cross-sectional view schematically showing that the optical sheet of FIG. 1 is deformed due to changes in the surrounding environment such as temperature and / or humidity changes, and curls are generated. The same reference numerals as in the above-described drawings indicate the same member or parts of the same member.

Referring to FIG. 2, the optical sheet 10 including the diffusion film 13, the reflective polarizer 11, and the λ / 4 retardation film 12 is bent in the thickness direction. Reflective polarizing plate 11 is composed of a plurality of cholesteric liquid crystal film, and the final production by UV curing, not the stretching process, so after curing completely does not occur curl itself due to temperature or humidity changes Do not. Therefore, as described above, the optical sheet 10 may be bent in one direction, which may be attributed to different thermal contraction rates of the λ / 4 retardation film 12 and the diffusion film 13.

The present invention provides an optical sheet having a reflective polarizer with curl prevention.

In addition, the present invention provides a backlight unit in which defects are reduced by employing the optical sheet.

In addition, the present invention provides a liquid crystal display device that can maintain a high quality by employing the optical sheet.

The present invention to solve the above problems,

A first optical film;

A reflective polarizer disposed on the first optical film; And

A second optical film disposed on the reflective polarizing plate,

The first optical film and the second optical film provide optical sheets having substantially the same thermal contraction rate.

According to an embodiment of the present invention, the first optical film and the second The thermal shrinkage difference of the optical film is within 0.01% to 0.3%.

According to another embodiment of the present invention, the first optical film is a diffusion film, and the second optical film is a λ / 4 retardation film.

According to another embodiment of the present invention, the optical sheet further includes at least one optical film disposed between the reflective polarizing plate and the second optical film.

According to another embodiment of the present invention, the first optical film and the second optical film is a diffusion film, the optical sheet, λ / 4 phase difference film disposed between the reflective polarizing plate and the second optical film It further includes.

According to another embodiment of the present invention, the reflective polarizer is a type in which a CLC (cholesteric liquid crystal) film is laminated, a type in which an isotropic and anisotropic material-containing film is alternately laminated, and complementary in a matrix of isotropic or anisotropic material. In general, it is any one type of dispersion of islands of anisotropic or isotropic material.

According to another embodiment of the present invention, the diffusion film is a coating of organic or inorganic light diffusion particles on at least one surface of a PET (polyethylene terephthalate) or PC (polycarbonate) film, the λ / 4 retardation film is a PC It is a film.

According to another embodiment of the present invention, it further comprises a compensation film disposed on the lambda / 4 phase difference film or between the lambda / 4 phase difference film and the diffusion film.

The present invention also provides a backlight unit having an optical sheet according to any one of the embodiments to solve the above problems.

In addition, to solve the above problems, the present invention provides a liquid crystal display device having an optical sheet according to any one of the embodiments.

According to the present invention, an optical sheet having a reflective polarizer with curl prevention can be provided.

In addition, according to the present invention, a backlight unit having reduced defects may be provided by employing the optical sheet.

In addition, according to the present invention, a liquid crystal display device capable of maintaining a high quality by employing the optical sheet can be provided.

Next, an optical sheet having a reflective polarizer according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

3 is a cross-sectional view of an optical sheet 20 according to an embodiment of the present invention having a reflective polarizer 21, a λ / 4 retardation film 22, a compensation film 23, and a diffusion film 24.

Referring to FIG. 3, the reflective polarizer 21 is a type in which a CLC (cholesteric liquid crystal) film is laminated, a type in which an isotropic and anisotropic material-containing film is alternately laminated, and complementarily in a matrix of isotropic or anisotropic material. It is one of the types which disperse | distributed the island of anisotropic or isotropic material. For example, in the case where the CLC film is laminated, a cholesteric liquid crystal film having different selective reflection wavelength regions is generally used so that the selective reflection wavelength region covers all visible light regions (380 to 770 nm). At least three sheets are laminated.

The λ / 4 retardation film 22 is disposed on one surface of the reflective polarizer 21 to convert circularly polarized light into linearly polarized light. The λ / 4 retardation film 22 preferably has a refractive index in the thickness direction greater than that in the planar direction, but the present invention is not limited thereto. Although the thermal contraction rate (δ) of the λ / 4 retardation film 22 used in the present embodiment varies greatly depending on the material of the retardation film, in this embodiment, the PC (polycarbonate) having a thermal contraction rate (δ) of 0.5 to 0.7% A film of material can be preferably used.

In particular, since the liquid crystal coated reflective polarizer 21 is made of an anisotropic material having optically two refractive indices of ordinary reflective index and extra-ordinary reflective index, the path of the light according to the incident angle of the incident light. And birefringence change. Therefore, the liquid crystal display device employing the liquid crystal coated reflective polarizing plate 21 has a color change depending on the viewing direction, that is, a change in contrast ratio and gray scale inversion. Occurs. Therefore, a compensation film 23 including a material having an image refractive index and an abnormal refractive index of the liquid crystal coated reflective polarizing plate 21 is disposed on the lambda / 4 phase difference film 22 to compensate for this. The phenomenon can be improved. However, the present invention is not limited thereto, and the compensation film 23 may be disposed at various other positions such as disposed between the reflective polarizer 21 and the λ / 4 retardation film 12. Specifically, the compensation film 23 is isotropic on the film surface, and has a larger refractive index in the thickness direction of the film. In addition, the compensation film 23 has a negligible thermal contraction rate with respect to the temperature / humidity change.

The diffuser film 24 is disposed on the other side of the reflective polarizer 21, that is, the opposite side to the λ / 4 retardation film 22, to evenly diffuse the light emitted from the backlight unit and to brighten and brighten the light guide plate (not shown). It serves to hide. In addition, the diffusion film 24 also serves to protect the reflective polarizer 21 from physical and / or chemical changes.

On the other hand, the diffusion film 24 is generally polypropylene, polyethylene, polyethylene terephthalate (PET), or polycarbonate (PC) and the like, preferably a light diffusion layer on one or both sides of the PET or PC base film (not shown) It is manufactured by forming (not shown). For example, a homopolymer obtained by using styrene, melamine formaldehyde, and / or an acrylic monomer in a binder resin such as a single or copolymer of an acrylic resin, a polyester resin, a urethane resin, an epoxy resin, or a melamine resin. Or organic particles such as copolymer particles or inorganic particles such as calcium carbonate, barium sulfate, titanium dioxide, aluminum hydroxide, silica, glass, talc, mica, white carbon, magnesium oxide, zinc oxide, and air on one side of the PET substrate. It can be manufactured by coating by air knife method, gravure method, Meyer bar method, reverse roll method, or spray (spray) method.

The thermal shrinkage δ of the diffusion film 24 has a great influence on the appearance deformation of the optical sheet 20. Therefore, when selecting the diffusion film 23, the thermal shrinkage rate δ is considered to be the final product. Important to reduce curl or deformation.

The diffusion film 24 may vary from 0.01 to 1.0% in heat shrinkage depending on the base film. It can be selected from various materials to suit the purpose of the curl prevention of the present invention.

δ₂₃). 따라서, 온도 및/또는 습도 등 주위환경에 어떤 변화가 있 더라도 광학시트(20)에 심각한 컬이 생기지 않으며, 이러한 광학시트(20)를 채용한 백라이트 유니트의 불량이 감소되어 액정표시장치가 고화질을 유지할 수 있게 된다. In this embodiment, the difference between the thermal contraction rate (δ) of the λ / 4 retardation film 22 and the diffusion film 24 is within 0.01% to 0.3%, so that the thermal contraction rate (δ) between the two films should be substantially the same. (I.e. δ ₂₂

δ ₂₃ ). Therefore, no significant curl is generated in the optical sheet 20 even if there is any change in the surrounding environment such as temperature and / or humidity, and the defect of the backlight unit employing the optical sheet 20 is reduced, so that the liquid crystal display device may have high quality. It can be maintained.

δ₃₄

δ₃₅), 도 3의 광학시트와는 달리 확산필름이 더 추가되었지만, 실질적으로 광학시트(30)의 컬이 발생하지 않게 된다. 미설명된 도면의 참조부호 31 및 33은 각각 반사형 편광판 및 보상필름을 나타낸다.In another embodiment of the present invention illustrated in FIG. 4, the thermal contraction rates of the λ / 4 retardation film 32 and the two diffusion films 34 and 35 are substantially the same (that is, δ _32).

δ ₃₄

δ ₃₅ ), but unlike the optical sheet of FIG. 3, a diffusion film is added, but the curl of the optical sheet 30 is substantially not generated. Reference numerals 31 and 33 in the unexplained drawings denote reflective polarizers and compensation films, respectively.

δ₃₄

δ₃₅), 반사형 편광판(31) 및 λ/4 위상차 필름(32)을 사이에 두고 배치된 2장의 확산필름(34, 35)의 열수축률이 동일하여, 광학시트(30)의 컬이 발생하지 않게 된다. 본 명세서에서 앞서 도시된 도면에서와 동일한 참조부호는 동일한 부재 또는 동일한 부재의 부분을 나타낸다.The embodiment of FIG. 5 differs from the embodiment of FIG. 4 in that the thermal contraction rates of the λ / 4 retardation film 32 and the two diffusion films 34 and 35 are not substantially the same (ie, δ _32).

δ ₃₄

δ ₃₅ ), the heat shrinkage of the two diffusing films 34 and 35 arranged with the reflective polarizing plate 31 and the λ / 4 retardation film 32 interposed therebetween, resulting in curling of the optical sheet 30. You will not. In the present specification, the same reference numerals as in the above-described drawings indicate the same members or parts of the same members.

Such adjustment of thermal contraction rate (δ) can be achieved by appropriately adjusting the material of each film.

In this embodiment, the difference in thermal contraction rate between the λ / 4 retardation film 32 and each of the diffusion films 34 and 35 is within 0.3%. When the difference in thermal contraction rate according to the temperature change is out of the range, it is not preferable because the curl generation is deepened and the size of the generated curl increases.

The present invention is not limited to the optical sheet having the above configuration, but may be applied to various other optical sheets having a reflective polarizing plate.

Since the specific configuration and operation of the backlight unit and the liquid crystal display having the same are well known in the art, a detailed description thereof will be omitted herein.

The present invention will be described in detail with reference to the following Examples. However, the present invention is not limited only to the following examples.

Example 1 Preparation of Optical Sheet 1

The optical sheet 1 having the constitution of FIG. 3 was manufactured by the following manufacturing method.

Preparation Example 1 Preparation of Reflective Polarizing Plate

As described in Preparation Example 1 of Korean Patent No. 766179, cholesteric liquid crystal films having a center wavelength of 470 nm, 535 nm, and 600 nm in the selective reflection region were prepared, and the films were laminated to prepare a reflective polarizer. The prepared reflective polarizer was 9 microns thick.

Preparation Example 2 With λ / 4 Retardation Film

On one surface of the reflective polarizing plate prepared in Preparation Example 1, a pressure sensitive adhesive (PSA) was coated, the thickness was 50 microns, the phase difference was 138 nm, and the thermal contraction rate (150 ° C X 30min) was 0.58 depending on temperature change. (Lambda) / 4 phase (s) difference film of PC material which is% is crimped | bonded and attached at normal temperature.

Preparation Example 3 Preparation of Compensation Film and Lamination

In the case of the reflective polarizer manufactured in Preparation Example 1, the color of the reflective polarizer is changed according to the viewing angle due to the use of cholesteric liquid crystal. Thus, a compensation film which is isotropic on the film plane and has a larger refractive index in the thickness direction of the film was laminated on the reflective polarizer prepared as follows.

As in Preparation Example 1, the vertical alignment liquid crystal (Merck) was roll coated on the alignment film-treated polyethylene terephthalate (PET) film by the roll coating method, and then the solvent was dried in an oven at 85 ° C. to orient the liquid crystal, and then 300 W. (Center wavelength 360nm) UV was irradiated with a UV lamp to prepare a vertical alignment liquid crystal compensation film. The compensation film thus produced has a negligible heat shrinkage rate for temperature / humidity changes. This compensation film was pressure-bonded to the lambda / 4 phase difference film of the film prepared in Preparation Example 2 using an adhesive, and then irradiated with UV and laminated to remove the PET film as the base film.

Preparation Example 4 Attachment of Diffusion Film

In the film stack composed of such a reflective polarizer, a λ / 4 retardation film and a compensation film, first, on the other side of the reflective polarizer, that is, on the opposite side to the λ / 4 retardation film, the thickness is 100 μm and the thermal shrinkage is 0.64%. Phosphorus diffusion film was attached. As a result, an optical sheet (1) having a thermal shrinkage difference of λ / 4 retardation film and a diffusion film of 0.06% was obtained.

Example 2: Preparation of Optical Sheet 2

In the film laminate composed of the same reflective polarizing plate, lambda / 4 phase difference film and the compensation film as prepared in Preparation Examples 1 to 3, to have the structure of Figure 4, first, one surface of the reflective polarizing plate, that is, on the compensation film On the surface, a diffusion film 1 having a thickness of 100 µm and a heat shrinkage ratio of 0.62% was attached.

Subsequently, a diffuser film 2 having a thickness of 100 μm and a thermal contraction rate of 0.64% due to temperature change was attached to the other surface of the reflective polarizer, that is, the surface opposite to the λ / 4 retardation film.

As a result, the optical sheet 2 having the structure of FIG. )

Example 3: Preparation of Optical Sheet 3

By the same method as in Example 2, except that a diffusion film 1 having a thickness of 100 μm and a heat shrinkage rate of 0.92% and a diffusion film 2 having a thickness of 100 μm and a heat shrinkage rate of 0.94% were used. The optical sheet 3 having the structure of FIG. 5 in which the difference in thermal contraction rate between the diffusion film 1 and the diffusion film 2 is 0.02% and the difference in thermal contraction rate between the lambda / 4 phase difference film and the diffusion film 1 is 0.34% Prepared.

Comparative Example 1: Preparation of Optical Sheet (4)

Except for using the diffusion film 2 having a thickness of 100 μm and a heat shrinkage rate of 0.94% (that is, the heat shrinkage of the diffusion film and the lambda / 4 phase difference film is different) and the method of Example 1 By the same method, the optical sheet 4 having the structure of FIG. 6 in which the difference in thermal contraction rate between the lambda / 4 phase difference film and the diffusion film is 0.36% was prepared.

Comparative Example 2: Preparation of Optical Sheet (5)

By the same method as in Example 2, except that a diffusion film 1 having a thickness of 100 μm and a heat shrinkage rate of 0.62% and a diffusion film 2 having a thickness of 100 μm and a heat shrinkage rate of 0.94% were used. The optical sheet 4 having the structure of FIG. 7 in which the difference in thermal contraction rate between the diffusing film 1 and the diffusing film 2 is 0.34% and the difference in thermal contraction rate between the λ / 4 retardation film and the diffusing film 1 is 0.04% Prepared.

In the present example and the comparative example, a diffusion film having a thickness of 100 μm was used. However, even if a diffusion film having a different thickness is used according to the intended use, it can be applied as long as it has the same thermal contraction rate.

Experimental Example 1 Measurement of Thermal Shrinkage

The film specimens having a length of about 200 mm and a width of 10 mm were placed in an air circulating oven maintained at 150 ° C. and heat-treated at no load for 5 minutes. The specimen was taken out of the oven and left to stand at room temperature for 1 hour to measure the length, and the heat shrinkage percentage (%) was calculated according to the following equation (1).

[Equation 1]

Thermal Shrinkage (%) = [(L-L ') / L] X 100

In the above formula, L is the length of the specimen before heat treatment, and L 'is the length of the specimen after heat treatment.

Experimental Example 2: Calculation of the difference in thermal contraction rate between two films (first film and second film)

The difference (%) of thermal contraction rate between the two films was calculated according to Equation 2 below.

[Equation 2]

Difference in Thermal Shrinkage (%) = [(δ _f2 -δ _f1 ) / δ _f1 ] X 100

Wherein δ _f1 and δ _f2 is the thermal contraction rate of the first film and the thermal contraction rate of the second film, respectively, and δ _f2 ≧ δ _f1 .

Experimental Example 3: Measurement of curl size (Δ)

For each of the optical sheets (1), (2), (3), (4) and (5) prepared as described above, the size Δ of the curl was measured in the following manner.

First, as shown in FIG. 8A, the optical sheet 40 is cut into 10 cm x 10 cm (width x length), the optical sheet 40 is placed on a glass plate (not shown), and then four corners are cut. As a reference, the incision was made along the diagonal cut line CL toward the center from the point 1 cm apart in the transverse and longitudinal directions, respectively. Each of the optical sheets thus prepared was stored for one week in an environment with a relative humidity of 90% and a temperature of 65 ° C.

Subsequently, as shown in FIG. 8B, the separation distance from the smoothest surface of the optical sheet 40 among the curled portions formed along the incision line CL to the farthest portion is measured, and the curled distance measured at each corner is measured. It was made to size. In addition, four sizes of curls measured at each corner were averaged and the value was taken as the size (Δ) of the curl of the final optical sheet 40.

The results of the curl size (Δ) measurement are shown in Table 1 below.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Size of curl (Δ), mm 0.7 0.5 0.6 3.2 2.8

Referring to Table 1, Examples 1, 2, and 3 in which the optical shrinkage of the optical films disposed on one surface of the reflective polarizer and the optical film disposed on the other side of the optical film were adjusted to be substantially the same. In this case, it can be seen that the size (Δ) of the generated curl is smaller than that of Comparative Examples 1 and 2 that are not.

Although the present invention has been described above with reference to the drawings and embodiments, these are merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. . Therefore, the protection scope of the present invention should be defined by the appended claims.

1 is a cross-sectional view of an optical sheet according to the prior art having a reflective polarizer, a λ / 4 retardation film, and a diffusion film.

FIG. 2 is a cross-sectional view schematically illustrating a state where curling occurs due to deformation of the optical sheet of FIG. 1 due to changes in the surrounding environment such as temperature and / or humidity changes.

3 is a cross-sectional view of an optical sheet according to an embodiment of the present invention having a reflective polarizer, a λ / 4 retardation film, a compensation film, and a diffusion film.

4 to 7 are cross-sectional views of an optical sheet according to another embodiment of the present invention having a reflective polarizer, a λ / 4 retardation film, a compensation film, and a diffusion film.

8A and 8B illustrate a method of measuring the degree of curl generated in an optical sheet according to a change in temperature / humidity.

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

10, 20, 30, 40: optical sheet 11, 21, 31: reflective polarizer

12, 22, 32: lambda / 4 phase difference film 13, 24, 34, 35: diffusion film

Compensation Film: 23, 33 40a: Curl

CL: Incision Δ: Size of curl

δ: thermal shrinkage

Claims (10)

A first optical film; A reflective polarizer disposed on the first optical film; And A second optical film disposed on the reflective polarizing plate, And the first optical film and the second optical film have substantially the same thermal contraction rate. The method of claim 1, The first optical film and the second The thermal shrinkage difference of the optical film is an optical sheet, characterized in that within 0.01% to 0.3%. The method of claim 1, The first optical film is a diffusion film, the second optical film is an optical sheet, characterized in that the lambda / 4 phase difference film. The method of claim 1, And at least one optical film disposed between the reflective polarizer and the second optical film. The method of claim 4, wherein The first optical film and the second optical film is a diffusion film, the optical sheet further comprises a λ / 4 phase difference film disposed between the reflective polarizing plate and the second optical film. The method of claim 1, The reflective polarizer is a type in which a CLC (cholesteric liquid crystal) film is laminated, a type in which an isotropic and anisotropic material-containing film is alternately laminated, and an island of isotropic or isotropic material complementarily in a matrix of isotropic or anisotropic material. ) Is an optical sheet, characterized in that any one type of dispersion. The method according to claim 3 or 5, The diffusion film is a coating of organic or inorganic light diffusion particles on at least one surface of a PET (polyethylene terephthalate) or a PC (polycarbonate) film, the λ / 4 retardation film is an optical sheet, characterized in that the PC film. The method according to claim 3 or 5, And a compensation film disposed on the lambda / 4 phase difference film or between the lambda / 4 phase difference film and the diffusion film. A backlight unit comprising the optical sheet according to any one of claims 1 to 6. A liquid crystal display device comprising the optical sheet according to any one of claims 1 to 6.

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