JPH10186109A - Diffusion film, its production and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film having a large effect for widening a visual angle to a liquid crystal display device to be inexpensively and easily manufactured. SOLUTION: When the film is a uniaxially oriented amorphous and thermoplastic resin, the film is provided with craze in a direction rectangular to the oriented direction so that the average length L of the craze is 1μm to 50μm, the average width D is >=1μm and the ratio of length to width (L/D) is >=1. The chromaticity of transmitted light is 0 to 6 when the color coordinates in L*a*b* display system is b*. Thus the film is arranged on the liquid crystal display device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffusion film used for a liquid crystal display and a method for manufacturing the same. Further, the present invention relates to a liquid crystal display device having the viewing angle characteristics improved by disposing the diffusion film.

[0002]

2. Description of the Related Art Liquid crystal displays (LCDs) have become widespread in word processors, notebook computers, monitors, and the like because of their features of being thin, light, and low in power consumption. Both the active matrix drive type LCD and the simple matrix drive type LCD have been able to obtain image quality exceeding CRT when viewed from the front due to technological improvements so far, but contrast when viewed from an oblique direction. Since the viewing angle characteristics such as a decrease and a change in hue are not sufficient as compared with the CRT, this improvement is strongly desired.

[0003] The viewing angle of LCDs has been improved by various methods. Among them, a method of expanding an image in the frontal direction in all directions by using a microlens and a method of reducing scattering as shown in Japanese Patent Application Laid-Open No. 8-292322. There is a method of using a direction-dependent optical film to increase the viewing angle by scattering. Japanese Patent Application Laid-Open No. H07-146403 discloses a method of generating craze in a direction parallel to the molecular orientation by contacting a thermoplastic resin film with a blade. It has been disclosed.

[0004]

However, the method using a microlens and the method described in Japanese Patent Application Laid-Open No. 8-292322 are effective for both modes of LCD, but are not suitable for industrial production. It is not always satisfactory because it requires the technology described above and is relatively expensive. Further, the film disclosed in JP-A-7-146403 is suitable for use in viewing angle control (privacy filter) in which the craze size is large and becomes opaque from an oblique direction, but the effect of widening the viewing angle of the LCD is obtained. I can't.

[0005]

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on a film which is inexpensive and can be easily manufactured and has a large effect of improving the viewing angle of an LCD, and as a result, yielded an amorphous thermoplastic resin. 80% or more of stress 100
By performing uniaxial stretching at a deformation rate of 30 mm / sec or less under a stress of less than 30%, a diffusion film having a craze having a specific shape in a direction orthogonal to the orientation direction is obtained, and this diffusion film has a large effect of improving the viewing angle of the LCD. This led to the present invention.

That is, the present invention is as follows. (1) A uniaxially stretched amorphous thermoplastic resin film having crazes in a direction orthogonal to the orientation direction, and having an average length (L) of 1 μm or more and 50 μm or less,
The average width (D) is 1 μm or more, the length to width ratio (L / D) is 1 or more, and the chromaticity of transmitted light is L ^* a ^*.
diffusing film, wherein the b ^* in the color system of the color coordinate b ^* is 0 to 6. (2) Amorphous thermoplastic resin is reduced to 80% or more of yield stress
The method for producing a diffusion film according to the above (1), wherein the film is uniaxially stretched at a deformation rate of 30 mm / sec or less under a stress of less than 0%. (3) A liquid crystal display device in which the diffusion film according to the above (1) is disposed on the upper or lower surface of an upper polarizing plate. Hereinafter, the present invention will be described in detail.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION As the amorphous thermoplastic resin film used in the present invention, polycarbonate, polyarylate, polystyrene, polymethyl methacrylate and the like can be mentioned, but polycarbonate is preferred from the viewpoint of transparency and workability.

[0008] In these amorphous thermoplastic resin films, crazes are generated by the action of stress exceeding the critical stress. By uniaxially stretching the amorphous thermoplastic resin film, craze occurs in a direction orthogonal to the orientation direction. Factors that affect the size, shape, and density of the craze include the stretching ratio, stretching temperature, deformation speed, and resin molecular weight. In addition, adding a filler or a plasticizer at the time of producing the film, or creating minute defects on the film surface by irradiating ultraviolet rays or corona treatment change the expression and size of craze and change the optical characteristics.

[0009] The average length of the craze is high definition LC
In order not to adversely affect the image quality of D, the thickness needs to be 50 μm or less, and preferably 20 μm or less. 50μ
LCD display becomes rough when it exceeds m
Image quality is deteriorated. In addition, when white light is incident on the diffusion film, blue light having a short wavelength is particularly scattered, and the parallel transmitted light may become yellow. This is a phenomenon that occurs when the size of the craze is as small as the wavelength of visible light, and is not preferable because it causes a decrease in the visibility of the LCD. In order to prevent the transmitted light from being colored, the average length and average width of the craze must be sufficiently larger than the wavelength of visible light. That is, the craze of the diffusion film of the present invention has an average length (L) of 1 μm to 50 μm and an average width (D) of 1
μm or more, and the ratio of length to width (L / D) is 1 or more.

[0010] Since craze is an orientation phenomenon, the size of craze is affected by the magnitude of applied stress, time and temperature. The craze can be developed by stretching under a stress of 80% or more and less than 100% of the yield stress, and the craze grows sufficiently by stretching at a deformation speed of 30 mm / sec or less to reduce coloring of transmitted light. be able to. Specifically, the stretching stress and the deformation speed are controlled in the above-mentioned range by the stretching temperature, the stretching ratio, and the stretching speed.

According to the above method, the degree of coloring (yellow) is
Or with a chromaticity meter^*a^*b ^*Color coordinates b of the color system
^*(JIS Z8729), the value of which is 6 or less
A film is obtained, and if it is 6 or less, the visibility of the LCD decreases.
Is acceptable.

The haze value of the diffusion film is required to be 20% or more in order to obtain sufficient scattering for improving the viewing angle of the LCD, but the haze value must be 40% or less in order to secure luminance and not lower contrast. No.

Since the diffusion film of the present invention can be produced by stretching, a uniform and large area can be easily obtained.
The stretching method can be any method such as a roll-to-roll stretching method and a tenter method. The transverse stretching method by the tenter method is suitable because a large force can be continuously and stably applied, and is excellent in homogeneity. For example, when the chip is cut at 30 cm × 25 cm, which is equivalent to a 15-inch monitor size, the in-plane variation in haze can be obtained with a uniformity of ± 2% or less. b ^* is also obtained with a homogeneity within ± 0.5. In the present invention, the length and width of the craze, the chromaticity, and the haze are average values at four corners and five points at the center of a 30 cm × 25 cm diffusion film.

By arranging the diffusion film of the present invention on the upper or lower surface of the upper polarizing plate, the viewing angle of the liquid crystal display can be improved. When the diffusion film is disposed on the upper surface of the upper polarizing plate and used, an additional function can be provided on the surface of the diffusion film. For example, a transparent protective film for preventing scratches may be attached to the surface of the diffusion film which is the outermost surface, or a hard coat layer for preventing scratches may be provided. Further, an anti-glare layer which forms fine irregularities on the surface to prevent the reflection of external light and irregularly reflects the external light, or an anti-reflection layer composed of a multilayer thin dielectric film may be formed. Further, a transparent protective film having an anti-reflection layer formed thereon may be laminated, or an anti-reflection layer may be formed on the hard coat layer. Further, when the diffusion film of the present invention does not exhibit the retardation value or when the orientation direction is arranged so as to be parallel or orthogonal to the absorption axis of the upper polarizing plate even if the retardation value is exhibited, it is disposed on the lower surface of the upper polarizing plate. And diffused reflection of external light by the diffusion film can be reduced.

[0015]

According to the present invention, a diffusion film can be easily manufactured industrially, and by arranging the film, the viewing angle of a liquid crystal display device can be improved.

[0016]

The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples. The evaluation was performed by the following method. The following (1) length and width of the craze, (2) chromaticity, and (3) haze value of a 30 cm × 25 cm diffusion film equivalent to a 15-inch monitor size.
Measurements were made at five points at the corners and the center, and the average value was displayed. (1) Length and width of craze of diffusion film (μ)
m): Optical microscope (OPTIPHOT2-POL Ni)
(manufactured by kon Co., Ltd.), a range of 100 μm × 100 μm was observed at 400 × magnification at the measurement points, and the average value of the five measurement points was displayed. (2) Chromaticity of diffusion film: Shimadzu self-recording spectrophotometer (UV-
2200 (manufactured by Shimadzu Corporation), and L ^* a ^* b
^* Represented by a color system (JIS Z8729). (3) Haze value of the diffusion film: Measured using a haze meter (manufactured by Haze Computer, Suga Test Instruments Co., Ltd.). (4) Confirmation of viewing angle improving effect of LCD: STN type L
CD (contrast is 17.8 in front, 40 ° 1.8 above)
7, lower 40 ° 0.93, and the chromaticity of white display at the front was determined by using (x, y) = (0.29491, 0.31655) in the XYZ color system. (5) Measurement of LCD brightness and chromaticity: Determined by an instantaneous multi-photometry system (MCPD2000, manufactured by Otsuka Electronics Co., Ltd.).

Example 1 The thickness obtained by the solvent casting method was 140 μm.
m was uniaxially stretched 3.0 times at 168 ° C. at a deformation rate of 25 mm / sec by a tenter method. Craze average length 7 μm, average width 1 μm, haze 3
A diffusion film of 0% was obtained. Chromaticity is L ^* 84.3
6, a ^* 0.12 and b ^* 2.96. The film was arranged on the upper surface of the upper polarizing plate so that the orientation method was in the vertical direction of the LCD, and the contrast in the front and the upper and lower directions of 40 ° and the decrease in front luminance and change in chromaticity in white display were measured. The front luminance in the display is 6 when not used.
The contrast was 8.8%, the contrast was 18.1 in front, 4.6 above, and 3.9 below, showing a viewing angle improving effect. The chromaticity of white display is (
x, y) = (0.29787, 0.31819), and there was no problem in visibility.

Example 2 The thickness obtained by the solvent casting method was 70 μm.
Was uniaxially stretched 2.0 times at 80 ° C. and 12 mm / sec by a tenter method.
Craze average length 20 μm, average width 1 μm, haze 25
% Diffusion film was obtained. Chromaticity L ^* 77.5, a ^*
0.20, b ^* 0.75. The film was arranged on the lower surface of the upper polarizing plate so that the orientation method was in the vertical direction of the LCD, and the contrast in the front and upper and lower directions of 40 ° and the decrease in front luminance and change in chromaticity in white display were measured. The front luminance in display is 74.5 when not used.
%, Contrast is 13.8 in front, 3.2 upper, 3.1 lower
And there was a visual angle improving effect. The chromaticity of the white display is (x, y
) = (0.29523, 0.31712), and there was no problem in visibility.

Comparative Example 1 The thickness obtained by the solvent casting method was 140 μm.
m was uniaxially stretched 2.5 times at 163 ° C. at a deformation rate of 46 mm / sec by a tenter method. Average length of craze 15 μm, average width 0.5 μm,
A diffusion film with a haze of 23% was obtained. Chromaticity L ^* 84.
83, a ^* 0.03, b ^* 8.73. The film was arranged in the same manner as in Example 1, and the front and the contrast in the vertical direction of 40 ° were measured. As a result, the front luminance in white display was 75.8% when not used, and the contrast was 1
3.8, 3.2, and 2.1, indicating a visual angle improving effect. The chromaticity of white display is (x, y) = (0.30956, 0.32959
) Was yellowish and problematic.

Comparative Example 2 The thickness obtained by the solvent casting method was 140 μm.
The polycarbonate film of m was pulled by a tenter so as to be 1.01 times at room temperature, and was brought into contact with ethanol in a state where tension was applied in the lateral direction. A film having an average craze length of 1.1 mm, an average width of 10 μm, and a haze value of 4% was obtained. Chromaticity L ^* 95.8, a ^* -0.18, b
^* 0.52. The film was placed in the same manner as in Example 1, and the contrast in the front and in the vertical direction of 40 ° was measured.
1%, contrast 17.9 in front, 1.9 on top, 0.
9 and no visual angle improving effect was observed. The chromaticity of white display was (x, y) = (0.29505, 0.31685), which was no problem.

[0021]

[Table 1] CR: Contrast Luminance: Relative intensity with the luminance of the front white display when the diffusion film is not used as 100

[0022]

[Table 2]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view when a diffusion film is disposed on the upper surface of an upper polarizing plate of an STN LCD.

FIG. 2 is a cross-sectional view when a diffusion film is arranged on the lower surface of an upper polarizing plate of an STN LCD.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diffusion film 2 Upper polarizing plate 3 Upper retardation plate 4 STN-LCD panel 5 Lower retardation plate 6 Lower polarizing plate 7 Condensing lens sheet 8 Backlight system

Claims (5)

[Claims] 1. An amorphous thermoplastic resin film which is uniaxially stretched and has a craze in a direction orthogonal to an orientation direction, and the craze has an average length (L) of 1 μm or more and 50 μm or more.
m, the average width (D) is 1 μm or more, the length to width ratio (L / D) is 1 or more, and the chromaticity of the transmitted light is
A diffusion film having a color coordinate b ^* of 0 to 6 in L ^* a ^* b ^* color system. 2. The diffusion film according to claim 1, wherein the haze value is 20% or more and 40% or less. 3. The production of a diffusion film according to claim 1, wherein the amorphous thermoplastic resin is uniaxially stretched at a deformation rate of 30 mm / sec or less under a stress of 80% or more and less than 100% of a yield stress. Method. 4. The method according to claim 3, wherein the uniaxial stretching is transverse stretching by a tenter method. 5. A liquid crystal display device comprising the diffusion film according to claim 1 disposed on the upper or lower surface of an upper polarizing plate.