JP2007094112A - Diffusion plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the warpage of a diffusion plate that is mounted on a direct type backlight for a liquid crystal display device and disturbs an image by contacting the liquid crystal panel.
SOLUTION: The warp d of the diffusion plate 1 mounted on the direct type backlight is convex on the back side, and the warp d per 1 m width is set to 0.1 mm or more and less than 0.5 mm. And
[Selection] Figure 1

Description

  The present invention relates to a diffusion plate for a direct backlight of a liquid crystal display device.

  In recent years, liquid crystal display devices are rapidly expanding as large-sized and thin display devices replacing CRTs. Since the liquid crystal itself does not emit light, a back light source called a backlight is always integrated.

  There are two main types of backlights. One is a linear light source typified by a cold-cathode tube placed on the end surface of a transparent resin plate called a light guide plate, and the light guide plate surface is illuminated to make a liquid crystal back light source. This is a so-called edge light type backlight. The other is a so-called direct type backlight in which a large number of linear light sources are arranged and a diffusion plate is arranged thereon, and the linear light source directly becomes the back light source of the liquid crystal.

  Edge-light type backlights are widely used in notebook computer screens, LCD monitors, mobile phone screens, etc. in response to the strong demand for miniaturization and thinning. However, since light passes through a transparent resin plate called a light guide plate, it is difficult to increase the screen size and brightness, and it is difficult to apply when the size is 20 inches or more as in a large TV (television).

  On the other hand, direct-type backlights have not been able to meet the demands for miniaturization and thinning, but the market has been growing in recent years because it is very easy to handle large screens and high brightness by increasing the number of linear light sources. It is growing rapidly as a backlight for large LCD TVs that are expanding.

  Diffusers used in direct type backlights scatter light from a linear light source, and the light source is required not to be seen through. Various developments have been made in response to the rapid increase. Many of the developments aim at high transmission and high diffusion, and control the kind, particle size, and blending amount of the light scattering fine particles (see, for example, Patent Documents 1 to 3).

  In recent years, liquid crystal displays are becoming larger and larger in screen, and the diffusion plate mounted on the direct type backlight of the liquid crystal display warps due to moisture absorption, dehydration, heat, etc., and comes into contact with the liquid crystal panel, distorting the liquid crystal image. The problem of end up occurring.

  Further, as a result of various studies, it has been found that the cause of the warpage of the diffusion plate is largely dependent on the moisture absorption of the diffusion plate, and studies using low moisture absorption resin materials as the diffusion plate material are proceeding (for example, (See Patent Documents 4 and 5).

Japanese Patent Laid-Open No. 01-172801 Japanese Patent Laid-Open No. 02-194058 Japanese Patent Application Laid-Open No. 07-100755 JP 2001-342263 A JP 2004009524 A

  However, the use of a low moisture-absorbing resin as a diffuser plate certainly reduces the hygroscopic warpage of the diffuser plate when mounting a direct type backlight, and the above problems are being greatly improved, but prevents contact with the liquid crystal panel. It is not enough for this, and further improvements and countermeasures are urgently needed.

  The present invention solves the above-mentioned problems, and an object of the present invention is a diffusion plate mounted on a direct type backlight of a liquid crystal display device, which prevents contact with the liquid crystal panel and prevents disturbance of the liquid crystal image. It is intended to provide a diffusion plate that can be used.

  As a result of intensive studies, the present inventor prevents contact between the diffuser plate and the liquid crystal panel, which is a problem by preliminarily attaching a slight warp to the diffuser plate mounted on the direct type backlight in a certain direction. As a result, the present invention has been completed.

  That is, the diffusion plate according to the present invention for achieving the above object is a diffusion plate mounted on a direct backlight of a liquid crystal display device arranged in the order of a reflection sheet, a linear light source, a diffusion plate, and a liquid crystal panel. The warpage of the diffusion plate is convex on the back side, and the warpage per 1 m of width is 0.1 mm or more and less than 0.5 mm.

  The diffusion plate according to the present invention can be mounted on a direct backlight of a liquid crystal display device, and can realize a high-quality liquid crystal image with little warpage during actual use, no contact with a liquid crystal panel, and no disturbance.

  An embodiment of the diffusion plate according to the present invention will be specifically described with reference to the drawings. FIGS. 1A and 1B are a perspective explanatory view and a side explanatory view showing a configuration of a diffusion plate according to the present invention.

  The direct type backlight of the liquid crystal display device on which the diffusion plate 1 according to the present invention is mounted is a module in which a reflection sheet, a linear light source, the diffusion plate 1 shown in FIG. That is.

  The diffusing plate 1 is made of a light-transmitting resin such as acrylic resin, styrene resin, MS resin (methacrylate ester and styrene copolymer resin), polycarbonate resin, polyethylene, polypropylene, AS resin (polystyrene, acrylonitrile and styrene). Copolymer resin), ABS resin (acrylonitrile, butadiene, styrene copolymer resin), PET (polyethylene terephthalate; polyester), COP (cycloolefin polymer), alphamethylstyrene copolymer, polylactic acid, fluororesin, phenol resin, It is made of a resin obtained by singly or copolymerizing an epoxy resin, polyurethane, or the like, and has a plate shape with a plate thickness of 1.0 mm or more and 2.0 mm or less.

Since the diffusion plate 1 needs a light scattering function to scatter light from a linear light source, light scattering particles such as acrylic crosslinked fine particles, styrene crosslinked fine particles, silicone crosslinked fine particles, fluorine fine particles, Glass fine particles, SiO ₂ fine particles, calcium carbonate, barium sulfate, titanium oxide, alumina, talc (inorganic powder obtained by finely pulverizing ore called talc), mica, and the like are blended alone or in combination.

  The particle size of the light-scattering fine particles may be arbitrarily selected between an average particle size of 0.01 μm and 100 μm, and the shape is also true sphere, ellipse, sphere, irregular shape, needle shape, flake (petal) , Hollow, cubic, triangular pyramid, etc.

  The blending amount of the light scattering fine particles to be blended in the resin varies depending on the particle size of the fine particles and the thickness of the diffusion plate 1, but is preferably 0.001 wt% or more and 30 wt% or less, more preferably 0.01. % By weight or more and 20% by weight or less.

  The diffusion plate 1 is usually formed into a plate shape by various molding techniques such as extrusion sheet molding, injection molding, and cast molding. The surface of the diffusing plate 1 may be smooth or matte, but the diffusing plate 1 enhances the light scattering effect and prevents adhesion with various optical films often installed between the diffusing plate 1 and the liquid crystal panel. The surface is more preferably a mat.

  As a method of forming the surface of the diffusion plate 1 in a mat shape, a surface of the mold plate is provided with a mat-like unevenness and the shape is transferred, and a light scattering agent is lifted on the surface of the diffusion plate 1 Various methods such as a method of forming a surface mat layer by bonding a layer containing a large amount of a diffusing agent to the surface of the diffusion plate 1 can be used. Examples of the method for forming the surface mat layer include a coextrusion method, a film sticking method, and a multilayer injection molding method.

  The conventional diffusion plate 1 has a warp d per 1 m width of the diffusion plate of 0 mm or more and less than ± 0.1 mm, that is, the diffusion plate 1 without the warp d is an excellent diffusion plate 1 with good quality. I have been told. However, the diffuser plate 1 in the direct type backlight of the liquid crystal display device is a so-called high-quality diffuser plate 1 of a conventional type that is convex on the front side due to moisture absorption, dehydration, heat, etc. It was difficult to completely prevent the warpage d.

  As shown in FIG. 1, the diffuser plate 1 of the present invention is characterized in that a slight warpage d is applied in a predetermined direction in advance, and its shape is convex on the back side (that is, convex on the side opposite to the liquid crystal panel side). Further, the size of the warp d per 1 m width of the diffusing plate 1 is preferably only 0.1 mm or more and less than 0.5 mm. Here, as shown in FIG. 1, the size of the warp d per 1 m width of the diffusing plate 1 refers to the convex inner surface end 1a of the diffusing plate 1 from the central portion 1c of the convex inner surface of the diffusing plate 1. The length of the perpendicular line drawn down to the straight line connecting 1b.

  Since such a diffusion plate 1 can maintain the shape by resisting the force that warps convexly on the front side (that is, convex on the liquid crystal panel side), the diffusion plate 1 is convex on the back side (that is, opposite to the liquid crystal panel side). Convex to the side). However, if the warp d is large, the diffusing plate 1 is wavy and the image is disturbed. Therefore, the warp d must be very small.

  As a result of many experiments, the present inventor required that the diffusion plate 1 be warped by 0.1 mm or more per 1 m width in order to prevent the diffusion plate 1 from coming into contact with the liquid crystal panel. In order to prevent image distortion due to the above, it has been found that the size of the warp d should be less than 0.5 mm, and more preferably set to 0.3 mm or less.

  As a method of attaching the warp d to the diffuser plate 1, for example, a method of forming the warp d by changing the cooling rate of the front and back sides of the diffuser plate 1 by changing the mold temperature on the front and back sides of the diffuser plate 1 during sheet forming. Or by changing the cooling rate on the front and back sides of the diffusion plate 1 by making a difference in the uneven shape which is the mat shape on the surface of the diffusion plate 1, and reheating the diffusion plate 1 without the warping d And a method of sticking various films on the surface of the diffusion plate 1 and correcting the warpage d of the diffusion plate 1 by its bonding force. d must be able to be formed.

  The following description is based on specific examples and comparative examples.

  The warpage d of the diffusion plate 1 was evaluated as follows. A commercially available 37-inch type liquid crystal television (manufactured by Sharp Corporation) is modified, and the diffusion plate 1 produced in each of the following examples and comparative examples is installed inside the television. After that, the power is turned on with the black screen remaining without image (with the direct backlight turned on) and left for 24 hours. The black screen after 24 hours is visually observed to evaluate image disturbance. When the diffusing plate 1 is warped and touches the liquid crystal panel, the black screen is disturbed and white contact unevenness is generated, which can be easily determined visually.

[Example 1]
A resin (A) in which acrylic resin ("Delpet LP-1" manufactured by Asahi Kasei Chemicals Co., Ltd.) is blended with 1% by weight of spherical polymethylsilsesoxane fine particles having an average particle diameter of 2 μm as silicone-based crosslinked fine particles as a light scattering agent; Similarly, a resin (B) in which 15% by weight of scaly talc having an average particle diameter of 20 μm as a light scattering agent is blended in an acrylic resin (“Delpet LP-1” manufactured by Asahi Kasei Chemicals Corporation) by a coextrusion sheet molding method. A three-layer laminated sheet having a thickness of 1.5 mm was prepared by laminating the resin (B) by 40 μm on both sides of (A). At this time, the diffusion plate 1 was brought into contact with the front and back surfaces, and a warp d was formed on the diffusion plate 1 by giving a difference of 15 ° C. on the front and back surfaces to the temperature of the mold roll for cooling the diffusion plate 1.

  The warp d of the diffusing plate 1 is convex on the back side (that is, convex on the side opposite to the liquid crystal panel side), and the size of the warp d is 0.2 mm per 1 m width. The total light transmittance of the produced diffusion plate 1 is 60% when measured according to JISK7105, and the surface glossiness of the front and back surfaces of the diffusion plate 1 is measured according to JISK7105. there were.

  The diffuser plate 1 produced in this way was mounted on the above-mentioned 37-inch liquid crystal television, and the backlight was turned on for 24 hours to check the image disturbance, that is, the presence or absence of the warp d of the diffuser plate 1. There was no problem at all. The evaluation results are shown in Table 1 below together with comparative examples.

[Example 2]
The same operation as in Example 1 was performed except that the temperature difference between the front and back sides of the diffusion plate 1 of the mold roll was changed to 20 ° C. The warp d of the produced diffusion plate 1 is convex on the back side (that is, convex on the opposite side to the liquid crystal panel side), and the size of the warp d is 0.3 mm per 1 m width, and the surface gloss of the front and back surfaces of the diffusion plate 1 The degree was 20% on the front surface and 30% on the back surface as measured according to JISK7105. The evaluation results are shown in Table 1 below.

[Example 3]
The same procedure as in Example 1 was performed except that the temperature difference between the front and back sides of the diffusion plate 1 of the mold roll was changed to 10 ° C. The warp d of the manufactured diffusion plate 1 is convex on the back side (that is, convex on the side opposite to the liquid crystal panel side), and the size of the warp d is 0.1 mm per 1 m width, and the surface gloss of the front and back surfaces of the diffusion plate 1 When measured according to JISK7105, the degree was 20% on the front surface and 23% on the back surface. The evaluation results are shown in Table 1 below.

[Example 4]
The same operation as in Example 1 was performed except that the thickness of the diffusion plate 1 was set to 2.0 mm. The warp d of the manufactured diffusion plate 1 is convex on the back side (that is, convex on the side opposite to the liquid crystal panel side), and the size of the warp d is 0.1 mm per 1 m width, and the surface gloss of the front and back surfaces of the diffusion plate 1 The degree was 40% on the front surface and 60% on the back surface as measured according to JISK7105. The evaluation results are shown in Table 1 below.

[Comparative Example 1]
The same operation as in Example 1 was performed except that the temperature difference between the front and back sides of the diffusion plate 1 of the mold roll was 5 ° C. The produced diffusion plate 1 has a warp d of 0 mm and has no warp d, and the surface gloss of the front and back surfaces of the diffusion plate 1 is measured in accordance with JISK7105. Met.

  As in Example 1, the diffusion plate 1 was mounted on a 37-inch liquid crystal television, and the backlight was turned on for 24 hours to check whether the image was distorted, that is, whether the diffusion plate 1 was warped d. Later, white irregularities appeared in the center of the screen in about 4 hours. This is a problem that is caused by the disturbance of the image formed by the diffusion plate 1 being warped and contacting the liquid crystal panel. The evaluation results are shown in Table 1 below together with each example.

[Comparative Example 2]
The same procedure as in Example 1 was performed except that the mold roll was molded at the same temperature without creating a temperature difference between the front and back sides of the diffusion plate 1. The warp d of the manufactured diffuser plate 1 is convex on the front side (that is, convex on the liquid crystal panel side), the magnitude of the warp d is 0.1 mm per 1 m width, and the surface glossiness of the front and back surfaces of the diffuser plate 1 is JISK7105. When measured according to the above, it was 22% on the front surface and 20% on the back surface. The evaluation results are shown in Table 1 below.

[Comparative Example 3]
The same operation as in Example 1 was performed except that the temperature difference between the front and back sides of the diffusion plate 1 of the mold roll was set to 30 ° C. The warp d of the manufactured diffusion plate 1 is convex on the back side (that is, convex on the side opposite to the liquid crystal panel side), and the size of the warp d is 0.6 mm per 1 m width, and the surface gloss of the front and back surfaces of the diffusion plate 1 The degree was 30% on the front surface and 50% on the back surface as measured according to JISK7105. The evaluation results are shown in Table 1 below.

[Comparative Example 4]
The same procedure as in Example 1 was performed except that the thickness of the diffusion plate 1 was changed to 2.0 mm and the temperature difference between the front and back sides of the diffusion plate 1 of the mold roll was changed to 5 ° C. The produced diffusion plate 1 has a warp d of 0 mm and is finished in the diffusion plate 1 having no warpage d as in Comparative Example 1, and the surface glossiness of the front and back surfaces of the diffusion plate 1 is measured in accordance with JISK7105. The front surface was 50% and the back surface was 53%. The evaluation results are shown in Table 1 below.

  As an application example of the present invention, it can be suitably used as a direct-type backlight diffusion plate for liquid crystal display devices.

(A), (b) is the perspective explanatory drawing and side explanatory drawing which show the structure of the diffusion plate which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diffusing plate 1a, 1b ... Convex part inner surface edge 1c ... Convex part inner surface center part d ... Warpage

Claims (1)

A diffusion plate mounted on a direct type backlight of a liquid crystal display device arranged in the order of a reflection sheet, a linear light source, a diffusion plate, and a liquid crystal panel,
A diffusion plate for a direct type backlight, wherein the warp of the diffuser plate is convex on the back side, and the warp per 1 m of width is 0.1 mm or more and less than 0.5 mm.

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