JP2010064418A - Optical sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element which improves crease in an optical sheet used for controlling illumination optical path in a backlight unit for a display and used for a most obverse surface of liquid crystal panel side. <P>SOLUTION: The optical sheet is used for controlling illumination optical path in a backlight unit for a display, is produced by an extrusion molding process, and satisfies the formula (1): 0.8<A/B<1.2, where A is the linear expansion coefficient of the optical sheet in the flow direction at 75°C±5°C and B is the linear expansion coefficient of the optical sheet in the direction normal to the flow direction at 75°C±5°C. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention illuminates, from the back side, a liquid crystal panel in which a transmissive / non-transmissive lens sheet and a display optical sheet in pixel units or a display element in which a display pattern is defined according to a transparent state / scattering state is arranged The present invention relates to a backlight unit and a display device.

  In recent years, liquid crystal display devices using TFT liquid crystal panels and STN liquid crystal panels have been commercialized mainly for color notebook PCs (personal computers) in the OA field.

  In such a liquid crystal display device, a so-called backlight method in which a light source is arranged on the back side (observer side) of the liquid crystal panel and the liquid crystal panel is illuminated with light from the light source is employed.

  As a backlight unit employed in this type of backlight system, a light source lamp such as a cold cathode fluorescent lamp (CCFT) is roughly divided within a flat light guide plate made of acrylic resin having excellent light transmittance. There are a “light guide plate light guide method” for reflecting (a so-called edge light method) and a “direct type method” that does not use a light guide plate.

  As a liquid crystal display device on which a light guide plate light guide type backlight unit is mounted, for example, the one shown in FIG. 4 is generally known.

  This is provided with a liquid crystal panel 72 sandwiched between polarizing plates 71 and 73 at the top, and a light guide plate 79 made of a transparent base material such as a substantially rectangular plate-like PMMA (polymethyl methacrylate) or acrylic on the lower surface side. Is installed, and a diffusion film (diffusion layer) 78 is provided on the upper surface (light emission side) of the light guide plate.

  Further, a scattering reflection pattern portion for efficiently scattering and reflecting the light introduced into the light guide plate 79 in the direction of the liquid crystal panel 72 is provided on the lower surface of the light guide plate 79 by printing or the like. A reflection film (reflection layer) 77 is provided below the scattering reflection pattern portion (not shown).

Further, the light guide plate 79 is provided with a light source lamp 76 at the side end, and further covers the back side of the light source lamp 76 so that the light from the light source lamp 76 can be efficiently incident on the light guide plate 79. Thus, a high-reflectance lamp reflector 81 is provided. The scattering reflection pattern portion is formed by printing, drying, and forming a mixture of white titanium dioxide (TiO ₂ ) powder in a solution such as a transparent adhesive in a predetermined pattern, for example, a dot pattern. The light incident on the light plate 79 is imparted with directivity and guided to the light exit surface side, which is a device for increasing the brightness.

  Further, recently, a prism film (prism layer) having a light condensing function is provided between the diffusion film 78 and the liquid crystal panel 72 as shown in FIG. ) 74 and 75 are proposed. The prism films 74 and 75 are configured to collect light emitted from the light exit surface of the light guide plate 79 and diffused by the diffusion film 78 on the effective display area of the liquid crystal panel 72 with high efficiency.

However, in the apparatus illustrated in FIG. 4, the control of the viewing angle is left only to the diffusibility of the diffusion film 78, which is difficult to control, and the center in the front direction of the display is bright and becomes darker toward the periphery. This characteristic is inevitable. For this reason, when the liquid crystal screen is viewed from the side, the luminance is greatly reduced, and the light utilization efficiency is reduced.
Furthermore, in the apparatus using the prism film illustrated in FIG. 5, two prism films are required, which not only causes a large decrease in the amount of light due to the absorption of the film but also increases the cost due to the increase in the number of members. It was also.

  On the other hand, in the direct type, a display device such as a large liquid crystal TV in which the light guide plate is difficult to use is used.

  As a direct-type liquid crystal display device, a device illustrated in FIG. 6 is generally known. In this, a liquid crystal panel 72 sandwiched between polarizing plates 71 and 73 is provided on the upper side, and is emitted from a light source 51 made of a fluorescent tube or the like on the lower surface side thereof and diffused by an optical sheet such as a diffusion film 82. The light is condensed on the effective display area of the liquid crystal panel 72 with high efficiency. In order to efficiently use the light from the light source 51 as illumination light, a reflector 52 is disposed on the back surface of the light source 51.

  However, even in the apparatus illustrated in FIG. 6, the control of the viewing angle is left only to the diffusibility of the diffusion film 82, which is difficult to control, and the center in the front direction of the display is bright and becomes darker toward the periphery. This characteristic is inevitable. For this reason, when the liquid crystal screen is viewed from the side, the luminance is greatly reduced, and the light utilization efficiency is reduced. Furthermore, in the case of using a prism film, since the number of prism films is two, not only the light amount is greatly decreased due to absorption of the film but also the cost is increased due to an increase in the number of members.

  If the distance between the light sources 51 is too wide, uneven brightness tends to occur on the screen, and the number of light sources 51 cannot be reduced, leading to an increase in power consumption and an increase in cost.

  By the way, in such a liquid crystal display device, light weight, low power consumption, high luminance, and thinning are strongly demanded as market needs, and accordingly, a backlight unit mounted on the liquid crystal display device is also light. In addition, low power consumption and high brightness are required.

  In particular, in a color liquid crystal display device that has recently made remarkable progress, the panel transmittance of the liquid crystal panel is remarkably lower than that of a monochrome-compatible liquid crystal panel. It is essential to obtain power consumption.

However, as described above, it is difficult to say that the conventional apparatus sufficiently satisfies the demand for high luminance and low power consumption, and the user has low price, high luminance, high display quality, and low power consumption. The development of a backlight unit capable of realizing a power liquid crystal display device is awaited.
Special table 2008-515026

Attempts have been made to realize the functions of a plurality of optical sheets used in the backlight unit with a smaller number of optical sheets. The development of optical sheets that have both light condensing and diffusing functions has been actively conducted.
However, when an optical sheet having a light collecting function is positioned on the outermost surface on the panel side, there is a problem that the optical sheet is very noticeable when the optical sheet is wrinkled due to changes in the surrounding environment, particularly heat when the light source is turned on. .
In particular, wrinkles are likely to occur in a sheet prepared by an extrusion method.
Although there is a method of increasing the thickness of the optical sheet in order to prevent the generation of wrinkles, there is a problem of light loss and warpage due to an increase in cost and an increase in thickness, and this is not a complete solution.
The present invention was devised by paying attention to a sheet prepared by an extrusion method, and an object of the present invention is to provide an optical sheet that is advantageous in suppressing the generation of wrinkles without increasing the thickness. There is to do.

In order to achieve the above object, an invention according to claim 1 is an optical sheet manufactured by extrusion molding and used for illumination light path control of a backlight unit for a display, wherein the optical sheet has a flow direction in the extrusion molding. The optical sheet is characterized in that a linear expansion coefficient A at 75 ° C. ± 5 ° C. and a linear expansion coefficient B at 75 ° C. ± 5 ° C. in the direction perpendicular to the flow direction of the optical sheet satisfy Expression (1).
0.8 <A / B <1.2 Formula (1)

  The invention of claim 2 is characterized in that the optical sheet has a matrix-like structure having a light collecting function on the surface.

  The invention according to claim 3 is characterized in that the optical sheet has a linear structure on the surface, in which a plurality of protrusions having a light collecting function and extending linearly are arranged in parallel.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the optical sheet has a structure that exhibits a light diffusing function on the surface, or exhibits a light diffusing function by containing particles in the sheet material. The optical sheet according to claim 1.

  According to a fifth aspect of the present invention, there is provided a display backlight, comprising: a light source and at least the optical sheet according to any one of the first to fourth aspects on a back surface of an image display element that defines a display image. Is a unit.

  As the light source, a cold cathode tube, an LED, a semiconductor laser, or the like can be used.

  The invention of claim 6 comprises an image display element comprising a liquid crystal display element that defines a display image in accordance with transmission / shading in pixel units, a light source, and a backlight unit according to claim 5. Display.

As a result of intensive research, the present inventors have made a great difference in the linear expansion coefficient between the sheet flow direction (MD) and the direction perpendicular to the flow direction (TD). I found out.
Here, the sheet flow direction (MD) is the direction in which the product flows on the production line of the extrusion process.
As a result of further earnest research, the cause of wrinkling of the optical sheet was considered to be that the expansion and contraction ratios of the MD and TD of the optical sheet differed under heat and distortion occurred.
The expansion ratio of MD and TD of the optical sheet can be confirmed by the linear expansion coefficient.
According to the present invention in which the difference between the linear expansion coefficients of MD and TD of the optical sheet produced by the extrusion method is reduced, that is, the present invention in which the difference between the linear expansion coefficients of MD and TD of the optical sheet is suppressed within a predetermined range. According to, it became possible to eliminate wrinkles.
The adjustment work for reducing the difference in the linear expansion coefficient between the MD and TD of the optical sheet can be performed at the time of manufacturing the extrusion process, and can also be performed after the product is manufactured.
The reason why the value of 75 ° C. ± 5 ° C. is used as the linear expansion coefficient of MD and TD is that the backlight unit of the liquid crystal television is about 75 ° C. ± 5 ° C. when in use. Applications are not limited to liquid crystal televisions, and are widely applied to various devices other than liquid crystal televisions.

  Embodiments of the present invention will be described below.

  2 and 3 show a cross section of a backlight configuration example using the optical sheet of the present invention. Light K from the light source 15 enters the diffusion plate 5. Thereafter, the light reaches the incident surface of the optical sheet 2 or 1 from the exit surface of the diffusion plate 5. Finally, the light is emitted as L from the emission surface of the optical sheet 1. L reaches the liquid crystal layer 19 sandwiched between the polarizing plates 21. The light transmitted through here is emitted to S and is visually recognized by an observer. Note that not only the optical sheet 2 but also the number of optical sheets may be appropriately increased between the optical sheet 1 and the diffusion plate 5. In any configuration, the optical sheet 1 of the present invention is located on the outermost surface on the liquid crystal layer side.

  FIGS. 1 (a-1), (a-2), (a-3), (b), and (c) are diagrams showing a configuration example of the optical sheet of the present invention. (a-1) (a-2) (a-3) shows an example of arrangement of MD and TD, and (b) and (c) show examples of the shape of the optical sheet. As described above, the wrinkles of the optical sheet are caused by the difference between the linear expansion coefficients of MD and TD generated by extrusion. Therefore, the optical sheet of the present invention which has improved this is in any orientation with respect to the shape. TD may be arranged. Further, the optical sheet shape may be appropriately changed according to the housing of the display.

  As the main material of the optical sheet, for example, polycarbonate, acrylic-styrene copolymer, polystyrene, styrene-butadiene-acrylonitrile copolymer, or cycloolefin polymer may be used. Moreover, you may have the transparent particle disperse | distributed in the main material, and the refractive index of these main materials and the refractive index of a transparent particle differ. The difference between the refractive index of the main material and the refractive index of the transparent particles is preferably 0.01 or more. If the difference in refractive index is smaller than this, sufficient light scattering performance cannot be obtained. Further, the refractive index difference is 0.5 or less. The transparent particles preferably have an average particle size of 0.5 to 30.0 μm.

  As the transparent particles, transparent particles made of an inorganic oxide or transparent particles made of a resin can be used. For example, examples of the transparent particles made of an inorganic oxide include particles made of silica, alumina or the like. The transparent particles made of resin include acrylic particles, styrene particles, styrene acrylic particles and cross-linked products thereof; melamine-formalin condensate particles, PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy resin), FEP (tetra Examples thereof include fluorine-containing polymer particles such as fluoroethylene-hexafluoropropylene copolymer), PVDF (polyfluorovinylidene), and ETFE (ethylene-tetrafluoroethylene copolymer), and silicone resin particles. These transparent particles may be used as a mixture of two or more. Alternatively, the plate-like member has a structure in which a main material has a fine cavity containing air, and diffusion performance may be obtained by a difference in refractive index between the main material and air.

  The optical sheet may have fine unevenness on the surface, and the surface may have light diffusibility. Here, examples of the fine unevenness include a convex cylindrical shape, a lens shape, and a triangular prism shape, but are not limited thereto, and the light diffusion function is compared before the fine unevenness is given. However, the shape is not limited to the above as long as the shape is improved. In addition, fine irregularities can improve appearance characteristics such as optical adhesion, unevenness, and Newton rings.

  The optical sheet may have a multilayer structure or may include a transparent layer.

  The optical sheet has a light collecting function by providing a structure on the surface. As a structure having a light collecting function, a prism, a lens, a microlens, or a combination of these is well known. In recent years, as shown in Japanese Translation of PCT International Publication No. 2008-515026, many shapes that have not been applied to the definition of the structure so far have been proposed, but any optical sheet having a condensing function can be used as well.

  The optical sheet is manufactured by an extrusion method, and those having a thickness of 12 μm or more and 1 mm or less can be used.

Here, adjustment of the linear expansion coefficients of MD and TD when manufactured by a normal extrusion method will be described. That is, an adjustment operation for reducing the difference in the linear expansion coefficient between MD and TD after the optical sheet becomes a product will be described.
The optical sheet melt-extruded from the die is molded with a mold having a desired shape and size, cooled by a plurality of rolls having different temperatures so that the temperature gradually decreases, and wound.
The optical sheet is stretched by applying tension to the MD of the line while the temperature is high, and the TD does not receive any particular force.
And it is cooled in this state.
For this reason, when heat is applied to the created optical sheet, a force that shrinks and restores the stretched MD acts, and the linear expansion coefficient decreases.
Therefore, if heating is repeated several times and the internal stress in the stretched state disappears, the linear expansion coefficient of MD approaches the value of the original material.
As described above, even after being commercialized, the linear expansion coefficient of MD can be adjusted by repeatedly performing heating, and the difference between the linear expansion coefficients of MD and TD can be within the range of formula (1). Become. There is no such movement in TD.

Further, the difference in the linear expansion coefficient between MD and TD of the optical sheet of the present invention may be adjusted during production so as to be in the range of the formula (1).
In this case, the MD tension applied to the optical sheet in the above-mentioned line must be reduced.
The optical sheet melt-extruded from the die is first laminated with the substrate sheet, cooled in the laminated state, sufficiently cooled, peeled off from the substrate sheet, and wound.
Unlike normal methods, the tension of the line is received together with the base sheet until cooling, so that the elongation of MD is relaxed and the difference in the linear expansion coefficient between MD and TD can be kept within the range of formula (1). Become.
Regarding the linear expansion coefficients of MD and TD, adjustment work performed at the time of manufacture and adjustment work performed after the product is manufactured may be used in combination.
As the base sheet, polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc., synthetic resins such as polyamide, polycarbonate, polypropylene, polyolefins such as poly-4-methylpentene 1 (TPX), and laminated sheets thereof Etc. can be used.

  In addition, if a reverse plate shape having a desired shape is given to the base sheet in advance, a desired shape and structure can be given to the optical sheet.

  The diffusion plate can be made of the same main material as that of the optical sheet, and may also be configured to include the above-described transparent particles. The refractive index of these main materials and the refractive index of transparent particles need to be different. The difference between the refractive index of the main material and the refractive index of the transparent particles is preferably 0.01 or more. If the difference in refractive index is smaller than this, sufficient light scattering performance cannot be obtained. Further, the refractive index difference may be 0.5 or less. In addition, since the light incident on the optical layer needs to be transmitted while being scattered, the average particle size of the transparent particles is preferably 0.5 to 30.0 μm. Alternatively, the main material may have a structure having fine cavities containing air, and the diffusion performance may be obtained by the difference in refractive index between the main material and air.

  The light source side optical sheet used together with the optical sheet of the present invention is appropriately used, such as a diffusion sheet and a prism sheet, which are well known in the art.

  The optical sheet produced as described above can improve wrinkles even when used on the panel side surface as compared with the optical sheet. When used in a backlight, a display having a desired display performance can be provided by using the optical member of the present invention or another optical member in combination.

  Examples will be described below.

(Production methods of Examples 1, 2, 3, 4 and Comparative Examples 1, 2)
A base sheet in which a prism shape having a pitch of 66 μm and an apex angle of 90 ° was formed on a PET with a UV curable resin was prepared. A thermoplastic polycarbonate resin sheet is melted, and the resin sheet is extruded into a sheet shape from a die by an extruder, laminated with the base sheet before the sheet cools and hardens, peels off from the base sheet after cooling. An extruded sheet having a lenticular lens was obtained. The thickness was 320 um. The line tension was adjusted to obtain extruded sheets with different MD linear expansion coefficients.
As the thermoplastic polycarbonate, M1201Z and ML1103 from Teijin Chemicals Ltd. were appropriately blended and used to obtain an extruded sheet having diffusibility. The Hz at 320 um was adjusted to 15%.
The extruded sheet was cut into a true square of 730 mm × 310 mm from the center to obtain an optical sheet. The long side direction of the optical sheet is MD.
In this manner, Examples 1, 2, 3, and 4 and Comparative Examples 1 and 2 were manufactured.

(Conventional manufacturing method)
A first mold roll cut into a shape of a convex cylindrical lens having a pitch of 66 μm was disposed in the vicinity of the extruder. A thermoplastic polycarbonate resin sheet was melted, molded by the extruder, and molded by the first mold roll before the sheet cooled and cured to obtain an extruded sheet having a lenticular lens. The thickness was 320 um.
As the thermoplastic polycarbonate, M1201Z and ML1103 from Teijin Chemicals Ltd. were appropriately blended and used to obtain an extruded sheet having diffusibility. The Hz at 320 um was adjusted to 15%.
The extruded sheet was cut into a true square of 730 mm × 310 mm from the center to obtain an optical sheet. The long side direction of the optical sheet is MD.
A conventional example was produced in this way.

(Measurement of linear expansion coefficient)
A measurement sample was cut into a 4 mm wide string from the MD and TD of each sheet.
The linear expansion coefficient was measured using a quartz tension jig of TMA (EXS TC6000PC station manufactured by SEIKO). While applying a weight of 50 mN to the measurement sample, a TMA-temperature curve of a 15 mm length portion was measured. The linear expansion coefficient in the range of 75 ° C. ± 5 ° C. was calculated. The results are shown in Table 1.

(Confirmation of wrinkles)
The produced optical sheet was incorporated into a 32 type display with a commercially available diffusion plate (Teijin Chemicals 65HLW), turned on for 1 hour, and visually checked for wrinkles. This was the initial state. The display in which no wrinkles were recognized in the initial state was put in an environment of 60 ° C. for 24 hours while the display was turned on. At this time, the environment where the optical sheet was placed was 75 ° C. ± 5 ° C. After 24 hours, the lamp was turned on and returned to room temperature, and wrinkles were visually confirmed every hour for 24 hours.
The results are shown in Table 1.
In Examples 1, 2, 3, and 4, no wrinkles occurred in the optical sheet, whereas in the conventional example and Comparative Examples 1 and 2, wrinkles occurred in the optical sheet.

Explanatory drawing which shows the structural example of the optical sheet of this invention. Explanatory drawing which shows the cross section of the backlight structural example using the optical sheet of this invention. Explanatory drawing which shows the cross section of the backlight structural example using the optical sheet of this invention. Explanatory drawing which shows the structural example of the liquid crystal display device by a prior art. Explanatory drawing which shows the structural example of the liquid crystal display device by a prior art. Explanatory drawing which shows the structural example of the liquid crystal display device by a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical sheet 2 of this invention Optical sheet 5 other than this invention Diffuser 10 Optical member 15 Light source 17 Reflector 19 Liquid crystal layer 21 Polarizing plate K Light from a light source
L Light emitted from the optical member S Display viewing direction

Claims (6)

An optical sheet manufactured by extrusion and used for illumination light path control of a backlight unit for display,
The linear expansion coefficient A at 75 ° C. ± 5 ° C. in the flow direction of the optical sheet during extrusion molding and the linear expansion coefficient B at 75 ° C. ± 5 ° C. in the direction perpendicular to the flow direction of the optical sheet satisfy the equation (1). Features
Optical sheet.
0.8 <A / B <1.2 Formula (1) The optical sheet has a matrix-like structure having a light collecting function on the surface,
The optical sheet according to claim 1. The optical sheet has a linear structure on the surface having a light collecting function and a plurality of linearly extending protrusions arranged in parallel.
The optical sheet according to claim 1. The optical sheet has a structure having a light diffusing function on the surface, or has a light diffusing function by containing particles in the sheet material,
The optical sheet according to any one of claims 1 to 3. On the back of the image display element that defines the display image,
A display backlight unit comprising at least a light source and the optical sheet according to claim 1. An image display element comprising a liquid crystal display element that defines a display image according to transmission / shading in pixel units;
A display comprising a light source and the backlight unit according to claim 5.

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