JP2013051135A - Light guide plate, and backlight unit and display device using this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light guide plate capable of maintaining stiffness even if the thickness is thin, without deteriorating an optical performance, and having a sufficient reliability, and to provide a backlight unit for liquid crystal display and a display device which use the same.SOLUTION: The light guide plate 7 guiding incident light toward a display element: has a translucent first resin layer 17 and a translucent second resin layer 18 which are laminated in two-layer structure and are different in thickness and kind; forms a first optical element 15 on a light emission surface 17a of the first resin layer 17; and forms a second optical element 16 on the opposite surface to the first resin layer 17, of the second resin layer 18. Then, the first resin layer 17 has a bending strength larger than that of the second resin layer 18. Furthermore, the water absorption rate of the first resin layer 17 is made smaller than that of the second resin layer 18.

Description

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

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 (CCFL) 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 reflection, 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. 9 is generally known.
In such a liquid crystal display device, a liquid crystal panel 72 sandwiched between polarizing plates 71 and 73 is provided above the backlight unit. The backlight unit includes a light source lamp 76, a light guide plate 79 that reflects light from the light source lamp 76 to the liquid crystal panel 72, and a diffusion film (diffusion layer) 78. The light guide plate 79 is made of a transparent base material such as PMMA (polymethyl methacrylate) or acrylic having a substantially rectangular plate shape, and is disposed on the lower surface side of the liquid crystal panel 72. A diffusion film (diffusion layer) 78 is provided on the upper surface (light emission side) of the light guide plate 79.

  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.

A light source lamp 76 is attached to the side end portion of the light guide plate 79. Further, the light source lamp 76 is covered with the back side of the light source lamp 76 so that the light from the light source lamp 76 is efficiently incident on the light guide plate 79. 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.

  Furthermore, recently, in order to increase the light utilization efficiency and increase the brightness, a prism film (prism layer) having a light condensing function 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.

On the other hand, as a liquid crystal display device on which a direct type backlight unit is mounted, the one shown in FIG. 11 is generally known.
In such a liquid crystal display device, a liquid crystal panel 72 sandwiched between polarizing plates 71 and 73 is provided on the upper portion, and the lower surface thereof is emitted from a light source 51 made of a fluorescent tube, an LED, etc. The light diffused by the sheet 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.

In the current backlight market, there are strong demands for cost reduction, low power consumption, and thinning. In particular, regarding the problem of thinning, it is easy to realize a light guide plate light guide method as a backlight method. For this reason, in recent years, the light guide plate light guide method has been increasingly adopted particularly for thinning.
Although this method has the great advantage of being thin, it has been found that the backlight unit has a disadvantage in terms of reliability compared to the direct type. In addition, since the position of the light source is close to the optical member such as the light guide plate or the optical sheet in this method, when the light source is turned on, heat is generated from the light source, and the temperature of the optical member such as the light guide plate or the optical sheet adjacent to the light source rises. . Then, temperature variation occurs in the plane of the optical member depending on the distance from the light source. As a result, wrinkles, deflection, warpage, and the like occur in the optical member, reducing reliability.

If wrinkles, deflection, warpage, or the like occurs in an optical member such as a light guide plate or an optical sheet, the display quality of the display device is degraded. This is because the optical member has functions such as condensing, diffusing, and light guiding, and therefore, when wrinkles or deflections occur, the brightness changes and the viewing angle changes. In particular, when the light guide plate is warped, the end portion of the light guide plate on which the light from the light source enters is shifted from the position of the light source, so that the light is not sufficiently incident and the light loss increases. As a result, the portion where the warpage occurs is darker because the luminance is lower than that of the surrounding area, which leads to a reduction in display quality.
In addition, when the warp or deflection of the optical member is further increased, a situation where the optical member comes into contact with the liquid crystal panel also occurs. When the optical member comes into contact with the liquid crystal panel, the image may be disturbed, or when a stronger force is applied, the liquid crystal panel may be damaged.
From the above, the optical member such as the light guide plate needs to have sufficient reliability. However, as the backlight unit becomes thinner and the thickness of the optical member becomes thinner, sufficient reliability can be obtained. It's getting harder.

  Patent Document 1 proposes a method of improving the warpage of a diffusion plate that touches a liquid crystal panel and disturbs an image by controlling the warpage of the diffusion plate mounted on the direct type backlight. However, in this method, when the diffusion plate becomes thin, the influence of heat due to temperature increases, and when the liquid crystal backlight is used upright, it is impossible to prevent the plate from being bent to the liquid crystal panel side by its own weight. . In particular, this tendency becomes remarkable when the light source is turned on or the surrounding environment becomes high temperature. Moreover, since the distance between the diffusion plate and the liquid crystal panel becomes smaller as the liquid crystal backlight becomes thinner, this method alone cannot be used.

  Patent Document 2 proposes a method of approximating the linear expansion coefficient of an optical sheet mounted on a backlight device and reducing the wrinkle of the sheet due to a difference in elongation due to heat. Controlling the linear expansion coefficient is effective in improving the reliability of the optical member. However, the linear expansion coefficient alone is not the only wrinkle improvement effect, and does not fundamentally increase the rigidity of the optical member.

  As described above, there is no specific solution to achieve sufficient reliability while achieving both cost reduction and optical performance, and wrinkle visibility is achieved by stacking an optical sheet such as a diffusion sheet with a strong diffusion effect on the forefront side. There is a situation where reliability is ensured by reducing the thickness of the optical member or maintaining a certain thickness of the optical member. Therefore, a fundamental solution for realizing cost reduction while having optical performance and reliability and display quality is required.

JP 2007-94112 A Japanese Patent Laid-Open No. 2005-50802

By the way, it is difficult to maintain the reliability required for the light guide plate as the cost is reduced and the thickness of the member is reduced.
Currently, the light guide plate is supported by a method of maintaining a certain thickness by maintaining a certain thickness or a method of changing the support shape of the light guide plate by changing the housing shape. If the thickness is maintained, it cannot be reduced in thickness and cost.
In addition, the effect of reducing the wrinkles and deflection of the light guide plate can be confirmed by examining the support method of the light guide plate, but the support method needs to be changed depending on the design of the backlight unit, and the light guide plate itself has a certain degree of reliability. Otherwise, it has been found that reliability cannot be solved by the support method alone.

As described above, when the backlight unit for liquid crystal display is made thinner, the light guide plate is inevitably made thinner. However, in the light guide plate light guide method, the distance to each member of the backlight is close, so it is easily affected by temperature, and the possibility of contact with the liquid crystal panel is increased, so that sufficient reliability can be obtained. It's getting harder.
Accordingly, the present invention provides a light guide plate that can maintain rigidity even when the thickness is thin without degrading optical performance and has sufficient reliability, a backlight unit for liquid crystal display using the same, and a display device. Objective.

  The invention of claim 1 is a light guide plate that guides incident light toward a display element, and includes a first resin layer and a second resin layer having different thicknesses and types laminated in a two-layer structure. A first optical element is formed on a light emitting surface of the first resin layer opposite to the second resin layer, and a second optical layer is formed on the surface of the second resin layer opposite to the first resin layer. An optical element is formed, and the first resin layer has a higher bending strength than the second resin layer.

  According to a second aspect of the present invention, in the light guide plate according to the first aspect, the first and second optical elements have a concavo-convex shape arranged in a one-dimensional direction or a two-dimensional direction.

  According to a third aspect of the present invention, in the light guide plate according to the second or third aspect, the thickness of the first resin layer is larger than the thickness of the second resin layer.

  According to a fourth aspect of the present invention, in the light guide plate according to any one of the first to third aspects, the water absorption rate of the resin material used for the first resin layer is used for the second resin layer. It is characterized by being smaller than the water absorption rate of the resin material.

  A fifth aspect of the present invention is the light guide plate according to any one of the first to fourth aspects, wherein the first resin layer and the second resin layer are produced by extrusion.

  A sixth aspect of the present invention is a backlight unit comprising a light source and the light guide plate according to any one of the first to fifth aspects.

  According to a seventh aspect of the invention, in the backlight unit of the sixth aspect, the light source is a cold cathode tube, an LED, an EL, or a semiconductor laser.

  The invention of claim 8 is a display device, comprising a liquid crystal display element that defines a display image according to transmission / light-shielding in pixel units, and the backlight unit of claim 6 or 7. To do.

  According to the light guide plate of the present invention, the backlight unit using the same, and the display device, the first resin layer with the first optical element and the second resin layer with the second optical element, which have different bending strengths. The two-layer structure is laminated so that the optical performance is not lowered, the rigidity is maintained even when the thickness is thin, the warpage is reduced by making it difficult to bend, and the contact with the liquid crystal panel is prevented, It is possible to prevent the image from being disturbed, and at the same time, it is possible to further increase the optical performance while maintaining the optical performance by adjusting the arrangement method when laminating the resin layers, the thickness of each layer, and the water absorption rate of the resin. Reliability can be ensured.

It is a schematic sectional drawing which shows the structure of the display apparatus containing the light-guide plate of this invention. It is explanatory drawing which shows an example of the light-guide plate concerning this invention. It is explanatory drawing which shows the other example of the light-guide plate concerning this invention. It is explanatory drawing which shows the further another example of the light-guide plate concerning this invention. It is explanatory drawing regarding the reliability requested | required of the light-guide plate of this invention. It is explanatory drawing of the extrusion method at the time of producing the light-guide plate of this invention. It is explanatory drawing which shows the Example and comparative example of shaping property in the light-guide plate of this invention, bending strength, and reliability evaluation. It is explanatory drawing which shows the verification example of the resin water absorption rate and curvature shape in the light-guide plate of this invention. It is explanatory drawing which shows the structure of the display apparatus by the conventional light-guide plate light guide system. It is explanatory drawing which shows the other structure of the display apparatus by the conventional light-guide plate light guide system. It is explanatory drawing which shows other structure of the display apparatus by the conventional light-guide plate light guide system.

(First embodiment)
Next, embodiments of a light guide plate of the present invention and a display device using the light guide plate light guide system using the same will be described in detail with reference to the drawings.
As shown in FIG. 1, a light guide plate in the present embodiment and a light guide plate light guide display device 1 using the same include a backlight unit 2 and a liquid crystal panel (liquid crystal display element) 3 as an image display element. I have.
The backlight unit 2 includes a lamp house 6 and a light guide plate 7.
The lamp house 6 includes a plurality of light sources 4 made of LEDs arranged at a predetermined interval, for example, and a reflection plate 5 that is disposed on the back surface of the light source 4 and reflects emitted light on the back surface side. The light source 4 is not limited to an LED, and a cold cathode tube (CCFL), EL, LED, semiconductor laser, or the like can be employed.

The light guide plate 7 guides light entering from the light source 4 to the front surface 7 a facing the liquid crystal panel 3, and covers the back surface 7 b and the side peripheral surface 7 c of the light guide plate 7 in the reflection plate 5. The light source 4 is disposed on the front side in the light irradiation direction.
The liquid crystal panel 3 is configured by sandwiching a liquid crystal element 10 between polarizing plates 9 and 9, and the liquid crystal panel 3 is disposed to face the front surface of the light guide plate 7. Further, between the diffusion plate 7 and the liquid crystal panel 3, optical sheets 12, 13, and 14 for condensing and diffusing the light transmitted through the diffusion plate 7 are disposed.

In the embodiment shown in FIG. 1, a prism sheet having a high light condensing effect is used for the optical sheet 12. Moreover, a diffusion sheet having a high effect of diffusing light is used for the optical sheets 13 and 14.
The combination of such optical sheets is not limited to the combination shown in the present embodiment, and the optical sheets 12, 13, and 14 can all be combined with the same optical sheet, or all can be combined with different optical sheets. Is also possible. In addition, the number of optical sheets stacked is not limited to three, and may be one, two, or four or more stacked, and the types of optical sheets used and the stacking order are limited. There is nothing. In addition to prism sheets and diffusion sheets, combinations of microlens sheets with a light-diffusion effect and lens sheets with shapes in two directions are conceivable, and the optical performance obtained depends on the type and stacking order. Come.

In addition, the combination of the optical sheets affects the reliability of the backlight unit. If not only the light guide plate but also the optical sheet undulates and irregularities are generated in the screen, it may be visually recognized as luminance unevenness and display quality may deteriorate. Examples of countermeasures against wrinkles and deflection of the optical sheet include a method of placing an optical sheet with a high diffusion effect such as a diffusion sheet on the top surface of the optical sheet on the most liquid crystal panel side, making it difficult to recognize uneven brightness, A common method is to install a high optical sheet and reduce the generation of wrinkles themselves.
From the above, the optical sheets can be freely combined from the viewpoint of optical performance, reliability, and cost.

Next, the light guide plate 7 shown in the present embodiment will be described in detail with reference to FIG. 2 schematically showing the structure of the light guide plate.
The light guide plate 7 has a translucent first resin layer 17 and a second resin layer 18 which are laminated in a two-layer structure and have different thicknesses and types.
Further, a surface opposite to the second resin layer 18 of the first resin layer 17 is used as a light emitting surface 17a, and light incident from the light source 4 arranged to face the side end surface 7a of the light guide plate 7 is confined in the light emitting surface 17a. A first optical element 15 having a lenticular lens shape is formed. Further, on the surface 18a opposite to the first resin layer 17 of the second resin layer 18, the light incident from the light source 4 disposed opposite to the side end surface 7a of the light guide plate 7 is front of the light guide plate 7 (on the optical element 15 side). The second optical element 16 having a shape different from the first optical element 15 is formed, in which the microlens shape having an effect of guiding light to the liquid crystal display element and raising the light in the direction of the liquid crystal display element is concave.
Further, the bending strengths of the resins used for the first resin layer 17 and the second resin layer 18 are different, and the resin used for the first resin layer 17 is used for the second resin layer 18. It has a bending strength greater than that of the resin. The thickness of the first resin layer 17 is thicker than the thickness of the second resin layer 18.

Here, the reliability required for the light guide plate 7 will be described with reference to FIG.
FIG. 5 is a schematic view of the case where the backlight unit 2 is installed upright based on the actual use.
In FIG. 5A, the light guide plate 7 is supported without bending, and the distance between the light guide plate 7 and the liquid crystal panel 3 is constant. From this state, when the temperature in the backlight unit 2 increases due to the lighting of the light source or the increase in the surrounding environment, the light guide plate 7 extends due to linear expansion due to the temperature difference. Then, as shown in FIG. 5B, the light guide plate 7 is warped in a convex shape on the liquid crystal panel 3 side in order to allow the extension. This is because, on the opposite side of the liquid crystal panel 3 side, there is a housing made of a reflector 5 and metal, and the light guide plate 7 is fixed by a frame or the like, so that a warp that is convex on the reflector side occurs. do not do. In particular, the strength of the light guide plate greatly depends on the strength of the first resin layer 17 on the side that is pulled when pressure is applied, that is, on the liquid crystal panel surface side. Therefore, the resin used for the first resin layer 17 on the emission surface side must have a large bending strength.

  In addition, since the bending strength of the resin is related to the molecular weight of the resin, as a result, the shapeability of the optical element applied to the light guide plate 7 is also affected. If the molecular weight of the resin is large, the bending strength tends to be strong. Therefore, in order to increase the bending strength of the light guide plate 7, a resin having a large resin molecular weight is selected. However, particularly when the light guide plate 7 is produced by an extrusion method or the like, the shape shaping property of the optical element is deteriorated if the molecular weight of the resin is large. This is because when the resin is melted for shape shaping, the higher the molecular weight, the harder the resin, so that when the mold roll is transferred and shaped, the resin tends not to enter the mold tip. Therefore, in order to achieve high formability, it is better to select a resin that is soft, that is, has a low molecular weight.

  As described above, in the light guide plate 7 of the present invention, the optical elements 15 and 16 are formed on both the light emitting surface side and the other surface corresponding to the opposite surface. Among them, the optical element 16 on the other surface side, which is the opposite surface of the light emitting surface, exhibits the effect of raising the light incident from the light source 4 disposed on the side edge of the light guide plate 7, and thus the shape is intended. Whether or not it can be shaped as it is is important and greatly affects the optical performance of the light guide plate 7. When the light guide plate 7 is produced by an extrusion method, the shapeability of the optical elements 15 and 16 is greatly influenced by the molecular weight of the resin. Therefore, it is effective to use a resin having a low bending strength and a good shaping property on the other surface which is opposite to the light emitting surface of the light guide plate 7 which requires a more accurate shaping. .

  FIG. 3 shows that the thickness distribution of each resin layer is uniform in the plane with respect to the thickness 21 of the entire light guide plate. FIG. 4 shows that the thickness distribution of the first and second resin layers 17 and 18 is not uniform in the plane with respect to the thickness 21 of the entire light guide plate. Here, the thickness 19 of the first resin layer 17 must be thicker than the thickness 20 of the second resin layer 18. This is because the light guide plate of the present invention is intended to increase the rigidity of the light guide plate and reduce deflection and warpage, so that the influence of the first resin layer 17 using a resin having a high bending strength is strengthened. It is. In the light guide plate 7 of the present invention, the first resin layer 17 may be thicker than the thickness 20 of the second resin layer 18, but the thickness of the first resin layer 17 as thick as possible can increase the strength of the entire light guide plate. Good for. However, if the thickness of the second resin layer 18 is too thin, the thickness of the layer is not stable, resulting in poor appearance during molding, or the shape of the optical element 16 applied to the second resin layer 18 is not stable. Things may happen. Therefore, the thickness of the first resin layer 17 and the thickness of the second resin layer 18 can be adjusted in consideration of the required strength and moldability of the light guide plate 7.

Next, the hygroscopicity of the light guide plate 7 due to the humidity effect will be described with reference to FIG.
In FIG. 5A, the light guide plate 7 is supported without bending, and the distance between the light guide plate 7 and the liquid crystal panel 3 is constant.
On the other hand, in FIG. 5B, the light guide plate 7 has a convex warp on the liquid crystal panel 3 side. As the light guide plate 7 bends, the optical sheets 12, 13, and 14 installed on the liquid crystal panel 3 side with respect to the light guide plate 7 are similarly bent, and a phenomenon of contacting the liquid crystal panel 3 occurs. In the liquid crystal panel 3, when pressure is applied from the outside, the image is disturbed and the display quality is lowered. Further, since the liquid crystal panel 3 may be damaged when a stronger pressure is applied, the contact between the liquid crystal panel 3 and the light guide plate 7 must be prevented. Therefore, it is effective to improve the reliability of the backlight unit by controlling the warpage by changing the water absorption rate of the front and back surfaces of the light guide plate 7 and preventing the liquid crystal panel 3 from coming into contact.

  For this purpose, the water absorption rate of the resin used in the first resin layer 17 of the light guide plate 7 is preferably smaller than the water absorption rate of the resin used in the second resin layer 18. This is because when the backlight unit 2 is installed in a high humidity environment, the difference in the water absorption rate between the front and back sides of the light guide plate 7 reduces the warpage and deflection caused by its own weight and hygroscopicity as much as possible. The amount of warpage can be reduced as compared with the case of using only. Therefore, in order to suppress warping and deflection in a high humidity environment, a resin having as low a water absorption rate as possible is selected, and the resin used for the first resin layer 17 on the liquid crystal panel 3 side is a second resin. What is necessary is just to make it have a smaller water absorption than the resin used for the layer 18. FIG. Thereby, since the amount of warpage of the convex shape on the liquid crystal panel 3 side can be reduced, the reliability is improved.

As a material for molding the light guide plate 7, a material having optical transparency with respect to the wavelength of light emitted from the light source unit is used. For example, a plastic material usable for an optical member can be used.
Examples of this material include polyester resin, acrylic resin, polycarbonate resin, polystyrene resin, MS (acrylic and styrene copolymer) resin, polymethylpentene resin, thermoplastic resin such as cycloolefin polymer, or polyester acrylate. And transparent resins such as oligomers such as urethane acrylate and epoxy acrylate. Further, depending on the application, fine particles may be dispersed in the transparent resin.
The material of the light guide plate 7 of the present invention can be appropriately selected from these materials, from the optical performance such as the refractive index and transparency of the resin, the moldability and the shapeability of the optical element. From the viewpoint of enhancing, it is preferable to select a resin having as large a bending strength as possible from the materials among them.

The first optical element 15 is not limited to the lenticular lens shape shown in FIG. 2 and may have other shapes. As examples of the shape, a conical shape, a polygonal pyramid shape, a columnar shape, a polygonal columnar shape, and a microlens shape may be present continuously in a one-dimensional direction or a two-dimensional direction.
The first optical element 15 has a brightness increasing effect for increasing the light use efficiency of the light guide plate itself, a light confinement effect for increasing the straightness of light incident from the end face of the light guide plate 7, and the opposite of the light exit surface side. The shape of the optical element can be appropriately selected depending on the performance required of the light guide plate 7 such as a concealing effect that reduces the visibility of the second optical element 16 shaped on the surface side.
Further, these shapes may be arranged regularly or irregularly. When regularly arranged, uniform optical characteristics can be provided over the entire surface of the optical member. However, since regular arrangement may cause moire between the optical sheet laminated above the light guide plate 7 and other members such as a liquid crystal panel, it is necessary to fully consider regularity. is there.

On the other hand, when irregularly arranging the shape of the optical elements, it is possible to solve the problem of moiré that is most concerned when the optical elements are regularly provided. Can be used. However, irregularity may cause uneven optical performance in the surface and cause unevenness, so it is necessary to sufficiently consider irregularity.
The shapes of the first and second optical elements may be roughened by sandblasting or corrosion. This is because the surface is roughened so that the surface of the light guide plate 7 is not easily noticeable, the diffusion effect due to minute unevenness, and the optical sheet installed on the liquid crystal panel 3 side of the light guide plate 7 The effect etc. which prevent optical close_contact | adherence etc. can be acquired.

  Further, the optical element 16 applied to the second resin layer 18 of the light guide plate 7 is not limited to the microlens shape shown in FIG. As an example of the shape, a prism shape, a cone shape, a polygonal pyramid shape, a columnar shape, or a polygonal columnar shape may exist continuously in a one-dimensional direction or a two-dimensional direction. The optical element provided on the other surface side plays a very important role in order to guide the light from the light source disposed on the end face of the light guide plate to the liquid crystal panel side. Can be selected as appropriate from the effect of raising the light that guides the light from the front to the front and the visibility of the optical element.

As for the arrangement of the first optical element 17, the arrangement method is selected according to the light emission efficiency from the light source to the front and the luminance distribution of the light. When all optical elements having the same shape are used as shown in FIG. 2, the light extraction effect per optical element is the same, and therefore represents the ratio of the area of the optical element 17 provided per unit area. By changing the area ratio according to the distance from the light source, the luminance obtained in the front direction is arranged to be as uniform as possible. Therefore, when the same shape is used, the optical element 17 is generally arranged with the area ratio changed in the plane. Also, when multiple optical elements with different shapes are used, the light extraction effect of each optical element is different, so the arrangement is unified within the surface of the light guide plate, and uniform luminance is obtained in the front direction by changing the shape. It becomes possible.
From the above, the first and second optical elements can be appropriately selected with respect to the shape and arrangement of the optical elements.

Next, a method for producing the light guide plate of the present invention will be described. The light guide plate of the present invention is produced by a coextrusion method.
Use diamond tool with various lens shapes for mold roll, and if the cross-sectional shape is triangular or lenticular lens shape, use diamond tool with various lens shapes to cut the mold roll to support various lens shapes A part to be made is produced.
Further, as a method for producing a mold having a portion corresponding to a hemispherical or elliptical lens shape, a laser method and a cutting method can be given. In the laser method, a black resin is uniformly applied to the surface of the mold roll, and after irradiating the laser, the entire mold roll is attached to an acid solution to corrode the laser irradiated portion and mold the portion corresponding to the optical projection. Is the method. The cutting method is a method that can intermittently press the center of a cutting tool whose tip shape is an aspherical shape against a die roll to produce a portion corresponding to the optical protrusion.

In addition to the laser method and the cutting method, a method for producing the mold roll includes a method using sand blasting and a forming method using bead dispersion.
The sand blasting method is a method in which glass beads or the like are sprayed directly on the mold surface to make the mold surface uneven. The bead dispersion method is a method for producing a reverse plate from a sheet in which glass beads are closely packed in a flat shape.
The method for producing the mold roll is appropriately selected depending on the required surface condition because the suitable molding method differs depending on the uneven shape, the density of the unevenness, the material of the mold roll, and the like. It is not necessary to adopt only one method for producing the mold roll, and two or more methods may be adopted. Moreover, you may produce by the production methods other than the above.

Next, a light guide plate is formed using a mold roll. The light guide plate can be manufactured by an extrusion method, a casting method, or an injection method. When this production method is used, the thickness of the plate-like member for producing the light guide plate can be produced with a thickness of 12 μm to 5 mm. When the thickness is less than 12 μm, there is no rigidity that can withstand the processing by the above-described manufacturing method, and when the thickness exceeds 5 mm, there is no flexibility that can withstand the processing. However, when the light guide plate of the present invention is mounted on a backlight unit and used, it is particularly preferably 0.5 mm or more and 4 mm or less. This is because the relationship between the thickness of the light guide plate and the size of the light source greatly affects the optical performance of the backlight unit. If the size of the light source is large relative to the thickness of the light guide plate, the amount of light that leaks outside without being incident on the end face of the light guide plate increases, and the amount of light that is guided to the liquid crystal display element is extremely reduced. It has been found that the optical performance of the light unit is greatly reduced.
On the other hand, if the thickness of the light guide plate is larger than that of the light source, the cost of the light guide plate is increased. For this reason, it is desirable that the size of the light source disposed on the end face of the light guide plate and the thickness of the light guide plate be the same as much as possible. Therefore, the current thickness of the light guide plate is not only from the viewpoint of reliability and cost but also from 0.5 mm to 4 mm due to the problem of the size of the light source.
In addition, although the typical preparation example was demonstrated about the light-guide plate 7, it is also possible to produce using materials, structures, processes, etc. other than the above.

Next, a method for producing an optical sheet will be described. The optical sheet can be molded by pressing it against a mold like the light guide plate of the present invention.
Examples of the method for producing the mold roll include the cutting method and the laser method, as in the method for producing the mold roll in the light guide plate of the present invention. Here, as a method for producing the mold, either the cutting method or the laser method may be used, or both may be used. Either of them may be produced first. The production of the mold roll is not limited to the method described above, and the method is appropriately selected depending on the shape and accuracy.

  Next, the optical sheet 12 is molded using a mold roll. The optical sheet 12 can be manufactured by an extrusion method, a casting method, or an injection method. As the plate-like member for producing the optical sheet 12, one having a thickness of 12 μm or more and 1 mm or less can be used. When the thickness is less than 12 μm, there is no rigidity capable of withstanding the processing by the manufacturing method described above, and when the thickness exceeds 1 mm, there is no flexibility capable of withstanding the processing.

The optical sheet 12 may be manufactured by a UV curing method.
When produced by the UV curing method, a UV curable resin is applied onto a base that is a sheet-like base material, a mold having a desired shape is pressed, and then UV irradiation is performed to irradiate the base and the optical protrusions. An optical sheet composed of optical elements is obtained.
Moreover, when it has 2 or more types of different lens shapes, you may shape | mold each shape separately, and may shape | mold integrally. In the case of molding various lenses and a base, a diffusing agent such as a filler can be dispersed inside and molded.
In addition, an application layer such as a hard coat or an antistatic layer may be provided on the surface of the optical member on which the optical element is formed or on the opposite surface. The hard coat layer is often applied mainly for the purpose of preventing damage to the surface of the optical sheet. However, in particular, when the hard coat layer is provided on either the exit surface or the entrance surface, the linear expansion coefficient is different, which causes warpage or the like. In particular, since the temperature difference is large in the housing and warping is likely to occur, it is necessary to fully consider environmental characteristics.

As a material for molding the optical member 12, a material having optical transparency with respect to the wavelength of light emitted from the light source unit is used. For example, a plastic material usable for the optical member can be used.
Examples of this material include polyester resin, acrylic resin, polycarbonate resin, polystyrene resin, MS (acrylic and styrene copolymer) resin, polymethylpentene resin, thermoplastic resin such as cycloolefin polymer, or polyester acrylate. And transparent resins such as radiation curable resins composed of oligomers such as urethane acrylate and epoxy acrylate, or acrylates. Further, depending on the application, fine particles may be dispersed in the transparent resin.

Examples will be described below.
(Experiment 1: Bending strength and reliability verification)
As the resin for the light guide plate in this experiment, two kinds of acrylic resins made by Mitsubishi Rayon having different bending stresses were used. Resin A has a bending strength of 95 MPa and a water absorption rate of 0.3% when immersed for 24 hours, and resin B has a bending strength of 140 MPa and a water absorption rate of 0.3% when immersed for 24 hours. Using these resins, a light guide plate having a single layer, two layers, and three layers was prepared, and shape forming evaluation, bending strength evaluation, and reliability evaluation of the shapes on both sides were performed.
A lenticular lens shape having a width of 150 μm and a height of 50 μm is arranged on the entire surface as an optical element on the exit surface side, and a microlens shape having a diameter of 50 μm and a height of 20 μm is provided as an area with respect to the distance from the light source. Light guide plates arranged at different rates were produced. Regarding the shape shaping of the first and second optical elements, the die roll formed grooves corresponding to the lenticular lens shape by a cutting method. A mold roll for forming a light guide plate having a lenticular lens shape on the base surface was prepared by setting a mold roll on a precision cutting machine and cutting with a diamond tool having a lenticular lens shape at the tip. In addition, the microlens shape of the first and second optical elements was formed corresponding to the microlens shape by a cutting method. A die roll was set on a precision cutting machine, and the center of the aspherical tool was intermittently pressed against the die roll to produce a portion corresponding to the microlens shape. It produced as mentioned above.

  A mold roll corresponding to the shape of the lenticular lens disposed on the exit surface side was installed on the mold pressing roll 37 as shown in FIG. A mold roll corresponding to the microlens shape formed on the first and second resin layers was installed in the extruder 35 as a mold forming roll 36. Only the acrylic resin A or only the acrylic resin B, or both resins are melted and molded by the extruder 35, and before the acrylic resin is cooled and cured, the mold forming roll 36 and the mold pressing roll 37 are used. Each was molded, and light guide plates having a lenticular lens shape in the first resin layer and a microlens shape in the second resin layer were obtained. All the light guide plates have a thickness of 2.0 mm.

(Production of Comparative Example 1)
A light guide plate produced using only resin A was set as Comparative Example 1.
(Production of Comparative Example 2)
A light guide plate produced using only resin B was referred to as Comparative Example 2.
(Production of Example 1)
A light guide plate having a two-layer structure in which the resin A was used for the first resin layer on the emission surface side and the resin B was used for the second resin layer opposite to the emission surface side was produced as Example 1. At this time, the thickness of each layer was adjusted so that the thickness of the first resin layer and the first resin layer was 8: 2.
(Production of Comparative Example 3)
A light guide plate having a two-layer structure in which the resin B was used for the first resin layer on the emission surface side and the resin A was used for the second resin layer opposite to the emission surface side was produced as Comparative Example 3. At this time, the thickness of each layer was adjusted so that the thickness of the first resin layer and the second resin layer was 8: 2.
(Production of Comparative Example 4)
Using resin A and resin B, a light guide plate having a three-layer structure was produced. A light guide plate produced using resin A on the outermost surface side and resin B inside was used as Comparative Example 4. At this time, the thickness of each layer was adjusted to be 1.5: 7: 1.5 from the emission surface side.
(Production of Comparative Example 5)
Using resin A and resin B, a light guide plate having a three-layer structure was produced. A light guide plate produced by using the resin B on the outermost surface side and using the resin A inside was designated as Comparative Example 5. At this time, the thickness of each layer was adjusted to be 1.5: 7: 1.5 from the emission surface side.

(Experiment 1: Optical element formability evaluation method)
It was confirmed how much the shape of the lenticular lens arranged on the exit surface side and the shape of the microlens arranged on the surface opposite to the exit surface side were shaped on the light guide plate. The lenticular lens shape and the microlens shape are observed with a Keyence laser microscope, and the shaping ratio indicates the ratio of the size of the optical element shape actually shaped on the light guide plate to the design value. Expressed numerically. When the shaping rate is 99% or more, it is acceptable (◯), and when it is less than 99%, it is unacceptable (x).
(Experiment 1: Bending strength / reliability evaluation method)
The produced light guide plate was installed on an LG 23-inch monitor, and further, a total of three optical sheets, two KIMOTO diffusion sheets and a 3M prism sheet, were stacked on the light guide plate. A glass plate was set as an alternative to the liquid crystal panel. The glass plate has a 2cm square hole in the center of the screen so that the distance of the light guide plate can be confirmed. The monitor was put into an environmental test machine in an upright state where it was actually used, and the interior of the test machine was set to 60 ° C. and 95%. After 24 hours, the light guide plate was observed from the hole at the center of the glass plate, and it was confirmed whether or not the liquid crystal panel substitute glass and the light guide plate were in contact. When the glass plate and the light guide plate are not in contact, it is acceptable (◯), and when the glass plate and the light guide plate are in contact, it is unacceptable (x).
Moreover, bending strength was measured about the produced light-guide plate. The bending strength was measured according to JIS K7171.

(Experiment 1: Evaluation result)
FIG. 7 shows the evaluation results of Experiment 1. Regarding the formability, the lenticular lens shape which is an optical element provided on the exit surface side was 100% for both resin A and resin B. However, the shape of the microlens, which is an optical element provided on the side opposite to the exit surface side, was 100% formability with resin A, but when resin B was used, the formability was 80% or less for both. The formability was rejected (x). In addition, the layer structure did not affect the formability.
Next, regarding the reliability, there was no contact between the light guide plate of Comparative Example 2 and the glass plate installed as an alternative to the liquid crystal panel only in the environment of 60 ° C. and 95% for 24 hours. Other light guide plates were in contact with a glass plate installed as an alternative to a liquid crystal panel in an environment of 60 ° C and 95% for 24 hours, and then contacted even if left in a room temperature environment of 20 ° C and 65% for 2 hours. It remained.
From the above, Example 1 was the only light guide plate that obtained good results in both shape shaping and reliability.

(Experiment 2: Resin water absorption rate and warpage shape verification)
In this experiment, resins having different bending strength and water absorption were prepared, a light guide plate having a two-layer structure was prepared, and the warpage shape of the light guide plate was verified. Resin A is an acrylic resin manufactured by Mitsubishi Rayon, and has a bending strength of 95 MPa and a water absorption of 0.3% when immersed for 24 hours. Resin B is an acrylic resin manufactured by Mitsubishi Rayon, which has a bending strength of 140 MPa and a water absorption of 0.3% when immersed for 24 hours. Resin C is polystyrene made by PS Japan, and has a bending strength of 105 MPa and a water absorption of 0.1% when immersed for 24 hours. Using these resins, a light guide plate having a two-layer structure was produced, and the warped shape was verified.
The optical element provided on the exit surface side and the other surface side opposite to the exit surface side has the same shape as in Experiment 1. The optical element on the exit surface side has a lenticular lens shape with a width of 150 μm and a height of 50 μm on the entire surface. A light guide plate was prepared in which microlens shapes having a diameter of 50 μm and a height of 20 μm were arranged as the optical elements on the exit surface side and the other surface side while changing the area ratio with respect to the distance from the light source. The method for producing the mold and the method for producing the light guide plate are the same as in Experiment 1. All the light guide plates have a thickness of 2.0 mm.
(Production of Example 2)
Using two types of resins, resin A and resin C, a light guide plate having a two-layer structure was produced. Resin C having a water absorption rate of 0.1% was used on the emission surface side, and resin A having a water absorption rate of 0.3% was used on the surface opposite to the emission surface. At this time, the thickness of each layer was adjusted so that the thickness of the resin layer on the emission surface side and the resin layer on the other surface side was 8: 2.
(Production of Comparative Example 6)
Using two types of resins, resin B and resin C, a light guide plate having a two-layer structure was produced. Resin B having a water absorption rate of 0.3% was used on the emission surface side, and resin C having a water absorption rate of 0.1% was used on the surface opposite to the emission surface. At this time, the thickness of each layer was adjusted so that the thickness of the resin layer on the emission surface side and the resin layer on the other surface side was 8: 2.

(Experiment 2: Warpage shape evaluation method)
The light guide plates of Example 1, Example 2, and Comparative Example 6 were each placed on the flat plate in the environmental test machine so that the exit surface side was down, and the distance from the flat plate at the end of the light guide plate was measured with a ruler. . There are a total of 8 measurement positions consisting of 4 locations at the 4 corners of the light guide plate and 4 locations at the center of the long and short sides. Among them, the place where the distance from the flat plate was the largest was recorded as the numerical value of the warpage amount of the light guide plate.
Next, the inside of the environmental tester is set to a high temperature and high humidity environment of 60 ° C. and 95%, the same measurement is carried out after 24 hours, and the same measurement is performed when left in a room temperature and humidity environment of 20 ° C. and 65% for 2 hours. Carried out.

(Experiment 2: Evaluation results)
The evaluation results of Experiment 2 are shown in FIG. The initial value of the warpage amount of the light guide plate before the operation of the environmental test machine was 0 mm. Then, the amount of warping after leaving for 24 hours in a high-temperature and high-humidity environment of 60 ° C. and 95% is 2 mm during the test, and 1 mm after returning to the normal temperature environment, compared with Example 1 and Comparative Example 6. The amount of warpage was small and good results were obtained.

DESCRIPTION OF SYMBOLS 1 ... Display apparatus 2 ... Backlight unit 3 ... Liquid crystal panel 4 ... Light source 5 ... Reflector plate 6 ... Lamp house 7 ... Light guide plate 9 ... Polarizing plate 10 ... Liquid crystal element 12, 13, 14 ... Optical sheet 15 ... 1st optical element 16 ... 2nd optical element 17 ... 1st resin layer 18 ... 2nd optical element 19 ... 1st resin layer thickness 20 ... 2nd resin layer thickness 21 ... Thickness 35 of the whole light-guide plate ... Extruder 36 ... Mold formation Roll 37 ... Die pressing roll

Claims (8)

A light guide plate that guides incident light toward the display element,
Having a translucent first resin layer and a second resin layer of different thickness and type laminated in a two-layer structure;
A first optical element is formed on a light exit surface of the first resin layer opposite to the second resin layer;
A second optical element is formed on a surface of the second resin layer opposite to the first resin layer;
The first resin layer has a higher bending strength than the second resin layer;
A light guide plate characterized by that.   The light guide plate according to claim 1, wherein the first and second optical elements have an uneven shape arranged in a one-dimensional direction or a two-dimensional direction.   The light guide plate according to claim 1, wherein a thickness of the first resin layer is larger than a thickness of the second resin layer.   4. The water absorption rate of the resin material used for the first resin layer is smaller than the water absorption rate of the resin material used for the second resin layer. The light guide plate described.   The light guide plate according to any one of claims 1 to 4, wherein the first resin layer and the second resin layer are produced by extrusion processing. A light source and the light guide plate according to any one of claims 1 to 5,
Backlight unit characterized by that.   The backlight unit according to claim 6, wherein the light source is a cold cathode tube, an LED, an EL, or a semiconductor laser. A liquid crystal display element that defines a display image according to transmission / light shielding in pixel units, and the backlight unit according to claim 6 or 7.
A display device.

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