Optical sheet, backlight unit, image display device, illumination device, and method for manufacturing optical sheet

Abstract

The present invention provides an optical sheet having high light transmittance and high light diffusion characteristics as well as improved operating characteristics. An optical sheet according to one aspect of the present invention has a base layer containing a polyester as a main component, in which the base layer has a first raised portion on at least one surface, and a value obtained by multiplying an aspect ratio of the first raised portion by 1000 is less than 15. In a preferred aspect, the ratio SRz / SRa of the ten-point average roughness SRz to the average roughness SRa on the one surface is from 17 to 45 (inclusive).

Description

Technical Field

[0001] The present invention relates to an optical sheet, a backlight unit, an image display device, an illumination device, and a method for manufacturing an optical sheet. Background Art

[0002] An image display device (such as a liquid crystal display device) includes a display panel and a backlight unit provided on the back side of the display panel. There are various types of backlight units, such as edge-lit and direct-lit. These backlight units include a light source and an optical sheet that provides an optical function for the light emitted from the light source. When such an optical sheet is provided, the backlight unit can be made brighter and have uniform brightness.

[0003] As an optical sheet, for example, an optical sheet having a base layer and a functional layer (such as a light diffusion layer) laminated on the base layer is known. As the base layer, a polyester film having strong adhesiveness and thus improved handling characteristics is known (Japanese Patent Application Laid-Open No. 2014-043571).

[0004] Citation List

[0005] Patent Literature

[0006] PTL 1: Japanese Patent Application Laid-Open No. 2014-043571 Summary of the Invention

[0007] Technical Problem

[0008] The polyester film disclosed in PTL 1 is formed by biaxial stretching and has strong adhesiveness due to adjustment of the thermal shrinkage rate in the longitudinal direction and the surface crystallinity of the plasma-treated adhesive surface. However, the polyester film described in PTL 1 has stronger adhesiveness to other layers, but the handling characteristics of the film itself during manufacturing have not been studied.

[0009] In view of the above circumstances, an object of the present invention is to provide an optical sheet with improved handling characteristics.

[0010] Solution to the Problem

[0011] An optical sheet according to an aspect of the present invention made to achieve the above object has a base layer containing polyester as a main component, wherein the base layer has first convex portions on at least one surface, and a value obtained by multiplying the aspect ratio of the first convex portions by 1000 is less than 15.

[0012] Advantageous Effects of the Invention

[0013] The optical sheet according to an aspect of the present invention has excellent handling characteristics. Brief Description of the Drawings

[0014] Figure 1 Figure 1 is a schematic side view showing the internal structure of an image display device according to an embodiment of the present invention.

[0015] Figure 2 Figure 2 is a schematic cross-sectional view showing a partially enlarged surface of a base layer of a light diffusion sheet in a backlight unit of an image display device provided in Figure 1 .

[0016] Figure 3 Figure 3 shows a schematic side view of the structure of a light diffusion sheet including a layered conductive portion, Figure 3 (a) is a view showing the configuration of a light diffusion sheet containing an antistatic agent in a light diffusion layer, Figure 3 (b) is a view showing the configuration of a light diffusion sheet containing an antistatic agent in an adhesive layer. DETAILED DESCRIPTION

[0017] Description of Embodiments of the Present Invention

[0018] First, embodiments of the present invention will be listed and described.

[0019] An optical sheet according to one aspect of the present invention has a base layer containing polyester as a main component, wherein the base layer has first protrusion portions on at least one surface, and a value obtained by multiplying the aspect ratio of the first protrusion portions by 1000 is less than 15.

[0020] Accordingly, the operating characteristics can be improved.

[0021] A ratio SRz / SRa of a ten-point average roughness SRz to an average roughness SRa on the one surface may be 17 or more and 45 or less. In this configuration, the operating characteristics can be further improved.

[0022] The first protrusion portions may be formed of highly crystalline portions of polyester. In this configuration, it is easy to control such that the ratio SRz / SRa is within the above range.

[0023] The base layer may contain a plurality of particles, and may have a plurality of second protrusion portions caused by the plurality of particles on the one surface. That is, due to these particles, other portions of the concavo-convex shape constituting the average roughness SRa and the ten-point average roughness SRz can be formed. In this configuration, it is easy to control such that the ratio SRz / SRa is within the above range.

[0024] A value obtained by multiplying the aspect ratio of the second protrusion portions by 1000 may be 15 or more. Accordingly, it is possible to more easily set the ratio SRz / SRa within the above range.​​​​​​

[0025] The average roughness SRa can be 2.5 nm or more and 15 nm or less, and the ten-point average roughness SRz can be 107 nm or more and 410 nm or less. Accordingly, the operating characteristics of the optical sheet can be further improved.

[0026] In the transmission color tone of the optical sheet, the a value can be -1.0 or more, and the b value can be 5.0 or less. Accordingly, the transparency can be improved while improving the operating characteristics of the optical sheet.

[0027] The optical sheet may include a laminated conductive portion formed parallel to the surface of the sheet, and the thickness of the laminated conductive portion is 0.001 μm or more and 1 μm or less. Accordingly, charging of the optical sheet can be effectively suppressed and contamination inside the backlight can be effectively prevented. The conductive portion may contain various antistatic agents such as electron-conductive type, ion-conductive type, and conjugated electron type. Here, in order to effectively suppress charging of the optical sheet, the thickness of the laminated conductive portion is preferably 0.005 μm or more and 0.8 μm or less, and more preferably 0.01 μm or more and 0.6 μm or less.

[0028] The surface resistivity measured on at least one of the front surface and the back surface is preferably 1 × 10 2 Ω / sq or more and 10 × 10 15 Ω / sq or less. Accordingly, charging of the optical sheet can be effectively suppressed and contamination inside the backlight can be effectively prevented.

[0029] The surface resistivity measured on at least one of the front surface and the back surface is more preferably 1 × 10 2 Ω / sq or more and 10 × 10 13 Ω / sq or less. Accordingly, charging of the optical sheet can be more effectively suppressed and contamination inside the backlight can be more effectively prevented. Here, the lower limit value of the surface resistivity is preferably smaller, and in order to prevent contamination inside the backlight, the lower limit value of the surface resistivity can be 1 × 10 6 Ω / sq or 1 × 10 10 Ω / sq.

[0030] The polyester may be polyethylene terephthalate. Accordingly, an optical sheet having excellent molding characteristics, thermomechanical characteristics, and optical characteristics can be obtained, and the ratio SRz / SRa of the optical sheet can be easily set within the above range. In addition, when the polyester is polyethylene naphthalate or polybutylene terephthalate, an optical sheet having excellent molding characteristics, thermomechanical characteristics, and optical characteristics can be obtained.

[0031] The optical sheet may further have a light-diffusing layer laminated on the base layer. When the ratio SRz / SRa on one surface is within the above range, the optical sheet is suitable as the base layer of a light-diffusing sheet.

[0032] A backlight unit according to another aspect of the present invention includes the optical sheet.

[0033] The backlight unit includes the optical sheet having a base layer with improved operating characteristics, and thus is easy to manufacture.

[0034] An image display device according to still another aspect of the present invention includes the backlight unit.

[0035] The image display device includes the backlight unit, and thus is easy to manufacture.

[0036] A lighting device according to yet another aspect of the present invention includes the backlight unit.

[0037] The lighting device includes the backlight unit, and thus is easy to manufacture.

[0038] A method for manufacturing an optical sheet according to yet another aspect of the present invention includes: a step of pulverizing a molded product containing polyester as a main component, a step of forming pellets by extrusion molding using the powder pieces obtained in the pulverizing step, and a step of forming a base layer using these pellets.

[0039] The method for manufacturing an optical sheet may further include a step of recycling a molded product containing polyester as a main component.

[0040] In the method for manufacturing an optical sheet, a molded product containing polyester as a main component is recycled, and the recycled molded product is crushed into pellets to manufacture an optical sheet. Thus, an optical sheet having excellent operating characteristics can be easily manufactured using recycled resources.

[0041] Here, the term "main component" means the component having the largest content by mass, for example, a component having a content of 50% by mass or more. The term "average roughness SRa" means a value measured using a contact-type surface roughness measuring instrument in accordance with JIS-B0601 (2013). The term "ten-point average roughness SRz" means a value measured using a contact-type surface roughness measuring instrument in accordance with JIS-B0601 (2013). The aspect ratio is the ratio of the height of the protruding portion to the length of the bottom of the protruding portion (the length of the base layer in the thickness direction and the vertical direction) in an image of the surface including the base layer obtained by a contact-type surface roughness measuring instrument. The transmission color tone includes the a value and b value of the color tone defined in JIS-Z8722 (2009) and JIS-Z8730 (2009).

[0042] Details of Embodiments of the Present Invention

[0043] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Here, the accompanying drawings are schematic diagrams for explaining the present invention, and the shapes and scales of components (constructions) may be different from those of actual components.

[0044] <Image Display Device>

[0045] As Figure 1 shown, the image display device 1 includes a backlight unit 100 and a display panel 200 disposed on the front side (viewer side) of the backlight unit 100. In addition, the image display device 1 includes a housing (not shown) in which the backlight unit 100 is accommodated.

[0046] In the present embodiment, the image display device 1 is a liquid crystal display device. That is, the display panel 200 is a liquid crystal panel. The image display device 1 may be a portable display device such as a smart phone or a tablet terminal, or may be a relatively large display device such as a television or a personal computer.

[0047] [Backlight Unit]

[0048] The backlight unit 100 according to the present embodiment is a side - entry type backlight unit. The backlight unit 100 mainly includes a light source 110, a light guide plate 120 that receives light emitted from the light source 110 from the side and emits the light from the front surface, a light diffusion sheet 130 disposed on the light guide plate 120 (front side), and prism sheets (a first prism sheet 140 and a second prism sheet 150) disposed on the light diffusion sheet 130. The light guide plate 120, the light diffusion sheet 130, and the prism sheets 140 and 150 are all optical sheets. In the present embodiment, the light diffusion sheet 130 described below will be described as an optical sheet according to an aspect of the present invention, but the optical sheet may be other optical sheets such as the light guide plate 120 and the prism sheets 140 and 150. That is, any one or two or more of the light guide plate 120 and the prism sheets 140 and 150 may have the base layer 131 described below.

[0049] (Light Source)

[0050] The light source 110 is not particularly limited, and may be, for example, a light source in which a plurality of light - emitting diodes are arranged in a straight line. The light source 110 is disposed such that the emitted light is incident on the end surface of the light guide plate 120. Here, in Figure 1 it, the light source 110 is disposed such that it is in contact with the light guide plate 120, but the light source 110 may be disposed away from the light guide plate 120.

[0051] (Light Guide Plate)

[0052] The light guide sheet 120 has a substantially rectangular light-emitting surface as its upper surface, guides the light that has entered the interior from the end surface (light-incident end surface) toward the end surface on the side opposite to the light-incident end surface, and emits the light from the light-emitting surface to the light diffusion sheet 130. In the light guide sheet 120, a light reflection member may be provided on the end surface on the side opposite to the light-incident end surface and on the lower surface of the light guide sheet 120. In addition, the light guide sheet 120 may have a concavo-convex shape, such as a prism on the light-emitting surface. Here, in Figure 1 the light guide sheet 120 has a substantially rectangular cross section in the thickness direction along the light guiding direction, but the cross section may have a wedge shape or the like. Specifically, the lower surface may gradually approach the light-emitting surface from the light-incident end surface toward the opposite end surface.

[0053] (Light diffusion sheet)

[0054] The light diffusion sheet 130 diffuses the light emitted from the light guide sheet 120 and emits the light toward the prism sheets 140 and 150. The light diffusion sheet 130 in this embodiment includes a base layer 131, a light diffusion layer 132 laminated on the light-emitting surface of the base layer 131, and an anti-sticking layer 133 laminated on the light-incident surface of the base layer 131. The base layer 131 may have surface layers (not shown) on the light-emitting surface and the light-incident surface (the light-emitting surface and the light-incident surface may be collectively referred to as "surfaces" hereinafter). That is, the light diffusion layer 132 and the anti-sticking layer 133 may be laminated on the surface layers.

[0055] The light diffusion layer 132 and the anti-sticking layer 133 are not particularly limited and may have known configurations. For example, the light diffusion layer 132 includes first beads 134 having a high refractive index and a resin binder 135 for fixing the first beads 134 in a dispersed state, and improves the light diffusion characteristics of the light diffusion sheet 130. The anti-sticking layer 133 contains second beads 136 having a diameter smaller than the diameter of the first beads 134 of the light diffusion layer 132 and a resin binder 135 for separating and dispersing the second beads 136, and inhibits the light-incident surface of the light diffusion sheet 130 and the light-emitting surface of the light guide sheet 120 from adhering to each other.

[0056] <<Base layer>>

[0057] The base layer 131 is a transparent layer that can transmit light and is a resin layer containing polyester as a main component. The content of polyester in the base layer 131 is preferably 50% by mass or more, more preferably 80% by mass, and can be 100% by mass. The polyester is not particularly limited, and polyethylene terephthalate (PET) is preferred in order to easily form the first raised portion 131b and the second raised portion 131c described below. The raw material of the polyester can be a new raw material (virgin raw material), and considering environmental protection, a recycled raw material is preferred. A recycled optical film is particularly preferred. In addition, a mixed raw material in which a virgin raw material and a recycled raw material are mixed can be used. The mixing ratio of the virgin raw material and the recycled raw material in the mixed raw material is not particularly limited, and preferably the content of the recycled raw material is larger than the content of the virgin raw material. The virgin raw material can be derived from fossil fuels or plants.

[0058] When a recycled PET raw material is used to prepare the base layer of an optical sheet, the PET base layer is subjected to steps involving heating such as washing, extrusion, and polymerization. Therefore, thermally crystallized portions having a high degree of crystallinity tend to be formed inside the PET base layer. When there is some thermal crystallization on the surface of the base sheet, relatively gentle raised portions are formed on the surface of the base sheet. Therefore, in the optical sheet manufacturing step, when two or more base layers of optical sheets are stacked and operated, due to the presence of the relatively gentle raised portions on the surface of the base layer, the surface frictional resistance can be reduced, the occurrence of adhesion where the base layers adhere to each other can be reduced, and excellent handling characteristics can be obtained.

[0059] The ratio SRz / SRa of the ten-point average roughness SRz to the average roughness SRa on the surface 131a of the base layer 131 is 17 or more and 45 or less. The average roughness SRa is preferably 2.5 nm or more and 15 nm or less, and the ten-point average roughness SRz is preferably 107 nm or more and 410 nm or less. When the average roughness SRa and the ten-point average roughness SRz are within the above ranges, the handling characteristics of the base layer 131 can be further improved, and thus the handling characteristics of the light diffusion sheet 130 can be further improved.

[0060] Specifically, when an uneven shape is formed on the surface 131a such that the average roughness SRa and the ten-point average roughness SRz are controlled within the above ranges, the surface frictional resistance can be reduced. During the manufacturing process of the optical sheet, the base layer whose conveying surface contacts the conveying roller. In this case, in the base layer having a smooth surface, the surface frictional resistance during conveyance increases. On the other hand, if the surface has too many steep unevennesses, when the surface contacts the conveying roller, the protruding portions may be scraped off, dust may be generated, and the conveying roller may be contaminated. On the other hand, since the base layer 131 in the light diffusing sheet 130 has an uneven shape appropriately controlled on the surface 131a, the surface frictional resistance can be minimized during conveyance and the contamination of the conveying roller can be reduced. In addition, during manufacturing, the base layer is rolled up, so these surfaces may come into contact with each other. In the base layer having a smooth surface, the surfaces may stick to each other, but the optical sheet (base layer 131) has an uneven shape appropriately controlled on the surface 131a, so the occurrence of such sticking can be reduced. When the surface wear resistance and sticking are reduced, the yield rate of the base layer 131 can be increased, it is easy to manufacture the light diffusing sheet 130 (optical sheet) and the backlight unit 100, and the manufacturing cost can be reduced. In addition, when the uneven shape is appropriately controlled, even if some scratches appear on the surface 131a of the base layer 131, the deterioration of the optical function due to these scratches can be reduced. In this way, the base layer 131 having an uneven shape appropriately controlled on the surface 131a can improve the operating characteristics.

[0061] When the light diffusing sheet 130 includes the base layer 131, the light diffusing characteristics can be improved. That is, when the base layer 131 is used in the light diffusing sheet, due to the uneven shape of the surface 131a, the light diffusing characteristics of the light diffusing sheet 130 can be improved.

[0062] Preferably, both the light incident surface and the light exit surface of the base layer 131 satisfy the ratio SRz / SRa, and more preferably, the base layer 131 further satisfies the average roughness SRa and the ten-point average roughness SRz. In the base layer 131, due to the components described below, a desired uneven shape can be formed on the surface 131a. Therefore, it is easy to form uneven shapes on both the light incident surface and the light exit surface, wherein the ratio SRz / SRa, the average roughness SRa, and the ten-point average roughness SRz are controlled within the desired ranges.

[0063] To further improve the operating characteristics of the base layer 131, the lower limit of the ratio of the ten-point average roughness SRz to the average roughness SRa, SRz / SRa, is more preferably 25, and even more preferably 27. On the other hand, to reduce the risk of scratches on other components, etc., due to the protruding portions formed on the surface 131a being too steep, the upper limit of the ratio SRz / SRa is more preferably 38, and even more preferably 37.

[0064] If SRz / SRa is set as above, in the optical sheet manufacturing step, when two or more base layers of optical sheets are stacked and operated, since irregularities (such as relatively gentle convex portions) are formed on the surface of the base layer, the surface friction resistance can be reduced, the occurrence of adhesion where the base layers adhere to each other can be reduced, and excellent operating characteristics can be obtained.

[0065] The base layer 131 may contain a plurality of particles. The particles can be either inorganic particles or organic particles. Examples of the particles include alumina, silica, and calcium carbonate. The lower limit of the content concentration of the particles in the polyester is preferably 10 ppm, more preferably 30 ppm, still more preferably 50 ppm. The upper limit of the content concentration is preferably 10000 ppm, more preferably 9000 ppm, still more preferably 8000 ppm.

[0066] In addition, the lower limit of the average particle size of the particles is preferably 0.005 μm, more preferably 0.010 μm, still more preferably 0.015 μm. The upper limit of the average particle size is preferably 50 μm, more preferably 40 μm, still more preferably 30 μm.

[0067] As Figure 2 shown, the base layer 131 has a surface 131a, and the surface has a plurality of first convex portions 131b formed by highly crystalline portions. In addition, the base layer 131 may also have a plurality of second convex portions 131c caused by a plurality of particles on the surface 131a. That is to say, on the surface 131a, an uneven shape can be formed due to different factors.

[0068] The plurality of first convex portions 131b formed by the highly crystalline portions are relatively gentle convex portions, and the value obtained by multiplying the aspect ratio by 1000 is preferably less than 15. The upper limit of this value is more preferably 10, still more preferably 8, and particularly preferably 7. The lower limit of this value is not particularly limited and can be, for example, 1, 1.5, or 2. Here, the aspect ratio is the value obtained by dividing the height of the convex portion by the length of the bottom of the convex portion.

[0069] The presence of the first convex portion 131b of the optical sheet is determined as follows. First, in order to observe the surface 131a of the base layer 131, other layers laminated on the surface 131a of the base layer 131 of the optical sheet are peeled off by the method described below. Here, when the surface 131a is exposed, it is not necessary to peel off other layers. In order to image the unevenness on the surface 131a of the base layer 131, a contact surface roughness measuring instrument is used. At the center of the base layer 131, 81 measurements are performed in the optical sheet forming direction, where the measurement length is 0.5 mm, the measurement interval (pitch) is 5 μm, and the stylus pressure is 100 μN. That is, a rectangle with a long side of 0.5 mm and a short side of 0.4 mm is defined as an observation area, and it is detected whether there is a first convex portion 131b having a predetermined aspect ratio. Since the first convex portion 131b does not necessarily exist uniformly on the base layer 131, the presence of the first convex portion 131b is determined by observing a total of 10 observation areas.

[0070] In order to improve the operating characteristics of the base layer 131, in the first convex portion 131b formed on the surface 131a, the lower limit of the ratio of the first convex portion 131b having an aspect ratio within the above range is preferably 40%, more preferably 50%, still more preferably 70%. On the other hand, the upper limit of this ratio can be 100%.

[0071] The ratio of the first convex portion 131b of the optical sheet is calculated as follows. First, in order to observe the surface 131a of the base layer 131, other layers laminated on the surface 131a of the base layer 131 of the optical sheet are peeled off by the method described below. Here, when the surface 131a is exposed, it is not necessary to peel off other layers. As described above, in order to image the unevenness on the surface 131a of the base layer 131, a contact surface roughness measuring instrument is used. At the center of the base layer 131, 81 measurements are performed in the film forming direction of the optical sheet, where the measurement length is 0.5 mm, the measurement interval (pitch) is 5 μm, and the stylus pressure is 100 μN. A rectangle with a long side of 0.5 mm and a short side of 0.4 mm is defined as an observation area, and a total of 10 observation areas are observed. Among these 10 observation areas, the number of observation areas in which one or more first convex portions 131b having a predetermined aspect ratio exist is defined as the ratio of the first convex portion 131b.

[0072] Here, in order to further improve the operating characteristics of the base layer 131, the lower limit of the width of the bottom of the first convex portion 131b is preferably 15 μm, more preferably 20 μm. In addition, the upper limit of the width of the bottom of the first convex portion 131b is not particularly limited, and can be, for example, 40 μm or 30 μm.

[0073] Here, in order to further improve the operating characteristics of the base layer 131, the upper limit of the height of the first protrusion portion 131b is preferably 0.20 μm, more preferably 0.15 μm. Additionally, the lower limit of the height of the first protrusion portion 131b is not particularly limited and can be, for example, 0.05 μm or 0.07 μm.

[0074] Here, in order to further improve the operating characteristics of the base layer 131, the lower limit of the full width at half maximum of the first protrusion portion 131b is preferably 10 μm, more preferably 12 μm. Additionally, the upper limit of the full width at half maximum of the first protrusion portion 131b is not particularly limited and can be, for example, 20 μm or 15 μm.

[0075] The plurality of second protrusion portions 131c caused by the plurality of particles are steep protrusion portions, and the value obtained by multiplying the aspect ratio by 1000 is preferably 15 or more. The upper limit of this value is not particularly limited and can be, for example, 70, 60, or 50.

[0076] In order to improve the operating characteristics of the base layer 131, among the second protrusion portions 131c formed on the surface 131a, the lower limit of the proportion of the second protrusion portions 131c with the aspect ratio within the above range is preferably 40%, more preferably 50%, still more preferably 70%. On the other hand, the upper limit of this proportion can be 100%. The method for calculating the proportion of the second protrusion portions 131c is the same as the method for calculating the proportion of the first protrusion portions 131b.

[0077] When such a plurality of first protrusion portions 131b and a plurality of second protrusion portions 131c are provided, the average roughness SRa, the ten-point average roughness SRz, and the ratio SRz / SRa can be easily set within the above ranges.

[0078] In the light diffusing sheet 130, the shape of the surface 131a can be changed by the particles that may be contained in the base layer 131. It is also preferable that the average roughness SRa and SRz on the surface 131a of the base layer 131 satisfy any one of the following conditions (a) or (b). Here, when the base layer 131 does not contain particles, condition (a) is easily satisfied, and when the base layer 131 contains particles, condition (b) is easily satisfied.

[0079] (a) 2.5 nm ≤ average roughness SRa < 8 nm, and 107 nm ≤ ten-point average roughness SRz < 200 nm

[0080] (b) 8 nm ≤ average roughness SRa ≤ 15 nm, and 200 nm ≤ ten-point average roughness SRz ≤ 410 nm

[0081] In condition (a), the lower limit of the average roughness SRa can be 3 nm or 3.5 nm. In condition (a), the upper limit of the average roughness SRa can be 7 nm or 6 nm. In condition (a), the lower limit of the ten-point average roughness SRz can be 110 nm or 115 nm. In condition (a), the upper limit of the ten-point average roughness SRz can be 170 nm or 150 nm.

[0082] In condition (b), the lower limit of the average roughness SRa can be 9 nm or 10 nm. In condition (b), the upper limit of the average roughness SRa can be 14 nm or 13 nm. In condition (b), the lower limit of the ten-point average roughness SRz can be 250 nm or 300 nm. In condition (b), the upper limit of the ten-point average roughness SRz can be 405 nm or 403 nm.

[0083] The average thickness of the base layer 131 is not particularly limited, preferably 10 μm or more and 400 μm or less, more preferably 20 μm or more and 130 μm or less. When the average thickness is within the above range, the base layer has a strength that can reduce the occurrence of bending and the light diffusing sheet 130 can be made thin. Here, the term "average thickness" refers to the average of the thicknesses at any 10 points.

[0084] The base layer 131 preferably has a transmission color tone with an a value of -1.0 or more and a b value of 5.0 or less. When the a value and b value of the transmission color tone satisfy the above values, the color difference of the base layer 131 can be reduced and the transparency can be improved. The upper limit of the a value and the lower limit of the b value of the transmission color tone are not particularly limited, and preferably the upper limit of the a value is 1.0 and the lower limit of the b value is -1.0.

[0085] As described above, the base layer 131 can have a surface layer interposed between the light diffusing layer 132 and the anti-sticking layer 133. The surface layer can include, for example, a binder and particles dispersed in the binder.

[0086] As the binder contained in the surface layer, either an organic binder or an inorganic binder can be used, but it is preferable to use an organic binder because it is easy to process into a roll when manufacturing the light diffusing sheet 130. An organic binder is an organic compound having a covalent bond and a molecule composed of two or more types of atoms as the minimum unit, and is preferably a phenolic resin, a silicone resin, a nylon resin, polyethylene, polyester, polyolefin, vinyl resin, acrylic resin, cellulose resin, etc., and two or more of them may be included. More preferably, considering the coatability on the base layer 131, the organic binder contains a polyester or an acrylic resin that can select appropriate functional groups. In addition, it is still more preferable that the organic binder has a crosslinked structure in order to improve heat resistance, hydrolysis resistance, and wear resistance. Examples of the organic binder having a crosslinked structure include polyurethane, polyurea, melamine resin, polyamide, and polysilane.

[0087] As the particles contained in the surface layer, either or both of organic particles and inorganic particles can be used. The inorganic particles are not particularly limited, and examples thereof include particles made of the following: silica, colloidal silica, alumina, cerium dioxide, kaolin, talc, mica, calcium carbonate, barium sulfate, carbon black, zeolite, titanium oxide, and various metal oxides. The particle size of the particles depends on the dispersibility of the particles and the thickness of the binder, and particles having various particle sizes can be used, and preferably, the lower limit value of the average particle size of at least one kind of particle is 0.2 nm, and the upper limit value thereof is 2000 nm. The upper limit value of the average particle size is more preferably 1000 nm. Here, the particle size is the average particle size, which is the average value of the effective size (diffraction scattering size) calculated by the laser diffraction scattering method.

[0088] The lower limit of the average thickness of the surface layer is preferably 0.001 μm, more preferably 0.010 μm. The upper limit of the average thickness is preferably 10 μm, more preferably 2 μm. When the thickness is set within this range, the base layer 131 can be made thin, and the first convex portion 131b and the second convex portion 131c can be effectively formed on the surface of the surface layer.

[0089] With respect to 100 parts by mass of the binder in the surface layer, the lower limit of the addition amount of the particles is preferably 0.01 part by mass, more preferably 0.1 part by mass. The upper limit of the addition amount is preferably 100 parts by mass, more preferably 50 parts by mass. When the addition amount of the particles is within the above range, it is possible to prevent the manufacturing cost of the base layer 131 from rising unnecessarily and prevent the light transmittance of the base layer 131 from decreasing.

[0090] Even if the base layer 131 has a surface layer, an uneven shape is presented on the surface of the surface layer. That is, the surface 131a of the base layer 131 having the surface layer has a ratio SRz / SRa of 17 or more and 45 or less.

[0091] Here, when the light diffusion layer 132, the anti-sticking layer 133, and the surface layer or other layers are laminated on the surface of the base layer 131, these layers are peeled off, and then the surface of the base layer 131 is analyzed to observe the unevenness of the base layer 131 itself. As a method for peeling off the layers laminated on the base layer 131, for example, the following method can be used: The optical sheet including the base layer 131 is immersed in a container and appropriately heated under vibration by an ultrasonic washer, and the container contains methyl ethyl ketone (MEK), toluene, butyl acetate, acetone, methyl p-hydroxybenzoate, ethyl p-hydroxybenzoate, propyl p-hydroxybenzoate, etc., or a mixture thereof.

[0092] (Prismatic lens)

[0093] The prismatic lenses 140 and 150 improve the light output performance of the backlight unit 100 by reflecting the light emitted from the light diffusion sheet 130. The backlight unit 100 according to the present embodiment includes two prismatic lenses 140 and 150. The first prismatic lens 140 provided on one side of the light diffusion sheet 130 reflects the light incident from the light diffusion sheet 130 and emits the light to the second prismatic lens 150. The second prismatic lens 150 reflects the light incident from the first prismatic lens 140 so that the light propagates substantially perpendicular to the light incident surface of the display panel 200, and emits the light toward the display panel 200.

[0094] The prismatic lenses 140 and 150 include substantially plate-shaped (sheet-shaped) base portions 141 and 151 having a light incident surface and a light exit surface, and a plurality of prism portions 142 and 152 provided on the light exit surfaces of the base portions 141 and 151 and extending in one direction. The plurality of prism portions 142 and 152 are raised portions arranged in parallel rows on the light exit surfaces of the prismatic lenses 140 and 150. The cross-sectional shape of the prism portions 142 and 152 in the thickness direction of the prismatic lenses 140 and 150 is not particularly limited and may be semi-circular, semi-elliptical, triangular, etc. The first prismatic lens 140 and the second prismatic lens 150 are preferably arranged such that the ridge lines of the prism portions 142 and 152 are perpendicular to each other in a plan view.

[0095] The height (thickness of the prismatic lens) from the light incident surface of the prismatic lenses 140 and 150 to the apex of the prism portions 142 and 152 is not particularly limited, and is preferably 50 μm or more and 200 μm or less, more preferably 100 μm or more and 180 μm or less. The interval (pitch) between the prism portions 142 and 152 is preferably 30 μm or more and 100 μm or less, more preferably 40 μm or more and 60 μm or less.

[0096] <Method for manufacturing an optical sheet>

[0097] As described above, the optical sheet is preferably formed from recycled polyester raw material or a mixed raw material containing recycled polyester raw material. When using recycled raw material that has undergone a thermal history during the process of forming virgin raw material polyester into a molded product, the crystallinity tends to be uneven, and the first convex portion 131b tends to be formed. A method for manufacturing an optical sheet using recycled raw material will be described below. The method for manufacturing an optical sheet mainly includes: a step of recovering a molded product containing polyester as a main component, a step of pulverizing the molded product recovered in the recovery step, a step of forming pellets by extrusion molding using the powder pieces obtained in the pulverizing step, and a step of forming a base layer using the pellets. In addition, the method for manufacturing an optical sheet includes a step of washing the powder pieces obtained in the pulverizing step.

[0098] [Recovery step]

[0099] In the recovery step, a molded product containing polyester as a main component is recovered. The molded product containing polyester as a main component is not particularly limited as long as it is a transparent molded product, and examples thereof include other used optical sheets and PET bottles.

[0100] [Pulverizing step]

[0101] In the pulverizing step, the molded product recovered in the recovery step is pulverized. The pulverizing method is not particularly limited, and known pulverizer devices that can break the molded product into powder pieces can be used.

[0102] [Washing step]

[0103] In the washing step, the powder pieces obtained in the pulverizing step are washed. The washing method is not particularly limited, and known washing machines that can wash the powder pieces using a detergent or the like can be used.

[0104] The molded product recovered in the recovery step can be washed. That is, the molded product recovered in the recovery step can be washed, and the washed molded product can be pulverized.

[0105] [Pellet forming step]

[0106] In the pellet forming step, pellets are formed by extrusion molding using the powder pieces obtained in the pulverizing step. Specifically, the powder pieces are heated and melted, and the molten material is formed into pellets by an extruder. In the pellet forming step, additives for adjusting the optical properties of the obtained optical sheet can be added. In order to easily and reliably form the above-mentioned first convex portion on the obtained base layer, the lower limit of the extrusion temperature in the pellet forming step is preferably 250°C, more preferably 265°C. On the other hand, the upper limit of the extrusion temperature is not particularly limited, and is preferably, for example, 310°C in order to prevent the generation of decomposition gas.

[0107] [Step of forming the base layer]

[0108] In the step of forming the base layer, pellets are used to form the base layer. For example, in the step of forming the base layer, the pellets are used to form the base layer by extrusion molding. In the step of forming the base layer, virgin raw materials may be added to adjust the optical properties of the obtained base layer. The extrusion temperature in the extrusion molding is equal to or higher than the melting point temperature of the polyester. To improve the ease and reliability of forming the first raised portion, the lower limit of the extrusion temperature is preferably 250°C, more preferably 265°C. On the other hand, the upper limit of the extrusion temperature is not particularly limited and is preferably, for example, 310°C to prevent the generation of decomposition gases.

[0109] [Advantages]

[0110] In the optical sheet, since an uneven shape with a ratio SRz / SRa of the ten-point average roughness SRz to the average roughness SRa of 17 or more and 45 or less is formed on at least one surface 131a of the base layer 131, it is possible to more easily adhere to other layers laminated on this surface, and it is possible to reduce adhesion and surface wear caused by contact with other members. Therefore, the optical sheet can have improved handling characteristics. In addition, since the optical sheet has an uneven shape, it can have improved light diffusion characteristics.

[0111] In the method for manufacturing an optical sheet, since the optical sheet is manufactured from raw materials obtained by recycling molded products containing polyester as a main component, the optical sheet can be manufactured from recycled resources.

[0112] [Other embodiments]

[0113] The embodiments disclosed herein are merely examples in all respects and should not be considered restrictive. The scope of the present invention is not limited to the configuration of the embodiments, but is defined by the claims and is intended to cover equivalents of the scope of the claims and all modifications falling within the scope of the claims.

[0114] Even if the optical sheet is the above-mentioned light diffusion sheet, the specific configuration of the light diffusion sheet is not particularly limited. For example, the light diffusion sheet does not need to include the above-mentioned anti-sticking layer.

[0115] The specific configuration of the backlight unit is not limited to the configuration described in the embodiments. For example, the backlight unit may be a direct-lit type.

[0116] In addition, the backlight unit may be provided in a lighting device.

[0117] The light source of the backlight unit does not necessarily have to be an LED light source. In addition, the number of light sources may be one instead of multiple. The light source may be a line light source instead of a point light source.

[0118] The backlight unit may include optical sheets other than a light guide plate, a light diffusing sheet, and a prism sheet, or may not include any of the light guide plate, the light diffusing sheet, and the prism sheet. The backlight unit may include one, or three or more prism sheets, or may include a plurality of light diffusing sheets.

[0119] The optical sheet having a base layer can be used not only as a light diffusing sheet but also as a light guide plate, a prism sheet, a polarizing plate, a reflective sheet, a viewing angle limiting sheet, etc. Accordingly, excellent operating characteristics can be expected when stacking and operating the same type of the above optical sheets or when using different types of optical sheets to fabricate the backlight unit.

[0120] In the optical sheet, as long as the ratio SRz / SRa of the ten-point average roughness SRz to the average roughness SRa on at least one surface of the base layer is within the above range, the uneven shape on the surface does not necessarily have to be formed by highly crystalline portions and particles.

[0121] The prism portion of the prism sheet can be formed on the light incident surface of the base portion.

[0122] The recycled polyester raw material in the method for manufacturing an optical sheet is not limited to the polyester raw material obtained by physical recycling, but can be the polyester raw material obtained by chemical recycling.

[0123] Examples

[0124] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0125] A variety of optical sheets (thickness: 50 μm) manufactured under different conditions using PET as a raw material were prepared. The average roughness SRa and the ten-point average roughness SRz on the surface of the optical sheet were measured, and their ratio SRz / SRa was calculated. Table 1 shows the manufacturing conditions of the optical sheet, the measured values, and the calculation results.

[0126] [Table 1]

[0127]

[0128] Here, in Table 1, the yield rate is evaluated as ◎ when the defect rate caused by scratch defects in the test step is less than 2%, as ○ when the defect rate is 2% or more and less than 5%, and as Δ when the defect rate is 5% or more and less than 10%.

[0129] As shown in Table 1, it can be understood that all the optical sheets 1 to 6 have an SRz / SRa of more than 17 and not more than 45, and the defect rate caused by scratch defects in the test step is less than 5%, which indicates a high yield. Additionally, it can be understood that all the optical sheets 1, 2, 3, and 6 have an SRz / SRa of more than 25 and not more than 37, and the defect rate caused by scratch defects in the test step is less than 2%, which indicates an even higher yield. In some of the optical sheets 1 to 6, particles are added to the PET substrate, and when the particles are included, the SRa value tends to be relatively higher than when the particles are not included. Therefore, SRa alone is not regarded as a suitable index for selecting an optical sheet with excellent operating characteristics. On the other hand, for example, the optical sheets 1, 2, 3, and 6 have an SRz / SRa of more than 25 and not more than 37 and have a higher yield. Thus, even when particles are added to the PET substrate, SRz / SRa is a suitable index for selecting an optical sheet with excellent operating characteristics.

[0130] In addition, as shown in Table 1, the optical sheets 1, 2, 3, and 6 made from recycled PET raw materials have an SRz / SRa of more than 25 and not more than 37 and have a higher yield. This is considered to be because, in the optical sheets made from recycled PET raw materials, since irregularities (such as relatively gentle raised portions) are formed on the surface of the base layer, when stacking and operating the base layers of two or more optical sheets in the manufacturing step, the surface frictional resistance decreases, and thus the occurrence of adhesion where the base layers adhere to each other can be reduced.

[0131] Next, on the surface of the optical sheet 2, three first raised portions and three second raised portions are selected. For each first raised portion, the value obtained by multiplying the aspect ratio by 1000 is less than 15, and for each second raised portion, the value obtained by multiplying the aspect ratio by 1000 is 15 or more, and the value obtained by multiplying the aspect ratio by 1000 is calculated based on its height and the width of the bottom. Additionally, the full width at half maximum thereof is measured, and the flatness ratio (full width at half maximum / width of the bottom) is calculated. The results are shown in Table 2. Here, the full width at half maximum is the width in the direction perpendicular to the thickness direction of the optical sheet at the position of half the height of the first raised portion and the second raised portion.

[0132] [Table 2]

[0133]

[0134] It can be understood from Table 2 that the value obtained by multiplying the aspect ratio by 1000 is less than 15 for the first raised portion caused by the high crystallinity of the PET raw material, and is 15 or more for the second raised portion caused by the particles. Additionally, the flatness ratios of both the first raised portion and the second raised portion are approximately 0.5, specifically 0.4 or more and 0.7 or less.

[0135] In addition, the transmission hues of the optical sheets 1 to 6 shown in Table 1 were measured, and their a-values and b-values were calculated. The results are shown in Table 3.

[0136] [Table 3]

[0137]

[0138] In the results shown in Table 3, in all of these optical sheets, the a-value of the transmission hue is 1.0 or more, and the b-value of the transmission hue is 5.0 or less. When the a-value and b-value of the transmission hue satisfy the above numerical values, the color difference of the base layer can be reduced and the transparency can be improved. As described above, there are no particular limitations on the upper limit of the a-value and the lower limit of the b-value of the transmission hue. Preferably, the upper limit of the a-value is 1.0, and the lower limit of the b-value is -1.0.

[0139] Figure 3 The schematic side view showing the structure of the light diffusing sheet 130 including the layered conductive portion is shown, Figure 3 (a) is a view showing the configuration of the light diffusing sheet 130 in which an antistatic agent is contained in the light diffusing layer 132, Figure 3 (b) is a view showing the configuration of the light diffusing sheet in which an antistatic agent is contained in the adhesive layer 137. That is, in Figure 3 the aspect shown in (a), the light diffusing layer 132 functions as the layered conductive portion, while in Figure 3 the aspect shown in (b), the adhesive layer 137 functions as the layered conductive portion. Here, Figure 3 the aspect shown is merely an example, and the layered conductive portion can be provided on any one of the front surface, the rear surface, and the inside of the optical sheet.

[0140] As described above, when the sheet surface of the optical sheet includes a conductive portion, charging of the optical sheet can be effectively suppressed and contamination inside the backlight can be effectively prevented. Such a conductive portion can be a layered conductive portion formed in parallel on the surface of the sheet, and its thickness can be 0.001 μm or more and 1 μm or less.

[0141] Examples 7 to 13 are light diffusing sheets including the optical sheets 1 to 6 shown in Table 1, with or without a layered conductive portion.

[0142] As Figure 3 shown, Examples 7 and 12 are light diffusing sheets 130 in which a light diffusing layer 132 containing no antistatic agent is provided on the surfaces of the optical sheets 1 and 5 (both having a thickness of 50 μm). The light diffusing layer 132 is formed by wet coating an acrylic resin added with styrene beads having a particle size of 1 μm to 15 μm.

[0143] As Figure 3As shown, Examples 8, 9, 10, and 11 are light diffusion sheets 130 having a light diffusion layer 132 containing an antistatic agent provided on the surfaces of optical sheets 2, 3, 4, and 4 (all having a thickness of 50 μm). The light diffusion layer 132 is formed by wet coating an acrylic resin added with an ion conductive type antistatic agent and styrene beads having a particle size of 1 μm to 15 μm. The addition amount of the antistatic agent is 1.5 parts by mass with respect to 100 parts by mass of the resin forming the light diffusion layer 132.

[0144] As shown in FIG. 4, in Example 13, an adhesive layer 137 containing an antistatic agent was provided between the optical sheet 6 (having a thickness of 50 μm) and the light diffusion layer 132. That is, the light diffusion sheet 130 of Example 13 has a three-layer structure including a base layer 131, an adhesive layer 137, and a light diffusion layer 132. The adhesive layer 137 contains an ion conductive type antistatic agent and a primer for adhering the base layer 131 and the light diffusion layer 132. In addition, the light diffusion layer 132 is formed by wet coating an acrylic resin added with styrene beads having a particle size of 1 μm to 15 μm.

[0145] The surface resistance value [Ω / sq] of the surface of the light diffusion sheet 130 on which the light diffusion layer 132 was formed in Examples 7 to 13 was measured. The results are shown in Table 4.

[0146] [Table 4]

[0147]

[0148] In the results shown in Table 4, in the two light diffusion sheets (Examples 7 and 12) without a layered conductive portion, the surface resistance value exceeded the upper limit value of the measurement limit of 9.90 × 10 15 Ω / sq, showing a surface resistance value that was too high to measure. Accordingly, it can be evaluated that the surface resistance value of the light diffusion sheet without a layered conductive portion is much higher than 10 × 10 15 Ω / sq. On the other hand, it can be understood that all the light diffusion sheets (Examples 8 to 11 and 13) having a layered conductive portion have a low surface resistance value of about 10 11 Ω / sq, have strong conductivity, that is, have an antistatic effect.

[0149] That is, the surface resistivity measured on at least one of the front surface and the back surface is preferably 1 × 10 2 Ω / sq or more and 10 × 10 15 Ω / sq or less. Accordingly, charging of the optical sheet can be effectively suppressed and contamination inside the backlight can also be effectively prevented.

[0150] In addition, the surface resistivity measured on at least one of the front surface and the back surface is more preferably 1 × 10 2 Ω / sq or more and 10 × 10 13 Ω / sq or less. Accordingly, charging of the optical sheet is more effectively suppressed and contamination inside the backlight is also more effectively prevented.

[0151] Industrial Applicability

[0152] As described above, the optical sheet of the present invention can have improved operating characteristics, and thus can be suitably used for a backlight unit of an image display device or the like.

[0153] List of Reference Numerals

[0154] 1 Image display device

[0155] 100 Backlight unit

[0156] 110 Light source

[0157] 120 Light guide sheet

[0158] 130 Light diffusion sheet

[0159] 131 Base layer

[0160] 131a Surface

[0161] 131b First raised portion

[0162] 131c Second raised portion

[0163] 132 Light diffusion layer

[0164] 133 Anti-sticking layer

[0165] 134 First bead

[0166] 135 Resin binder

[0167] 136 Second bead

[0168] 137 Adhesive layer

[0169] 140 First prism sheet

[0170] 141, 151 Base portion

[0171] 142, 152 Prism portion

[0172] 150 Second prism sheet

[0173] 200 Display panel

Claims

1. An optical sheet having a base layer containing polyester as a main component, Among them, wherein the base layer has first convex portions on at least one surface, and a value obtained by multiplying the aspect ratio of the first convex portions by 1000 is less than 15.

2. The optical sheet according to claim 1, Among them, wherein a ratio SRz / SRa of a ten-point mean roughness SRz to an average roughness SRa on the one surface is 17 or more and 45 or less.

3. The optical sheet according to claim 1, Among them, wherein the first convex portions are formed of highly crystalline portions of the polyester.

4. The optical sheet according to claim 1, Among them, wherein the base layer contains a plurality of particles and has a plurality of second convex portions caused by the plurality of particles on the one surface.

5. The optical sheet according to claim 4, Among them, wherein a value obtained by multiplying the aspect ratio of the second convex portions by 1000 is 15 or more.

6. The optical sheet according to claim 1, Among them, wherein the average roughness SRa is 2.5 nm or more and 15 nm or less, and the ten-point mean roughness SRz is 107 nm or more and 410 nm or less.

7. The optical sheet according to claim 1, Among them, wherein the transmission hue has an a value of -1.0 or more and a b value of 5.0 or less.

8. The optical sheet according to claim 1, Among them, wherein the optical sheet includes a laminated conductive portion formed parallel to the surface of the optical sheet, and the thickness of the laminated conductive portion is 0.001 μm or more and 1 μm or less.

9. The optical sheet according to claim 1, Among them, The surface resistivity measured on at least one of the front surface and the back surface is 1 × 10 2 Ω / sq or more and 10 × 10 15 Ω / sq or less.

10. The optical sheet according to claim 9, Among them, The surface resistivity measured on at least one of the front surface and the back surface is 1 × 10 2 Ω / sq or more and 10 × 10 13 Ω / sq or less.

11. The optical sheet according to claim 1, Among them, wherein the polyester is polyethylene terephthalate.

12. The optical sheet according to claim 1, further including a light diffusing layer laminated on the base layer.

13. A backlight unit including the optical sheet according to any one of claims 1 to 12.

14. An image display device including the backlight unit according to claim 13.

15. A lighting device including the backlight unit according to claim 13.

16. A method for manufacturing an optical sheet, the method including: a step of pulverizing a molded product containing polyester as a main component; a step of forming pellets by extrusion molding using the pulverized pieces obtained in the pulverizing step; and a step of forming a base layer using these pellets.

17. The method for manufacturing an optical sheet according to claim 16, the method further including a step of recycling the molded product.

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