JP2004295080A - Optical diffusion sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical diffusion sheet having high light beam transmissivity, emitting uniform diffused light whose variance of luminance is restrained to be little, made excellent in shielding property that the dot of a light transmission plate is shielded, restraining the occurrence of wrinkles caused by thermal elongation and contraction, preventing a gum phenomenon from occurring in the case of manufacture, and having no possibility that it damages the light transmission plate of a backlight unit and a lens film, and to provide the optical diffusion sheet capable of realizing the further improvement of the shielding property and the further restraint of the wrinkles caused by the thermal elongation and contraction. <P>SOLUTION: The optical diffusion sheet 1 is obtained by integrating a surface layer 1b composed of light transmissive resin on the upside and the downside of a core layer 1a composed of light transmissive resin in which an optical diffusion agent 1c is contained by 15 to 35 wt.%, and constituted so that minute ruggedness 1d is formed on both of its front and back surfaces. The optical diffusion agent is contained also in the surface layer 1b, and it is good to make the content different from that of the core layer 1a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light diffusion sheet used for a backlight unit of a liquid crystal display, an illuminated signboard, a lighting cover, an arcade, a daylighting board, a blindboard of a balcony, and the like.

  A general backlight unit of a liquid crystal display includes a light guide plate having dots for light diffusion printed on a back surface thereof, a light source (a cold cathode tube or the like) disposed on one or both sides of the light guide plate, and a light guide plate. It is composed of a light diffusion sheet stacked on top, a lens film (prism sheet) stacked on or above or below this light diffusion sheet.

  The light diffusion sheet incorporated in such a backlight unit uniformly diffuses the light from the light guide plate, prevents the dots from being seen on the display screen, and suppresses the light loss to diffuse the light to the liquid crystal panel surface. It plays the role of releasing uniformly.

  As such a light diffusion sheet, the present applicant is a light diffusion sheet having a specific range of average surface roughness and surface area ratio, which is a single-layer sheet made of a light-transmitting resin having irregularities formed on both surfaces. A light diffusing sheet in which a single layer sheet further contains a light diffusing agent has been proposed (Patent Document 1).

  These light diffusion sheets have advantages such as suppressing light loss and emitting uniform diffused light with little variation in luminance. In particular, light diffusion sheets containing a light diffusing agent are concealable. Was good, and generation of wrinkles due to thermal expansion and contraction could be suppressed.

  However, the light diffusing sheet containing no light diffusing agent has a problem that the concealing property of making the dots on the light guide plate invisible is not sufficient, and wrinkles due to thermal expansion and contraction are likely to occur. On the other hand, a light diffusion sheet containing a light diffusing agent has good concealing properties and is unlikely to cause wrinkles due to thermal expansion and contraction. However, a light transmitting resin containing the light diffusing agent is extruded into a sheet. After that, if unevenness is formed on both surfaces with a crimping roll, the light diffusing agent adheres to the periphery of the extrusion opening of the extruder and grows into large particles at the time of extrusion (so-called eye-dye phenomenon). Since a linear mark due to the particles is formed on the surface of the extruded sheet, there is a problem that the area around the extrusion port must be cleaned once every several hours to remove the adhered particles. In addition, when a light diffusion sheet containing a light diffusion agent is incorporated into a backlight unit of a liquid crystal display, the light diffusion agent protruding from both sides of the light diffusion sheet may damage the light guide plate and the lens film of the backlight unit. .

On the other hand, even if the above-mentioned problem caused by the light diffusing agent is neglected to some extent, it is a fact that further improvement of the concealing property of the light diffusing sheet and further suppression of wrinkles due to thermal expansion and contraction are strongly demanded. .
JP 2002-221612 A

  The present invention has been made in view of the above circumstances, has high light transmittance, emits uniform diffused light with little variation in luminance, has good concealing properties for hiding dots of a light guide plate, and generates wrinkles due to thermal expansion and contraction. To provide a light diffusing sheet that does not cause eye tanning phenomenon during production and does not damage the light guide plate or lens film of the backlight unit, and further improves concealment properties and wrinkles due to thermal expansion and contraction. It is an object of the present invention to provide a light diffusion sheet capable of achieving further suppression of the above.

  In order to solve the above-mentioned problems, the light diffusion sheet of the present invention has a surface layer made of a light-transmitting resin integrated above and below a core layer made of a light-transmitting resin containing 15 to 35% by weight of a light diffusing agent. Characterized in that fine irregularities are formed on both front and back surfaces thereof.

  In the light diffusing sheet of the present invention, the light diffusing agent is contained in the surface layer, and the content thereof may be different from that of the core layer, or may be different from the light diffusing agent contained in the core layer. The agent may be contained in the surface layer in an amount of 10 to 40% by weight. Then, the thickness of the core layer is 50 to 200 μm, the thickness of the entire sheet is preferably 60 to 300 μm, and the arithmetic average roughness of the front and back surfaces is preferably in the range of 0.5 to 10.0 μm, It is desirable that the surface area ratio of both the front and back surfaces be in the range of 1.0010 to 1.5000, and that the surface area ratio of one surface serving as a light exit surface be equal to or greater than the surface area ratio of the opposite surface serving as a light entrance surface. Further, in the light diffusing sheet of the present invention, the light diffusing agent contained in the core layer is an inorganic particle such as talc, and the light transmitting resin of the core layer and the surface layer is preferably polypropylene, and further, In the case of a light diffusing sheet in which the surface layer also contains a light diffusing agent, the light diffusing agent contained in the core layer is an inorganic particle such as talc excluding glass particles, and the light transmission between the core layer and the surface layer. Preferably, the conductive resin is polypropylene, and the light diffusing agent contained in the surface layer is glass particles. And it is desirable that the above-mentioned polypropylene has a crystallinity of 30 to 80%.

Incidentally, for the "arithmetic mean roughness" in the present invention means a value measured based on JIS B 0601, the "surface area ratio", the area S ₀ when the measuring surface is assumed to be a flat surface It refers to the ratio of the actual surface area S (S / S ₀ ).

  The light diffusion sheet of the present invention, for example, using a three-layer co-extrusion molding machine, simultaneously extruding the light-transmissive resin containing a light-diffusing agent, at the same time, overlying the light-transmissive resin without a light-diffusing agent. By extruding the sheet, a sheet having a three-layer laminated structure is formed, and then manufactured by a method such as forming fine irregularities on both the front and back surfaces of the sheet with a crimping roll or the like. When a light-transmitting resin containing no light-diffusing agent is overlaid and co-extruded on the upper and lower sides of a light-transmitting resin containing the so-called light-diffusing agent, the light-diffusing agent does not adhere to the periphery of the extrusion port and grow into large particles, so-called, No linear marks due to the eye repelling phenomenon are attached to both sides of the sheet. Accordingly, frequent cleaning of the extrusion port is not required, and co-extrusion molding can be performed efficiently, so that productivity is greatly improved.

  In addition, when a surface layer not containing a light diffusing agent is laminated and integrated above and below a core layer containing a light diffusing agent as in this light diffusing sheet, the light diffusing agent may protrude from both front and back surfaces of the sheet. Since there is no light guide plate, the fear of damaging the light guide plate and the lens film when incorporated in the backlight unit is eliminated.

  When the light diffusion sheet of the present invention is incorporated in a backlight unit and superimposed on a light guide plate, most of the light traveling while appropriately reflecting the inside of the light guide plate has fine irregularities formed on the back surface of the sheet. By the action, light enters the sheet substantially uniformly from the entire back surface of the sheet, so that the light loss is reduced and there is almost no partial variation in the amount of incident light. The light that has entered the sheet is diffused by a light diffusing agent contained in the core layer in an amount of 15 to 35% by weight, and further diffused and emitted by the fine irregularities formed on the surface of the sheet. Therefore, this light diffusion sheet has a high light transmittance, has no variation in luminance of diffused light emitted, and has good concealing properties.

  When the core layer contains a light diffusing agent in an amount of 15 to 35% by weight as in the light diffusing sheet of the present invention, the linear expansion coefficient of the core layer decreases and the elastic modulus of the core layer increases. Even if the temperature of the diffusion sheet is increased by the heat of the light guide plate, generation of wrinkles is suppressed.

  In the light diffusing sheet of the present invention, the surface layer also contains a light diffusing agent with a different content from the core layer, or a light diffusing agent different from the light diffusing agent contained in the core layer. In the case where 10 to 40% by weight is contained, the linear expansion coefficient of the surface layer is reduced and the elastic modulus of the surface layer is increased by the light diffusing agent of the surface layer, whereby the linear expansion coefficient of the entire light diffusion sheet is reduced. In addition, since the elastic modulus increases, the generation of wrinkles due to the heat of the light guide plate is further suppressed. When the surface layer contains a light diffusing agent, the above-described problem of the so-called eye repelling phenomenon and the problem of damaging the light guide plate of the backlight and the like are restored, but these problems are caused by the light contained in the surface layer. By selecting and adjusting the type and content of the diffusing agent, it can be minimized.

  Further, the light diffusion sheet of the present invention in which the thickness of the core layer is 50 to 200 μm and the thickness of the entire sheet is 60 to 300 μm has good concealing properties despite high light transmittance and luminance, and thermal expansion and contraction. Wrinkles are less likely to occur. When the thickness of the core layer is less than 50 μm and the thickness of the entire sheet is less than 60 μm, the light transmittance and the luminance are further improved, but the linear expansion coefficient is increased, the rigidity is reduced, and wrinkles are easily generated, and the concealment is performed. There is a possibility that the property may also be reduced. Conversely, if the thickness of the core layer exceeds 200 μm and the thickness of the entire sheet exceeds 300 μm, wrinkles are less likely to occur and concealment is improved, but disadvantages such as a decrease in light transmittance and luminance occur.

  In the light diffusion sheet of the present invention, when the arithmetic average roughness of the front and rear surfaces on which fine irregularities are formed is in the range of 0.5 to 10.0 μm, the light diffusion sheet is guided when the light diffusion sheet is stacked on the light guide plate. Since the roughness is such that light can easily enter from the light plate, light loss is greatly reduced and variation in the amount of incoming light is eliminated. Moreover, since the arithmetic average roughness of 0.5 to 10.0 μm is suitable for diffusing light, the effect of diffusing light emitted from the light diffusion sheet is further improved.

  Further, in the light diffusing sheet of the present invention, when the surface area ratio of both front and back surfaces is in the range of 1.0010 to 1.5000, the unevenness of both front and back surfaces is distributed with fineness suitable for light input and light diffusion. Therefore, the amount of incident light and the diffusivity are improved, and in particular, when the surface area ratio of one surface serving as the light output surface is equal to or greater than the surface area ratio of the opposite surface serving as the light incident surface, the unevenness of the one surface is reduced. It becomes finer than the opposite surface and can emit light while diffusing light uniformly and sufficiently.

  In the light diffusion sheet of the present invention, it is particularly preferable that the light diffusion agent contained in the core layer is inorganic particles such as talc, and the light transmitting resin of the core layer and the surface layer is polypropylene. The diffusion sheet has high light transmittance, high haze value, high luminance, low linear expansion coefficient, and high elastic modulus, as demonstrated by the experimental data described below, so that it has excellent concealment, brightness, and protection. It has wrinkle properties and the like. And in the case of a light diffusion sheet in which a light diffusing agent different from the light diffusing agent of the core layer is also contained in the surface layer, the light diffusing agent contained in the core layer is inorganic particles such as talc other than glass particles. Preferably, the light transmitting resin of the core layer and the surface layer is polypropylene, and the light diffusing agent contained in the surface layer is glass particles. Such a light diffusion sheet has an advantage that wrinkle resistance is further improved, as evidenced by experimental data described later. A light diffusion sheet using polypropylene having a crystallinity in the range of 30 to 80% as a light-transmitting resin for the core layer and the surface layer has a high rigidity at high temperature, is hard to wrinkle, has a large amount of transmitted light, and has high brightness. A high light diffusion sheet can be obtained.

  Hereinafter, typical embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to only these embodiments.

  FIG. 1 is a sectional view showing an embodiment of the light diffusion sheet according to the present invention.

  The light diffusion sheet 1 is a sheet having a three-layer structure in which surface layers 1b, 1b made of a light-transmitting resin are laminated and integrated above and below a core layer 1a made of a light-transmitting resin containing a light diffusing agent 1c. Then, fine irregularities are formed on the front and back surfaces 1d, 1d.

  Examples of the light-transmitting resin of the core layer 1a include polycarbonate, polyester, polyethylene, polypropylene, polyolefin copolymer (for example, poly-4-methylpentene-1 or the like), polyvinyl chloride, and cyclic polyolefin (for example, having high total light transmittance). Thermoplastic resins such as cyclopolyolefins), acrylic resins, polystyrene, and ionomers are preferably used. Among these, polypropylene has good heat resistance, and when the degree of crystallinity is increased, the elastic modulus is improved, and thermal deformation and wrinkles are less likely to occur, and the difference in refractive index from the light diffusing agent is reduced due to the increase in refractive index. It is very preferably used because it has the advantages of increasing the amount of transmitted light and increasing the luminance. In particular, polypropylene having a crystallinity of 30 to 80% has high rigidity and a refractive index of about 1.48 to 1.52 which is close to the refractive index (1.54) of talc which is preferably used as a light diffusing agent. Therefore, when the core layer 1a is formed in combination with talc, a light diffusion sheet having a large total light transmission amount and a high luminance can be obtained. More preferred crystallinity of polypropylene is 50-60%.

  As the light-transmitting resin of the surface layer 1b, the same light-transmitting resin as that of the core layer 1a is used, and polypropylene is particularly preferably used.

  The light diffusing agent 1c contained in the core layer 1a has a role of diffusing light and a role of suppressing thermal expansion and contraction to prevent the occurrence of wrinkles, and has a different light refractive index from the light transmitting resin of the core layer 1a. Inorganic particles, metal oxide particles, and organic polymer particles are used alone or in combination. Examples of the inorganic particles include glass (A glass (soda-lime glass), C glass (borosilicate glass), E glass (low alkali glass)), silica, mica, synthetic mica, calcium carbonate, magnesium carbonate, barium sulfate, talc, Particles such as montmorillonite, kaolin clay, bentonite, and hectorite; metal oxide particles such as titanium oxide, zinc oxide, and alumina; and organic polymer particles such as acrylic beads, styrene beads, and benzoguanamine. Etc. are used. Among these, inorganic particles having a low linear expansion coefficient are preferably used from the viewpoint of suppressing thermal expansion and contraction of the core layer 1a. In particular, inorganic particles such as talc particles, especially talc particles have an aspect ratio of 3 to 500. It is large and reduces the linear expansion coefficient of the core layer 1a. In addition, by acting as a crystal nucleating agent for the polypropylene, the crystal grain size can be finely and uniformly dispersed while increasing the crystallinity of the polypropylene. And the elastic modulus can be improved, and the mechanical strength of the core layer 1a can be improved with a small amount of addition. In addition, the glass particles are transparent in themselves and transmit light well, so that the light transmission of the core layer 1a can be made favorable, and it is preferable to obtain a sheet with improved luminance. The shape of the light diffusing agent may be any shape such as a sphere, a plate, and a fiber.

  As these light diffusing agents 1c, those having an average particle size of 0.1 to 100 μm, preferably 0.5 to 50 μm, more preferably 1 to 30 μm are used. If the particle size is smaller than 0.1 μm, the particles are easily aggregated and the dispersibility is poor. Even if the particles can be dispersed uniformly, the light wavelength is larger and the scattering efficiency deteriorates. Therefore, particles having a size of 0.5 μm or more, more preferably 1.0 μm or more, are preferred. On the other hand, when the particle size is larger than 100 μm, the light scattering becomes non-uniform, the light transmittance is reduced, and the particles are undesirably seen. Therefore, particles up to 50 μm in size, and even up to 30 μm in size, are preferred.

  The content of the light diffusing agent 1c in the core layer 1a needs to be 15 to 35% by weight, and is preferably 18 to 30% by weight. When the amount is less than 15% by weight, light diffusion becomes insufficient, so that the concealing property is reduced. In addition, the suppression of thermal expansion and contraction becomes insufficient, so that the sheet is likely to be wrinkled. On the other hand, if the content exceeds 35% by weight, the light transmittance and brightness of the core layer 1a decrease due to scattering, reflection, and absorption by the light diffusing agent 1c. Therefore, a display using a backlight unit incorporating such a light diffusing sheet is used. Illuminates the display from behind.

  The thickness of the core layer 1a is desirably 50 to 200 μm, and the overall thickness of the light diffusion sheet is desirably 60 to 300 μm. The thickness of the surface layer 1b may be determined in a range that satisfies the thickness condition of the core layer 1a and the thickness condition of the entire sheet at the same time. When the thickness of the core layer 1a is thinner than 50 μm and the thickness of the entire sheet is thinner than 60 μm, as described above, the light transmittance and luminance of the light diffusion sheet are improved, but the linear expansion coefficient is increased and the elastic modulus is increased. As a result, wrinkles are likely to occur, and light diffusion is also weakened. Conversely, when the thickness of the core layer is greater than 200 μm and the thickness of the entire sheet is greater than 300 μm, thermal expansion and contraction of the light diffusion sheet is suppressed, wrinkles are prevented, and concealment is improved, but light transmittance and brightness are reduced. The display on the display is hard to see because of the decrease. The more preferable thickness of the core layer 1a is 60 to 180 μm, the more preferable thickness is 70 to 120 μm, the more preferable thickness of the whole is 80 to 200 μm, and the further preferable thickness is 100 to 150 μm.

  The thickness of the surface layer 1b may be determined in a range that simultaneously satisfies the thickness condition of the core layer 1a and the thickness condition of the entire sheet as described above, but the surface layer 1b protrudes from both upper and lower surfaces of the core layer 1a. Since it is formed to cover the particles of the light diffusing agent 1c, its thickness is desirably about 4 to 50 μm, preferably about 5 to 30 μm, and more preferably about 6 to 10 μm. When the surface layer 1b is thinner than 4 μm, the coating of the light diffusing agent 1c becomes insufficient, and a part of the light diffusing agent may be exposed on the front and back surfaces 1d. On the other hand, even if the surface layer 1b is formed thicker than 50 μm, the function and effect corresponding to the thickness cannot be obtained, which may cause wrinkles.

  The surface layer 1b may contain a light diffusing agent, which will be described later with reference to another embodiment shown in FIG.

  The front and back surfaces 1d, 1d of the light scattering sheet 1 are formed with fine irregularities so that the arithmetic average roughness Ra is 0.5 to 10.0 μm, preferably 0.6 to 8.0 μm. . As described above, since both surfaces having the arithmetic average roughness Ra of 0.5 to 10.0 μm easily enter light uniformly from the light guide plate and have high light transmittance, light loss is significantly reduced and uniformity is achieved. It becomes possible to emit strongly diffused light. The unevenness is desirably formed in a shape having a rounded tip. In this case, when the light scattering sheet 1 is incorporated in the backlight unit, there is a possibility that the upper and lower lens films 3 and the light guide plate 2 may be damaged. There is nothing. The arithmetic mean roughness Ra of the front surface and the arithmetic mean roughness Ra of the back surface may be the same or different.

  On the other hand, the surface area ratio due to the unevenness of the front and back surfaces 1d, 1d is in the range of 1.0010 to 1.5000, and the surface area ratio of the front surface 1d serving as the light emitting surface is opposite to the light receiving surface. It is desirable that the surface area ratio is equal to or larger than the surface area ratio. Specifically, the surface area ratio of the front surface 1d serving as the light emitting surface is set to 1.0050 to 1.5000, preferably 1.0200 to 1.3000, and the surface area ratio of the back surface 1d serving as the light incident surface is set to 1.0010 to 1. 1000, preferably 1.500 to 1.0200. In this way, light can enter the back surface 1d better, and there is an advantage that the light is strongly diffused and emitted on the front surface 1d.

  The light-diffusing sheet 1 having the above-described structure is prepared by, for example, using a three-layer co-extrusion molding machine, using a light-transmitting resin as a raw material containing a light-diffusing agent in an amount of 18 to 35% by weight. The mixture is extruded into a film or a sheet, and at the same time, a light-transmitting resin containing no light-diffusing agent is overlaid and extruded on the upper and lower sides, thereby exposing the surface layers 1b, 1b on and under the core layer 1a. Is molded into a three-layered sheet, and then passed between a pair of upper and lower crimping rolls or pressed by a press plate having fine irregularities, thereby forming the front and back surfaces 1d, 1d of the sheet. It is manufactured by forming fine irregularities on the surface. As described above, when the light-transmitting resin containing no light-diffusing agent is overlaid and co-extruded on the upper and lower sides of the light-transmitting resin containing the light-diffusing agent, the light-diffusing agent adheres to the periphery of the extrusion port and grows into large particles. Since the so-called eye repelling phenomenon is eliminated, and the linear marks do not adhere to both sides of the sheet, frequent cleaning of the extrusion port becomes unnecessary. Therefore, coextrusion molding can be performed efficiently for a long time continuously, so that productivity is greatly improved.

  Such a light diffusion sheet 1 is placed on a light guide plate 2 on a reflection sheet 5 as shown in FIG. 1, a lens film 3 is overlaid thereon, and a light source 4 is provided on a side of the light guide plate 2. When the backlight unit is assembled in this manner, the light diffusion sheet 1 comes into contact with the upper and lower lens films 3 and the light guide plate 2, but even with such contact, the light diffusion agent 1c is dispersed by the surface layers 1b, 1b. Since it does not protrude from the front and back surfaces 1d, 1d, the lens film 3 and the light guide plate 2 are not damaged.

  When the light from the light source 4 is incident on the light guide plate 2, as described above, the back surface 1 d having the irregularities of the light diffusion sheet 1 has an arithmetic average roughness and a surface area ratio suitable for the light incidence. Most of the light that travels while appropriately reflecting and exiting the inside of the sheet 1 enters the sheet 1 substantially uniformly from the entire back surface 1d of the sheet 1. The incident light is diffused by the light diffusing agent 1c of the core layer 1a, further diffused by the uneven surface 1d, and emitted toward the lens film 3. The light emitting surface 1d is already exposed. As described above, since it has an arithmetic average roughness suitable for diffusion and a surface area ratio larger than the back surface 1d, light diffusion is strong and uniform. Therefore, the light diffusion sheet 1 has a small light loss, can emit uniform scattered light, has no partial brightness variation, has good concealing properties, and does not see dots on the back surface of the light guide plate 2.

  Further, in the light diffusion sheet 1, since the core layer 1a contains 15 to 35% by weight of the light diffusion agent 1c, the coefficient of linear expansion is reduced and the elastic modulus is improved. , Heat expansion and contraction is suppressed and wrinkles are hardly generated.

  FIG. 2 is a cross-sectional view showing a light diffusion sheet according to another embodiment of the present invention.

The light diffusing sheet 10 has a light transmissive resin containing 10 to 40% by weight of a light diffusing agent 1e above and below the core layer 1a made of a light transmissive resin containing 15 to 35% by weight of a light diffusing agent 1c. This is a sheet in which the surface layers 1b, 1b are integrally formed, and the above-mentioned fine irregularities 1d, 1d are formed on both front and back surfaces thereof.

  As the light diffusing agent 1e for the surface layer 1b, any of the above-mentioned various light diffusing agents 1c to be contained in the core layer 1a can be used, but the same light diffusing agent for the core layer 1a and the surface layer 1b can be used. Is required to be different from each other, and when different light diffusing agents are to be contained, the respective contents may be the same or different. That is, assuming that the light diffusing agent 1c of the core layer 1a is, for example, talc powder, if the light diffusing agent 1e of the surface layer 1b is the same talc powder, the content may be different, and the light diffusing agent other than talc powder may be used. If they are the same, they may be the same or different.

  Preferred examples of the light diffusing agent 1e to be contained in the surface layer 1b include the above-mentioned organic polymer particles having a light refractive index different from that of the light-transmitting resin of the surface layer 1b, the above-mentioned glass particles having a low coefficient of linear expansion, and other inorganic particles. Is mentioned. Organic polymer particles and glass particles are smooth spheres, and there is no fear of damaging the lens film etc. of the backlight unit even if they protrude from the uneven surface 1d of the surface layer 1b. Harm is hardly observed even if it is contained in the surface layer 1b. Rather, the advantages of increasing the light condensing property, suppressing the occurrence of wrinkles due to thermal expansion and contraction, and further improving the light diffusivity are greater.

In particular, since the glass particles themselves have transparency, the light transmittance is not hindered by the surface layer 1b, and the glass particles are contained in an amount of 10 to 40% by weight, preferably 20 to 35% by weight to reduce the linear expansion coefficient. It is preferable that the brightness of the light diffusion sheet 10 is not lowered even if the surface layer 1b is lowered. Among these glass particles, the A glass particles are particularly preferable because the brightness of the light diffusion sheet 10 can be increased as compared with the E glass particles, as shown in Examples described later.

  The content of the light diffusing agent 1e in the surface layer 1b needs to be 10 to 40% by weight, and when the content is less than 10% by weight, the linear expansion coefficient of the surface layer 1b is reduced or the elastic modulus is reduced. Since the rise is insufficient, it is difficult to significantly suppress the generation of wrinkles due to the thermal expansion and contraction of the light diffusion sheet 10. On the other hand, when the content of the light diffusing agent 1e exceeds 40% by weight, the linear expansion coefficient and the elastic modulus are improved, but the light transmissivity is deteriorated and the luminance of the light diffusing sheet deteriorates.

  The light diffusing sheet 10 has the same structure as the light diffusing sheet 1 except that the light diffusing agent 1e is contained in the surface layer 1b, and the description of the same structure is omitted. I do.

  In the light diffusion sheet 10 having the above configuration, when the surface layer 1b contains the light diffusion agent 1e other than the glass particles, the total light transmittance is slightly lower than that of the light diffusion sheet 1 described above. Although there is a tendency, and in the case of multi-layer coextrusion molding, a slight dent phenomenon is also observed, but on the other hand, the light diffusing agent 1e contained in the surface layer 1b has a higher haze than the light diffusing sheet 1 described above. And the like, and furthermore, the generation of wrinkles can be remarkably suppressed. This advantage far outweighs the above-mentioned disadvantages, and thus has sufficient utility. In particular, when glass particles are contained as the light diffusing agent 1e, a remarkable effect that the generation of wrinkles can be suppressed without lowering the total light transmittance can be provided.

  Next, more specific examples and comparative examples of the present invention will be described.

  Using a three-layer co-extrusion molding machine, polypropylene containing 30% by weight of talc powder having an average particle diameter of 7.2 μm as a light diffusing agent is extruded into a sheet having a thickness of 116 μm, and light is diffused up and down. Polypropylene containing no agent was overlaid and co-extruded to a thickness of 7 μm to form a three-layer sheet having a total thickness of 130 μm. Then, the sheet was passed between upper and lower crimping rolls to produce a light diffusion sheet having fine irregularities on both front and back surfaces.

  The arithmetic mean roughness Ra of the above light diffusion sheet was measured using a DEKTAK II [manufactured by Japan Vacuum Engineering Co., Ltd.] in a measurement range of 5 mm in length. The Ra of the surface serving as the light emitting surface was 4.96 μm. The surface area ratio of the light diffusion sheet was measured using an ultra-depth shape measuring microscope VK-8550 [manufactured by Keyence Corporation]. The surface area ratio of the resulting surface was 1.0787.

  When the total light transmittance and the haze value of the above light diffusion sheet were measured using a haze meter NDH2000 (manufactured by Nippon Denshoku Industries Co., Ltd.), the total light transmittance was 87.9% and the haze value was 91. 6%.

The linear expansion coefficient of the above light diffusion sheet at 20 ° C. and 60 ° C. was measured using TMA8140C [manufactured by Rigaku Corporation]. The linear expansion coefficient at 20 ° C. was 3.6 × 10 ^−5. / ° C., and the linear expansion coefficient at 60 ° C. was 5.6 × 10 ⁻⁵ / ° C.

  Further, the above light diffusion sheet was measured for elastic modulus at 60 ° C. using DMS6100 [manufactured by Seiko Instruments Inc.]. As a result, it was 2,110 MPa.

  When the crystallinity of the polypropylene of the sheet was measured by the method of differential thermal analysis (DSC) (measurement conditions: temperature rise 10 ° C./min, 140 to 220 ° C.), the crystallinity was 44.7%. Was.

Next, the light diffusion sheet was placed on the light guide plate of a backlight unit for a liquid crystal display, and the light source was turned on. A luminance meter [ft- made by Minolta Co., Ltd.] was placed at a distance of 20 cm from the light diffusion sheet. 1 ° p], the luminance was 1302 cd / m ² . At the same time, whether or not the dots on the back surface of the light guide plate were concealed was visually observed. As a result, the dots were completely concealed and could not be visually recognized, and the dot concealability was good. Further, when the presence or absence of wrinkles was visually observed, no wrinkles were generated.

  The results of these measurements and observations are summarized in Table 1 below.

  A light diffusing sheet was produced in the same manner as in Example 1 except that the content of the talc powder of the light diffusing agent was changed to 21% by weight. This sheet was examined for total light transmittance, haze value, luminance, linear expansion coefficient at 20 ° C. and 60 ° C., dot concealing property, and presence or absence of wrinkles as in Example 1. The results are shown in Table 1 below.

[Comparative Examples 1 and 2]
A light diffusion sheet was produced in the same manner as in Example 1, except that the content of the talc powder as the light diffusing agent was changed to 40% by weight (Comparative Example 1) and 12% by weight (Comparative Example 2). For these light diffusion sheets, the total light transmittance, the haze value, the luminance, the linear expansion coefficient at 20 ° C. and 60 ° C., the elastic modulus at 60 ° C., the dot hiding property, and the presence or absence of wrinkles were examined in the same manner as in Example 1. . The results are shown in Table 1 below. The luminance, linear expansion coefficient, and elastic modulus of the light diffusion sheet of Comparative Example 1 could not be measured because the light diffusion sheet was broken.

  In Table 1, ○ indicates that the dot concealing property is good and the dot visibility is difficult, and X indicates that the dot concealing property is poor and the dot visibility is possible.

  It can be seen from Table 1 that the higher the talc content, the lower the linear expansion coefficient of the light diffusion sheet, the higher the elastic modulus, and the less the occurrence of wrinkles. The sheets having a talc content of 30% by weight and 21% by weight like the sheets of Examples 1 and 2 and falling within the range of the content of the light diffusing agent of the present invention have no wrinkles. The talc content as small as 12% by weight, such as the sheet of Comparative Example 2, which is less than the range of the light diffusing agent of the present invention, while the core layer does not crack due to the weakening of the core layer, Although the layer was not weakened, it was found that wrinkles occurred due to a high linear expansion coefficient and a low elastic modulus. Further, the sheet having a talc content as high as 40% by weight, which exceeds the content range of the light diffusing agent of the present invention, such as the sheet of Comparative Example 1, is liable to break because the core layer is brittle, and lacks practicality. You can see that.

  It can be seen from Table 1 that as the talc content increases, the reflection and diffusion of light increase, so that the total light transmittance and luminance gradually decrease and the haze value gradually increases. The light diffusion sheets of Examples 1 and 2 each containing 30% by weight and 21% by weight of talc have appropriate total light transmittance, luminance and haze value, respectively, because the reflection and diffusion of light are appropriate. A sheet having good dot concealing properties and uniform brightness, whereas a sheet having a talc content as high as 40% by weight as in the sheet of Comparative Example 1, is excellent in concealing properties, but has all light transmission. It can be seen that the sheet has poor brightness because of the low rate. A sheet having a talc content as low as 12% by weight, such as the sheet of Comparative Example 2, is excellent in brightness and brightness, but has a low light diffusion and a low haze value, and thus has poor dot concealing properties. You can see that.

  At the same time, polypropylene containing 30% by weight of talc powder was extruded into a sheet having a thickness of 124 μm, and at the same time, polypropylene not containing a light diffusing agent was coextruded to a thickness of 3 μm above and below to form a sheet. Then, the light diffusion sheet of Example 3 having fine irregularities on both the front and back surfaces was produced by passing this sheet through a crimping roll different from the rubber roll of Example 1.

  At the same time, polypropylene containing 30% by weight of talc powder is extruded into a sheet having a thickness of 112 to 115 μm, and at the same time, polypropylene containing no light diffusing agent is co-extruded to a thickness of 6 to 7 μm above and below the sheet. Molded. Then, this sheet was passed through a crimping roll different from the rubber roll of Example 1 to produce a light diffusion sheet of Example 4 having fine irregularities on both front and back surfaces.

  At the same time, polypropylene containing 30% by weight of talc powder was extruded into a sheet having a thickness of 93 to 96 μm, and at the same time, polypropylene not containing a light diffusing agent was coextruded to a thickness of 15 μm above and below to form a sheet. . The sheet was passed through a crimping roll different from the rubber roll of Example 1 to produce a light diffusion sheet of Example 5 having fine irregularities on both front and back surfaces.

  At the same time, polypropylene containing 30% by weight of talc powder was extruded into a sheet having a thickness of 90 to 95 μm, and at the same time, polypropylene containing no light diffusing agent was coextruded to a thickness of 18 μm above and below to form a sheet. . Then, this sheet was passed through a crimping roll different from the rubber roll of Example 1 to produce a light diffusion sheet of Example 6 having fine irregularities on both front and back surfaces.

  At the same time, polypropylene containing 30% by weight of talc powder is extruded into a sheet having a thickness of 77 to 80 μm, and at the same time, polypropylene not containing a light diffusing agent is coextruded to a thickness of 20 to 21 μm above and below the sheet. Molded. Then, this sheet was passed through a crimping roll different from the rubber roll of Example 1 to produce a light diffusion sheet of Example 7 having fine irregularities on both front and back surfaces.

  At the same time, polypropylene containing 30% by weight of talc powder was extruded into a sheet having a thickness of 68 μm, and at the same time, polypropylene not containing a light diffusing agent was coextruded to a thickness of 31 μm above and below to form a sheet. Then, this sheet was passed between embossing rolls different from the rubber roll of Example 1 to produce a light diffusion sheet of Example 8 having fine irregularities on both front and back surfaces.

  Regarding the light diffusion sheet having a total thickness in the range of 117 to 131 μm obtained in Examples 3 to 8, as in Example 1, the total light transmittance, the haze value, the luminance, and the light transmittance at 20 ° C. and 60 ° C. The linear expansion coefficient, the elastic modulus at 60 ° C., the dot concealing property, the crystallinity, the arithmetic average roughness, and the presence or absence of wrinkles were examined. The results are listed in Table 2 below.

  In Table 2, .largecircle. Indicates that the dot concealing property is good and the visibility of the dot is difficult, and .DELTA. Indicates that the dot concealing property is slightly poor and the dot is slightly visible.

  From Table 2, it can be seen that in the light diffusion sheets of Examples 3 to 7 in which the total thickness is 117 to 131 μm, as the thickness of the core layer becomes thinner and the thickness of the surface layer becomes thicker, the total light transmittance of the light diffusion sheet and It can be seen that the luminance is improved and the haze value is reduced. The sheets of Examples 4 to 7 in which the thickness of the core layer is in the range of 77 to 115 μm and the total thickness is in the range of 117 to 131 μm are all excellent in total light transmittance, luminance and haze. Further, since the sheet has a low coefficient of linear expansion and a high modulus of elasticity, it is understood that the sheet has good dot concealing properties and uniform wrinkles without wrinkles. On the other hand, when the thickness of the core layer is as thin as 68 μm as in the light diffusion sheet of Example 8, although the brightness and the brightness are excellent, the light diffusion is slightly weakened and the linear expansion coefficient is increased, so that the dot concealing property is reduced. It was found that the core layer was not sufficiently exhibited, wrinkles were slightly generated, and the thickness of the core layer could not be further reduced. On the other hand, the light diffusion sheet of Example 3 in which the thickness of the core layer is the largest is excellent in dot concealing properties and does not cause wrinkles, but has a lower total light transmittance than other sheets. It can be seen that the sheet has slightly lower brightness and brightness. As described above, the characteristics of the light diffusion sheet differ depending on the thickness of the core layer. The thickness of the core layer that can exhibit the most balanced characteristics is about 70 to 120 μm. It can be seen that the light diffusion sheet having a smaller amount of the light source should be used in a light source having a large calorific value and the light diffusion sheet having a thicker core layer should be used in a light source having a large heat generation amount.

  Using a three-layer co-extrusion molding machine, a polypropylene containing 21% by weight of talc powder having an average particle diameter of 7.2 μm as a light diffusing agent is extruded into a sheet having a thickness of 108 μm, and simultaneously, Polypropylene containing 30% by weight of E glass particles having an average particle size of 10 μm as a light diffusing agent different from the light diffusing agent is superposed and coextruded to a thickness of 11 μm to form a three-layer structure having a total thickness of 130 μm. A sheet was formed. Then, the sheet was passed between upper and lower crimping rolls to produce a light diffusion sheet having fine irregularities on both front and back surfaces. In the same manner as in Example 1, the light diffusion sheet was examined for total light transmittance, haze value, luminance, linear expansion coefficient at 20 ° C. and 60 ° C., elastic modulus at 60 ° C., dot concealability, and presence or absence of wrinkles. Is shown in Table 3 below.

  Instead of the E glass particles, a light diffusion sheet having a three-layer structure with a total thickness of 130 μm was prepared in the same manner as in Example 9 except that 30% by weight of A glass particles having an average particle size of 18 μm was contained. In the same manner as in Example 1, the sheet was examined for total light transmittance, haze, luminance, linear expansion coefficient at 20 ° C. and 60 ° C., elastic modulus at 60 ° C., dot hiding properties, and the presence or absence of wrinkles. The results are shown in Table 3 below.

  Using a three-layer co-extrusion molding machine, a polypropylene containing 21% by weight of talc powder having an average particle diameter of 7.2 μm as a light diffusing agent is extruded into a sheet having a thickness of 108 μm, and at the same time, light is diffused up and down. Polypropylene containing no agent was overlaid and co-extruded to a thickness of 11 μm to form a three-layer sheet having a total thickness of 130 μm. Then, the sheet was passed between upper and lower crimping rolls to produce a light diffusion sheet having fine irregularities on both front and back surfaces. As in Example 1, the light diffusion sheet was examined for total light transmittance, haze value, luminance, linear expansion coefficient at 20 ° C. and 60 ° C., elastic modulus at 60 ° C., dot hiding property, and presence or absence of wrinkles. It is shown in Table 3 below.

  In Table 3, .largecircle. Indicates that the dot concealing property is good and it is difficult to visually recognize the dot, and .DELTA. Indicates that the dot concealing property is slightly poor and the dot is slightly visible.

  Referring to Table 3, the light diffusion sheets of Examples 9 and 10 containing 30% by weight of E glass particles and A glass particles in the surface layer are different from the light diffusion sheets of Example 11 containing no glass particles in the surface layer. In comparison, although the total light transmittance and luminance did not change much, the coefficient of linear expansion was reduced and the elastic modulus was improved, which proved that the wrinkle resistance was further improved. No wrinkles occurred. In addition, the light diffusion sheets of Examples 9 and 10 containing particles of E glass and A glass have a higher haze value than the light diffusion sheets of Example 11 containing no glass particles. It can be seen that the dot concealability of the sample is further improved. Further, when comparing the sheet of Example 9 containing the E glass particles with the sheet of Example 10 containing the A glass particles, the total light transmittance and luminance of the sheet of Example 10 are slightly higher. From this, it can be seen that it is better to include A glass particles in order to obtain a bright light diffusion sheet.

It is sectional drawing which shows one Embodiment of the light-diffusion sheet which concerns on this invention. It is sectional drawing which shows other embodiment of the light-diffusion sheet which concerns on this invention.

Explanation of reference numerals

REFERENCE SIGNS LIST 1 light diffusion sheet 1 a core layer 1 b surface layer 1 c light diffusion agent 1 d front and back surfaces of light diffusion sheet 1 e light diffusion agent contained in surface layer 2 light guide plate 3 lens film (prism sheet)
4 Light source

Claims (10)

  A sheet in which a surface layer made of a light-transmitting resin is integrated above and below a core layer made of a light-transmitting resin containing 15 to 35% by weight of a light diffusing agent. Light diffusion sheet being formed.   The light diffusing sheet according to claim 1, wherein the light diffusing agent is also contained in the surface layer, and its content is different from that of the core layer.   The light diffusion sheet according to claim 1 or 2, wherein a light diffusing agent different from the light diffusing agent contained in the core layer is contained in the surface layer in an amount of 10 to 40% by weight.   The light diffusion sheet according to claim 1, wherein the thickness of the core layer is 50 to 200 μm, and the thickness of the entire sheet is 60 to 300 μm.   The light diffusion sheet according to any one of claims 1 to 3, wherein the arithmetic mean roughness of both front and back surfaces is in a range of 0.5 to 10.0 µm.   The light diffusion sheet according to any one of claims 1 to 3, wherein the surface area ratio of the front and back surfaces is in the range of 1.0010 to 1.5000.   7. The light diffusion sheet according to claim 6, wherein the surface area ratio of one surface serving as a light exit surface is equal to or greater than the surface area ratio of the opposite surface serving as a light entrance surface.   The light diffusing sheet according to claim 1, wherein the light diffusing agent contained in the core layer is inorganic particles such as talc, and the light transmitting resin of the core layer and the surface layer is polypropylene.   The light diffusing agent contained in the core layer is inorganic particles such as talc excluding glass particles, the light transmitting resin of the core layer and the surface layer is polypropylene, and the light diffusing agent contained in the surface layer is glass. The light diffusion sheet according to claim 2 or 3, which is a particle.   The light diffusion sheet according to claim 8, wherein the crystallinity of the polypropylene is 30 to 80%.

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