JP2009014776A - Reflective sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reflective sheet having a high reflective performance, facilitating the manufacture by thermoforming and capable of improving the reflection efficiency. <P>SOLUTION: The reflective sheet includes at least two layers of a surface layer portion and an inner layer portion, and the surface layer portion has a minute three-dimensional structural face. This reflective sheet is characterized in that the surface layer portion includes 80 vol.% or more polypropylene resin A and has the maximum thickness of 2 μm-200 μm, the inner layer portion includes a polypropylene resin A and at least one type of a resin B incompatible with the polypropylene resin, the phase separation structure of the inner layer portion is a sea-island structure comprising a sea phase of the polypropylene resin A and an island phase of the resin B, a basis weight is 30-500 g/m2 and a density is 0.35-0.85 g/m3. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a reflective sheet, and more particularly to a reflective sheet suitable for a reflective material used for a backlight of a liquid crystal display device.

  Since the liquid crystal itself does not emit light, a light source is required to use the liquid crystal as a display device. A liquid crystal display device is composed of a liquid crystal panel composed of a liquid crystal, an alignment plate, an electrode, a polarizing plate, and the like, and a device for irradiating the panel with light, generally a lighting device called a backlight, and directs light from the lamp toward the screen. A reflective sheet is used for efficient reflection.

  In general, the backlight of a liquid crystal display device is roughly classified into two types: an edge light type backlight and a direct type backlight.

  Edge light type backlights are backlights that are often used in small liquid crystal display devices used in mobile phones and personal digital assistants. Light sources such as light emitting diodes and cold cathode ray tubes, acrylic resins, etc. In many cases, the light guide plate is formed of a transparent resin in a wedge shape and a reflector disposed on the side surface of the light guide plate opposite to the liquid crystal panel. In the edge light type backlight, light from a light source such as a light emitting diode or a cold cathode ray tube disposed on the end face of the light guide plate is incident from the end face of the light guide plate. Light incident on the light guide plate exits from the side surface of the light guide plate in the process of passing through the light guide plate. Light exiting from the liquid crystal panel side of the light guide plate illuminates the liquid crystal panel, but light exiting from the side opposite to the liquid crystal panel side of the light guide plate cannot illuminate the liquid crystal panel A light reflecting plate is installed on the side of the light guide plate opposite to the liquid crystal panel, and the light coming out from the side of the light guide plate is reflected to the liquid crystal panel to effectively use the light from the light source. Usually, the liquid crystal panel is irradiated.

  The direct type backlight is a backlight in which a plurality of light source lamps such as cold cathode ray tubes are arranged side by side on the opposite side of the display surface of the liquid crystal panel, and is used for a large-screen liquid crystal display device used in large televisions. It is done. In large-screen liquid crystal display devices, the brightness of the light source lamp is limited when trying to increase the brightness of the edge-light type backlight to a level that satisfies the brightness, so a direct-type backlight that uses multiple light source lamps is usually used. ing. Since the light from the light source lamp is also irradiated on the side opposite to the liquid crystal side, it is possible to effectively irradiate the liquid crystal panel with the light from the light source by providing a reflective sheet on the side opposite to the liquid crystal side of the light source lamp. This is generally done for backlights.

  Recently, not only televisions but also personal computers often display moving images, and liquid crystal display devices are required to be brighter. For this reason, in a backlight used for a liquid crystal display device, a reflective sheet having a reflectance of 90% or more is often used. For higher reflection performance, the sheet has a higher reflectance and is bent. Developments such as ingenuity are underway. Further, in order to make the liquid crystal display device brighter, the output of a light source such as a cold cathode ray tube tends to increase. For this reason, the temperature of the backlight in use tends to be higher. For this reason, the resin used for the reflective sheet is required to have a heat resistance of approximately 80 ° C., which is close to the heat resistant temperature of the liquid crystal substance. For this reason, the reflective sheet used for the backlight of a liquid crystal display device is required to be a reflective sheet of a resin composition that is easily formed into a sheet and excellent in heat resistance. Furthermore, in a backlight used for a large-screen liquid crystal display device such as a large television, a large-area reflective sheet is illuminated with intense light over a long period of time. For this reason, there is a demand for a reflective sheet that is less likely to undergo discoloration or alteration due to light from the light source, and that does not easily undergo deformation such as warping due to temperature rise or moisture absorption over a long period of time.

  In recent years, a backlight in which a large number of light-emitting diodes are arranged on the same surface as a grid instead of a cold cathode ray tube has been used in a direct backlight of a large-screen liquid crystal display device. In this case, since the brightness of each light emitting diode is high, there is a problem that a lamp image of the diode tends to remain on the display unit. For this reason, this lamp image can be obtained by reflecting the light once on the diffusion plate directly above the light emitting diode and reflecting the reflected light again on the reflection sheet, or by increasing the distance between the light emitting diode and the diffusion plate. The correspondence which makes it erase as much as possible is made.

  It is well known that a resin sheet containing pores and bubbles inside is reflected in light and appears white or shows pearly luster when irradiated with light. The reason why the resin containing holes and bubbles in the interior reflects light well is considered as follows. Since the refractive index of the resin is approximately 1.4 to 1.6 and the refractive index of air is about 1, the reflectance of light generated by the difference in refractive index between the resin and air is about 4% per reflection. Only. However, in a resin sheet containing a large number of holes and bubbles inside, there are a large number of interfaces between the resin and air inside, so that the light irradiated to the sheet is reflected many times inside the sheet. As a result, in the resin sheet containing a large number of holes and bubbles inside, the irradiated light is mostly reflected inside the sheet, and as a result, the reflectivity of the entire sheet is considered to increase.

  In addition, since many holes and bubbles contained in the resin often have different shapes and sizes, the light reflected at the interface between the holes and bubbles is not reflected in one direction. The direction of the reflected light is different for each hole or bubble. For this reason, reflection when light is applied to a resin sheet containing a large number of holes and bubbles therein tends to be diffuse reflection in which incident light is reflected in all directions. As a sheet of a resin composition containing pores and bubbles inside, (1) by stretching the resin to which the inorganic powder is added, the interface between the resin and the inorganic powder is cleaved, and pores are formed inside the resin. There are known ones formed and (2) one obtained by dissolving a pressurized inert gas in a resin and then foaming it by reducing the pressure to form bubbles inside the resin. As the resin sheet of (1), for example, in Patent Document 1, a polyethylene terephthalate resin containing 5 to 30 wt% of fine calcium carbonate is melt extruded and biaxially stretched, and the void ratio calculated from the density is 7 to 30. % White polyethylene terephthalate sheet is disclosed. In addition, as the resin sheet of (2), for example, in Patent Document 2, an inert gas such as carbon dioxide gas was dissolved in a thermoplastic polyester under a pressurized atmosphere, and then heated and foamed under normal pressure. A light reflecting sheet containing fine bubbles therein is disclosed. On the other hand, with respect to diffuse reflection, reflection in which the angle at which light is incident on the reflection surface and the reflection angle are symmetric is called regular reflection, and the reflection surface has a specular shape. As the regular reflection by the resin sheet, (3) a polyester resin sheet whose surface is coated with a substance having a high reflectance such as silver by a method such as vapor deposition is known. The reflection sheet in this example is a sheet that causes specular reflection, but in a direct type backlight used for a large screen liquid crystal display device such as a large television, light from a light source lamp in which a plurality of light source lamps are arranged is used. It is known that spots are likely to occur in the brightness of the liquid crystal screen due to interference. For this reason, a reflection sheet that causes diffuse reflection is often used for the direct type backlight.

  As a white resin sheet in which pores are formed without adding inorganic particles, Patent Document 3 discloses a white biaxially stretched polyolefin sheet containing 65 to 93% by weight of polypropylene and 5 to 20% by weight of an incompatible resin. Although disclosed, it can be used as a white sheet used for printing paper, labels, etc., but it is difficult to obtain a high reflectivity that can be used for backlights of liquid crystal display devices. .

  Patent Document 4 discloses a resin composition containing 50% by volume or more and less than 80% by volume of a polypropylene resin and 20% by volume or more and less than 50% by volume of a resin phase-separated from the polypropylene resin at a temperature at which the polypropylene resin can be stretched. A reflective sheet is disclosed. For this, a reflection sheet having a high reflectance of 90% or more is provided with an ordinary simple resin stretching production apparatus without containing inorganic powder.

The conventional reflective sheet is usually used in the form of a flat plate, but for the purpose of increasing the reflection efficiency, the case is used after being mechanically bent, cut-folded, or processed into a two-dimensional curved surface. There are also many. However, in order to further improve the reflection performance, it has been proposed to process the reflection sheet into an optimum shape according to the usage pattern of the backlight. For example, in the case of a light-emitting diode (LED) backlight system, in order to improve the reflection efficiency, a reflection sheet is also proposed in which a portion immediately below the LED is bent into a concave shape in accordance with the arrangement of many LEDs arranged like a grid. ing. By devising the shape and controlling the reflection direction, the reflection efficiency increases, and if the lamp image such as LED becomes easy to disappear, the distance between the lamp and the diffuser can be shortened, and the thinner backlight unit Can also be formed. On the other hand, edge-light type backlights are used for small liquid crystal display devices such as mobile types, so that the display device itself is particularly required to be thin, and the entire sheet is thick when processed three-dimensionally. It is not preferable. Therefore, a method for controlling the reflection direction by providing a fine three-dimensional structure surface on the sheet surface has been proposed. For example, in Patent Document 5, light emitted in an oblique direction from the back surface of the light guide plate is reflected on an optical sheet having a concavo-convex pattern having a metal thin film formed on the surface, and then re-enters the back surface of the light guide plate vertically. An optical sheet to be made has been proposed. In order to perform fine shaping for controlling the reflection direction, for example, there is a method in which UV curing resin is used and this resin is UV shaped on the substrate surface. However, this is complicated and expensive, and there are problems such as adhesion to the substrate surface. As an easy and inexpensive method, there is a hot compression molding method in which a sheet is heated to a temperature equal to or higher than the thermal deformation temperature and then press compression molding with a shaping die or roll compression molding with a roll having a shaping die. However, since a conventional reflective sheet obtained by biaxially stretching a sheet made of an inorganic material and polypropylene has a large thermal shrinkage at the thermoforming temperature, it is difficult to obtain a molded product having a desired shape. In addition, there is a problem that the reflection performance is deteriorated by compressing and molding the sheet, or the holes or bubbles formed by stretching are crushed. In addition, the reflection sheet obtained by biaxial stretching of polyethylene terephthalate or its micro-foaming also causes surface deformation by compression molding due to problems such as large deformation when the temperature is raised to the softening temperature due to the properties of polyethylene terephthalate resin. difficult.
Japanese Patent Publication No. 6-89160 Japanese Patent No. 2925745 Japanese Patent No. 3139510 International Publication No. 2005/096036 Pamphlet Japanese Patent No. 3928395

  The present invention has been made in view of the above points, and is a reflective sheet in which the sheet surface is finely shaped by an easy and inexpensive thermoforming process, and the reflection efficiency is further increased by the finely shaped surface. An object of the present invention is to provide a reflective sheet that can be used.

The reflective sheet of the present invention is a reflective sheet composed of at least two layers of a surface layer portion and an inner layer portion, wherein the surface layer portion contains 80% by volume or more of polypropylene resin (A), and the maximum thickness of the surface layer portion is 2 μm. The inner layer portion includes a polypropylene resin (A) and at least one resin (B) that is incompatible with the polypropylene resin, and the phase separation structure of the inner layer portion is a polypropylene resin (A ) And an island phase of the resin (B), wherein the reflecting sheet has a basis weight of 30 to 500 (g / m ² ) and a density of 0.00. 35 to 0.85 (g / cm ³ ), and the surface layer portion has a fine three-dimensional structure surface.

  In the reflection sheet of this invention, it is preferable that the polypropylene resin (A) of the said inner layer part is 50 volume%-80 volume%, and the said resin (B) is 20 volume%-50 volume%.

  In the reflective sheet of this invention, it is preferable that the elasticity modulus of the said resin (B) in the temperature which can extend | stretch the polypropylene resin (A) of the said inner layer part is larger than this polypropylene resin (A).

  In the reflective sheet of this invention, it is preferable that the isotactic index (mmmm pentad%) of the polypropylene resin (A) of the said surface layer part is 55-85 mol%.

  In the reflective sheet of this invention, it is preferable that polycarbonate resin is included as said resin (B).

In the reflecting sheet of the present invention, it is preferable that the surface layer portion containing an inorganic particle of ^{0.1g / m 2 ~5g / m 2} . In this case, it is preferable that the inorganic particles include at least one selected from the group consisting of zinc oxide, titanium dioxide, barium sulfate, and calcium carbonate.

  In the reflective sheet of the present invention, the fine structure surface is preferably a reflective surface for controlling the reflection direction.

  In the reflection sheet of this invention, it is preferable that the cross section containing the vertex of one structure which comprises the said fine structure surface is a substantially triangular, The vertex angle is 30 degrees-150 degrees.

  In the reflective sheet of the present invention, it is preferable that the base of the triangle is 10 to 1000 μm and the height is 10 to 150 μm.

  In the reflective sheet of the present invention, the average total reflectance is preferably 90% or more.

  In the reflective sheet of the present invention, it is preferable that the fine structure surface is obtained by thermoforming.

  The reflection sheet of the present invention has a fine three-dimensional shape on the surface thereof compared to a conventional reflection sheet, thereby controlling the reflection direction and providing more effective reflection efficiency.

The present invention will be specifically described below.
(Reflective sheet structure)
The reflection sheet of the present invention may be composed of at least two layers of a surface layer portion and an inner layer portion, for example, may have a three-layer structure of surface layer portion / inner layer portion / surface layer portion. It may have a layer. Moreover, when comprised from two layers of a surface layer part and an inner layer part, let the layer used for the light source side of a reflection sheet be a surface layer part, and let the layer used on the opposite side to a light source be an inner layer part. Further, in the case of being composed of three or more layers, the surface layer portion and the inner layer portion described in the present invention may be included in the three or more layers. For example, when a light-resistant layer is provided on the outermost layer, the outermost layer Part (light-resistant layer) / surface layer part / inner layer part may be employed. In the reflection sheet of the present invention in which the surface layer portion has a fine three-dimensional structure surface, the structure of the surface layer portion is important.

(Fine three-dimensional structure surface)
The reflective sheet of the present invention is characterized in that the surface layer portion has a fine structure surface. The maximum thickness of the surface layer portion is 2 μm to 200 μm, and the thickness is determined by the shape of the fine structure surface. In the case of a shape with a large depth, the thickness of the surface layer portion is increased. In the case of a shape deeper than the thickness of the surface layer portion, the shape may extend to the inner layer portion, but there is no particular problem if there is no problem in use without substantially affecting the reflection performance substantially. However, in order to obtain molding processability, uniformity of shape, and uniform reflectivity, it is preferable that the desired shape is formed with the resin of the surface layer portion without extending the shape to the inner layer portion. The thickness of the surface layer portion of the reflection sheet before shaping is determined to an appropriate value from the shape in which it is shaped. That is, the thickness (h1) of the surface layer part before molding is preferably equal to or greater than the average thickness (h2) of the shaped part after molding (h1 ≧ h2). This relationship is shown in FIG. The average thickness (h2) of the shaped portion refers to the thickness when unevenness due to shaping is flattened. In FIG. 1, (a) shows the shape before shaping | molding, FIG.1 (b) shows the shape after shaping | molding. In the figure, reference numeral 1 indicates a surface layer portion, and reference numeral 2 indicates an inner layer portion.

  The shape of the fine structure is not particularly limited as long as the fine structure surface is a reflection surface for controlling the light reflection direction. For example, the shape of the cross section including the apex of one structure constituting the fine structure surface may be, for example, a substantially triangular shape, a semicircular shape, or an arc shape. In addition, one element of the structure is, for example, a quadrangular pyramid illustrated in FIG. 2, and this is regularly arranged on the sheet surface to form one fine structure surface. In this case, the quadrangular pyramid may be conical, or may be a polygonal pyramid such as a triangular pyramid, a pentagonal pyramid, or a hexagonal pyramid. Further, as shown in FIG. 3, the shape of the cross section including the apex may be, for example, a triangle, and the same shape in one direction in the plane and the shape regularly arranged in the plane. Furthermore, one shape may be formed by combining the shapes exemplified above. Further, a Fresnel lens-like structural surface made of an aggregate having a saw-shaped cross section is also preferable in terms of controlling the light reflection direction.

  In the preferred fine structure surface of the present invention, the cross section including the vertex of one structure constituting the fine structure surface is substantially triangular, and the apex angle is 30 ° to 150 °. The meaning of the substantial triangle is included in the substantial triangle, although the vertex of the triangle may be rounded to some extent in the molding process. As the shape of the triangle, an isosceles triangle or a right triangle is selected according to the purpose. More preferably, the angle is 50 ° to 120 °. In this case, the length of the base of the triangle is preferably 10 μm to 1000 μm in consideration of the effects of diffuse reflectance and fine shape. More preferably, it is 30 micrometers-500 micrometers. The height of the triangle is preferably 10 μm to 150 μm in consideration of reflection performance and the like. More preferably, it is 30 micrometers-100 micrometers.

  The surface layer portion contains 80% by volume or more of the polypropylene resin (A), and the maximum thickness is 2 μm to 200 μm in consideration of the effect of the three-dimensional structure and the reflection performance. Preferably they are 10 micrometers-100 micrometers, More preferably, they are 20 micrometers-80 micrometers. The inner layer portion is composed of a composition containing a polypropylene resin (A) and at least one resin (B) that is incompatible with the polypropylene resin (A), and the sea phase of the polypropylene resin (A) It has a sea-island structure composed of an island phase of resin (B) that is incompatible with polypropylene resin. In this case, the polypropylene resin (A) in the surface layer portion and the polypropylene resin (A) in the inner layer portion may be exactly the same, or different types of polypropylene resins may be used as long as the polypropylene resins exemplified below. Also good. In addition, although this invention is comprised from the at least 2 layer of a surface layer part and an inner layer part, as long as this requirement is satisfy | filled, the structure of the 3 layers or more may be sufficient. For example, the third layer can be provided outside the inner layer portion, that is, on the back surface portion. The third layer may have the same composition as the surface layer portion or a different composition.

(Polypropylene resin (A))
The polypropylene resin (A) is a polypropylene resin such as a homopolymer of propylene or a copolymer with a monomer such as ethylene that can be copolymerized with propylene. The polypropylene resin (A) is preferably a polypropylene resin having a melt flow rate of 0.1 to 10 g / min measured at a temperature of 230 ° C. and a load of 21.2 N by the method of JIS K7210. The melt flow rate is 0. From the viewpoint of discoloration due to the load of the extruder and the heat of the resin composition when the polypropylene resin is melt-molded. It is preferable that it is lg / min or more, and it is preferable that it is 10 g / min or less from a viewpoint of the viscosity and moldability of resin. The proportion of the surface layer portion of the polypropylene resin (A) in the composition of the surface layer portion is 80% by volume or more, preferably 90% by volume or more, more preferably 95%, from the viewpoints of fine formability and reflection performance of the molded product. Volume% or more. Further, from the viewpoint that a sharp shape is easily obtained at the time of heat forming, the polypropylene resin (A) of the surface layer portion is preferably a polypropylene resin having an isotactic index (mmmm pentad%) of 55 to 85 mol%. More preferably, it is 60-70 mol%. Isotactic index (mmmm pentad%) Isotactic in pentad units in polypropylene molecular chains measured using nuclear magnetic resonance spectra with isotope carbon (13C-NMR) according to the method published in Macromolecules, 6, 925 (1973) by Zambelli et al. It is a fraction. In other words, it is the fraction of propylene monomer units in which five propylene monomer units are continuously meso-bonded. As a preferable specific example of the polypropylene resin (A) of the surface layer portion of the present invention, there is a transparent type of “Prime TPO” manufactured by Prime Polymer Co., Ltd.

  The proportion of the polypropylene resin (A) in the entire composition is preferably 50% by volume or more from the viewpoint of tension and stretchability during resin stretching of the inner layer portion. Further, in order to obtain a reflective sheet having a high average total reflectance of 90% or more by stretching the sheet from which the composition has been extruded to form holes in the sheet, a polypropylene resin (A ) Is preferably less than 80% by volume, more preferably less than 70% by volume. The average total reflectivity here refers to an average value in both directions when the total reflectivity when light having a wavelength of 550 nm is incident from each of the MD direction and the TD direction of the sheet is measured.

(Polypropylene resin (A) and incompatible resin (B))
Examples of the resin (B) (hereinafter also simply referred to as “resin (B)”) that is phase-separated from the polypropylene resin at a temperature at which the polypropylene resin constituting the inner layer can be stretched include polyethylene resin, polystyrene resin, and polymethyl methacrylate. Examples thereof include resins, polycarbonate resins, polymethylpentene resins, polycycloolefin resins such as polynorbornene resins, polyester resins, and polyamide resins. Among these resins, a resin having a higher elastic modulus at a temperature at which the polypropylene resin can be stretched is more preferable than the polypropylene resin. Examples thereof include resins and polyamide resins. Among these resins, at least one kind of resin is preferably used by being melt mixed with a polypropylene resin, more preferably a polycarbonate resin or a polyamide resin, and most preferably a polycarbonate resin.

In this invention, it is preferable that the quantity of resin (B) in the composition which comprises an inner layer part is 20 volume% or more and less than 50 volume%. The resin (B) is preferably less than 50% by volume of the entire resin composition from the viewpoint of reducing the stretching tension. From the viewpoint of increasing the number of holes and the hole volume of the sheet to obtain a higher average total reflectance, the resin (B) is preferably 20% by volume or more, and preferably 30% by volume or more of the entire resin composition. In addition, when prescribing the resin composition, the conversion from weight% to volume% can be calculated from the density of the basic characteristics of each resin. For example Density of polypropylene resin 0.89~0.91g / cm ^3, the density of the polycarbonate resin is 1.2 g / cm ^3, it can be easily converted from these values.

  The polycarbonate resin which is a preferable example of the resin (B) of the present invention can be used alone or in combination from aromatic polycarbonate, linear polycarbonate and branched polycarbonate. The polycarbonate resin is preferably a polycarbonate resin having a melt flow rate of 0.1 to 50 g / lOmin measured by a method of JISK7210 at a temperature of 300 ° C. and a load of 11.8 N. From the viewpoint of making the mixing with the polypropylene resin uniform, the melt flow rate of the polycarbonate resin is preferably 0.1 g / lOmin or more, and from the viewpoint of easily forming pores during stretching, the melt flow rate is 50g / l0min or less. Is preferred.

  An example of the resin (B) other than the polycarbonate resin is a polyamide resin. The polyamide resin can be used alone or in combination among polyamide 66, polyamide 6, polyamide 610, polyamide 612, polyamide 11, polyamide 12, aromatic polyamide and the like. The polyamide resin is preferably a polyamide resin having a melting point of 300 or less from the viewpoint of dispersibility when extruded by an extruder.

  In the present invention, a polystyrene resin can be used for the inner layer portion in addition to the polypropylene resin (A) and the resin (B). As the polystyrene resin, a polystyrene resin having a melt flow rate of 0.1 to 20 g / lOmin measured at a temperature of 200 ° C. and a load of 49 N by the method of JISK7210 is preferable. By adding 5% by volume or less of polystyrene resin to the entire resin composition, the rotational torque of the extruder for melting and mixing the entire resin composition can be reduced without significantly impairing the transparency of the entire resin composition. The effect of making the process and equipment for manufacturing the reflective sheet simpler, such as reducing the tension when the sheet is stretched to generate holes in the sheet, is provided. From the viewpoint of sufficiently obtaining the effect of reducing the rotational torque of the extruder and reducing the stretching tension of the sheet, and from the viewpoint of obtaining an optically uniform sheet, the ratio of the polystyrene resin to the entire resin composition is 5% by volume or less. Is preferred.

  In the present invention, a resin composition in which a polypropylene resin (A), a resin (B) and, if necessary, a polystyrene resin are mixed is used for the reflective sheet. You may add a light stabilizer, a heat stabilizer, a nucleating agent, an antistatic agent, etc.

(Inorganic particles)
The reflective sheet of this invention can add a new function by including an inorganic particle in a surface layer part. For example, silica fine particles or the like can be added as a lubricant to make the reflective sheet slip easily with the rollers and guide rails of the backlight assembly apparatus. In addition, metal oxide particles such as zinc oxide and titanium dioxide can be added for the purpose of preventing yellowing of the sheet due to ultraviolet rays. Prevention of yellowing of the sheet is particularly important for liquid crystal display material applications. Furthermore, the reflection performance can be further increased by adding titanium dioxide, barium sulfate, calcium carbonate or the like which contributes to the improvement of the reflection performance. In particular, by adding inorganic particles having the reflective performance, it is possible to obtain a reflective sheet having a composite reflectivity having a reflection at the inner layer portion and a reflectivity at the surface layer portion. It can be controlled. Added amount when the surface layer portion containing an inorganic particle, when considering the workability of the fine molding, ^{0.1g / m 2 ~5g / m 2} . Preferably ^{0.3g / m 2 ~3g / m 2} , more preferably ^{0.5g / m 2 ~1.5g / m 2} . In this case, the particle diameter of the inorganic particles is 0.01 μm to 5 μm, preferably 0.015 μm to 1 μm. In addition to the inorganic particles, an organic ultraviolet absorber can be added for the purpose of preventing yellowing of the reflection sheet by ultraviolet rays. Amount in this case is ^{0.1g / m 2 ~5g / m 2} , and preferably from ^{0.3g / m 2 ~2g / m 2} .

  Further, in the present invention, inorganic particles are not positively added to the inner layer portion, but can be added as long as the moldability is not impaired. For example, in order to prevent yellowing of the reflection sheet by ultraviolet rays, metal oxide particles such as zinc oxide and titanium dioxide or organic ultraviolet absorbers are added in an amount of about 0.1 to 1% by weight. Furthermore, you may add a light stabilizer, a heat stabilizer, a nucleating agent, an antistatic agent, etc. as needed. In this case, the particle diameter of the metal oxide is 0.01 to 10 μm, preferably 0.015 to 1 μm. In addition, inorganic particles having reflective performance such as titanium dioxide, barium sulfate, calcium carbonate and the like can also be added. The addition amount in this case is 1 to 30% by weight, and the particle size is 0.01 to 10 μm, preferably 0.015 to 1 μm.

(Raw sheet forming method)
The reflective sheet of the present invention first produces an original sheet having a laminated structure. For production of the raw sheet, a coextrusion method using two extruders, ie, a main extruder for the inner layer and a sub extruder for the surface layer is usually employed. Each extruder is usually a twin screw extruder in terms of improving the kneadability and dispersibility of a plurality of components, but may be a single screw extruder if necessary. As the forming die, a co-extrusion laminated die such as a feed block die or a multi-manifold die is used, and the inner layer portion and the surface layer portion are laminated and extruded in the die. In order to stabilize the amount of the resin composition to be extruded, a gear pump may be used between the extruder and the die. In the main extruder, the polypropylene resin (A) 50 volume% or more and less than 80 volume% and the resin (B) 20 volume% or more and less than 50 volume% are melt-mixed, and the resin (B) A resin composition having a so-called sea-island structure in which the islands are dispersed is extruded. In this case, the resin (B) that is harder than the polypropylene resin (A) is caused by the shearing force due to the rotation of the screw when passing through the gap between the cylinder and the screw or the gap between the screws inside the extruder. It is finely divided from a normal resin pellet having a size of about several mm to a dispersed phase having a size of about several μm. By appropriately setting the shape of the screw of the extruder, the cylinder temperature, the number of rotations of the screw, and the like, the average value and distribution of the size at which the resin (B) is divided can be adjusted. In the sub-extruder, a resin containing 80% by volume or more of the polypropylene resin (A) and an inorganic powder added as necessary are melt-mixed and extruded. Moreover, it is preferable to set the operating conditions of the extruder and the die so that the temperature of the extruded resin is in the range of 200 ° C to 300 ° C.

(Biaxial stretching method)
In the present invention, preferably, the laminated sheet extruded from the die is cooled and solidified by a cooling roller or the like, and then stretched by a stretching machine. In a cooling roller, it is preferable to set the temperature and speed of a cooling roller so that it may become the range of 20 to 150 degreeC. In the stretching step, it is preferable to perform stretching at as low a temperature as possible in order to generate holes in the reflective sheet. In the case of stretching at a high temperature, there is a tendency that generation of holes is less likely to occur inside the sheet as compared with the case of stretching at a low temperature. Therefore, it is preferable to make the stretching ratio larger than when stretching at a low temperature.

  The resin extruded from the main extruder that forms the inner layer portion is cleaved by this, the interface between the polypropylene resin (A) and the resin (B) is cleaved to generate holes in the sheet, and the thickness of the sheet is set to a desired value. It can be made as thin as possible. On the other hand, the resin extruded from the sub-extruder forming the surface layer portion does not substantially form a sea-island structure having pores inside by this stretching, and is thinned according to the stretching ratio.

  In the present invention, a normal biaxial stretching method can be employed. That is, longitudinal / horizontal sequential biaxial stretching, lateral / longitudinal sequential biaxial stretching, simultaneous biaxial stretching, and after these biaxial stretching, re-stretching can be performed in either or both of the longitudinal and lateral directions. Preferably, longitudinal and lateral sequential biaxial stretching or simultaneous biaxial stretching is used. Longitudinal and lateral biaxial stretching is a longitudinal stretching process in which a sheet is passed through a plurality of rollers with different speeds to stretch the sheet in the flow direction, and a transverse direction in which the sheet is stretched in the width direction using a clip tenter or the like. Stretching step. Simultaneous biaxial stretching is a method in which a flow direction and a width direction are simultaneously stretched using a pantograph stretching machine or the like. More preferred is the most general longitudinal and transverse sequential biaxial stretching method. The stretching ratio of the biaxial stretching is 1.5 times or more in each of the MD direction and the TD direction, and the area stretching ratio is from 3 times to 50 times. Preferably, MD and TD are each 2 times or more and the area magnification is 4 times or more and 30 times or less.

  Of the resins (B) constituting the inner layer portion, a resin having a modulus of elasticity at a temperature at which the polypropylene resin can be stretched is larger than that of the polypropylene resin. The reason is considered as follows. In the present invention, the sheet of the resin composition is stretched at a temperature at which the polypropylene resin can be stretched, and the interface between the resin (B) phase and the polypropylene resin (A) phase in the resin composition is cleaved. A hole is formed inside. When the elastic modulus of the resin (B) is larger than the elastic modulus of the polypropylene resin (A) at the temperature at which the sheet is stretched, the deformation amount due to the stretching force of the resin (B) phase is larger than the deformation amount of the polypropylene resin (A) phase. Since it is small, it is considered that the interface between the resin (B) phase and the polypropylene resin (A) phase is more easily cleaved. Furthermore, it is considered that the fact that the elastic modulus at a temperature at which the polypropylene resin of the resin (B) can be stretched is larger than that of the polypropylene resin greatly contributes to the fine shaping processability by thermoforming. Ordinary biaxially stretched polypropylene is deformed due to large thermal shrinkage when the temperature is raised to the vicinity of the stretching temperature. However, the presence of a resin (B) having an elastic modulus greater than the polypropylene resin in the inner layer portion near the stretching temperature. It is considered that the thermal deformation is suppressed and the thermoformability is improved. Further, the presence of the resin (B) also improves the heat resistance during use as a reflective material, and contributes to an increase in the strength of the reflective sheet.

(Method of forming fine structure surface)
The biaxially stretched sheet is subjected to post-treatment such as heat treatment as necessary to impart thermal dimensional stability, and then a microstructured surface is formed. For this molding, press molding or roll molding is preferably employed. These moldings can be applied to both batch molding from a single wafer type and continuous molding from a roll shape.

In press molding, a mold on which a fine structure surface is formed is fixed to one side of the press surface, the sheet is set on the mold surface, and then compressed. The press temperature in this case is 140 to 160 ° C., 1 second to 10 minutes press time, the press pressure is a 1 to 100 kg / cm ^2, these molding conditions, the thickness of the sample, forming speed, die shape, press Appropriate conditions are set according to the later cooling method. Also, if the press surface on which the mold is set is heated to the molding temperature and the other press surface is cooled, the back side of the sheet is kept in a cooled state, so cooling after pressing becomes faster and molding It is preferable from the viewpoint that the speed can be increased and thermal deformation of the inner layer portion during press working can be suppressed.

  4 and 5 show examples of the roll forming method. In the example shown in FIG. 4, the shape of the roll surface is changed to a sheet by passing the sheet 1 between a pair of rolls composed of a roll 2 and a cooling roll 3 that are heated to a molding temperature with a fine shape. Compressed heat transfer. The transferred sheet is further cooled by a roll 4. The cooling roll may be a metal or elastic rubber roll. In the example shown in FIG. 5, a cooling metal belt is passed through the cooling rolls 3, 4, and 5, and cooling and compression are performed by the metal belt 6. In this method, since cooling and compression are performed over the roll surface, molding can be performed more efficiently. The roll temperature, roll speed, and compression pressure are set to appropriate conditions from the sample thickness, mold shape, cooling method after compression, and the like.

In the reflecting sheet of the present invention, basis weight, characterized in that it is a 30 to 500 g / m ^2. In this range, it is possible to form a sufficient number of holes or bubbles for obtaining reflectivity with sufficient strength and rigidity for molding a reflective sheet having a fine three-dimensional structure surface. For this reason, a high reflectance can be obtained over the total reflection surface including a fine three-dimensional structure for reflection. The preferred basis weight is 50 to 400 g / m ² , more preferably 100 to 350 g / m ² .

Moreover, in the reflective sheet of this invention, a density is 0.35-0.85 g / cm < ³ >, It is characterized by the above-mentioned. In this range, it is possible to make a large volume of holes or bubbles in a state where the amount of resin does not decrease, and a high reflectance can be obtained. In addition, since the amount of resin is sufficient, thermal melting does not occur in part during thermoforming, thermoforming becomes easy, and the strength as a reflective sheet is sufficient. For this reason, a preferable density is 0.40-0.80 g / cm < ³ >, More preferably, it is 0.45-0.75 g / cm < ³ >.

Examples carried out to clarify the effects of the present invention will be described below.
<Evaluation method>
First, the physical property items to be evaluated for the reflective sheet and the evaluation method will be described.

(1) Thickness The thickness of the reflective sheet was measured using a thickness meter manufactured by Peacock.

(2) Total light reflectivity / average total reflectivity The total light reflectivity of the reflective sheet was determined by using a spectrophotometer UV-3150 manufactured by Shimadzu Corporation and an integrating sphere sample stand. The relative reflectance with the reflectance of (Spectralon) made as 100% was measured in the wavelength range of 400 nm to 700 nm. For the average total reflectance, the measured value when light was incident from each of the MD direction and the TD direction of the sheet with respect to light having a wavelength of 550 nm was obtained, and the arithmetic average value of both was taken as the average total reflectance.

(3) Basis weight of inner layer part A sheet is cut into a 50 mm square, and its weight is first measured. Subsequently, the weight of the surface layer part was calculated from the thickness of the surface layer part, the constituent resin, the kind of additive, and the amount, and the weight of the surface layer part was subtracted from the total weight to obtain the basis weight of the inner layer part by calculation.

(4) Density of inner layer part The thickness of an inner layer part is calculated | required from the sample which calculated | required the basic weight of said inner layer part, and the volume of an inner layer part is calculated. The density was determined by calculation from this volume and the basis weight.

<Die and press molding method used in Examples>
FIG. 6 is a schematic view showing a cross section of the molding die used in this example. As shown in FIG. 3, the shape of the shaping surface is a triangle having a cross-sectional shape including a vertex with a vertex angle of 63 °, a base length of 100 μm, and a height of 50 μm, and this shape is the same in one direction in the surface. It is a simple shape and is regularly arranged in a prism shape in the plane. A flat plate shape having a shaping surface of 100 × 100 mm was used for the mold.

Using a compression molding machine having a press surface of 30 cm square, the upper press surface was cooled to 25 ° C., and the lower press surface was heated to 150 ° C. The molding die was placed on the lower press surface, and after sufficiently preheating with the heat of the press surface, the pre-molding reflecting sheet cut to 120 × 120 mm was set on the die surface and compressed. The compression molding was performed under the conditions of a compression time of 5 minutes and a compression pressure of 50 kg / cm ² .

(Physical properties of reflective sheet)
In the reflection sheet of the present invention, light reflection occurs at the interface between air and resin using the difference in refractive index between air and resin. Since the refractive index of the resin is approximately 1.4 to 1.6 and the refractive index of air is about 1, the reflectance of light caused by the difference in refractive index between the resin and air is at most about 4 per reflection. % Only. However, by providing a structure including a large number of holes and bubbles inside the sheet, a large number of interfaces can be present. As a result, reflection inside the sheet is repeated many times, and a high reflectance can be obtained. Therefore, in the reflective sheet of the present invention, regarding the reflective performance, a structure including a large number of holes and bubbles in the inner layer portion, that is, a phase separation structure is important. The structural requirements that determine the phase separation structure are the basis weight and density of the inner layer portion in the reflective sheet of the present invention.

Example 1
As an inner layer portion, a raw material resin obtained by mixing 62% by volume (55% by weight) of a polypropylene resin (Prime Polymer E-105GM) and 38% by volume (45% by weight) of a polycarbonate resin (Asahi Kasei Wonderlight PC110) Using a co-rotating twin screw extruder with a cylinder bore of 25 mm and a cylinder to bore ratio of 48, the cylinder temperature was melted at 250 ° C and the screw rotation speed was 100 rpm, and the temperature was adjusted to 250 ° C. Was introduced into a two-layer, three-layer die having a lip width of 400 mm through a gear pump, and a polypropylene resin (Prime Polymer Prime TPO resin E-2900) was used as the surface layer part using a single screw extruder with a cylinder diameter of 30 mm. Through a gear pump melted at a cylinder of 210 ° C. and adjusted to a temperature of 210 ° C. It was introduced to seed three-layer die. In the laminated die, two kinds of resins were laminated and extruded from a sheet die having a lip clearance of 2.0 m. The extruded molten laminated resin was taken up by a pair of cooling pinch rollers set at 80 ° C., and the resin was cooled and solidified while pulling the molten resin in the extrusion direction to produce a laminated raw sheet having a thickness of 1.9 mm. The obtained laminated sheet was a three-layer laminated sheet having an inner layer portion of 1.4 mm and a surface layer portion of 0.25 mm on the front and back sides. In addition, the isotactic index (mmmm pentad%) by the 13C-NMR measurement of the polypropylene resin used for the surface layer part was 66 mol%.

The resulting sheet was stretched 3 times in the extrusion direction (MD direction) at a temperature of 155 ° C using a roll longitudinal stretching machine, and then stretched 3 times in the width direction (TD direction) at a temperature of 155 ° C using tenter transverse stretching. And a white sheet stretched 3 × 3 times was obtained. The average total reflectance of this white sheet was 97.0%, and the basis weight and density of the inner layer portion were 175 g / m ² and 0.46 g / cm ³ , respectively. The total thickness including the surface layer portion was 440 μm, and the thickness of each layer was a surface layer portion 33 μm, an inner layer portion 380 μm, and a back surface layer portion 27 μm.

  The obtained white sheet was cut into 120 × 120 mm squares and subjected to compression molding with a mold having a shaping surface as shown in FIG. 6 to obtain a reflection sheet having the shaping surface transferred to the surface layer portion. The obtained molded product was a uniform molded product having no molding spots as in the mold shape.

The basis weight of the inner layer portion of the obtained molded product is 175 g / m ² , the density is 0.65 g / cm ³ , the average total reflectance on the shaping surface side is 96.7%, and the shaping shape Later, it had a high reflectance as a reflective sheet. Moreover, when the reflection angle of the reflected light with respect to the incident light by the variable-angle high-speed spectrophotometer is measured, strong reflected light is observed in the direction along the shape of the shaping surface, and the direction of the reflected light can be controlled by this fine shaping. I understood that.

(Example 2)
In Example 1, as the surface layer part, the same as Example 1 except that 5% by weight of titanium oxide was added to a polypropylene resin (Prime Polymer Prime TPO resin E-2900) and a pelletized resin was used in advance. Thus, a white sheet stretched 3 × 3 times by stretching in the longitudinal and lateral directions was obtained. The average total reflectance of this white sheet was 98.4%, and the basis weight and density of the inner layer portion were 178 g / m ² and 0.46 g / cm ³ , respectively. The total thickness including the surface layer portion was 445 μm, and the thickness of each layer was a surface layer portion 32 μm, an inner layer portion 390 μm, and a back surface layer portion 23 μm. The surface layer portion contained 1.6 g / m ² of titanium oxide.

  Similarly, the obtained white sheet was cut into a 120 × 120 mm square and subjected to compression molding using a mold having a shaping surface in FIG. 6 to obtain a reflection sheet having the shaping surface transferred to the surface layer portion. The obtained molded product was a uniform molded product having no molding spots as in the mold shape.

The basis weight of the inner layer portion of the obtained molded product is 175 g / m ² , the density is 0.66 g / cm ³ , the average total reflectance on the shaping surface side is 98.0%, and even after the shaping In addition to having a high reflectance as a reflective sheet, the average total reflectance was improved by 1.3% by including titanium oxide in the surface layer portion. Similarly, when the reflection angle of the reflected light with respect to the incident light by the variable-angle high-speed spectrophotometer was measured, strong reflected light was observed in the direction along the shape of the shaping surface.

(Comparative Example 1)
An original fabric sheet having a thickness of 1.7 mm was manufactured with the same polymer and composition as the inner layer portion of the reflective sheet of Example 1 and having a layer configuration of only the inner layer portion. Next, in the same manner as in Example 1, a white sheet was obtained that was stretched 3 × 3 times by stretching in the longitudinal and lateral directions. This white sheet had an average total reflectance of 97.5%, a thickness of 470 μm, and a basis weight and a density of 220 g / m ² and 0.47 g / cm ³ , respectively.

  Similarly, the obtained white sheet is cut into a 120 × 120 mm square and subjected to compression molding using a mold having a shaping surface as shown in FIG. 6 to obtain a reflection sheet having the shaping surface transferred to the surface layer portion. It was. The resulting molded product had a surface that was roughly shaped according to the shape of the mold, but the resin flow was insufficient, the apex corners of the triangle were rounded, and each side was partially curved. It was seen that the molded product was uneven throughout.

In addition, the basis weight of the obtained molded product is 220 g / m ² , the density is 0.7 g / cm ³ , the average total reflectance on the shaping surface side is 97.0%, and it is also reflected after shaping. The rate was high.

(Comparative Example 2)
In Example 1, 83.4% by volume (50% by weight) of polypropylene resin (Prime Polymer Co., Ltd. E-105GM) and 16.6% by volume of barium sulfate having an average particle diameter of 0.8 μm (true specific gravity: 4.5) % (50 wt%) was used, the surface layer portion was the same as in Example 1, and a laminated raw sheet having a thickness of 1.9 mm was obtained as in Example 1. The obtained laminated sheet was a three-layer laminated sheet having an inner layer portion of 1.4 mm and a surface layer portion of 0.25 mm on the front and back sides.

  Next, in the same manner as in Example 1, a white sheet was obtained that was stretched 3 × 3 times by stretching in the longitudinal and lateral directions. The average total reflectance of this white sheet was 95.2%.

  Similarly, the obtained white sheet is cut into a 120 × 120 mm square and subjected to compression molding using a mold having a shaping surface as shown in FIG. 6 to obtain a reflection sheet having the shaping surface transferred to the surface layer portion. It was. The obtained shaping surface was a relatively good molding surface, but the compression was large due to the collapse of the internal pores, and the thickness of the sheet after molding was 220 μm. In addition, wrinkles were generated due to partial heat shrinkage, and some uneven portions were observed on the shaped surface. The average total reflectance on the shaping surface side of the molded product was 91.5%, and the reflectance was greatly reduced.

  The present invention is not limited to the embodiment described above, and can be implemented with various modifications. For example, the dimensions, materials, and the like in the above-described embodiment are illustrative, and can be changed as appropriate. In addition, various modifications can be made without departing from the scope of the present invention.

  Since the reflective sheet of the present invention has higher reflection performance than a conventional reflective sheet and has a fine three-dimensional shape on the surface, it is suitably used as a reflective material for a backlight of a liquid crystal display device or the like.

It is a figure which shows the relationship of the thickness of the surface layer part before and behind shaping | molding of this invention, (a) is a figure which shows before shaping | molding, (b) is sectional drawing of the sheet | seat after shaping | molding. It is a figure which shows the example of the shape of the three-dimensional microstructure of this invention. It is a figure which shows the example of the shape of the three-dimensional microstructure of this invention. It is a figure which shows the example of the roll forming method for manufacturing the reflective sheet of this invention. It is a figure which shows the example of the roll forming method for manufacturing the reflective sheet of this invention. It is a figure which shows the cross section of the metal mold | die used for the shaping form of this invention in the Example.

Explanation of symbols

1 Sheet 2 Roll 3, 4, 5 Cooling roll 6 Metal belt

Claims (12)

A reflection sheet composed of at least two layers of a surface layer portion and an inner layer portion, wherein the surface layer portion contains 80% by volume or more of polypropylene resin (A), the maximum thickness of the surface layer portion is 2 μm to 200 μm, and the inner layer A sea phase in which the part contains a polypropylene resin (A) and at least one resin (B) that is incompatible with the polypropylene resin, and the phase separation structure of the inner layer part is composed of the polypropylene resin (A) And the island phase of the resin (B), the basis weight of the reflective sheet is 30 to 500 (g / m ² ), and the density is 0.35 to 0.85 (g). / Cm ³ ), and the surface layer portion has a fine three-dimensional structure surface.   The reflective sheet according to claim 1, wherein the polypropylene resin (A) in the inner layer portion is 50 vol% to 80 vol%, and the resin (B) is 20 vol% to 50 vol%.   The reflection according to claim 1 or 2, wherein the elastic modulus of the resin (B) at a temperature at which the polypropylene resin (A) of the inner layer portion can be stretched is larger than the elastic modulus of the polypropylene resin (A). Sheet.   The reflective sheet according to any one of claims 1 to 3, wherein an isotactic index (mmmm pentad%) of the polypropylene resin (A) of the surface layer part is 55 to 85 mol%.   The reflective sheet according to any one of claims 1 to 4, wherein the resin (B) includes a polycarbonate resin. Reflecting sheet according to any one of claims 1 to 5, wherein the surface layer portion, characterized in that it comprises inorganic particles of ^{0.1g / m 2 ~5g / m 2} .   The reflective sheet according to claim 6, wherein the inorganic particles include at least one selected from the group consisting of zinc oxide, titanium dioxide, barium sulfate, and calcium carbonate.   The reflection sheet according to any one of claims 1 to 7, wherein the fine structure surface is a reflection surface for controlling a reflection direction.   The cross section including the apex of one structure constituting the microstructure surface is substantially a triangle, and the apex angle is 30 ° to 150 °. Reflective sheet as described in 1.   The reflective sheet according to claim 9, wherein the triangle has a base of 10 to 1000 μm and a height of 10 to 150 μm.   The reflective sheet according to any one of claims 1 to 10, wherein the average total reflectance is 90% or more.   The reflective sheet according to claim 1, wherein the microstructured surface is thermoformed.

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