JP5388505B2 - Reflective film for liquid crystal display - Google Patents

Description

  The present invention relates to a reflective film for a liquid crystal display device used as a reflective film for a light source in a backlight unit of the liquid crystal display device.

  The backlight unit of a liquid crystal display device has a backlight method in which a light source is placed on the back surface of the display device and a sidelight method in which a light source is placed on the side surface. In either method, light from the light source escapes to the back surface of the screen. In order to prevent this, a reflective film is installed on the back. This reflective film is required to be thin and have a high reflectance. As this reflective film, a white polyester film containing fine bubbles inside the film is used.

JP 63-62104 A Japanese Patent Publication No. 8-16175 JP 2000-37835 A JP 2005-125700 A JP 2004-50479 A

  Many liquid crystal televisions include a backlight type backlight unit, but a backlight unit with higher luminance is required in order to cope with higher luminance of the television. However, there is a limit only to improving the reflectance of the reflective film.

  In the backlight method, when the specular reflection of the reflecting plate is strong, the light returns to the light source arranged on the reflecting film in the liquid crystal display device, and the light does not reach the display surface, resulting in light loss. It may cause a decrease in brightness. The present invention relates to a reflective film for a liquid crystal display device that collects the reflected light to prevent the reflected light from returning to the light source in the liquid crystal display device and does not impair the amount of light that reaches the display surface. It is an object of the present invention to provide a reflective film for a liquid crystal display device capable of obtaining the above.

  That is, the present invention comprises a white film and a transparent particle layer comprising transparent particles provided thereon and a binder covering the transparent particles, and the transparent particle layer is between the surface of the white film and the surface of the transparent particle layer. For a liquid crystal display device comprising 2 to 30 transparent particles having a particle diameter of 5 to 100 μm in a direction perpendicular to the film surface, and the thickness of the binder covering the transparent particles on the outermost surface of the transparent particle layer being 0.01 to 5 μm It is a reflective film.

  According to the present invention, there is provided a reflective film for a liquid crystal display device that collects the reflected light to prevent the reflected light from returning to the light source in the liquid crystal display device and does not impair the amount of light reaching the display surface. A reflective film for a liquid crystal display device capable of obtaining luminance can be provided.

Hereinafter, the present invention will be described in detail.
[White film]
The white film in the present invention is a film made of a thermoplastic resin and having a white colorant or a void-forming substance contained in the film so as to exhibit a white color. As the colorant or void forming substance, for example, inorganic particles and organic particles can be used.

  The light reflectance of the white film is preferably 95% or more, more preferably 96% or more, and particularly preferably 97% or more as the reflectance at a wavelength of 550 nm. The white film may be a single layer film or a laminated film. From the viewpoint of obtaining high light reflectance and mechanical strength, a laminated film composed of a layer containing a relatively large amount of voids and a layer containing a relatively small amount of voids or no voids is preferable. Examples of the thermoplastic resin constituting the film include polyester, polyolefin, and polystyrene. Polyester is preferred from the viewpoint of achieving both mechanical properties and thermal stability.

[polyester]
When using polyester as a thermoplastic resin of a white film, polyester which consists of a dicarboxylic acid component and a diol component is used as polyester. Examples of the dicarboxylic acid component include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid, and sebacic acid. Examples of the diol component include ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol.

  Among these polyesters, aromatic polyesters are preferable, and polyethylene terephthalate is particularly preferable. Polyethylene terephthalate may be a homopolymer, but a copolymer is preferred. In particular, when a laminated film composed of a layer containing a relatively large amount of voids and a layer containing a relatively small amount of voids or no voids is used as a white film, it is used for a layer containing a relatively large amount of voids. The polyester is preferably a copolymerized polymer. In that case, the ratio of the copolymerization component is, for example, 3 to 20 mol%, preferably 4 to 15 mol%, more preferably 5 to 13 mol%, based on the total dicarboxylic acid component. By setting the proportion of the copolymer component within this range, excellent film forming properties can be obtained even for a layer containing a relatively large amount of voids, and a laminated film having excellent thermal dimensional stability can be obtained.

  When inorganic particles are used as the colorant or void forming substance, white inorganic particles are preferred as the inorganic particles. Examples of the white inorganic particles include barium sulfate, titanium dioxide, silicon dioxide, and calcium carbonate particles. The average particle diameter of the inorganic particles is, for example, 0.2 to 3.0 μm, preferably 0.3 to 2.5 μm, and more preferably 0.4 to 2.0 μm. By using inorganic particles in this range, it can be dispersed moderately in the polyester, the particles are less likely to aggregate, and a film without coarse protrusions can be obtained, and at the same time, the film surface is not too rough, Glossiness can be controlled within an appropriate range. The inorganic particles may have any particle shape, for example, a plate shape or a spherical shape. The inorganic particles may be subjected to a surface treatment for improving dispersibility.

  When organic particles are used as the colorant or void-forming substance, resin particles that are incompatible with polyester are used as the organic particles. As the organic particles, silicone resin particles and polytetrafluoroethylene particles are preferable. The average particle diameter of the organic particles is, for example, 0.2 to 10 μm, preferably 0.3 to 8 μm, and more preferably 0.4 to 6 μm. By using the organic particles in this range, it is possible to obtain a film that can be appropriately dispersed in the polyester, that the particles do not easily aggregate, and that does not have coarse protrusions.

[Transparent particle layer]
In the present invention, a transparent particle layer is provided on the white film.
The transparent particle layer is composed of transparent particles and a binder that covers the transparent particles and holds it on a white film.

  What is important in the present invention is that the transparent particle layer comprises 2 to 30 transparent particles having a particle diameter of 5 to 100 μm in a direction perpendicular to the film surface between the surface of the white film and the surface of the transparent particle layer. And the thickness of the binder which covers the transparent particle of the outermost surface of this transparent particle layer is 0.01-5 micrometers.

  Transparent particles having a particle diameter of 5 to 100 μm constituting the transparent particle layer are held on the white film by being bonded to each other by a binder and also to the white film. The number of transparent particles in the direction perpendicular to the film surface from the surface of the white film to the outermost surface of the transparent particle layer is 2 to 30 transparent particles having a particle diameter of 5 to 100 μm, preferably 2.5 to 30, more preferably 3-30. The number of the transparent particles can be confirmed by observing the cross section of the reflective film of the present invention with a microscope, for example, and counting the number of transparent particles having a particle diameter of 5 to 100 μm. More precisely, this number is the number of transparent particles that draw a straight line in a direction perpendicular to the white film surface between the surface of the white film and the outermost surface of the transparent particle layer, and the straight line crosses. However, only transparent particles having a particle diameter of 5 to 100 μm are counted here. Thirty straight lines are drawn at an arbitrary position on the film, and the number of particles crossed by the 30 straight lines is counted and averaged to obtain a film surface between the surface of the white film and the outermost surface of the transparent particle layer. The number of transparent particles in the vertical direction.

  If the number of transparent particles is less than two, the reflected light cannot be sufficiently directed, and the amount of light that reaches the display surface by returning light to the light source disposed above the reflective film in the backlight unit. The effect of preventing the decrease in brightness is low, and a sufficient brightness increase effect cannot be obtained. On the other hand, when it exceeds 30, the adhesiveness of the binder is deteriorated, the transparent particle layer cannot be supported on the white film, and surface defects are generated.

  The thickness of the binder covering the transparent particles on the outermost surface of the transparent particle layer is 0.01 to 5 μm, preferably 0.02 to 4.5 μm, and more preferably 0.03 to 4 μm. The thickness of the binder is a thickness of the binder measured along a line drawn from the center of the transparent particle located on the outermost surface of the transparent particle layer to the outermost surface of the transparent particle layer. When the thickness of the binder is less than 0.01 μm, the adhesiveness between the transparent particles is low, and a stable transparent particle layer cannot be formed. On the other hand, if it exceeds 5 μm, the lens effect of the transparent particles is impaired, and the effect of increasing the brightness is small.

[Transparent particles]
In the present invention, a transparent particle having a particle diameter of 5 to 100 μm is required as a component constituting the transparent particle layer, and the average particle diameter is, for example, 5 to 100 μm, preferably 7 to 70 μm as the transparent particles for forming the transparent layer. Can be used. The transparent particle layer may contain transparent particles having an individual particle size of less than 5 μm or transparent particles having a particle size of more than 100 μm, but these particles should be smaller and include transparent particles having a particle size of more than 100 μm. Preferably not. The transparency of the transparent particles means that the light transmittance in the visible light region is, for example, 50% or more, preferably 60% or more, and more preferably 70% or more.

  As the transparent particles, either organic particles or inorganic particles can be used. Examples of the organic particles include acrylic particles, silicone particles, and styrene particles. Of these, acrylic particles and styrene particles are preferred because they hardly absorb light in the visible light region. Glass particles can be exemplified as the inorganic particles.

  The transparent particles are composed of a curved surface or a shape composed of a curved surface and a plane in order to collect light. As this shape, for example, a spherical shape, a rugby ball shape, or a convex lens shape can be used. In order to effectively improve the luminance, an aspect ratio of 3 or less is preferable, and an aspect ratio of 1.2 or less is more preferable. A particularly preferred shape is a spherical particle. The aspect ratio is major axis / minor axis. The particle diameter of the transparent particles is a value obtained by taking the average of the major axis and the minor axis when the transparent particles are not spherical.

  The size of the transparent particles constituting the transparent particle layer is preferably 5 to 100 μm, more preferably 5 to 50 μm, particularly preferably 7 to 40 μm, and most preferably 8 to 30 μm in terms of average particle diameter. By having an average particle diameter in this range, aggregation is unlikely to occur, the directivity of light can be controlled, and a reflective film that exhibits a good appearance with no particle dropout or streak-like coating defects can be obtained. it can.

[binder]
The transparent particle layer is composed of a transparent particle and a binder that covers the surface of the transparent particle, adheres the transparent particles to each other, and supports the surface of the white film. As the binder, acrylic resins, polyester resins, polyurethane resins, polyester amide resins, polyolefin resins, copolymers or blends thereof can be used.
In the binder, an isocyanate-based, melamine-based, or epoxy-based crosslinking agent may be blended to form a crosslinked structure.

The most preferable binder is an acrylic resin. And it is preferable that a transparent particle layer consists of 30-70 weight% of acrylic particles and 70-30 weight% of acrylic binders on the basis of the weight of a transparent particle layer.
The reflective film for a liquid crystal display device of the present invention preferably has a reflectance of 96% or more.

[Production method]
Hereinafter, an example of the method for producing the reflective film of the present invention will be described. In this example, a laminated film is used as the white film. In addition, as this white film, the film marketed by the name of Teijin Tetron UX02-225 (made by Teijin DuPont Film) can be used, for example.

  The polyester used for the laminated white film is preferably filtered using a nonwoven fabric type filter having an average opening of 10 to 100 μm, preferably an average opening of 20 to 50 μm, and made of stainless steel fine wires having a wire diameter of 15 μm or less. By performing this filtration, it is possible to suppress agglomeration of particles that are usually agglomerated and become coarse agglomerated particles, and to obtain a laminated film with few coarse foreign matters.

The filtered polyester composition is extruded from a die to a multilayer state by a simultaneous multilayer extrusion method using a feed block in a molten state to produce a laminated unstretched sheet.
The laminated unstretched sheet extruded from the die is cooled and solidified by a casting drum to form a laminated unstretched film. This laminated unstretched film is heated by roll heating, infrared heating or the like, and stretched in the longitudinal direction to obtain a laminated longitudinally stretched film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The stretching is preferably performed at a temperature equal to or higher than the glass transition point (Tg) of the polyester. The draw ratio is preferably 2.2 to 4.0 times, more preferably 2.3 to 3.9 times in both the longitudinal direction and the direction orthogonal to the longitudinal direction (hereinafter referred to as the transverse direction). If it is less than 2.2 times, the thickness unevenness of the film is deteriorated, and a good film cannot be obtained.

  Subsequently, the laminated film after the longitudinal stretching is sequentially subjected to lateral stretching, heat setting, and thermal relaxation to form a laminated biaxially oriented film. These processes are performed while the film is running. The transverse stretching process starts from a temperature higher than the glass transition point (Tg) of the polyester. Although the temperature rise in the transverse stretching process may be continuous or stepwise (sequential), the temperature is usually raised sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone. The transverse stretching ratio is preferably 2.5 to 4.5 times, more preferably 2.8 to 3.9 times, although it depends on the required characteristics of this application. If it is less than 2.5 times, the thickness unevenness of the film is deteriorated and a good film cannot be obtained, and if it exceeds 4.5 times, breakage tends to occur during film formation. Hereinafter, the melting point is abbreviated as Tm.

  The film after transverse stretching is heat-treated at a temperature of (Tm-20 ° C.) to (Tm-100 ° C.) with a constant width or a width reduction of 10% or less while lowering both ends to reduce the heat shrinkage rate. Good. When the heat treatment temperature is higher than (Tm−20 ° C.), the flatness of the film is deteriorated and the thickness unevenness is increased, which is not preferable. If it is lower than (Tm-100) ° C., the heat shrinkage rate may increase, which is not preferable. Further, in order to adjust the heat shrinkage, both ends of the film being held can be cut off, the take-up speed in the film vertical direction can be adjusted, and the film can be relaxed in the vertical direction. As a means for relaxing, the speed of the roll group on the tenter exit side is adjusted. As the rate of relaxation, the speed of the roll group is reduced with respect to the film line speed of the tenter, preferably 0.1 to 2.5%, more preferably 0.2 to 2.3%, particularly preferably 0.3. The film is relaxed by performing a speed reduction of ˜2.0% (this value is referred to as “relaxation rate”), and the longitudinal heat shrinkage rate is adjusted by controlling the relaxation rate. Further, the width of the film in the horizontal direction can be reduced in the process until both ends are cut off, and a desired heat shrinkage rate can be obtained.

As a coating liquid for forming a transparent particle layer on the surface of the white film on the reflective layer side, a predetermined amount of a coating liquid in which transparent particles and a binder are dispersed in a solvent is applied using a coating apparatus, and the temperature is 120 ° C. The reflective film for a liquid crystal display device of the present invention can be obtained by drying in an oven whose temperature is set stepwise. Coating weight, as the wet coating amount before drying is preferably 10 to 50 g / ^{m 2,} more preferably a 15 to 40 g / ^{m 2.}
As the coating apparatus, a die coating apparatus, a gravure roll coating apparatus, and a bar coating apparatus can be used.

Hereinafter, the present invention will be described in detail by way of examples. Each characteristic value was measured by the following method.
(1) Relative luminance The luminance of the display device when used as a reflector in a liquid crystal display device was evaluated. Remove the reflective film from the backlight of Sony's 32-inch TV (BRAVIA KDL-32V2500), install the reflective film to be evaluated, and use a luminance meter (Model MC-940, manufactured by Otsuka Electronics) The luminance was measured at a measurement distance of 500 mm from the front of the center.
Relative luminance was calculated | required by the following formula using the brightness | luminance calculated | required with the white film before coating of a transparent particle layer, and the reflective film after coating, respectively.
Relative brightness = (Brightness of reflective film after coating) / (Brightness of white film before coating) x 100 (%)
If the relative luminance is 101.0% or more, it can be said that an excellent luminance increasing effect as a backlight was obtained.

(2) Average particle size of particles in white film The particle size distribution of the particles was obtained with a particle size distribution meter (LA-950, manufactured by Horiba, Ltd.), and the particle size at d50 was defined as the average particle size.

(3) Average particle diameter of transparent particles Using an S-4700 field emission scanning electron microscope manufactured by Hitachi, Ltd., arbitrarily measuring 100 particles before being added to the film raw material composition at a magnification of 1000 times ( In the case of other than spherical (determined by major axis + minor axis) / 2), the average particle size was determined.

(4) Aspect ratio of transparent particles Using an S-4700 field emission scanning electron microscope manufactured by Hitachi, Ltd., 30 exposed particles were arbitrarily observed at a magnification of 500 times. The average value was calculated.
Aspect ratio = major axis / minor axis

(5) Number of laminated transparent particles The slices cut using a microtome in the thickness direction of the film were observed. Using a S-4700 field emission scanning electron microscope manufactured by Hitachi, Ltd. at a magnification of 3000 times, a particle diameter of 5 to 100 μm passing through a straight line perpendicular to the film surface from the surface of the white film to the surface of the transparent particle layer. The number of transparent particles was counted (a in FIG. 1). The number of transparent particles having a particle diameter of 5 to 100 μm was counted by observing 30 points, and an average value of 30 points was taken.

(6) Binder thickness For transparent particles having a particle diameter of 5 to 100 μm located on the outermost surface of the transparent particle layer, a straight line is drawn from the center of the transparent particle toward the outermost surface of the transparent particle layer, and along this line. The thickness of the binder (b in FIG. 1) was measured. Measurement was performed on 30 transparent particles having a particle diameter of 5 to 100 μm located on the outermost surface, and an average value of the 30 particles was taken.

(7) Reflectance An integrating sphere was attached to a spectrophotometer (Shimadzu UV-3101PC), and the reflectance when the BaSO ₄ white plate was 100% was measured at a wavelength of 550 nm, and this value was defined as the reflectance.

[Example 1]
Reflection of a white film (Teijin Tetron UX02-225 made by Teijin DuPont Films) with a total film thickness of 225 μm composed of two layers, a reflective layer made of a polyester composition containing barium sulfate particles as a void-forming agent and a support layer made of polyester On top of the layer (reflectance 98.5%), using a Meyer bar with a count of 16 in a die coating apparatus, a coating solution having the composition shown in the following preparation recipe was applied with a wet thickness of 25 g / m ² . After coating with a coating amount, it was dried in an oven to obtain a reflective film.
Preparation recipe 1)
・ Transparent particles: Sekisui Plastics Industry MBX-30 (acrylic transparent particles) (35% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (23 wt%)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (2% by weight)
Organic solvent: butyl acetate (40% by weight)
The evaluation results of the obtained reflective film are shown in Table 1.

[Example 2]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1.
Preparation recipe 2)
・ Transparent particles: Sekisui Plastics Industry MBX-15 (acrylic transparent particles) (35% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (23 wt%)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (2% by weight)
Organic solvent: butyl acetate (40% by weight)

[Example 3]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1.
Preparation recipe 3)
-Transparent particles: Sekisui Plastics Industry MBX-50 (acrylic transparent particles) (32% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (25% by weight)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (3 wt%)
Organic solvent: butyl acetate (40% by weight)

[Example 4]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1.
Preparation recipe 4)
-Transparent particles: Sekisui Plastics Industry MBX-12 (acrylic transparent particles) (38 wt%)
-Acrylic binder: Nippon Shokubai U Double S2740 (25% by weight)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (3 wt%)
Organic solvent: ethyl acetate (34% by weight)

[Example 5]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1.
Preparation recipe 5)
・ Transparent particles: Sekisui Plastics Industry MBX-5 (acrylic transparent particles) (19% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (37% by weight)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (4% by weight)
Organic solvent: ethyl acetate (40% by weight)

[Comparative Example 1]
The reflective layer side of the white film was evaluated without coating the white film with the coating solution.

[Comparative Example 2]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1. The increase in brightness was small.
Preparation recipe 6)
-Transparent particles: Sekisui Plastics Industry MBX-15 (acrylic transparent particles) (10% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (48 wt%)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (2% by weight)
Organic solvent: butyl acetate (40% by weight)

[Comparative Example 3]
A reflective film was obtained in the same manner as in Example 1 except that the coating liquid was changed to a coating liquid having the composition shown in the following preparation recipe. The evaluation results of the obtained reflective film are shown in Table 1. The increase in brightness was small.
Preparation recipe 7)
・ Transparent particles: Sekisui Plastics MBX-15 (acrylic transparent particles) (2% by weight)
-Acrylic binder: Nippon Shokubai U Double S2740 (56 wt%)
・ Crosslinking agent: Nippon Polyurethane Industry Co., Ltd. Coronate HL (2% by weight)
Organic solvent: butyl acetate (40% by weight)

  The reflective film for a liquid crystal display device of the present invention can be suitably used as a reflective film for a liquid crystal display device, particularly as a reflective film for a backlight type liquid crystal display device in which a light source is placed on the back surface of a display device such as a liquid crystal television. .

It is a schematic diagram of a transparent particle layer.

Explanation of symbols

a: A straight line drawn in a direction perpendicular to the film surface from the surface of the white film to the surface of the transparent particle layer. The number of transparent particles having a particle diameter of 5 to 100 μm through which the straight line passes is counted.
b: The thickness of the binder along a straight line extending from the center of the transparent particle to the outermost surface of the transparent particle layer.

Claims (1)

  A transparent particle layer comprising a white film and transparent particles provided thereon and a binder covering the transparent particles, and the transparent particle layer is perpendicular to the film surface between the surface of the white film and the surface of the transparent particle layer. 2 to 30 transparent particles having a particle diameter of 5 to 100 μm in the direction, and the thickness of the binder covering the transparent particles on the outermost surface of the transparent particle layer is 0.01 to 5 μm, Reflective film.

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