CN114746779B - Light diffusion sheet and backlight unit for liquid crystal display device - Google Patents

Abstract

The present invention has been made to solve the problems of providing a light diffusion sheet having high damage prevention performance and excellent adhesion prevention performance. In one embodiment of the present invention, the light diffusion sheet has a plurality of protrusions in a scattered manner on the back surface, wherein the average height of the plurality of protrusions is 3.0 μm to 20.0 μm, and the coefficient of variation in height is 0.30. Another mode of the backlight unit for a liquid crystal display device according to the present invention includes a light guide plate or a light guide sheet for guiding light incident from an end surface to a surface side, one or more light sources arranged along the end surface of the light guide plate or the light guide sheet, and the light diffusion sheet arranged on the surface side of the light guide plate or the light guide sheet.

Description

Light diffusion sheet and backlight unit for liquid crystal display device

Technical Field

The present invention relates to a light diffusion sheet and a backlight unit for a liquid crystal display device.

Background Art

A liquid crystal display device such as a flat panel display includes, for example, a liquid crystal display panel having a liquid crystal box and a pair of polarizing plates stacked on both sides of the liquid crystal box; and a backlight unit disposed on the back side of the liquid crystal display panel to irradiate light to the liquid crystal display panel.

As the types of the backlight unit, there are side light type, direct type, etc. The side light type backlight unit comprises: a light guide plate or a light guide sheet, which guides the light incident from the end surface to the surface side; one or more light sources, which are arranged along the end surface of the light guide plate or the light guide sheet; a light diffusion sheet, which is arranged on the surface side of the light guide plate or the light guide sheet; and a prism sheet, which is arranged on the surface side of the light diffusion sheet.

In the edge-light type backlight unit, the back side of the light diffuser is in contact with the light guide plate or the light guide sheet. Therefore, in the edge-light type backlight unit, the back side of the light diffuser is sometimes partially in close contact with the light guide plate or the light guide sheet, resulting in adhesion. Since light is difficult to fully diffuse in the portion where the adhesion occurs, it becomes a cause of uneven brightness of the light emitted from the backlight unit.

From such a viewpoint, currently, a release layer having beads dispersed in an adhesive is provided as the back layer of a light diffusion sheet (see Japanese Patent Application Laid-Open No. 2004-252353).

Prior art literature

Patent Document 1: Japanese Patent Publication No. 2004-252353

However, according to the conventional anti-adhesion layer described in the above-mentioned publication, it is difficult to control the protrusion height of the beads. Therefore, there is a situation where the protrusion height of the beads partially increases, and since the load applied to this part increases, the possibility of damage on the surface of the light guide plate or light guide sheet increases.

Summary of the invention

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a light diffusion sheet having high scratch prevention performance and excellent anti-sticking performance and a backlight unit for a liquid crystal display device.

A light diffusion sheet according to one aspect of the present invention, which is made to solve the above problems, includes a plurality of protrusions in a scattered pattern on the back surface, wherein the average height of the plurality of protrusions is 3.0 μm to 20.0 μm, and the coefficient of variation of the height is 0.30 or less.

Since the coefficient of variation of the heights of the plurality of protrusions is below the upper limit, the optical diffuser can suppress the bias of the load applied to the plurality of protrusions. In addition, since the average height of the plurality of protrusions is within the above range, the optical diffuser can suppress adhesion to other optical components arranged on the back side of the optical diffuser, and can suppress damage to the surface of the optical component caused by contact with the plurality of protrusions. Therefore, the optical diffuser has high damage prevention performance and excellent anti-adhesion performance.

Preferably, the protrusion has a truncated cone-shaped main body and a dome-shaped front end portion continuous with the front end side of the main body. In this way, the protrusion has a truncated cone-shaped main body and a dome-shaped front end portion continuous with the front end side of the main body, thereby increasing the strength of the plurality of protrusions, reducing the contact area between the protrusions and the optical component, and suppressing the occurrence of adhesion. Thus, even when used in a small portable terminal such as a smart phone or a tablet terminal, a sufficiently high-definition image can be displayed.

The area ratio of the plurality of protrusions on the back surface is preferably 2% to 80%. By setting the area ratio of the plurality of protrusions on the back surface within the above range, the scratch prevention performance and the sticking prevention performance can be improved.

When the size of the light emitting area is set to S [inch] and the area occupied by the plurality of protrusions on the back surface of the light emitting area is set to R [%], the following conditions (1) or (2) are satisfied.

(1) When S＜7, 60≤R

(2) When 7≤S≤15, 2.0≤R≤30.0

This can further improve the scratch prevention performance and the sticking prevention performance.

When the size of the light emitting area is set to S [inch] and the average radius of the circle into which the three protrusions close to each other on the back surface of the light emitting area enter is set to r [μm], the following conditions (1) or (2) are satisfied.

(1) When S＜7, r≤40

(2) When 7≤S≤15, 40≤r≤400

This can further improve the scratch prevention performance and the sticking prevention performance.

Another embodiment of the present invention is a backlight unit for a liquid crystal display device that is made to solve the above-mentioned problem, and comprises: a light guide plate or a light guide sheet that guides light incident from the end face to the surface side; one or more light sources that are arranged along the end face of the light guide plate or the light guide sheet; and the light diffusion sheet that is arranged on the surface side of the light guide plate or the light guide sheet.

Since the backlight unit for a liquid crystal display device includes the light diffusion sheet, the backlight unit has high scratch prevention performance and excellent sticking prevention performance.

In addition, in the present invention, "surface" refers to the surface on the viewer's side when assembled in a liquid crystal display device, and "back side" refers to the surface on the opposite side of the surface. "Average height of multiple protrusions" refers to the average value of the heights of any 10 protrusions. "Coefficient of variation of the heights of multiple protrusions" refers to the value obtained by dividing the standard deviation of the heights of any 10 protrusions by the average height. "Light emission area" refers to the area overlapping with the display screen when viewed from above. "Average radius of the circle entered by three protrusions close to each other on the back side" means extracting any 10 protrusions, and for each extracted protrusion, finding the minimum radius of the circle on the back side entered by the protrusion and the other two protrusions other than the protrusion, and then excluding the average value of the maximum and minimum values among the 10 radii found.

As described above, the light diffusion sheet according to one embodiment of the present invention and the backlight unit for a liquid crystal display device according to another embodiment have high scratch prevention performance and excellent sticking prevention performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic end view of a backlight unit for a liquid crystal display device showing one embodiment of the present invention.

FIG. 2 is a schematic bottom view of a light diffusion sheet of the backlight unit for the liquid crystal display device of FIG. 1 .

FIG. 3 is a schematic side view showing a protrusion of the light diffusion sheet of FIG. 2 .

FIG. 4 is a schematic bottom view of the protrusion of FIG. 3 as viewed from the front end side.

FIG. 5 is a schematic partially enlarged bottom view showing the arrangement of protrusions of the light diffusion sheet of FIG. 2 .

FIG. 6 is a schematic side view showing a protrusion according to an embodiment different from the protrusion of FIG. 3 .

FIG. 7 is a schematic bottom view of the protrusion of FIG. 6 as viewed from the front end side.

Description of Reference Numerals

1 Backlight unit for liquid crystal display device (backlight unit)

2 Light guide

3 Light Source

4 Light diffuser

4a Back

5 Prism

5a Base material layer

5b Prism Array

6 Reflective sheet

11, 21 protrusion

11a, 21a Main body

11b, 21b front end

12. Base material layer

13 Light diffusion layer

13a Light Diffuser

13b Adhesive

21c Foot of the mountain

A Light emission area

D1 Diameter of the bottom surface of the protrusion

H1 Height of protrusion

r Mean radius

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described in detail with reference to appropriate drawings.

[First embodiment]

＜Light Diffuser＞

The backlight unit 1 for a liquid crystal display device of FIG. 1 (hereinafter also referred to as "backlight unit 1") is an edge-light type backlight unit. The backlight unit 1 comprises: a light guide plate 2 for guiding light incident from the end face to the surface side; one or more light sources 3 arranged along the above-mentioned end face of the light guide plate 2; and a light diffuser plate 4 arranged on the surface side of the light guide plate 2. The light diffuser plate 4 is directly stacked on the surface of the light guide plate 2 (without other components). The light diffuser plate 4 itself constitutes one embodiment of the present invention. In addition, the backlight unit 1 comprises: a prism sheet 5 arranged on the surface side of the light diffuser sheet 4; and a reflective sheet 6 arranged on the back side of the light guide plate 2. In addition, the backlight unit 1 may also be provided with other prism sheets on the surface side of the prism sheet 5.

(Light Diffuser)

The light diffuser 4 has a light incident surface facing the light guide sheet 2 and a light exit surface opposite to the light incident surface. The light diffuser 4 allows the light incident from the light incident surface to be emitted from the light exit surface. The light diffuser 4 has flexibility as a whole.

The light diffuser sheet 4 has a plurality of protrusions 11 on the back surface 4a in a scattered pattern. The light diffuser sheet 4 has a substrate layer 12 and a light diffuser layer 13 laminated on the surface side of the substrate layer 12, and a plurality of protrusions 11 are formed on the back surface of the substrate layer 12 in a scattered pattern. The light diffuser sheet 4 has the light diffuser layer 13 laminated directly on the surface of the substrate layer 12, and a plurality of protrusions 11 are formed directly on the back surface of the substrate layer 12. That is, the light diffuser sheet 4 does not have any other layer other than the substrate layer 12, the light diffuser layer 13 and the plurality of protrusions 11, and the back surface of the substrate layer 12 constitutes the back surface 4a of the light diffuser sheet 4. In the light diffuser sheet 4, the back surface of the substrate layer 12 and the outer surface of the plurality of protrusions 11 constitute the light incident surface, and the surface of the light diffuser layer 13 constitutes the light emitting surface. The light diffuser sheet 4 is arranged on the surface side of the light guide sheet 2 by partially contacting the surface of the light guide sheet 2 with the plurality of protrusions 11. The contacting portion between the plurality of protrusions 11 and the light guide sheet 2 is not fixed by an adhesive or the like.

[Base material layer]

The substrate layer 12 is transparent because it is necessary to allow light to pass through it. The substrate layer 12 is flexible. The substrate layer 12 has a synthetic resin as a main component. There is no particular limitation on the main component of the substrate layer 12, and examples thereof include polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, and weather-resistant vinyl chloride. Among them, polyethylene terephthalate having excellent transparency and high strength is preferred, and polyethylene terephthalate having improved bending properties is particularly preferred. In addition, the "main component" refers to the component with the largest content in terms of mass conversion, for example, a component with a content of 50% by mass or more.

The lower limit of the average thickness of the substrate layer 12 is preferably 10 μm, and more preferably 20 μm. On the other hand, the upper limit of the average thickness of the substrate layer 12 is preferably 200 μm, more preferably 100 μm, and further preferably 50 μm. If the above-mentioned average thickness is less than the above-mentioned lower limit, the strength of the substrate layer 12 becomes insufficient, and there is a possibility that the back side of the substrate layer 12 is partially in contact with the light guide sheet 2. On the contrary, if the above-mentioned average thickness exceeds the above-mentioned upper limit, it is possible that the requirement of thinning the light diffuser 4 is violated, the substrate layer 12 becomes heavier, and thus the load applied to the plurality of protrusions 11 becomes larger, and the possibility of damage to the surface of the light guide sheet 2 becomes higher.

[Protrusion]

2 to 4 , a plurality of protrusions 11 protrude from the back surface of the base layer 12 toward the light incident side (light guide sheet 2 side). The plurality of protrusions 11 are arranged irregularly on the back surface of the base layer 12 .

The average height H1 _AVG of the plurality of protrusions 11 is not less than 3.0 μm and not more than 20.0 μm. In addition, the coefficient of variation of the height H1 of the plurality of protrusions 11 is not more than 0.30. For the light diffusion sheet 4, for example, as described later, the plurality of protrusions 11 are formed by using a pressing roller having a plurality of recesses, the plurality of recesses having the reverse shape of the plurality of protrusions 11, thereby being able to form the protrusions 11 into a desired shape.

The lower limit of the average height H1 _AVG of the plurality of protrusions 11 is preferably 4.0 μm, and more preferably 5.0 μm. On the other hand, the upper limit of the average height H1 _AVG is preferably 16.0 μm, and more preferably 10.0 μm. If the average height H1 _AVG is less than the lower limit, adhesion to the light guide sheet 2 may not be sufficiently suppressed. On the contrary, if the average height H1 _AVG exceeds the upper limit, the surface of the light guide sheet 2 may be damaged due to contact with the plurality of protrusions 11, and the protrusions 11 may be easily broken.

The upper limit of the coefficient of variation of the height H1 of the plurality of protrusions 11 is preferably 0.20, more preferably 0.10, and further preferably 0.05. If the coefficient of variation exceeds the upper limit, the heights of the plurality of protrusions 11 become uneven, and the load is biased toward the protrusions 11 with a large height, which may cause damage to the surface of the light guide sheet 2. On the other hand, the lower limit of the coefficient of variation is not particularly limited, and may be 0, for example.

The lower limit of the average diameter D1 _AVG of the bottom surface (the boundary surface with the base material layer 12) of the plurality of protrusions 11 is preferably 10 μm, and more preferably 15 μm. On the other hand, the upper limit of the above average diameter D1 _AVG is preferably 40 μm, and more preferably 30 μm. If the above average diameter D1 _AVG is less than the above lower limit, there is a possibility that the strength of the plurality of protrusions 11 becomes insufficient. On the contrary, if the above average diameter D1 _AVG exceeds the above upper limit, there is a possibility that the contact area between the plurality of protrusions 11 and the light guide plate 2 becomes larger, resulting in adhesion. In addition, the "diameter" refers to the diameter converted to a circle of equal area. In addition, throughout this specification, the "average diameter" refers to the average value of any 10 diameters.

As shown in FIG. 3 and FIG. 4 , the protrusion 11 has a main body 11a in the shape of a truncated cone and a dome-shaped front end 11b continuous with the front end side of the main body 11a. The main body 11a decreases in diameter from the base end side (the substrate layer 12 side) toward the front end side. The protrusion 11 abuts against the light guide sheet 2 at the front end side of the front end 11b. For the light diffuser 4, since the protrusion 11 has the main body 11a, the strength of the protrusion 11 can be made sufficiently large even when the size of the protrusion 11 is made small. The light diffuser 4 has a dome-shaped front end 11b at the front end side of the main body 11a, thereby reducing the abutment area between the protrusion 11 and the light guide sheet 2, and can suppress the occurrence of adhesion. As a result, the light diffuser 4 can display a sufficiently high-definition image even when used in a small portable terminal such as a smart phone or a tablet terminal. In addition, "truncated cone" includes not only a cross section that is circular or substantially circular in a direction perpendicular to the height direction, but also an elliptical cross section. Furthermore, the term "dome-shaped" means that the outer edge on the distal end side is curved into an arcuate shape (circular arc shape, elliptical arc shape, etc.) convex toward the distal end side in any cross section passing through the apex and parallel to the height direction.

Preferably, the ratio of the height H1 of the protrusion 11 to the diameter D1 of the bottom surface is less than 1. That is, preferably, the protrusion 11 is flat with a small height H1 relative to the diameter D1. The protrusion 11 can easily increase its strength by reducing the ratio of the height H1 to the diameter D1. In addition, since the protrusion 11 has a dome-shaped front end portion 11b on the front end side of the truncated cone-shaped main body portion 11a, even if the ratio of the height H1 to the diameter D1 is reduced, the contact area between the protrusion 11 and the light guide sheet 2 can be prevented from becoming too large.

The outer surface of the front end portion 11b is formed continuously from the side surface of the main body portion 11a. The front end portion 11b can be configured to have a curvature radius that increases stepwise and/or continuously from the base end side toward the front end side. The light diffuser 4 can prevent the main body portion 11a from contacting the light guide sheet 2 by having the main body portion 11a be in the above-mentioned truncated cone shape. Therefore, even if the light diffuser 4 increases the curvature radius of the front end side of the front end portion 11b, the contact area between the protrusion 11 and the light guide sheet 2 can be suppressed to a smaller contact area. As a result, the light diffuser 4 can fully suppress adhesion to the light guide sheet 2, and can fully suppress damage to the surface of the light guide sheet 2 caused by contact with the plurality of protrusions 11.

The lower limit of the area occupied by the plurality of protrusions 11 on the back surface 4a of the light diffusion sheet 4 (the back surface of the substrate layer 12 in this embodiment) is preferably 2%. On the other hand, the upper limit of the above-mentioned area occupied is preferably 80%. If the above-mentioned area occupied is less than the above-mentioned lower limit, it may not be possible to fully suppress adhesion to the light guide sheet 2. On the contrary, if the above-mentioned area occupied exceeds the above-mentioned upper limit, the plurality of protrusions 11 become unnecessarily increased, and it is possible to cause damage to the surface of the light guide sheet 2.

As shown in FIG. 2 , the light diffuser 4 has a light emitting area A corresponding to the liquid crystal display screen of the liquid crystal display panel when assembled into the liquid crystal display device. The light emitting area A is, for example, rectangular, typically a long rectangle. Preferably, when the size of the light emitting area A is set to S [inch] and the area occupied by the plurality of protrusions 11 on the back surface 4a of the light emitting area A is set to R [%], the light diffuser 4 satisfies the following conditions (1) or (2).

(1) When S＜7, 60≤R

(2) When 7≤S≤15, 2.0≤R≤30.0

In addition, the light emitting area A is usually substantially equal to the plane area of the light diffuser 4. In the case where the light emitting area A is substantially equal to the plane area of the light diffuser 4, the light emitting area A can also be converted into the plane area of the light diffuser. In addition, 7 inches refers to a size with a diagonal length of 17.8 cm, and 15 inches refers to a size with a diagonal length of 38.1 cm.

In addition, when S is less than 7, the lower limit of R is more preferably 65. If R is less than the lower limit, the light diffuser sheet 4 may be damaged due to the fall of the device in which the light diffuser sheet 4 is assembled. On the other hand, the upper limit of R in this case is preferably 90, and more preferably 85. When R exceeds the upper limit, the effect of improving the anti-sticking function may not be obtained.

When 7≤S≤15, the lower limit of R is more preferably 3.0, and more preferably 5.0. On the other hand, in this case, the upper limit of R is more preferably 25.0, and more preferably 20.0. If R is less than the lower limit, there is a possibility that adhesion to the light guide sheet 2 cannot be sufficiently suppressed. On the contrary, if R exceeds the upper limit, there is a possibility that damage is generated on the surface of the light guide sheet 2.

As shown in FIG. 5 , it is preferred that the following conditions (1) or (2) are satisfied when the size of the light emitting area A is set to S [inch] and the average radius of the circle into which the three protrusions 11 close to each other on the back side 4a of the light emitting area A enter (the circle in contact with the side edges of the three protrusions 11) is set to r [μm].

(1) When S＜7, r≤40

(2) When 7≤S≤15, 40≤r≤400

In addition, when S is less than 7, the upper limit of r is more preferably 35. If r exceeds the upper limit, the light diffuser sheet 4 may be damaged due to the fall of the device in which the light diffuser sheet 4 is assembled. On the other hand, the lower limit of r in this case is preferably 5, and more preferably 10. When r is less than the lower limit, the effect of improving the anti-sticking function may not be obtained.

When 7≤S≤15, the lower limit of r is more preferably 50. In this case, the upper limit of r is more preferably 70, and further preferably 60. If r is less than the lower limit, there is a possibility that damage is generated on the surface of the light guide sheet 2. On the contrary, if r exceeds the upper limit, there is a possibility that adhesion to the light guide sheet 2 cannot be sufficiently suppressed.

The protrusions 11 do not contain particles such as resin beads. The protrusions 11 contain, for example, an active energy ray-curable resin or a synthetic resin that is the same as the main component of the base material layer 12 as a main component.

Examples of the active energy ray-curable resin include ultraviolet-curable resins. Examples of the ultraviolet-curable resin include polyurethane acrylates and acrylic resins. Examples of the acrylic resin include ultraviolet-curable polyester acrylates, ultraviolet-curable epoxy acrylates, and ultraviolet-curable polyol acrylates.

[Light diffusion layer]

The light diffusion layer 13 constitutes the outermost surface of the light diffusion sheet 4. The light diffusion layer 13 has a plurality of light diffusing agents 13a and their adhesives 13b. The light diffusing agents 13a are surrounded by adhesives 13b. The light diffusion layer 13 disperses a plurality of light diffusing agents 13a, thereby being able to diffuse light transmitted from the back side to the surface side roughly uniformly. In addition, the light diffusion layer 13 forms micro-concave and convex portions roughly uniformly on the surface through a plurality of light diffusing agents 13a, and each concave portion and convex portion of the micro-concave and convex portion is formed into a lens shape. The light diffusion layer 13 utilizes the lens effect of the micro-concave and convex to exert an excellent light diffusion function. Due to the light diffusion function, it has a refraction function that refracts the transmitted light to the normal direction side and a focusing function that macroscopically converges the transmitted light to the normal direction.

The light diffuser 13a is a microparticle having the property of diffusing light, and is typically a resin bead. As the main component of the light diffuser 13a, for example, acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyamide, polyacrylonitrile, etc. can be cited. Among them, acrylic resin with high transparency is preferred, and polymethyl methacrylate (PMMA) is particularly preferred.

The shape of the light diffusing agent 13 a is not particularly limited, and examples thereof include spherical, cubic, needle-like, rod-like, spindle-like, plate-like, flaky, and fibrous shapes. Among them, a spherical shape is preferred because of its excellent light diffusivity.

The adhesive 13b is formed by curing (crosslinking, etc.) a polymer composition containing a substrate polymer. The light diffuser 13a is fixed to the entire surface of the substrate layer 12 at a substantially equal density by the adhesive 13b. In addition, the polymer composition used to form the adhesive 13b may also be appropriately formulated with, for example, minute inorganic fillers, curing agents, plasticizers, dispersants, various leveling agents, antistatic agents, ultraviolet absorbers, antioxidants, viscosity modifiers, lubricants, light stabilizers, etc.

(Light guide sheet)

The light guide sheet 2 is formed in a substantially square shape in a plan view and in a plate shape (not a wedge shape) with a substantially uniform thickness.

The lower limit of the average thickness of the light guide sheet 2 is preferably 100 μm, more preferably 150 μm, and further preferably 200 μm. On the other hand, the upper limit of the average thickness of the light guide sheet 2 is preferably 600 μm, and more preferably 400 μm. If the average thickness of the light guide sheet 2 is less than the above lower limit, there is a possibility that the strength of the light guide sheet 2 becomes insufficient, and there is a possibility that it is difficult to allow the light from the light source 3 to fully enter the light guide sheet 2. On the contrary, if the average thickness of the light guide sheet 2 exceeds the above upper limit, there is a possibility that the requirement for thinning the backlight unit 1 is violated.

The light guide sheet 2 needs to be light-transmissive, so a transparent, especially colorless and transparent resin is used as the main component. The main component of the light guide sheet 2 is not particularly limited, but examples thereof include polycarbonate having excellent transparency and strength, and synthetic resins such as acrylic resin having excellent transparency and scratch resistance.

The light guide sheet 2 may have a concavo-convex shape such as a lenticular lens or a prism array on its surface so that the emitted light can be controlled.

(light source)

The light source 3 is arranged so that the irradiation surface is opposite to (or abuts against) the end surface of the light guide plate 2. As the light source 3, various light sources can be used, for example, a light emitting diode (LED) can be used. Specifically, as the plurality of light sources 3, a light source in which a plurality of light emitting diodes are arranged along the end surface of the light guide plate 2 can be used.

(Prism Sheet)

The prism sheet 5 has a substrate layer 5a and a prism array 5b, wherein the prism array 5b is composed of a plurality of convex prism portions stacked on the surface of the substrate layer 5a. The substrate layer 5a and the prism array 5b are transparent because they need to allow light to pass through. The substrate layer 5a and the prism array 5b are formed mainly of synthetic resin. The backlight unit 1 may also be configured with other prism sheets on the surface side of the prism sheet 5. In this case, it is preferred that the prism sheet 5 is orthogonal to the prism array of the other prism sheets.

(Reflective sheet)

Examples of the reflection sheet 6 include a white sheet obtained by dispersing a filler in a base resin such as polyester, and a mirror sheet obtained by depositing a metal such as aluminum or silver on the surface of a film made of polyester to improve regular reflectivity.

<Method for manufacturing light diffusion sheet>

An example of a method for manufacturing the light diffusion sheet 4 is described. The following describes a case where an ultraviolet curing resin is used as a main component of the plurality of protrusions 11. The method for manufacturing the light diffusion sheet includes: a step of preparing a substrate layer 12 (preparation step); a step of forming the plurality of protrusions 11 on the back side of the substrate layer 12 prepared in the preparation step (protrusion forming step); and a step of laminating a light diffusion layer 13 on the surface side of the substrate layer 12 (light diffusion layer laminating step).

(Protrusion Forming Step)

The protrusion forming step comprises: a step of applying a protrusion forming resin composition containing, for example, the above-mentioned ultraviolet curable resin on the back side of the substrate layer 12 (coating step); a step of pressing a pressing roller having a plurality of recessed portions on the outer peripheral surface which are the inverse shape of the plurality of protrusions 11 on the coating liquid applied by the above-mentioned coating step (pressing step); and a step of curing the protrusion forming resin composition by ultraviolet irradiation after the above-mentioned pressing step (curing step). In the above-mentioned pressing step, the plurality of protrusions 11 are transferred to the back side of the substrate layer 12. Preferably, the above-mentioned curing step is performed in a state where the above-mentioned pressing roller is pressed on the back side of the substrate layer 12. As a method of forming a plurality of recessed portions on the outer peripheral surface of the above-mentioned pressing roller, for example, a method of using laser irradiation and a method of etching after forming a copper plating layer on the outer peripheral surface of the pressing roller can be cited.

(Light diffusion layer lamination step)

In the light diffusion layer lamination step, after the protrusion formation step, a coating liquid containing a plurality of light diffusing agents 13 a and a binder composition is applied to the surface of the base layer 12 , and the applied coating liquid is dried and cured.

＜Advantages＞

In the light diffuser sheet 4, since the coefficient of variation of the heights of the plurality of protrusions 11 is below the above upper limit, it is possible to suppress the bias of the load applied to the plurality of protrusions 11. In addition, in the light diffuser sheet 4, since the average height H1 _AVG of the plurality of protrusions 11 is within the above range, it is possible to suppress adhesion to other optical components (light guide sheet 2) arranged on the back side of the light diffuser sheet 4, and it is possible to suppress damage to the surface of the optical component caused by contact with the plurality of protrusions 11. Therefore, the light diffuser sheet 4 has high damage prevention performance and excellent adhesion prevention performance.

Since the backlight unit 1 includes the light diffusion sheet 4 , it has high scratch prevention performance and excellent sticking prevention performance.

The method for manufacturing the light diffusion sheet can easily and reliably manufacture the light diffusion sheet 4 having excellent scratch prevention performance and sticking prevention performance.

[Other embodiments]

The embodiments disclosed this time are illustrative in all aspects and are not restrictive. The scope of the present invention is not limited to the configuration of the above embodiments, but is shown by the technical solutions, and is considered to include the meaning equivalent to the technical solutions and all changes within the scope.

For example, the plurality of protrusions may not be directly formed on the back side of the substrate layer. For example, the light diffuser may also have a back layer made of synthetic resin laminated on the back side of the substrate layer, and the back side of the back layer has a plurality of protrusions. In this case, the back side of the back layer constitutes the back side of the light diffuser.

The specific structure of the light diffusion layer is not limited to the structure described in the above embodiment. For example, the light diffusion layer may be formed with a concavo-convex shape on the outer surface by embossing or the like.

The above-mentioned protrusion does not necessarily have to be composed of only a truncated cone-shaped main body and a dome-shaped front end portion continuous with the front end side of the main body. With reference to Figures 6 and 7, a modified example of the above-mentioned protrusion is described. The protrusion 21 of Figures 6 and 7 has a truncated cone-shaped main body 21a, a dome-shaped front end portion 21b continuous with the front end side of the main body 21a, and a foot portion 21c continuous with the base end side of the main body 21a. As the main body 21a, the same structure as the main body 11a of the protrusion 11 of the light diffuser 4 of Figure 1 can be adopted. As the front end portion 21b, the same structure as the front end portion 11b of the protrusion 11 of the light diffuser 4 of Figure 1 can be adopted. The foot portion 21c expands in diameter in a trumpet shape from the front end side of the protrusion 21 toward the base end side. According to this structure, the light diffuser can increase the adhesion between the back surface and the plurality of protrusions.

It is preferred that the protrusion has a truncated cone-shaped main body and a dome-shaped front end portion continuous with the front end side of the main body. However, as a specific shape of the protrusion, for example, a configuration without a truncated cone-shaped main body may be adopted. In this case, the protrusion may have a main body in the shape of a cylinder, a polygonal column, a polygonal truncated cone, etc. instead of the truncated cone-shaped main body.

The backlight unit may include a light guide plate having an average thickness of more than 600 μm, for example, instead of the light guide sheet. The light guide plate may be in the form of a flat plate or in the form of a wedge in cross section.

The specific structure of the backlight unit is not limited to the structure described in the above-mentioned embodiment. The backlight unit may also have other optical sheets such as a microlens sheet, and may also have an upper light diffusion sheet with a lower diffusion function on the surface side of the prism sheet. In addition, the backlight unit may also be a direct-down type backlight unit. In addition, even if the backlight unit is a side-lit backlight unit, it does not need to be a single-sided side-lit backlight unit in which one or more light sources are arranged along only one end face of a light guide sheet or a light guide plate. It may be a double-sided side-lit backlight unit in which one or more light sources are arranged along a pair of opposite end faces of a light guide sheet or a light guide plate, or a full-surround side-lit backlight unit in which one or more light sources are arranged along each end face of a light guide sheet or a light guide plate.

The backlight unit can be used for relatively small portable terminals such as smartphones and tablet terminals, personal computers such as notebook computers, and relatively large display devices such as liquid crystal televisions.

[Example]

Hereinafter, the present invention will be described in further detail by way of examples, but the present invention is not limited to these examples.

[Examples 1 to 3, Comparative Examples 1 to 3]

A substrate layer with an average thickness of 50 μm and polyethylene terephthalate as the main component is prepared. While feeding the substrate layer between a pair of pressing rollers, an ultraviolet curable resin (resin composition for protrusion formation) is supplied to the back side of the substrate layer (the back side when assembled into a liquid crystal display device) at the portion that is about to reach the pair of pressing rollers. As a pressing roller in contact with the ultraviolet curable resin, a pressing roller having a plurality of recesses on the outer peripheral surface is used. After pressing the ultraviolet curable resin and the substrate layer with the pair of pressing rollers, the ultraviolet curable resin is cured by irradiating ultraviolet rays, and a plurality of protrusions that are the inverse shapes of the plurality of recesses are transferred to the back side of the substrate layer. In addition, a coating liquid containing a plurality of beads and an adhesive composition is applied to the surface side of the substrate layer, and by drying and curing the coating liquid, a light diffusion layer is laminated on the surface side of the substrate layer to obtain a light diffusion sheet. In Examples 1 to 3 and Comparative Examples 1 to 3, the average height of the plurality of protrusions is adjusted by changing the average depth of the recesses of the pressing roller. Table 1 shows the average heights of a plurality of protrusions and the coefficient of variation of the heights.

＜Damage prevention performance＞

The light diffusers of Examples 1 to 3 and Comparative Examples 1 to 3 were punched into rectangles of 10 cm × 20 cm, and three test pieces were made for each light diffuser. For these test pieces, a flat friction resistance tester (Tribo-Tester TYPE14 manufactured by Shinto Scientific Co., Ltd.) with an ASTM flat indenter was used. The prism sheet was attached to the lower part of the ASTM indenter, and the test piece was arranged so that multiple protrusions were opposite to the prism sheet. A damage test was performed at a moving speed of 50 mm/min, a moving distance of the indenter of 50 mm, and a load of 500 g. The test piece was evaluated for damage according to the following criteria. The evaluation results are shown in Table 1.

A: No damage was observed visually for two or more test pieces.

B: For two or more test pieces, damage was visually confirmed

＜Anti-sticking performance＞

The light diffusing sheets of Examples 1 to 3 and Comparative Examples 1 to 3 were left in an environment of a temperature of 40° C. and a humidity of 90% for 48 hours, and then the presence or absence of adhesion of these light diffusing sheets was measured in the following procedure.

A test piece obtained by cutting a light diffusion sheet into pieces of 13.5 cm × 22 cm was stacked on a side-light type backlight having a plane size of 13 cm × 21 cm and having an LED light source, a light guide sheet for guiding light irradiated from the LED light source, and a reflective sheet arranged on the back side of the light guide sheet. In addition, a transparent polyethylene terephthalate film having a thickness of 100 μm and a plane size of 13.5 cm × 22 cm was stacked on the surface of the test piece to obtain a stacked body, which was then placed in a polyethylene bag having an inner size of 20 cm × 25 cm. As the bag, a bag having an opening on one side and a sealable opening portion was used. With the stacked body placed therein, a vacuum exhaust hose was inserted from the opening portion of the bag, and then the opening portion was sealed.

A chromaticity and brightness meter "BM-7" manufactured by TOPCON TECHNOHOUSE was set at a position 50 cm away from the surface of the test piece in the normal direction, and the air in the bag was exhausted for 30 seconds with a diaphragm pump, and the brightness was measured with the chromaticity and brightness meter while the exhaust was stopped. After the measurement, the bag was further exhausted for 30 seconds with a diaphragm pump, and the brightness was measured. The brightness at the time of the initial measurement was set as L1 [cd/m ² ], and the brightness after the bag was exhausted for 30 seconds after the initial measurement was set as L2 [cd/m ² ], and the presence or absence of adhesion was evaluated according to the following criteria. The evaluation results are shown in Table 1.

A (no adhesion): (L2-L1)/L2＜0.14

B (with adhesion): (L2-L1)/L2≥0.14

Table 1

Average height of protrusions [μm] Coefficient of variation of protrusion height Damage prevention performance Anti-stick properties Example 1 4.0 0.20 A A Example 2 9.0 0.05 A A Example 3 16.0 0.10 A A Comparative Example 1 2.5 0.25 A B Comparative Example 2 22.0 0.19 B A Comparative Example 3 10.0 0.33 B A

<Evaluation Results>

As shown in Table 1, Examples 1 to 3 have excellent damage prevention and adhesion prevention performance because the average height of the plurality of protrusions is 3.0 μm or more and 20.0 μm or less and the coefficient of variation of the height is 0.30 or less. In contrast, Comparative Example 1 has insufficient adhesion prevention performance because the average height of the plurality of protrusions is small. Comparative Example 2 has insufficient damage prevention performance because the average height of the plurality of protrusions is large. Comparative Example 3 has insufficient damage prevention performance because the coefficient of variation of the height of the protrusions is large.

[Examples 4 to 7]

The average diameter of the bottom surface of the plurality of protrusions was set as shown in Table 2, and a light diffusion sheet was manufactured by the same procedure as in Example 1. In addition, the average diameter of the protrusions was adjusted by changing the opening diameter of the concave portion of the pressing roller. In addition, the average height of the plurality of protrusions in Examples 4 to 7 was 9.0 μm, and the coefficient of variation of the height was 0.15.

<Protrusion Strength>

The light diffusers of Examples 4 to 7 were punched into rectangles of 10 cm × 10 cm, and three test pieces were made for each light diffuser. A flat friction resistance tester (friction and wear tester TYPE40 manufactured by Shinto Science Co., Ltd.) with an ASTM indenter was prepared, and a light guide plate with a cylindrical lens formed on the upper surface was attached to the test bench of the flat friction resistance tester using an adhesive. In addition, the above-mentioned test piece was attached thereto using an adhesive in such a manner that a plurality of protrusions were opposite to the cylindrical lenses of the light guide plate, and the ASTM indenter was moved back and forth in the center of the test piece at a moving speed of 50 mm/min, a distance of 50 mm for the indenter to move, and a load of 500 g. The ASTM indenter was first moved back and forth 5 times in a direction parallel to the ridgeline of the cylindrical lens, and then the light guide plate was peeled off the test bench while being attached to the test piece and rotated 90°, and the light guide plate was attached to the test bench again using an adhesive, and then the ASTM indenter was moved back and forth 5 times in a direction orthogonal to the ridgeline of the cylindrical lens. After the reciprocating test, the test piece was peeled off from the light guide plate, the cylindrical lens surface of the light guide plate was visually confirmed, and the strength of the protrusion was evaluated according to the following criteria. The evaluation results are shown in Table 2. In addition, as an adhesive, a spray adhesive (Adhesive Spray) "Spray 55" manufactured by 3M was used so that when the test piece was peeled off from the light guide plate, the protrusion would not be damaged by the adhesive and the protrusion fragments would not adhere to the light guide plate.

A: No protruding fragments were observed

B: Protruding fragments were observed

<Anti-sticking properties>

The anti-sticking performance was evaluated by the same procedure and the same evaluation criteria as in Examples 1 to 3 and Comparative Examples 1 to 3. Table 2 shows the evaluation results.

Table 2

Average diameter [μm] Protrusion strength Anti-stick properties Example 4 10 A A Example 5 40 A A Example 6 5 B A Example 7 50 A B

<Evaluation Results>

As shown in Table 2, Examples 4 and 5, in which the average diameter of the plurality of protrusions is 10 μm or more and 40 μm or less, have high protrusion strength and excellent anti-adhesion performance. In contrast, Example 6 has a small average diameter of the plurality of protrusions, so the protrusion strength is low. In addition, Example 7 has a large average diameter of the plurality of protrusions, so the contact area of the protrusions is large, and the anti-adhesion performance is low.

[Examples 8 to 15]

The average thickness of the substrate layer was set to 38 μm, and the size of the light emitting area and the area occupied by the plurality of protrusions on the back of the substrate layer were set as shown in Table 3. The light diffusion sheet was manufactured by the same procedure as in Example 1. In addition, the area occupied by the plurality of protrusions was adjusted by changing the density of the concave portions of the pressing roller. In addition, the average height of the plurality of protrusions in Examples 8 to 15 was 10.0 μm, and the coefficient of variation of the height was 0.10.

< Damage prevention performance >

(Drop ball test)

For Examples 8 to 11, a drop ball test was performed using the following steps to evaluate the damage prevention performance. A polycarbonate film with a thickness of 475 μm imitating a light guide was laminated on the surface of a stainless steel plate with a thickness of 5 mm, and the light diffuser sheets of Examples 8 to 11 were laminated thereon in a manner such that multiple protrusions abutted against the light guide. In addition, two prism sheets with prism vertices protruding upward were laminated on the light diffuser in a manner orthogonal to each other's ridgeline directions. Next, after the liquid crystal display panel (color filter/liquid crystal layer/TFT substrate) was configured in a manner such that the TFT substrate and the prism sheet were opposite, a sphere made of nylon 66 with a diameter of 30 mm was dropped from a height of 100 cm above the liquid crystal display panel, and the light diffuser was visually confirmed to be damaged, and evaluated according to the following criteria. The evaluation results are shown in Table 3. In addition, for light diffusers with a small light emission area, most of them are used in smartphones, tablet terminals, etc., and it is important to evaluate whether there is damage caused by the fall of the terminal, so for Examples 8 to 11, a drop ball test was performed.

A: No damage was observed

B: Damage observed

The scratch prevention performance of Examples 12 to 15 was evaluated in the same procedure and evaluation criteria as in Examples 1 to 3 and Comparative Examples 1 to 3. The evaluation results are shown in Table 3.

＜Anti-sticking properties＞

The anti-sticking performance was evaluated by the same procedure and the same evaluation criteria as in Examples 1 to 3 and Comparative Examples 1 to 3. Table 3 shows the evaluation results.

Table 3

＜Evaluation Results＞

As shown in Table 3, Examples 8, 9, 10, 12, and 13 have excellent damage prevention and adhesion prevention performance by fully adjusting the occupancy rate of the multiple protrusions relative to the size of the light emitting area. In contrast, Example 14 has insufficient adhesion prevention performance because the occupancy rate of the multiple protrusions relative to the size of the light emitting area is small. In addition, Example 15 has insufficient damage prevention performance because the occupancy rate of the multiple protrusions relative to the size of the light emitting area is large. Example 11 has insufficient cushioning function against impact because the occupancy rate of the multiple protrusions relative to the size of the light emitting area is small, and the damage prevention performance using the drop ball test is insufficient.

[Examples 16 to 23]

The average thickness of the substrate layer was set to 38 μm, and the size of the light emitting area and the average radius of the circle into which the three protrusions approaching on the back of the substrate layer entered were set to be as shown in Table 4, and a light diffusion sheet was manufactured by the same procedure as in Example 1. In addition, the average radius of the circle into which the three protrusions approaching on the back of the substrate layer entered was adjusted by changing the configuration of the concave portion of the pressing roller. In addition, the average height of the plurality of protrusions in Examples 16 to 23 was 9.0 μm, and the coefficient of variation of the height was 0.15.

＜Damage prevention performance＞

Examples 16 to 19 were subjected to the same falling ball test as Examples 8 to 11, and Examples 20 to 23 were subjected to the same procedure and evaluation criteria as Examples 1 to 3 and Comparative Examples 1 to 3 to evaluate the scratch prevention performance. The evaluation results are shown in Table 4.

＜Anti-sticking properties＞

The anti-sticking performance was evaluated in the same procedure and evaluation criteria as in Examples 1 to 3 and Comparative Examples 1 to 3. Table 4 shows the evaluation results.

Table 4

＜Evaluation Results＞

As shown in Table 4, Examples 16, 17, 18, 20, and 21 have excellent damage prevention and adhesion prevention performance by fully adjusting the average radius of the circle into which three protrusions approach to the size of the light emitting area. In contrast, Example 22 has insufficient damage prevention performance because the average radius of the circle into which three protrusions approach to the size of the light emitting area is small, and Example 19 has insufficient cushioning function against multiple protrusions that impact because the average radius of the circle into which three protrusions approach to the size of the light emitting area is large, and thus the damage prevention performance in the drop ball test is insufficient. In addition, Examples 19 and 23 have insufficient adhesion prevention performance because the average radius of the circle into which three protrusions approach to the size of the light emitting area is large.

Industrial Applicability

As described above, the light diffusion sheet according to one embodiment of the present invention has high scratch prevention performance and excellent anti-sticking performance, and therefore is suitable for use in various liquid crystal display devices.

Claims (6)

1. A light diffusion sheet is characterized in that,

The light diffusion sheet has a plurality of protrusions in a scattered manner on the back surface,

The plurality of protrusions have an average height of 3.0 μm to 20.0 μm, an average diameter of 10 μm to 40 μm, a coefficient of variation in height of 0.30 or less,

When the size of the light emitting region is S inches and the occupied area ratio of the plurality of protrusions on the back surface of the light emitting region is R [% ], the following condition (1) or (2) is satisfied,

(1) When S is less than 7, 60 is less than or equal to R

(2) When S is more than or equal to 7 and less than or equal to 15, R is more than or equal to 2.0 and less than or equal to 30.0.

2. A light diffusion sheet according to claim 1, wherein,

The protrusion has a truncated cone-shaped body portion and a dome-shaped tip portion continuous with a tip side of the body portion.

3. A light diffusion sheet according to claim 1 or 2, wherein,

The area ratio of the plurality of protrusions on the back surface is 2% to 80%.

4. A light diffusion sheet according to claim 1 or 2, wherein,

When the size of the light emitting region is S inch and the average radius of the circle into which the three protrusions approaching on the back surface of the light emitting region enter is r μm, the following condition (1) or (2) is satisfied,

(1) When S is less than 7, r is less than or equal to 40

(2) When S is more than or equal to 7 and less than or equal to 15, r is more than or equal to 40 and less than or equal to 400.

5. A light diffusion sheet according to claim 3, wherein,

When the size of the light emitting region is S inch and the average radius of the circle into which the three protrusions approaching on the back surface of the light emitting region enter is r μm, the following condition (1) or (2) is satisfied,

(1) When S is less than 7, r is less than or equal to 40

(2) When S is more than or equal to 7 and less than or equal to 15, r is more than or equal to 40 and less than or equal to 400.

6. A backlight unit for a liquid crystal display device is characterized by comprising:

A light guide plate or a light guide sheet for guiding the light incident from the end face to the surface side;

One or more light sources disposed along the end face of the light guide plate or light guide sheet; and

The light diffusion sheet according to any one of claims 1 to 5, which is disposed on a surface side of the light guide plate or the light guide sheet.