JP2016126023A - Light diffusion sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rugged substrate for obtaining organic or inorganic photoelectric devices, such as light-emitting diodes, that feature improved light extraction efficiency and is free from problems like short circuit-related failures and life reduction.SOLUTION: An optical diffusion sheet 1 is characterized in that; a proportion of particles 22c having diameters no less than 0.5 μm and less than 3 μm to a dry solid content of a light diffusion layer 20 is no less than 2 mass%; a proportion of particles 22a having diameters exceeding 9 μm is no more than 15 mass%; some of the particles 22 form protrusions T on a surface of the light diffusion layer 20; total protrusion area St, defined as a sum of area of the protrusions T, occupies 20-50% of diffusion layer area Sa defined as area of the light diffusion layer 20; and film thickness at recesses formed between the protrusions T is 3-12 μm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light diffusing sheet, and more particularly to a light diffusing sheet that can be suitably used as a component such as an LED lighting apparatus, a laser display device, and a liquid crystal display.

  In lighting fixtures such as LED lighting fixtures, various display devices such as laser display devices and liquid crystal displays, and other various optical devices, a light diffusion sheet for diffusing light from the light source is provided in the light path or outside. The light is disposed on the light emission side.

  For example, in an LED lighting apparatus, in order to reduce glare peculiar to an LED light source, a light diffusion sheet is adhered to a light-transmitting cover such as glass or resin. In a laser display device, laser light from a laser light source is incident on an end portion of a light guide plate, and characters, figures, and the like are optically displayed from the light guide plate to the outside. In that case, a light diffusion sheet is disposed on the end face of the light guide plate on which the laser light is incident so that the light is scattered in the light guide plate. Further, not limited to the case of using laser light, a light diffusion sheet is disposed on the surface (display surface side) of the light guide plate in various optical display devices.

  The light diffusion sheet has many transparent microparticles (generally called microspheres and beads) on the surface (usually one side) of a base sheet (film) made of a soft resin such as PET (polyethylene terephthalate). In other words, those having a diffusion layer fixed with a binder are widely used (Patent Document 1).

JP 2000-275410 A

Conventional light diffusing sheets are prone to problems such as scratches on the light diffusing surface and a decrease in in-plane uniformity of optical characteristics due to bead drop during processing and assembly operations. For this reason, the product yield tends to decrease, and there is a problem in production efficiency. Further, in consideration of scratch resistance, it has been difficult to obtain appropriate optical characteristics as a light diffusion sheet.
An object of the present invention is to provide a light diffusing sheet having high scratch resistance and suitable optical properties.

In order to achieve the above object, the present invention employs the following configuration.
[1] A substrate and a light diffusion layer provided on the substrate,
The light diffusion layer includes a binder and a large number of particles having a refractive index difference between the binder and 0.05 to 0.50,
The ratio of the particles having an inner particle diameter of 0.5 μm or more and less than 3 μm to the dry solid content of the light diffusion layer is 2% by mass or more,
The proportion of particles having an inner particle diameter of 9 μm or more in the dry solid content of the light diffusion layer is 15% by mass or less,
Some of the particles form convex portions on the surface of the light diffusion layer,
The convex area ratio that the total convex area St, which is the total area of the convex parts, occupies the diffusion layer area Sa, which is the area of the light diffusion layer, is 20 to 50%,
The light diffusion sheet, wherein the thickness of the concave portions formed between the convex portions is 3 to 12 μm.
[2] The light diffusion sheet according to [1], wherein a ratio of a total particle area Sp, which is a total area of the particles in a plan view, to the diffusion layer area Sa is 100% or more.
[3] The light diffusion sheet according to [1] or [2], wherein the particles are one or both of particles mainly composed of an amino resin and particles mainly composed of alumina.
[4] The light diffusing sheet according to any one of [1] to [3], wherein the binder includes an ionizing radiation curable resin as a main component.

  The light diffusion sheet of the present invention has high scratch resistance and appropriate optical properties.

It is sectional drawing which shows typically the light-diffusion sheet of this invention. It is an example of a measurement of the area of a convex part. It is an example of measurement of the layer thickness in a recessed part.

[Configuration of light diffusion sheet]
A light diffusion sheet 1 according to an embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the light diffusion sheet 1 has a light diffusion layer 20 on one surface of a substrate 10. The light diffusion layer 20 includes a binder 21 and a large number of particles 22.
In Figure 1, for convenience of explanation, as a particle 22, shown particle 22a particle diameter is _{r a,} particle 22b particle diameter is _{r b,} the three kinds of particles having a particle 22c particle size of _{r c} Yes.

The difference in refractive index between the binder 21 and the particles 22 is 0.05 to 0.50, preferably 0.08 to 0.48, and more preferably 0.10 to 0.45.
When the difference in refractive index is equal to or greater than a predetermined value, a light scattering effect is obtained at the interface between the binder 21 and the particles 22. Moreover, since the difference in refractive index is equal to or less than a predetermined value, the binder 21 and the particles 22 can be easily selected from known materials. In addition, the refractive index in this invention is a refractive index in wavelength 589.3nm.

It is necessary to use particles 22 having different particle sizes in combination while balancing them. The proportion of the light diffusion layer 20 in the dry solid content is 2% by mass or more of particles having a particle size of 0.5 μm or more and less than 3 μm, and 15% by mass or less of particles having a particle size of 9 μm or more. is there.
Particles having a particle diameter of 0.5 μm or more and less than 3 μm are used for enhancing the scratch resistance of the light diffusion layer 20 and increasing the light diffusion power. The ratio of the light diffusion layer 20 to the dry solid content is 2% by mass or more, preferably 8% by mass or more, and more preferably 12% by mass or more.
In addition, since excessively containing particles having a particle size of 0.5 μm or more and less than 3 μm among the particles 22 causes a decrease in front illuminance and total light transmittance, it depends on the content of particles having other particle sizes. However, it is good to contain 30 mass parts or less with respect to the dry solid content of the light-diffusion layer 20 as a standard.

Among the particles 22, particles having a particle diameter exceeding 9 μm are used for increasing the front illuminance and the total light transmittance of the light diffusion layer 20. If this is excessively contained, the scratch resistance tends to be lowered. Therefore, the ratio of the light diffusion layer 20 to the dry solid content needs to be 15% by mass or less, and preferably 13% by mass or less. More preferably, it is 10 mass% or less.
In addition, if the below-mentioned convex part area ratio can be made more than predetermined value, the ratio of the particle | grains exceeding 9 micrometers of particle diameters may be zero mass%.

Of the particles 22, the content of particles having a particle diameter of 3 μm or more and 9 μm or less needs to be balanced with the content of the two kinds of particles, and the content should not be specified in general.
If the two kinds of particles have scratch resistance, light diffusibility, front illuminance, and total light transmittance within the desired ranges, it is not necessary to contain particles having a particle diameter of 3 μm or more but 9 μm or less. Since it becomes easy to adjust to, it is preferable to contain.
When particles having a particle diameter of 3 μm or more and 9 μm or less are contained, the proportion of the light diffusion layer in the dry solid content is preferably 10% by mass or more, more preferably 15% by mass or more, and more preferably 20% by mass or more. More preferably.
Moreover, it is preferable that it is 45 mass% or less, It is more preferable that it is 41 mass% or less, It is further more preferable that it is 38 mass% or less.
When the amount of particles having a particle diameter of 3 μm or more and 9 μm or less is increased, the front illuminance and the total light transmittance tend to be increased, and when excessively contained, the scratch resistance tends to be decreased.
In addition, about a light diffusivity, it can adjust to a desired light-diffusion angle by content balance with a particle | grain with a particle diameter of 0.5 micrometer or more and less than 3 micrometers.

  The ratio of the particles in the respective particle diameter ranges to the dry solid content of the light diffusion layer 20 can be adjusted by the mass ratio of the dry solid content of the binder 21 and the particles 22 and the particle size distribution of the particles 22. The particle size distribution of the particles 22 can be adjusted depending on what particle size distribution is used as particles (hereinafter referred to as “raw material particles”) used as the raw material of the coating liquid for forming the light diffusion layer 20. When a plurality of raw material particles having different particle size distributions are used in combination, it is easy to adjust the particle size distribution of the particles 22 in the coating liquid forming the light diffusion layer 20. In particular, when raw material particles having a narrow particle size distribution are used in combination, it is easy to adjust the particle size distribution of the particles 22 in the coating liquid.

Part of the particles 22 forms a convex portion T on the surface of the light diffusion layer 20. The convex portion T is recognized as an island-like substantially circular convex portion having a diameter of 2.5 μm or more by photographing the surface shape of the light diffusion layer 20 using an ultra-deep shape measuring microscope and performing circular particle analysis with image analysis software. Part.
Since the island-like substantially circular convex portion and the other boundary become dark, the image analysis software can recognize the boundary based on the contrast difference between light and dark. In the present invention, a region having a diameter of 2.5 μm or more among the regions surrounded by the boundary recognized as described above is defined as the convex portion T. Specific conditions for extracting the convex portion T will be described in detail in Examples described later.
The region having a diameter of 2.5 μm or more is defined as the convex portion T. In the case of a region having a diameter of less than 2.5 μm, the difference between the top (highest portion) of the convex portion and the height of the concave portion is slight, and the substantial convex portion. It was because it was not considered.

  It was examined by a preliminary test using a light diffusing sheet prepared in the same manner as in Example 1 to determine how much diameter it can be regarded as a substantial convex portion. In the preliminary test, about 20 areas each having a diameter of 1 μm to less than 1.5 μm, 1.5 μm to less than 2 μm, 2 μm to less than 2.5 μm, 2.5 μm to less than 3 μm, 3 μm to less than 3.5 μm, Using the ultra-deep shape measurement microscope, the area in the boundary and the profile in the thickness direction around it were measured. As a result, in the region having a diameter of less than 2.5 μm, the difference between the top (highest portion) of the convex portion and the height of the concave portion is less than 0.5 μm, and it was difficult to consider as a substantial convex portion. An area of 5 μm or more was defined as a convex portion T.

The ratio (hereinafter referred to as “convex area ratio”) of the total convex area St, which is the total area of the convex parts T, to the diffusion layer area Sa, which is the area of the light diffusion layer 20, is 20 to 50%. It is preferably 20 to 40%, and more preferably 20 to 30%.
Here, the area of the convex portion T is an area when the convex portion T is viewed in a plan view from a direction perpendicular to the substrate surface. The area of the convex portion T, the total convex portion area St per diffusion layer area Sa, and the convex portion area ratio (St / Sa × 100 [%]) can be automatically calculated by the image analysis software.

When the convex area ratio is equal to or greater than a predetermined value, it is easy to achieve both a sufficient diffusion angle and front illuminance. Moreover, it is easy to make abrasion resistance high because a convex part area ratio is below a predetermined value.
The convex area ratio can be adjusted by the mass ratio of the dry solid content of the binder 21 and the particles 22 and the ratio of relatively large particles in the particles 22.

The layer thickness in the recessed part formed between the said convex parts is 3-12 micrometers, it is preferable that it is 4-12 micrometers, and it is more preferable that it is 5-12 micrometers.
In the present invention, the concave portion means a region between the convex portions, that is, a region other than the convex portions. As shown in FIG. 1, the surface of the concave portion is slightly raised in the vicinity of the convex portion T. In the present invention, the layer thickness in the concave portion is a value obtained by averaging the smallest values of the layer thicknesses of the light diffusion layer 20 in a certain concave region, such as d ₁ and d ₂ in FIG. That is, it is the thickness of the light diffusion layer 20 in a portion where there is no bulging effect in the vicinity of the convex portion T.

The layer thickness in the concave portion can be adjusted by the dry solid content of the binder 21 per unit area, the mass ratio of the dry solid content of the binder 21 and the particles 22, and the ratio of relatively large particles in the particles 22. As the convex area ratio increases, the layer thickness in the concave portion tends to decrease. As shown in FIG. 1, since the binder 21 covers the surface of the particle 22 in the convex portion T, the larger the area of the convex portion T, the smaller the binder existing in the portion (concave portion) other than the convex portion T. is there.
When the layer thickness in the recess is equal to or greater than a predetermined value, the particles 22 are sufficiently fixed. In addition, when the layer thickness in the concave portion is equal to or less than a predetermined value, it is easy to obtain a sufficient convex area ratio without using a large amount of coarse particles having a particle diameter exceeding 9 μm.

The ratio (hereinafter referred to as “coverage”) of the total particle area Sp, which is the total area of the particles 22 in plan view, to the diffusion layer area Sa, which is the area of the light diffusion layer 20, is 100% or more. It is preferably 110% or more, more preferably 120% or more, and particularly preferably 130% or more. When the coverage is equal to or higher than a predetermined value, it is easy to increase the scratch resistance.
Further, the upper limit of the coverage is not particularly limited, but is preferably 350% or less, more preferably 300% or less, further preferably 280% or less, and particularly preferably 160% or less. preferable.

Particle 22a particle diameter is _{r a,} particle 22b particle diameter is _{r b,} particle 22c particle size of _{r c,} each in plan view the area Sp _a, Sp _b, by the following equation to Sp _c Can be represented.
Sp _a = π (r _a / 2) ²
Sp _b = π (r _b / 2) ²
Sp _c = π (r _c / 2) ²

Temporarily, there are only three types of particles 22: particles 22a, particles 22b, and particles 22c, and the numbers of particles 22a, particles 22b, and particles 22c existing per diffusion layer area Sa are N _a , N _b , and N _c , respectively. Then, a coverage is calculated | required with the following formula | equation.
Coverage (%) = Sp / Sa × 100
= {Sp _a × N _a + Sp _b × N _b + Sp _c × N _c } / Sa × 100
= 100π {(r _a / 2) ² × N _a + (r _b / 2) ² × N _b + (r _c / 2) ² × N _c } / Sa
Actually, since the particles 22 include particles having various particle diameters r _x , the ratio of the total value Sp _x × N _x of the entire particle size distribution obtained as the total particle area Sp can be obtained. That's fine.

[Constituent material of light diffusion sheet]
The base material 10 supports the light diffusing layer 20 and transmits light at the same time. The base material 10 only needs to be flexible enough to conform to various shapes. The material is not particularly limited, but usually polyethylene terephthalate, polyethylene naphthalate, polypropylene, polycarbonate, polyester, polyamide or the like is used. The thickness of the substrate 10 is not particularly limited, but is preferably 12 to 250 μm.

The binder 21 is not particularly limited as long as it has good adhesion to the particles 22 and has a predetermined refractive index difference from the particles 22.
For example, polyester resin, acrylic resin, polyvinyl chloride resin, ethylene-vinyl acetate copolymer resin, vinyl chloride-vinyl acetate copolymer resin, polyvinyl acetal resin, epoxy resin, melamine resin, phenol resin, and copolymer resins thereof Can be used.

The binder 21 preferably includes a curable resin as a main component, more preferably includes an ionizing radiation curable resin as a main component, and further preferably includes an ultraviolet curable resin as a main component.
The ionizing radiation curable resin is formed by curing a curable composition containing a photopolymerizable prepolymer, a photopolymerizable monomer, and a photopolymerization initiator by ionizing radiation irradiation. By using an ionizing radiation curable resin having a high coating film hardness, the light diffusion layer 20 becomes more difficult to be damaged.

The photopolymerizable prepolymer contained in the curable composition undergoes radical polymerization and / or cationic polymerization when a functional group introduced into the skeleton is irradiated with ionizing radiation. Among these, those that are cured by radical polymerization are preferred because of their high curing speed.
As the radical polymerizable prepolymer, an acrylic prepolymer having an acryloyl group is preferable.
Examples of the acrylic prepolymer include urethane acrylate, epoxy acrylate, melamine acrylate, and polyester acrylate.

The photopolymerizable monomer contained in the curable composition is preferably an acrylic monomer having an acryloyl group.
Examples of acrylic monomers include monofunctional acrylic monomers such as 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, butoxypropyl acrylate, 1,6-hexanediol acrylate, neopentyl glycol diacrylate, diethylene glycol acrylate, One or more types of bifunctional acrylic monomers such as polyethylene glycol diacrylate and hydroxypivalate ester neopentyl glycol acrylate, polyfunctional acrylic monomers such as dipentaerythritol hexaacrylate, trimethylpropane triacrylate and pentaerythritol triacrylate Can be mentioned.

Examples of the photopolymerization initiator contained in the curable composition include those that undergo radical polymerization by cleavage, those that undergo radical polymerization by extracting hydrogen, and those that undergo cationic polymerization by generating ions. A photoinitiator is suitably selected according to the prepolymer and monomer to be used.
Examples of radical photopolymerization initiators include benzoin ether, ketal, acetophenone, and thioxanthone. Examples of the cationic photopolymerization initiator include diazonium salts, diaryliodonium salts, triarylsulfonium salts, triarylbililium salts, benzylpyridinium thiocyanate, dialkylphenacylsulfonium salts, and dialkylhydroxyphenylphosphonium salts. These radical photopolymerization initiators and cationic photopolymerization initiators can be used alone or in combination.

The particles 22 are the main body for diffusing light incident on the light diffusion layer from the substrate 10 side. The particles 22 are not particularly limited as long as they have good adhesion to the binder 21 and have a predetermined refractive index difference from the binder 21.
The shape of the particle 22 is not limited, and a particle shape used for a general light diffusion sheet can be arbitrarily applied. However, a sphere (such as a true sphere or an elliptical sphere close thereto) is preferable.

Examples of the material for the particles 22 include inorganic pigments such as alumina, white alumina, and silica, and synthetic resin materials such as a cured product of an amino resin. Examples of amino resins include urea resins, melamine resins, guanamine resins, and co-condensation resins thereof.
These materials may be used alone or in combination of two or more. When used in combination, the particles 22 may be formed of a material in which two or more materials are mixed, or other materials are unevenly distributed with respect to one material, such as coating one material with another material. Thus, the particles 22 may be formed.
Among these, spherical composite cured melamine resin particles whose surface is coated with inorganic compound particles or in which inorganic compound particles are unevenly distributed in the vicinity of the surface are preferred because colloidal silica is the particle surface. Spherical composite cured melamine resin particles unevenly distributed in the vicinity are more preferable (see JP 2002-327036 A).

When the binder 21 is obtained by curing a curable composition containing an acrylic prepolymer, an acrylic monomer, and a radical photopolymerization initiator, the particles 22 are stabilized by ionizing radiation. Therefore, one or both of particles mainly composed of amino resin and particles mainly composed of alumina are preferable. In particular, it is preferable that the particle is mainly composed of an amino resin, or a particle composed mainly of an amino resin and a particle composed mainly of alumina are used in combination.
The ratio of one or both of the amino resin and alumina as the main component to the whole particle is preferably 5% by mass or more, and more preferably 10% by mass or more.
Among them, spherical composite cured melamine resin particles whose surface is coated with inorganic compound particles or inorganic compound particles are unevenly distributed near the surface are preferable, and colloidal silica is more preferably spherical composite cured melamine resin particles unevenly distributed near the particle surface ( Japanese Patent Laid-Open No. 2002-327036).

[Characteristics of light diffusion sheet]
The pencil hardness of the light diffusion sheet 1 based on JIS K5400-1990 is preferably H or more from the viewpoint of scratch resistance.
The preferable diffusion angle of the light diffusion sheet 1 varies depending on the purpose of use, but in order to have light diffusion, it is preferably more than 20 °, and more preferably 22 ° or more. Further, from the viewpoint of ensuring frontal illuminance, it is preferably 50 ° or less, more preferably 45 ° or less, and further preferably 40 ° or less.

The preferable front illuminance of the light diffusing sheet 1 varies depending on the purpose of use, but is preferably 65 or more, more preferably 70 or more, and further preferably 75 or more.
The total light transmittance of visible light (380 nm to 800 nm) of the light diffusion sheet 1 is preferably 65% or more, more preferably 70% or more, further preferably 80% or more, and particularly preferably 85% or more.

[Production method of light diffusion sheet]
The method for producing the light diffusion sheet 1 of the present embodiment is not particularly limited, but it is usually desirable to apply the following method.
That is, a particle 22 for forming the light diffusion layer 20, an uncured binder 21, a solvent (for example, toluene, methyl ethyl ketone) and the like are mixed in advance to prepare a coating solution for the light diffusion layer 20. The light diffusion layer 20 may be formed by applying the working liquid to the base material 10 by an appropriate coating means such as a bar coater, and then drying and curing.

[Mechanism of action]
The light diffusion sheet of the present invention has high scratch resistance and appropriate optical properties. The reason why such an effect is obtained is considered as follows.
That is, in the light diffusing sheet of the present invention, since the convex area ratio is not more than a predetermined value, the particles are sufficiently fixed by the binder. In addition, if there are many coarse particles having a particle diameter of more than 9 μm, even if a few particles fall off, a large damage occurs at a time, but such damage is reduced by the small number of coarse particles. In addition, since the coverage is high and the entire light diffusion layer is almost completely filled with particles, particles subjected to frictional force are difficult to move. In particular, when small particles having a particle diameter of less than 3 μm are present, it is considered that even if the large particles forming the convex portions receive a frictional force, the small particles can prevent the movement. Further, if not only small particles of less than 3 μm but also particles having a particle size of 3 to 9 μm are present, it is considered that even if particles having a particle size of more than 9 μm are present, the movement is more easily prevented.

  Further, since the convex area ratio is not less than a predetermined value and the small particles not forming the convex part also have a refractive index difference from the binder, a sufficient diffusion angle can be obtained. In addition, when a small particle of less than 3 μm is excessively added, only the diffusion angle becomes large and the front illuminance decreases, but in the present invention, the particle of 3 μm or more is used to such an extent that the convex area ratio becomes a predetermined value or more. Therefore, both a sufficient diffusion angle and front illuminance can be achieved.

  The light diffusing sheet of the present invention may be provided with a back layer for improving antistatic, curl prevention and handling on the surface where the light diffusing layer is not provided. Moreover, in order to be able to stick to a lighting fixture, a display, etc., you may provide an adhesive layer and an adhesive bond layer in a back surface. Further, an optical element such as a prism may be provided on the back surface in order to adjust the optical path.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to the following Example.

[Manufacture of light diffusion sheet]
The coating liquid having the composition shown in Table 1 is applied to one side of a transparent polyethylene tephrate film (“A4300” manufactured by Toyobo Co., Ltd., thickness 100 μm) so that the coating amount of the coating layer after drying is 8 g / m ^2. The sample was coated with a bar coater and dried at 160 ° C. to form a light diffusion layer before curing. Thereafter, ultraviolet irradiation (curing condition: high pressure mercury lamp 1000 mJ / cm ² ) treatment was performed to cure the light diffusion layer, and the light diffusion sheets of the examples and comparative examples were obtained.

Abbreviations and the like described in Table 1 are as follows. In the following, the particle size distribution of each particle in Table 1 (hereinafter referred to as “raw material particle”) is determined by a laser analysis method. Cumulative volume of 10%, 50%, a particle diameter corresponding to 90% as _{D 10,} D _{50, D 90,} and the average particle diameter a particle size of 50 vol% _{(D 50).}

Binder resin A: UV curable urethane acrylate resin (PC6840FT manufactured by DIC), refractive index 1.52 after curing.
Binder resin B: Dry curable acrylic resin (MH-101 manufactured by Fujikura Kasei Co., Ltd.), refractive index 1.49 after curing.
Binder solvent A: ethyl acetate.
Binder solvent B: methyl ethyl ketone.

OB20: manufactured by Nissan Chemical Industries, Ltd. Opto beads (registered trademark) 2000M, melamine resin / silica composite particles, refractive index 1.65, average particle diameter (D ₅₀ ) = 2.1 μm, D ₁₀ = 1.2 μm, D ₉₀ = 4.1 μm.
OB65: manufactured by Nissan Chemical Industries, Ltd., Opto beads 6500M, melamine resin / silica composite particles, refractive index 1.65, average particle size (D ₅₀ ) = 6.1 μm, D ₁₀ = 4.2 μm, D ₉₀ = 8.8 μm .
OB105: manufactured by Nissan Chemical Industries, Ltd., Opto beads 10500M, melamine resin / silica composite particles, refractive index 1.65, average particle diameter (D ₅₀ ) = 11.1 μm, D ₁₀ = 7.3 μm, D ₉₀ = 15.9 μm .

Alumina: manufactured by Sumitomo Chemical Co., Ltd., Advanced Alumina AA-1.5, Alumina crystal type particles, refractive index 1.76, average particle diameter (D ₅₀ ) = 2.6 μm, D ₁₀ = 1.3 μm, D ₉₀ = 5 0.0 μm.
PMMA: Mixture of MBX-5, MBX-12 and MBX-20 (MBX-5 / MBX-12 / MBX-20) which are polymethyl methacrylate particles (refractive index 1.49) manufactured by Sekisui Plastics Co., Ltd. 65/20/15). MBX-5 has an average particle diameter (D50) = 5 μm, D10 = 2.5 μm, and D90 = 7.5 μm. MBX-12 has an average particle size (D50) = 12 μm, D10 = 7.2 μm, and D90 = 18.1 μm. MBX-20 has an average particle diameter (D50) = 20 μm, D10 = 9.0 μm, and D90 = 26.0 μm.
Surface conditioner: BYK, BYK-UV3500, silicon surface conditioner.
Rheology control agent: BYK-410 manufactured by BYK.

  Table 2 shows the light diffusion sheets of each of the examples and comparative examples. The P / B ratio obtained by the following method, the particle content in the total solid content, the convex area ratio, the coverage, the layer thickness in the concave (simply Described as “layer thickness”). Moreover, about the light-diffusion sheet of each Example and a comparative example, the pencil hardness evaluated by the following method, a diffusion angle, front illuminance, and a total light transmittance are shown.

[P / B ratio]
The ratio of the total mass parts of the raw material particles in the coating liquid to the mass parts (dry solid content) of the binder resin was defined as the P / B ratio.

[Particle content in total solids]
From the particle size distribution of each raw material particle and the ratio (mass%) of each raw material particle (dry solid content) in the coating liquid (dry solid content), the particle size distribution of the entire coating liquid is calculated, and the light diffusion layer is dried. The ratio (mass%) of each of the particles having a particle diameter of 0.5 μm or more and less than 3 μm, particles having a particle diameter of 3 to 9 μm, and particles having a particle diameter exceeding 9 μm in the solid content was determined.

[Convex area ratio]
Using an ultra-deep shape measuring microscope VK-8500 (manufactured by Keyence Corporation), the surface shape was photographed at 400 times with respect to an area of 50 mm × 50 mm on the surface of the light diffusion layer. Next, circular particle analysis mode (particle brightness: dark, extraction method: automatic / manual, processing method: low speed, noise filter: yes, measurement using Asahi Kasei Engineering's dedicated image analysis software A Image-kun (registered trademark) A portion recognized as an island-like substantially circular convex portion having a diameter of 2.5 μm or more in an area of 50 mm × 50 mm in a diameter range: 2.5 μm or more, setting condition of light / dark threshold: 10 and particle overlap degree: 0) The total area was defined as the total convex area St in an area of 50 mm × 50 mm. This was converted into a value per 1 m ² and multiplied by 100 to obtain the convex area ratio (%).
FIG. 2 is an example of an image showing that A image-kun surrounds a portion recognized as an island-like substantially circular convex portion with a circle.

[Coverage]
The particle area Sp _x when a particle having a particle diameter r _x is viewed in plan is obtained by Sp _x = π (r _x / 2) ² . The number N _x of particles having a particle diameter r _x contained in the coating liquid used per unit area (1 m ² ) of the diffusion layer area Sa is the coating amount (g / m ² ) and the particles in the coating liquid. The total solid content concentration (mass%), specific gravity (g / cm ³ ), and particle size distribution of particles in the coating liquid.
With respect to the coating solution used for the diffusion layer area Sa of 1 m ² , the total of the values obtained for Sp _x × N _x of the entire particle size distribution was taken as the total particle area Sp, and the coverage was obtained from the following formula.
Coverage (%) = Sp (m ² ) / Sa (m ² ) × 100 = Sp × 100

[Layer thickness in recess]
The light diffusion sheet was cut into small pieces (width 5 mm × length 20 mm), and the small pieces were immersed in liquid nitrogen for 10 minutes for freezing treatment. The frozen small sample was cut along the width direction with a feather shaving blade to obtain a light diffusion layer cross section having a length of about 5 mm.
The cross section of the obtained light diffusion layer was photographed with an optical microscope OPTIPHOT (registered trademark) 200 (manufactured by Nikon) at a reflected light mode of 100 times (light reflected by the cross section using an objective lens of 100 times).
Image analysis made by Asahi Kasei Engineering Co., Ltd., with the smallest layer thickness in each of the five recesses (portions where no protrusions are formed on the surface) randomly extracted from the cross-sectional image of the light diffusion layer. Measurement was carried out in the shape analysis mode using dedicated software A image-kun (registered trademark), and the average was obtained.
FIG. 3 is an image showing an example of the smallest layer thickness in the concave region of the light diffusion layer cross-sectional image.

[Pencil hardness]
The pencil hardness was determined based on JIS K5400-1990. Specifically, the base material side of the light diffusing sheet was fixed on a 2 mm thick glass plate with a cellophane tape and measured using a pencil hardness meter COTES (manufactured by Coating Tester Kogyo Co., Ltd.).

[Diffusion angle and front illumination]
An LED light source with an irradiation angle of 3.5 degrees is arranged at a position 50 mm away from the base material on the side where the light diffusion layer is not provided, and the light axis has the highest light emission luminance. Incident light was perpendicular to the surface.
An illuminance curve was obtained by measuring the transmitted and scattered light at that time using a goniometer GENESISA (registered trademark) Gonio / FFP (manufactured by Genesia). The illuminance curve is a plotted curve with the horizontal axis as the light emission angle and the vertical axis as the relative illuminance.
Specifically, the relative illuminance when the illuminance of light perpendicularly emitted from the light diffusion sheet (the light emission angle of this light is 0 °) is 1, the X direction and the Y direction orthogonal to the X direction. A light emission angle of −90 ° to 90 ° was measured at 1 ° intervals to obtain an illuminance curve.
The full width at half maximum in each of the X direction and the Y direction (light emission angle range in which the relative illuminance is 1/2 or more) was obtained, and the average of these values was taken as the diffusion angle.
Further, for each of the X direction and the Y direction, the peak value in the entire transmitted / scattered light of the light transmitted and scattered within the range of the light emission angle of ± 90 ° was defined as the front illuminance.

[Total light transmittance]
Using a haze meter HM-150 (manufactured by Murakami Color Research Laboratory), the light of a standard D65 light source defined by CIE (International Commission on Illumination) is incident from the side of the light diffusion sheet where the light diffusion layer is not provided. A total light transmittance of visible light (380 nm to 800 nm) was obtained by a method defined in JIS K7361.

As shown in Table 2, the light diffusion sheets of the examples all had high pencil hardness. Moreover, the front illuminance was sufficiently maintained while having a diffusion angle exceeding 20 °. Further, the total light transmittance was a value with no problem as a light diffusion sheet.
On the other hand, in the light diffusion sheets of Comparative Examples 1 to 4, 6, and 7 having a low convex area ratio, either the diffusion angle or the front illuminance was insufficient. Further, Comparative Example 5 having a low convex area ratio and not containing particles having a particle diameter of 3 μm or more and 9 μm or less had insufficient pencil hardness.
Moreover, sufficient pencil hardness was not obtained for the light diffusion sheets of Comparative Examples 8 to 10 having a high content of particles having a particle diameter exceeding 9 μm. In addition, the light diffusion sheets of Comparative Examples 10 and 11 that did not contain particles having a particle diameter of 0.5 μm or more and less than 3 μm did not have sufficient pencil hardness and had a small diffusion angle. Moreover, sufficient pencil hardness was not obtained for the light diffusion sheets of Comparative Examples 12 and 13 having a high convex area ratio. Further, Comparative Example 14 in which the refractive index difference between the particles and the binder is small requires a very high convex area ratio in order to obtain a certain diffusion angle. Even though the diffusion angle is not sufficient, the pencil hardness is low. It was very inferior.

  DESCRIPTION OF SYMBOLS 1 ... Light-diffusion sheet, 10 ... Board | substrate, 20 ... Light-diffusion layer, 21 ... Binder, 22 ... Particle | grains, T ... Convex part

Claims (4)

A substrate, and a light diffusion layer provided on the substrate,
The light diffusion layer includes a binder and a large number of particles having a refractive index difference between the binder and 0.05 to 0.50,
The ratio of the particles having an inner particle diameter of 0.5 μm or more and less than 3 μm to the dry solid content of the light diffusion layer is 2% by mass or more,
The proportion of particles having an inner particle diameter of 9 μm or more in the dry solid content of the light diffusion layer is 15% by mass or less,
Some of the particles form convex portions on the surface of the light diffusion layer,
The convex area ratio that the total convex area St, which is the total area of the convex parts, occupies the diffusion layer area Sa, which is the area of the light diffusion layer, is 20 to 50%,
The light diffusion sheet, wherein the thickness of the concave portions formed between the convex portions is 3 to 12 μm.   2. The light diffusion sheet according to claim 1, wherein a ratio of a total particle area Sp, which is a total area of the particles in a plan view, to the diffusion layer area Sa is 100% or more.   The light diffusion sheet according to claim 1, wherein the particles are one or both of particles mainly composed of an amino resin and particles mainly composed of alumina.   The light diffusion sheet according to any one of claims 1 to 3, wherein the binder contains an ionizing radiation curable resin as a main component.