JP2667876B2 - Light diffusing plastic - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a light-diffusing plastic suitable for a member for the purpose of diffusing light such as a lighting cover, a lighting signboard, glazing, various displays or a transparent screen. is there.

(Prior art) Conventionally, as a light diffusing material such as a lighting cover and a display screen, a material in which inorganic or organic transparent fine particles are dispersed in a transparent plastic such as an acrylic resin or a styrene resin, or a transparent plastic surface is used. Materials and the like that have been roughened by some method are known, and it is also known to use them in combination. In recent years, many efforts have been made for higher performance materials, especially as the need for light-diffusing plastics, which require high performance such as screens for rear projection televisions, increases. The performance desired for these light diffusing materials is as wide as possible,
It is to spread light uniformly and brightly. However, since the amount of light emitted from the light source is constant, these requirements are in conflict with each other. Therefore, in practice, if necessary, change the concentration of the diffusing material, etc.
It is selected and used so as to have the most preferable brightness and spread. In a method of obtaining a light diffusing material by dispersing a light diffusing material in a transparent plastic, as an index for obtaining a suitable combination of the light diffusing material and the transparent plastic, the particle diameter of the light diffusing material fine particles and the light diffusing material fine particles are mainly used. And the difference in the refractive index of transparent plastic has been used. For example, Japanese Patent Publication No. 39-10515 discloses that the difference in refractive index from the transparent resin of the substrate is
A method is described in which crosslinked fine particles having a diameter of 0.05 to 0.3 and an average diameter of 0.5 to 5 μ are used as a light diffusing material.
JP-A-48-44333 describes a method of blending a crystal powder having a particle diameter of 10 μm or less having a refractive index larger than that of a base resin by 0.05 to 0.5, and further disclosed in JP-A-60-139758. JP-A-60-184559 proposes a refractive index difference of 0.02 to 0.1 and a particle size of 4 to 10 μ in addition to those having a refractive index difference of 0.02 to 0.1 and a particle size of 4 to 10 μ. 4762
As disclosed in Japanese Patent Application Laid-Open Publication No. H10-157, fine particles having a particle diameter of 4 to 50 μm and a refractive index of 0.02 to 0.1 larger than that of the base resin,
A method of using fine particles of 0.02 to 0.1 small is also described. Other Japanese Patent Publication No. 60-21662 or Japanese Unexamined Patent Publication No. 62-174
In Japanese Patent No. 261, the refractive index is 0.01 to 0.1 than that of the base resin.
A method of dispersing fine particles having a small average particle size of 1 to 10 μ has also been proposed. In addition, there are a wide variety of combinations of the base resin and the light diffusing material which are specifically described, and it is difficult to describe all of them. For example, when methacrylic resin (refractive index: 1.492) is used as the base resin, crystalline silica is used. (Refractive index 1.54), amorphous silica (refractive index 1.4
6), calcium carbonate (refractive index 1.58), aluminum hydroxide (refractive index 1.57), glass beads GB-210 (refractive index 1.5
21) Other refractive indices are unknown, other than glass sphere (refractive index 1.46), calcium fluoride (refractive index 1.43), lithium fluoride (refractive index 1.39), barium sulfate (refractive index 1.64), alumina powder (refractive index 1.7) However, magnesium oxide, titanium oxide,
Talc and crosslinked polymers are used, and polystyrene resin (refractive index 1.59) and polyvinyl chloride resin (refractive index 1.
Various inorganic fine particles are also used when the base resin is 55) or a polycarbonate resin (refractive index: 1.59). As described above, when the techniques that have been conventionally disclosed are arranged, a number of methods have been proposed for the refractive index difference and the particle size, but those having a very wide range have been proposed in various ways. At present, it is difficult to determine which combination is preferable. In fact, additional tests have shown that these existing technologies are still inadequate against the ever increasing demands for screens and the like for rear projection televisions in recent years.

(Problems to be Solved by the Invention) The present invention, in such a cluttered technical flow,
Providing a light diffusion material that has excellent light diffusion performance, that is, the maximum luminance Go is as large as possible, and the half-value angle (the angle at which the luminance decreases to half) is as large as possible, and the transmitted light does not have a reddish tint. It is intended to do so.

(Means for Solving the Problems) According to the present invention, in a plastic having a refractive index Ns, the following formula (I) and formula (II) 0.01 ≦ Np−Ns <0.02 (I) 7 ≦ d ≦ (Np− Ns) × 1200 (II) This is achieved by dispersing transparent fine particles having an average particle diameter d micron and a refractive index Np satisfying (II), and among the transparent fine particles, particles having voids therein are contained in all the transparent fine particles. It is more preferably achieved by setting the proportion to be 3% by weight or less, and / or by setting the proportion of the non-spherical particles of the above transparent fine particles in the total transparent fine particles to 10% by weight or less. When a methacrylic resin or a copolymer resin of methyl methacrylate (hereinafter referred to as MMA) and styrene is used as a substantially transparent plastic, MMA, styrene, alkyl acrylate and polyfunctional (meth) acrylate are used as fine particles. The above object can be achieved by using the crosslinked copolymer fine particles.

(Function) The present inventors have found that the difference in refractive index between the base transparent resin and the transparent fine particles,
The present invention has been accomplished by comprehensively examining the relationship between the particle size of transparent fine particles and the diffusion performance. That is, in the present invention,
The difference Np between the refractive index Ns of the base transparent resin and the refractive index Np of the transparent particles
-It is necessary that Ns is 0.01 or more and less than 0.02. Such a small difference in refractive index has been excluded in the conventional method on the assumption that the difference is too small and the diffusing ability is inferior. The present inventors have prepared various flat plates in which the difference in the refractive index between the base resin and the transparent fine particles and the particle size of the transparent fine particles are changed, and an angle analysis of the brightness of the light emitted from the front of the flat plate when a parallel ray is incident on the flat plate. Was measured, and the gain G was calculated from the illuminance on the flat plate surface and each luminance by the following formula.

The gain shows the maximum value in front of the diffuser plate, and as the angle formed with the normal line of the diffuser plate increases, the gain gradually decreases as shown in FIG. The gain maximum value and be represented by G ₀ is called the peak gain, the gain is to be represented by α is referred to as half-value angle angle is 1/2 of the peak gain, object of the present invention is a G ₀ and α either Is also to increase. However, in general, the light diffusion plate is not always used in the form of a simple flat plate, and often achieves its purpose by giving various lens shapes or by using another light diffusion imparting means such as surface treatment. , G ₀ , α, etc. are their types,
Since the performance varies depending on the degree, it is not appropriate to express the objective performance of the present invention by the values of G ₀ and α by such a measuring means. However, when a large combination of G ₀ and α measured with a flat light diffusion plate is employed in this way, it can be experimentally confirmed that excellent performance can be obtained even in other shapes. Mainly adopts a flat plate method that makes it easy to compare and evaluate the light diffusion performance of the light diffusion material. When the concentration of the light diffusing material is changed in a fixed combination of the substrate resin and the light diffusing material, G ₀ decreases and α increases with the increase of the density as shown in FIG.
Therefore, as a means for selecting a good light diffusing material, the concentration of the light diffusing material is changed to select a concentration at which a constant _G0 is obtained, and a material having a large half-value angle α at that concentration is a preferable light diffusing plastic. I thought. As a result of the experiment, it was surprisingly found that a refractive index difference of less than 0.02 resulted in a light-diffusing plastic having a remarkably high α. However, if the difference in refractive index is less than 0.01, the concentration of light diffusivity necessary for obtaining sufficient diffusivity becomes undesirably large, because it is not preferable. Another requirement is that the average particle size be in an appropriate range in order to obtain sufficient light diffusivity and large α with such a small difference in refractive index. First of all, it is necessary that the average particle diameter does not fall below 7 μm. When the average particle diameter does not reach 7 μm, when the particle concentration is low, a large amount of light travels in a limited solid angle in the straight direction, and this light is reddish. Increasing the particle concentration reduces the straight reddish light, but this anomalous light does not disappear until G ₀ is very low. This light is perceived as so-called scale when observed by human eyes.

The human eye can usually be distinguished at a viewing angle of 1 minute (= 1/60 degrees), and measuring this invisibility phenomenon with an optical measuring device (luminance meter) is comparable to that of the human eye. It is necessary to use a device having a viewing angle of.
Conventionally, there is a case in which there is no correlation between the luminance angle distribution data and the scale in literatures and the like, which is considered to be due to the difference in the viewing angle between the human eye and the luminance meter. At present, the visibility of commercially available luminance meters is 2, 1, 1/3, 0.2 degrees, etc., all of which are larger than 1/60 degrees. Considering that the viewing angle is as small as possible and stable measurement can be performed, the present inventors have developed a Minolta luminance meter nt1 manufactured by Minolta Co., Ltd. with a viewing angle of 1/3 °.
Measurements were taken using / 3 ° P. As a result, it was found that intense light concentrated in an extremely narrow angle range could be measured in response to the phenomenon of skein, and was reflected in the values of G ₀ and α to some extent. When the average particle diameter of the fine particles is less than 7 μm, it is necessary to increase the concentration of the light diffusing material until the maximum luminance becomes extremely low in order to prevent skein, and to obtain a light diffusion plate having no skein and a large gain. Not suitable for On the other hand, in general, the degree of light diffusion decreases as the difference in refractive index decreases,
Also, the smaller the average particle diameter, the smaller. Especially when the difference in the refractive index from the base resin is small as in the present invention, sufficient diffusion performance cannot be brought within a practical range unless the particle size is strictly controlled. In the present invention, the range is preferably such that the average particle diameter is (Np−Ns) × 1200 μm or less, preferably (Np−Ns) × 1000 μm or less as a function of the refractive index Ns of the base resin and the refractive index Np of the fine particles. If the average particle size exceeds this range, the concentration of the fine particles to be used becomes too large, so that it is not only economically disadvantageous in terms of production technology, but also a half value as compared with the case of a particle size within the range of the present invention. Corner becomes smaller.

There are various definitions for the average particle diameter, and the average particle diameter in the present invention is represented by a weight median diameter, that is, a particle diameter at which the cumulative weight fraction becomes 50% in a weight cumulative curve. Methods for measuring the particle size distribution include a Coulter counter method, a sedimentation method, a counting method using a micrograph or an electron micrograph, and the like, and any method may be used.

By applying the above-described range of the refractive index difference and the particle size, it has become possible to obtain a light-diffusing plastic having better performance than before. The present invention further provides a light-diffusing plastic having better performance by specifying the shape of the light-diffusing fine particles. The first characteristic is that the ratio of hollow fine particles in all fine particles is 3% by weight or less, and the second is that the ratio of non-spherical fine particles in all fine particles is 10% by weight. % Or less. Glass beads are generally well-known spherical transparent fine particles, but according to the study of the present inventors, by applying the refractive index difference and the particle size range as described above, it is possible to use glass beads. It has been found that the performance can be improved, but the performance is inferior to the crosslinked plastic beads having the same particle diameter and refractive index as described below. In addition, there are various crushed inorganic powders as fine particles substantially containing no hollow portion, and by using these in a range that satisfies the above conditions, an excellent light diffusing plastic can be obtained, but fine particles having a spherical shape can be obtained. who is, as compared with such amorphous particles, the half value angle α increases in the diffusion plate at a concentration giving the same peak gain G _0, further preferred. In the case of glass beads, etc., crushed fine particles and spherical fine particles are generally mixed, and in order to reduce the ratio of hollow beads, the performance is improved by mixing crushed fine particles with ordinary glass beads. Although it is possible and effective in practice, it is preferable that the ratio of the spherical fine particles is high as described above. The non-spherical fine particles are preferably 30% or less, more preferably 10% or less, and most preferably substantially free.

In order to satisfy the above-mentioned conditions of the difference in refractive index, particle diameter and shape, it is advantageous and preferable to use a polymer, particularly a crosslinked polymer, as the transparent fine particles. Spherical, non-hollow fine particles do not exist much other than the polymer, and in order to keep the refractive index difference within the above range, the refractive index of the beads is fixed, so that the refractive index of the base resin is suitable for the beads. It is necessary to adjust it, which affects the overall physical properties of the diffuser plate, which is not always advantageous. On the other hand, when a polymer, particularly a crosslinked polymer, is used as the transparent fine particles, it is possible and advantageous to arbitrarily design it according to the refractive index of the base resin. Generally, light-diffusing plastics
It is made by melt-kneading and extruding the light diffusing fine particles and the substrate resin, or by pressing, or, in some cases, dispersing the light diffusing fine particles in a polymerizable monomer or a partially polymerized polymerizable monomer syrup and polymerizing. . Therefore, it is necessary that the light diffusing fine particles do not change into an undesired shape due to melting, dissolution, etc. during the process of producing the light diffusing plastic. For this purpose, it is possible to use a method in which the molecular weight of the polymer is sufficiently high, but it is more preferable to provide appropriate crosslinking.

As the base resin of the light diffusing plastic, a transparent resin having a high light transmittance, that is, a methacrylic resin, a polystyrene resin, a polycarbonate resin, an epoxy resin, a polyvinyl chloride resin, or the like is used. Among them, methacrylic resin is a preferable resin because it is excellent in transparency, durability and physical properties. Normally, methacrylic resin has heat resistance, moldability,
In order to improve durability, etc., in addition to MMA, alkyl acrylate is copolymerized, a lubricant, an ultraviolet absorber is added, but the methacrylic resin here refers to all such resins mainly composed of MMA . When the base resin is a methacrylic resin, it is preferable to use, as the light diffusing agent, fine particles that are a cross-linking resin containing MMA, styrene, and a polyfunctional (meth) acrylate as essential components. Since ordinary methacrylic resin has a refractive index of about 1.49, the refractive index of the light diffusing agent fine particles is preferably about 1.50 to 1.51. Of course, the refractive index may be slightly higher or lower depending on the type of methacrylic resin as the base resin. In the present invention, the polyfunctional (meth) acrylate is a compound having two or more acryl groups or methacryl groups in one molecule, such as ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, tetraethylene glycol. (Poly) ethylene glycol di (meth) acrylate such as di (meth) acrylate and nonaethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, 1,3-butylene glycol di (meth) acrylate, tetramethylene glycol Glycol di (meth) acrylates such as di (meth) acrylate, hexamethylene di (meth) acrylate, neopentyl glycol di (meth) acrylate, and trimethylolpropane tri (meth) acrylate, There data pentaerythritol tetra (meth) polyhydric polyalcohols such as acrylates (meth) acrylate. These polyfunctional (meth) acrylates play a role in preventing the transparent fine particles from impairing the shape of the fine particles in the process of producing the light diffusing plastic, and 2% to 60% of the total monomers constituting the transparent fine particles. Is appropriately selected within the range.

When a copolymer of styrene and MMA is used as the base resin, the refractive index increases, so the effect as a lens increases,
May be advantageous. Even in such a case, it is desirable to use a crosslinked copolymer similar to the transparent fine particles used for the methacrylic resin. However, it may be adjusted to an appropriate refractive index according to the refractive index of the styrene-MMA resin. The polymer beads described above can be synthesized by the turbidity polymerization method. For example, it can be manufactured by using polyvinyl alcohol as a dispersant, finely dispersing the monomer by a disperser or the like, and then polymerizing, filtering, washing and drying.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1 39 parts of MMA, 3 parts of ethyl acrylate, 8 parts of styrene, 50 parts of ethylene glycol dimethacrylate, 0.6 part of lauroyl peroxide and 0.2 part of lauryl mercaptan were mixed with polyvinyl alcohol (PVA-420, manufactured by Kuraray Co., Ltd.).
It was mixed with 400 parts of water containing 4% and dispersed by a lab disperser. 70 ° C in a nitrogen atmosphere while stirring this liquid
For 120 minutes and 95 ° C. for 100 minutes. The resulting dispersion was repeatedly washed with excess water and water, and finally washed with methanol and then dried.

The particle size distribution of the crosslinked fine particles thus obtained was measured using a particle size distribution meter (MICRIN PHOTOSIZER SKA-5 manufactured by Seishin Enterprise).
000), the median weight diameter was 10.48 μm. The specific gravity at this time was the value (1.2003) measured separately by preparing a polymer having the same composition. The refractive index was 1.5078 as measured by a method of observing the movement of Pecke lines with a microscope.

Also, the beads were substantially all spherical and free of hollow particles.

Using the crosslinked beads, methacrylic resin plates having a thickness of 1 mm and containing beads at various weight fractions were prepared by the following method.

A mixture of 11.9 parts by weight of ethyl acrylate and 128.1 parts of MMA dissolves 60 parts by weight of acrylic resin beads (F-1000B manufactured by Kyowa Gas Chemical Industry Co., Ltd., F-1000B, refractive index: 1.4920) containing 8.5% of ethyl acrylate, and the acrylic resin syrup made. The required amount of the crosslinked beads was dispersed in this syrup while taking care not to aggregate. Dissolve 0.02 parts by weight of azobisisobutyronitrile in this, put it in two glass plates with gaskets welded, degas, and heat at 80 ° C for 2 hours and further at 120 ° C for 2 hours to polymerize. did. The thickness was adjusted to be 1 mm. After the completion of the polymerization, an acrylic plate containing fine particle beads was taken out from the glass plate.

The collimated light of the halogen lamp was incident on the fine-particle-containing acrylic resin plate (light diffusion plate) thus obtained from the rear. A luminance meter (Minolta luminance meter nt1 / 3 ° P) was installed 1.5 m ahead of the light diffusion plate, and the luminance was measured. The position of the luminance meter was shifted, the angle was changed, and the operation of measuring the same portion was repeated, and the angular distribution of luminance was measured. On the other hand, the illuminance at the position of the light diffusing plate was separately measured with an illuminometer, and the gain was calculated by the formula (VI) from the ratio of the luminance and the illuminance. Table 1 shows the values of the respective densities, where the frontal gain is G ₀ , and the angle at which the gain is 1/2 of G ₀ is α.

The results are plotted in a log-logarithmic graph in which the vertical axis is G ₀ and the horizontal axis is α, as shown in FIG. From this figure, G ₀
At a concentration of 20, α is 8.2 °. This value is larger than a value shown in a comparative example described later, and this light diffusion plate is an excellent material having a large diffusion half-value angle at a constant peak gain.

Example 2 In the same manner as in Example 1, MMA 41.3%, styrene 3.5%,
Crosslinked beads comprising 50% ethylene glycol dimethacrylate, 5.0% ethyl acrylate, and 0.2% lauryl mercaptan having a weight median diameter of 8.55 microns and a refractive index of 1.5035 were obtained. Using these crosslinked beads, a 1 mm thick acrylic resin light diffusion plate having various concentrations was prepared in the same manner as in Example 1.
Optical performance was measured.

The results are shown in Table 2. These G ₀ and α are plotted in FIG. From this figure, when the gain is 20, α is about 8.2 °
And good diffusion properties are shown.

Examples 3 and 4, Comparative Examples 1 and 2 Light diffusing plates were prepared using various fine particles and base resin as shown in Table 2 and the light diffusing performance was measured. Table 2 also shows the measurement results. As can be seen by comparing Example 3 with Comparative Example 1 and Example 4 with Comparative Example 2, even if the same fine particles are used, the difference in refractive index is within the range of the present invention.
Light diffusion performance can be improved. Examples 1, 2,
The results of Example 4 were slightly inferior to those of Example 3 because the fine particles of Example 4 contained about 10% of hollow fine particles.

(Effects of the Invention) According to the present invention, it has become possible to obtain a light-diffusing plastic having a high front luminance and a large diffusion half-value angle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the angular distribution of gain at each crosslinked bead concentration in Example 1, the vertical axis being a logarithmic scale,
The horizontal axis is shown on an equally spaced scale. FIG. 2 shows Examples 1-4.
7 is a graph showing peak gains and half-value angles of Comparative Examples 1 and 2, and both the vertical and horizontal axes are shown on a logarithmic scale.

Claims (7)

(57) [Claims] 1. In a transparent plastic having a refractive index Ns, the following formula (I) and formula (II): 0.01 ≦ Np−Ns <0.02 (I) 7 ≦ d ≦ (Np−Ns) × 1200 (II) A light-diffusing plastic obtained by dispersing transparent fine particles having an average particle size d (microns) and a refractive index Np satisfying the following. 2. The light diffusing plastic according to claim 1, wherein the ratio of the particles having pores in the transparent fine particles to the total amount of the fine particles is 3.0% by weight or less. 3. The light diffusing plastic according to claim 1 or 2, wherein the proportion of non-spherical particles in all the fine particles is 10% by weight or less. 4. The light-diffusing plastic according to claim 1, wherein the fine particles are a polymer. 5. The light-diffusing plastic according to claim 1, wherein the fine particles are a crosslinked polymer. 6. The method according to claim 1, wherein the transparent plastic is methacrylic resin and the fine particles are a copolymer of methyl methacrylate, styrene, alkyl acrylate and polyfunctional (meth) acrylate. 4. The light diffusing plastic according to item 3. 7. The method according to claim 1, wherein the transparent plastic is a copolymer resin of methyl methacrylate and styrene, and the fine particles are a copolymer of methyl methacrylate, styrene, alkyl acrylate and polyfunctional (meth) acrylate. The light-diffusing plastic according to item 1, 2, or 3.