JP2667878B2 - 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 sign, a glazing, various displays or a transmission screen. is there.

(Prior Art) Conventionally, as light diffusing materials for lighting covers, display screens, etc., inorganic or organic transparent fine particles are dispersed in transparent plastic such as acrylic resin or styrene resin, or the surface of transparent plastic is Materials and the like that have been roughened by some method are known, and it is 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,
It has excellent light diffusion performance. That maximize the maximum intensity G _0, and as large as possible of the half-value angle (an angle which the luminance decreased to 1/2), moreover that the transmitted light and to provide a light diffusing material never reddish Is.

(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): 0.02 ≦ | Np−Ns | ≦ 0.04 (I) 7 ≦ d Transparent spherical fine particles having an average particle diameter d (micron) satisfying <30 (II), a refractive index Np, and having a ratio of 3% by weight or less in all fine particles having pores therein. And the proportion of non-spherical particles in the transparent fine particles in all the transparent fine particles is set to 10% by weight or less. When methacrylic resin or methyl methacrylate (hereinafter referred to as MMA) and styrene copolymer resin is used as a substantially transparent plastic, MMA and polyfunctional (meth) acrylate, or MMA, styrene and polyfunctional ( The above object can be achieved by using fine particles of a cross-linked copolymer having a (meth) acrylate as a constituent component.

(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 absolute difference | Np-Ns | between the refractive index Ns of the base transparent resin and the refractive index Np of the transparent fine particles needs to be 0.02 or more and 0.04 or less. The present inventors have prepared various flat plates in which the refractive index difference between the base resin and the transparent fine particles and the particle size of the transparent fine particles are changed, and a parallel light beam is incident from the rear of the flat plate, and the angular distribution of the brightness of the light emitted forward. Was measured, and a gain G was calculated from the illuminance on the flat plate surface and each luminance by the following equation.

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 concentration 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. The experimental results found that α is obtained a considerably higher light diffusing plastic in G ₀ constant when the refractive index difference is 0.02 to 0.04.
Further, in the present invention, the average particle size of the light diffusing transparent fine particles needs to be 7 μm or more and less than 30 μm. If the average particle size is outside this range, α at a constant G ₀ will be small. Especially 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-ahead 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. Currently, commercially available luminance meters have viewing angles of 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 is possible, the present inventors have developed a Minolta luminance meter nt1 manufactured by Minolta with a viewing angle of 1/3 °.
Measurements were made at / 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 When the average particle size is 30 μm or more, the concentration of the fine particles to be used becomes too large, so that not only is it economically disadvantageous in terms of production technology, but also the half-value angle is smaller than in the case of a particle size within the range of the present invention. Get 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.

In the present invention, factors relating to the shape of the light diffusing fine particles are very important. That is, it is necessary that the light-diffusing fine particles have a proportion of hollow fine particles in the fine particles of 3% by weight or less, preferably substantially free of central fine particles and substantially spherical. 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. In the case of crosslinked polymer fine particles, the refractive index can be adjusted by the polymer composition, and by an appropriate polymerization method, substantially no hollow portion can be obtained, and substantially spherical fine particles can be obtained. . Generally, a light diffusing plastic is prepared by dissolving and kneading light diffusing fine particles and a base resin and then extruding or pressing.
Alternatively, in some cases, it is produced by a method of dispersing 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 fine particles which are a cross-linking resin containing MMA, styrene and a polyfunctional (meth) acrylate as a light diffusion agent. Since ordinary methacrylic resins have a refractive index of about 1.49, the refractive index of the light diffusing agent fine particles is preferably about 1.51 to 1.53. 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, for example, 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 In addition to glycol di (meth) acrylates such as di (meth) acrylate, hexamethylene di (meth) acrylate, neopentyl glycol (meth) acrylate, trimethylolpropane tri (meth) acrylate, pen There pentaerythritol tetra (meth) polyhydric alcohols polyvalent acrylates such as (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 a suspension 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 34 parts of MMA, 16 parts of styrene, 50 parts of ethylene glycol dimethacrylate and 0.2 part of lauroyl peroxide were added to 0.4% of polyvinyl alcohol (PVA-420 manufactured by Kuraray Co., Ltd.).
The resulting mixture was mixed with 300 parts of water and dispersed by a laboratory disperser. This liquid is stirred at 70 ° C in a nitrogen atmosphere.
Heated for 20 minutes at 95 ° C for 50 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 (Micron Photosizer SKA-5 manufactured by Seishin Enterprise).
000), the weight median diameter was 10.58 μm. The specific gravity at this time was a value (1.1891) separately measured by preparing a polymer having the same composition. The refractive index was 1.5174 as measured by a method of observing the movement behavior of the Becke line 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.

Acrylic resin beads obtained by copolymerizing 8.5% of ethyl acrylate in a mixture of 11.9 parts by weight of ethyl acrylate and 128.1 parts of MMA (Kyowa Gas Chemical Industry F-1000B, refractive index: 1.4920) 60
An acrylic resin syrup was prepared by dissolving parts by weight. The necessary amount of the above-mentioned crosslinked beads was dispersed in the syrup while being careful not to aggregate. Dissolve 0.02 parts by weight of azobisisobutyronitrile in this, put it in two glass plates equipped with gaskets, 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 operation of measuring the same portion was repeated by shifting the position of the luminance meter and changing the angle, and the angular distribution of luminance was measured. on the other hand,
The illuminance at the position of the separately-coated light diffusion plate was measured by an illuminometer, and the gain was calculated from the ratio of the luminance and the illuminance by the equation (VI). 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 7.8 °. 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.

Examples 2 to 4 and Comparative Examples 1 and 2 In the same manner as in Example 1, light diffusing plastics 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.

Example 5 The same glass beads GB as used in Comparative Example 1
-210 (manufactured by Toshiba Barotini) with specific gravity consisting of a mixture of 1,1,2,2-tetrabromoniethane and monochlorobenzene
When the glass beads were immersed in 2,424 liquids and centrifuged, the glass beads were separated into those that floated on the upper layer and those that settled on the lower part. Each phase was separated, washed, dried, put into a liquid of methylene iodide and carbon tetrachloride having a refractive index of about 1.521, and observed with an optical microscope. Although the boundaries of the beads were seen slightly through, the glass beads at the floating portion were observed with the core portion being black and clear, similar to the case where the hollow beads were similarly observed separately. The weight of the beads in the floating portion thus obtained was 4.7% of the weight of the beads before separation. Table 2 shows the results of the light diffusing plate obtained by the same method as in Example 1 using the glass beads in the settled portion obtained by this method as a light diffusing agent. Compared to the case of Comparative Example 1, the peak gain is
When it became 20, α was large and the performance as a light diffusing agent was improved.

(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, wherein the vertical axis is a logarithmic scale, and the horizontal axis is an equally spaced scale. FIG. 2 is a graph showing the peak gains and half-value angles of Examples 1 to 5 and Comparative Examples 1 and 2, both the vertical axis and the horizontal axis being shown on a logarithmic scale.

Claims (6)

(57) [Claims] 1. In a transparent plastic having a refractive index Ns, the following formula (I) and the following formula (II) are satisfied: 0.02 ≦ | Np−Ns | ≦ 0.04 (I) 7 ≦ d <30 (II) A light-diffusing plastic in which transparent spherical fine particles are dispersed, which has a refractive index Np and an average particle diameter d (μm) and has a content of voids of 3% by weight or less. 2. The light diffusing plastic according to claim 1, wherein the fine particles are polymer fine particles. 3. The light diffusing plastic according to claim 1, wherein the fine particles are crosslinked polymer fine particles. 4. The light diffusion according to any one of claims 1 to 3, wherein the proportion of non-spherical fine particles among all fine particles is 10% by weight or less. Plastic. 5. The light according to claim 1, wherein the transparent plastic is a methacrylic resin, and the fine particles are a copolymer of methyl methacrylate, styrene and a polyfunctional (meth) acrylate. Diffusible plastic. 6. The transparent plastic is a copolymer resin of methyl methacrylate and styrene, and the fine particles are a copolymer of methyl methacrylate and polyfunctional (meth) acrylate or a copolymer of methyl methacrylate, styrene and polyfunctional (meth) acrylate. The light diffusing plastic according to any one of claims 1 to 4, which is a polymer.