JP2007192861A - Light diffusing plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light diffusing plate which has high brightness and high light diffusion and which is light in weight. <P>SOLUTION: The light diffusion plate is obtained by molding a resin composition produced by blending one kind of aromatic vinyl resin (A), which is obtained by homopolymerization of an aromatic vinyl monomer and which has weight average molecular weight of 250,000 to 400,000, with one or more kinds of aromatic vinyl compound-unsaturated carboxylic acid ester compound copolymerized thermoplastic resins (B) which has weight average molecular weight of 250,000 to 400,000. The resin composition has phase separation structure, content of the unsaturated carboxylic acid ester compound (B) is 5 to 80 wt.%, whole light transmittance of the light diffusing plate is 40 to 70%, parallel light transmittance is 6.5% or less and thickness of the plate is 0.5 to 3mm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light diffusing plate that is widely used in applications such as lighting covers and displays.

  A light diffusing plate manufactured using resin as a main material and having light diffusibility can obtain a uniform luminance surface. For this reason, the light diffusing plate is widely used for applications such as an illumination cover, an internal illumination signboard, and a transmissive display. In particular, in recent years, the demand for a light diffusing plate as a surface light source body for a direct backlight of a liquid crystal display or a liquid crystal television is increasing.

  The light diffusing plate is required to scatter the light ray without making it travel as straight as possible and to suppress the light transmission loss due to the scattering as much as possible in order to suitably exhibit the above function.

  In order to express such required characteristics, the light diffusing plate has conventionally used inorganic particles such as calcium carbonate, barium sulfate, titanium oxide, silicon oxide, talc, mica, aluminum hydroxide, and magnesium oxide in a transparent resin. Light is scattered by the additive by the method of adding, the method of adding partially crosslinked polymer fine particles such as styrene polymer particles, acrylic polymer particles, siloxane polymer particles or the like, or the method of using these in combination. A technique has been adopted (see, for example, Patent Documents 1 to 3).

  In addition, methacrylic ester resins and amorphous cyclic olefin resins are used to produce light diffusing sheets with uniform diffusibility, excellent transparency, and bright diffused light. In addition, studies have been made with a sea-island structure (see, for example, Patent Documents 4 and 5).

  On the other hand, regarding the mixing of unsaturated carboxylic acid ester compound copolymerized thermoplastic resins, studies have been made on nacreous resins applying a phase separation structure (see Patent Document 6).

Japanese Patent No. 3195543 Japanese Patent No. 3195544 Japanese Patent No. 2512544 JP 2005-181825 A JP-A-10-111402 Japanese Patent Laid-Open No. 58-118841

  However, since the screen of liquid crystal displays or liquid crystal televisions has been enlarged and the diffusion plate for the backlight has been enlarged accordingly, when using inorganic particles having a large specific gravity, the problem of weight is an expensive crosslinking. When polymer fine particles are used, there is a problem of cost.

  Further, in the sea-island structure that has been studied so far, the size of the interface is too large, light diffusion is difficult, the backlight shielding property is low, and the diffusion property is not high.

  Further, the conventional diffusion sheet and the like are light and excellent in strength, and do not satisfy the function as a large light diffusion plate having high luminance and high light diffusion.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a light diffusing plate having high luminance, high light diffusion, light weight, and high strength.

  As a result of intensive studies, the present inventor has found that a light diffusing plate suitable for the above purpose can be obtained by using an interface between resins having different refractive indexes satisfying specific requirements as a light diffusing source. It came to be completed.

  That is, the present invention relates to one type of aromatic vinyl resin (A) obtained by homopolymerization and a copolymer thermoplastic resin (B) 1 obtained by copolymerizing an aromatic vinyl compound and an unsaturated carboxylic acid ester compound. A light diffusing plate obtained by molding a resin composition comprising a mixture of at least one type, wherein the content of the unsaturated carboxylic acid ester compound unit of the thermoplastic copolymer resin (B) is 5 to 80% by weight. The resin composition has a phase separation structure, the total light transmittance of the light diffusion plate is 40 to 70%, the parallel light transmittance is 6.5% or less, and the thickness of the plate is 0.5 to 3 mm. Is a light diffusion plate characterized by

  The aromatic vinyl resin (A) used in the present invention is obtained by homopolymerizing an aromatic vinyl compound. The copolymer thermoplastic resin (B) is obtained by copolymerizing an unsaturated carboxylic acid ester and an aromatic vinyl compound. The aromatic vinyl compound as a monomer used in the aromatic vinyl resin (A) and the copolymer thermoplastic resin (B) may be the same or different.

  Aromatic vinyl compounds include styrene, α-methylstyrene, p-methylstyrene, m-methylstyrene, o-methylstyrene, p-methoxystyrene, o-methoxystyrene, p-chlorostyrene, o-chlorostyrene, 4 -Vinylpyridine, 2-vinylpyridine and the like can be mentioned. Of these, styrene and α-methylstyrene are preferably used. In the copolymer thermoplastic resin (B), one or more of these aromatic vinyl compounds can be used.

  Examples of unsaturated carboxylic acid ester compounds as monomers used in the copolymerized thermoplastic resin (B) include methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, and acrylic acid. Examples include methyl, ethyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, t-butyl acrylate, and 2-ethylhexyl acrylate. Of these, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and t-butyl methacrylate are preferable, and methyl methacrylate is particularly preferable. These unsaturated carboxylic acid ester compounds can be used alone or in combination of two or more. Moreover, content of the unit derived from the unsaturated carboxylic ester compound in a copolymerization thermoplastic resin is the range of 5 to 80 weight%.

  The copolymer thermoplastic resin (B) includes a resin obtained by copolymerizing the above aromatic vinyl compound and an unsaturated carboxylic ester compound, and is preferably a styrene-methyl methacrylate copolymer resin.

  The aromatic vinyl resin (A) and copolymer thermoplastic resin (B) used in the present invention can be produced by polymerization by a known method. Both the weight average molecular weights of the aromatic vinyl resin (A) and the copolymer thermoplastic resin (B) are preferably 250,000 to 400,000. If it is less than 250,000, the fluidity becomes high and it becomes difficult to control the thickness of the sheet. Moreover, when it becomes 400000 or more, fluidity | liquidity is low, and the molding temperature must be raised and it colors.

  The resin composition used in the present invention is obtained by mixing one type of aromatic vinyl resin (A) and one or more types of copolymer thermoplastic resin (B). Among the resins mixed in the present invention, in at least two kinds of resins, the difference in the content of unsaturated carboxylic acid ester compound units is preferably 20 to 40% by weight. If it is less than 20% by weight, the compatibility is high and the refractive index difference is Δn ≦ 0.02, so that the diffusibility is deteriorated. On the other hand, if it exceeds 40% by weight, the compatibility is low and the transparency is deteriorated. The difference in the content of the unsaturated carboxylic acid ester compound unit is the content of the unsaturated carboxylic acid ester compound unit of the resin having the largest content of the unsaturated carboxylic acid ester compound unit in the copolymer thermoplastic resin (B). Usually, it becomes the difference in content as it is.

  In the resin composition used in the present invention, the aromatic vinyl resin (A) and the copolymer thermoplastic resin (B) need to be mixed in a finely dispersed state in order to satisfy the physical properties of optical properties and mechanical strength. There is. That is, it has a phase separation structure having a sea-island structure. Here, the dispersion state of each phase can be easily known by observing a sample selectively stained by a known method, that is, a staining method using ruthenium oxide, with a transmission electron microscope (for example, JS Trent, JIScheinbeim and PRCouchman, Macromolecules, 16,589 (1983) etc.).

  The length of the interface between the aromatic vinyl resin (A) and the copolymer thermoplastic resin (B) in the transmission electron micrograph image magnified 10,000 times of the resin composition is 1.5 μm or more per square μm. The thickness is often 2.0 μm or more. The upper limit is preferably 9 μm or less, and preferably 8 μm or less. If the thickness is 1.5 μm or less, the compatibility between the resins is not sufficient, so that each physical property cannot be satisfied. On the other hand, if it is 9 μm or more, the phase separation structure becomes too fine and the light diffusibility becomes insufficient. Usually, the phase separation structure of the resin composition has a sea-island structure, and the size of the island is about 0.2 to 5 μm, preferably about 0.5 to 1 μm in diameter. The length of this interface is calculated by dividing the length of the interface between the island and the sea in a cross section by the area of the cross section.

  The mixing ratio of the aromatic vinyl resin (A) and the copolymerized thermoplastic resin (B) is determined so as to have the phase separation structure. Preferably, it is in the range of 30 to 70% by weight of the aromatic vinyl resin (A) and 70 to 30% by weight of the copolymer thermoplastic resin (B).

  In the resin composition, if necessary, an ultraviolet absorber, an antioxidant, a heat stabilizer, a colorant, a plasticizer, a release agent, an antistatic agent, and a weathering aid, as long as the object of the present invention is not impaired. In addition, one or more additives such as an internal lubricant and an external lubricant may be blended.

  The manufacturing method of the light diffusing plate of the present invention is not particularly limited as long as the object of the present invention can be satisfied. For example, it can be obtained by molding the resin composition into a predetermined shape by injection molding, extrusion molding or the like.

  The light diffusing plate of the present invention has a thickness of 0.5 to 3 mm. If the thickness is less than 0.5 mm, the strength as a diffusion plate for backlight cannot be ensured, and if it exceeds 3 mm, the weight of the diffusion plate increases, so that it cannot cope with an increase in size and thickness.

  In the light diffusing plate of the present invention, the resin composition layer may be composed of a single layer, or may be composed of two or more layers.

  The light diffusion plate of the present invention has a total light transmittance of 40 to 70%, preferably 50 to 70%. When the total light transmittance is less than 40%, the amount of transmitted light is insufficient, so that a light diffusion plate with insufficient brightness is obtained. On the other hand, if the total light transmittance exceeds 70%, the light source is not concealed, resulting in a light diffusion plate with noticeable luminance unevenness caused by the image of the backlight light source. The parallel light transmittance is 6.5% or less, preferably 6% or less. If it exceeds 6.5%, the light source is not concealed, resulting in a light diffusion plate with noticeable luminance unevenness.

  Advantageously, the total light transmittance as a converted value of 2 mm thickness is 40 to 70%, preferably 50 to 70%. More advantageously, the total light transmittance as a converted value of 3 mm thickness is 40 to 70%, preferably 50 to 70%. The 2 mm thickness converted value or the 3 mm thickness converted value can be measured by molding the resin composition into a 2 mm or 3 mm thick plate shape, or can be calculated by calculating a conversion coefficient in advance. it can.

  The light diffusing plate of the present invention realizes high brightness and high light diffusion and light weight by using a diffusion source as an interface between resins. Depending on the above, various additives such as antistatic agents, antioxidants, flame retardants, light stabilizers, ultraviolet absorbers and fluorescent brighteners can also be used.

  The light diffusing plate of the present invention is used as a diffusing plate for a direct type backlight of a liquid crystal display or a liquid crystal television, for applications where a balance between light diffusing property and light transmitting property is required. It is suitable for applications that require

  Since the light diffusing plate according to the present invention can be obtained by blending resins having different refractive indexes at a specific ratio, the light diffusing plate is a light material having high luminance and high light diffusibility.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.

The physical properties of the light diffusing plate were evaluated by the following methods.
(1) Total light transmittance, parallel light transmittance It carried out based on the B method of JISK7105.
(2) Light diffusivity Four cold cathode tubes are arranged at intervals of 2 cm, a light diffusing plate sample is fixed in parallel on 10 mm, and the cold cathode tube, which is a light source, is visually transparent when it is transparent. The thing was shown by x.
(3) Interface length After ultra-thin sectioning, it was stained with ruthenium oxide. Photographing selectively dyed sample with a transmission electron microscope, and using a photograph magnified 10,000 times, stained aromatic vinyl resin part, unstained aromatic vinyl compound and unsaturated carboxylic acid The length of the interface of the sea-island structure with the copolymerized thermoplastic resin portion with the ester compound was measured for each island, and these were added together to calculate the total resin interface length per square μm.

(1) Preparation of resin composition Polystyrene (G-15L: manufactured by Toyo Styrene Co., Ltd., refractive index 1.592), methyl methacrylate-containing 20% styrene-methyl methacrylate copolymer resin (MS-200: Nippon Steel Corporation) Chemical Co., refractive index 1.573), methyl methacrylate containing 30% styrene-methyl methacrylate copolymer resin (MS-300: manufactured by Nippon Steel Chemical Co., Ltd., refractive index 1.564), containing methyl methacrylate Two or more types of 60% styrene-methyl methacrylate copolymer resin (MS-600: manufactured by Nippon Steel Chemical Co., Ltd., refractive index: 1.536) were kneaded and pelletized using a twin screw extruder. A thermoplastic resin composition was obtained.
(2) Preparation of diffusion plate by melt extrusion molding A diffusion plate having a thickness of 2 mm was prepared by melt extrusion molding of the thermoplastic resin composition.

  G-15L, MS-200, and MS-600 were mixed at a ratio of 50:40:10 (weight ratio), and first, this was kneaded and extruded into a pellet by a biaxial kneader (barrel temperature 200 to 240 ° C.). . Next, pellets were supplied to a single screw extruder having a T-die at the tip to obtain a sheet-like diffusion plate having a thickness of 2 mm. Table 1 shows the measurement results of various physical properties.

  A sheet was molded under the same conditions as in Example 1 except that G-15L and MS-300 were mixed at a ratio of 40:60 (weight ratio). Table 1 shows the measurement results of various physical properties.

  A sheet was molded under the same conditions as in Example 1 except that G-15L, MS-300, and MS-600 were mixed at a ratio of 50: 45: 5 (weight ratio). Table 1 shows the measurement results of various physical properties.

Comparative Example 1
A sheet was molded under the same conditions as in Example 1 except that MS-200 and MS-300 were mixed at a ratio of 80:20 (weight ratio). The evaluation results of the obtained light diffusion plate are shown in Table 1.

Comparative Example 2
A sheet was molded under the same conditions as in Example 1 except that G-15L, MS-200, and MS-600 were mixed at a ratio of 40:20:40 (weight ratio). The evaluation results of the obtained light diffusion plate are shown in Table 1.

Comparative Example 3
MS-200 was mixed with 1.0% by weight of silicone beads, melt-kneaded, and extruded to form a sheet having a thickness of 2 mm. The evaluation results of the obtained light diffusion plate are shown in Table 1.

Claims (5)

  Mix one type of aromatic vinyl resin (A) obtained by homopolymerization and one or more types of copolymer thermoplastic resin (B) obtained by copolymerizing an aromatic vinyl compound and an unsaturated carboxylic acid ester compound. A light diffusing plate obtained by molding the resin composition, wherein the content of the unsaturated carboxylic acid ester compound unit of the thermoplastic copolymer resin (B) is 5 to 80% by weight, and the resin composition is Light having a phase separation structure, light diffusion plate having total light transmittance of 40 to 70%, parallel light transmittance of 6.5% or less, and plate thickness of 0.5 to 3 mm Diffusion plate. The light diffusion plate according to claim 1, wherein the total length of the interface between the resins in the phase separation structure in the resin composition is 1.5 µm or more per 1 µm ² .   The light diffusing plate according to claim 1 or 2, wherein the aromatic vinyl resin (A) has a weight average molecular weight of 250,000 to 400,000 and the copolymerized thermoplastic resin (B) has a weight average molecular weight of 250,000 to 400,000.   The light diffusion according to any one of claims 1 to 3, wherein a difference in the content of unsaturated carboxylic acid ester compound units is 20 to 40% by weight in at least two kinds of resins forming the resin composition. Board.   The light diffusion plate according to any one of claims 1 to 4, wherein the aromatic vinyl resin (A) is polystyrene, and the copolymer thermoplastic resin (B) is a styrene-methyl methacrylate copolymer resin.