Light diffusion plate and direct type surface light source unit

Abstract

The present invention provides a light diffusion plate which contains a styrene resin composition, while having excellent long-term durability and strength.　The present invention provides a light diffusion plate which contains a styrene resin composition, wherein: the styrene resin composition contains a styrene resin (A), an antioxidant (B) and a light diffusing agent (C); the antioxidant (B) contains at least one of a phosphorus antioxidant (B-1), a phenolic antioxidant (B-2) and a phosphorus / phenolic antioxidant (B-3); relative to 100 parts by mass of the styrene resin, a total of from 0.001 to 0.5 part by mass of the phosphorus antioxidant (B-1) and the phosphorus / phenolic antioxidant (B-3), a total of from 0.001 to 0.5 part by mass of the phenolic antioxidant (B-2) and the phosphorus / phenolic antioxidant (B-3), and from 0.1 to 10 parts by mass of the light diffusing agent (C) are contained; and the content of t-butyl catechol contained in the styrene resin composition is from 0.1 to 10 μg / g.

Description

[Technical field]

[0001] The present invention relates to a light diffusion plate and a direct type surface light source unit. [Background technology]

[0002] The light diffusion plate is used in a direct type liquid crystal display device. A direct type liquid crystal display device is a display device including a liquid crystal cell and a backlight device arranged directly below the liquid crystal cell. The backlight device uses LEDs as a light source, and a light diffusion plate is arranged in front of the backlight device. A styrene-based resin composition is sometimes used as a resin constituting the light diffusion plate (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2010-134461 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in order to meet the demand for higher brightness, LEDs used as light sources have become more powerful, and the long-term durability of conventional light diffusion plates made of styrene-based resin compositions has become insufficient due to the increased heat generation, etc. Also, there are cases where the strength does not meet the requirements.

[0005] The present invention has been made in consideration of the above problems, and provides a light diffusion plate containing a styrene-based resin composition, which has excellent long-term durability and strength. [Means for solving the problem]

[0006] According to the present invention, there is provided a light diffusion plate containing a styrene-based resin composition, the styrene-based resin composition containing a styrene-based resin (A), an antioxidant (B), and a light diffusion agent (C), the antioxidant (B) containing at least one of a phosphorus-based antioxidant (B-1), a phenol-based antioxidant (B-2), and a phosphorus-phenol-based antioxidant (B-3), the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) being 0.001 to 0.5 parts by mass in total, the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) being 0.001 to 0.5 parts by mass in total, and the light diffusion agent (C) being 0.1 to 10 parts by mass, relative to 100 parts by mass of the styrene-based resin, and the content of t-butylcatechol contained in the styrene-based resin composition is 0.1 to 10 μg / g.

[0007] As a result of intensive research, the present inventors have found that when the contents of t-butylcatechol, phosphorus-based antioxidant, phenol-based antioxidant, phosphorus-phenol-based antioxidant, and light diffusing agent in a styrene-based resin composition are within predetermined ranges, the composition has excellent long-term durability and strength, and has completed the present invention.

[0008] Various embodiments of the present invention will be described below. The embodiments described below can be combined with each other. Preferably, the phosphorus-based antioxidant (B-1) is at least one selected from the group consisting of 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, tris(2,4-di-tert-butylphenyl)phosphite, 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane, tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4'diylbisphosphonite, and bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite. Preferably, the phenol-based antioxidant (B-2) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylenebis(oxyethylene)bis[3-(5-tert-butyl- a light diffusion plate comprising at least one member selected from the group consisting of pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane. Preferably, the phosphorus-phenol-based antioxidant (B-3) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine. Preferably, in the light diffusion plate, the light diffusing agent (C) is at least one selected from the group consisting of acrylic polymer crosslinked particles, styrene polymer crosslinked particles, and siloxane polymer crosslinked particles. Preferably, the styrene-based resin (A) has a weight average molecular weight (Mw) of 200,000 to 400,000 and a ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 1.0 to 3.0. Preferably, the light diffusion plate is a light diffusion plate for an LED light source. Preferably, a direct type surface light source unit has the above light diffusion plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Also, each feature can be an invention independently.

[0010] 1. Light diffusion plate The light diffusion plate according to one embodiment of the present invention is a light diffusion plate containing a styrene-based resin composition. The thickness of the light diffusion plate is not limited, but is, for example, 1 to 3 mm. In addition, the light diffusion plate may be laminated with an ultraviolet absorbing layer containing an ultraviolet absorbing agent or a light stabilizer.

[0011] The light diffusion plate according to one embodiment may have an antistatic agent applied to its surface. By applying the antistatic agent, the light diffusion plate can be used for a long period of time without a decrease in brightness because adhesion of dust and the like due to static electricity is suppressed after the light diffusion plate is attached to a backlight device. In addition, the light diffusion plate of the present invention may have a fine uneven shape such as embossing on both sides or one side, or may have a semicircular or elliptical lens shape or a prism shape on both sides or one side.

[0012] <Styrene-based resin composition> The styrene resin composition contains a styrene resin (A), a system antioxidant (B), and a light diffusing agent (C). The content of t-butylcatechol contained in the styrene resin composition is 0.1 to 10 μg / g. By setting the content in such a range, the long-term durability and strength are excellent. The content of t-butylcatechol is specifically 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, and 10 μg / g, and may be within a range between any two of the numerical values ​​exemplified here. If the content of t-butylcatechol is less than 0.1 μg / g, the effect of suppressing oxidation deterioration during thermoforming processing is small, so the strength of the light diffusion plate is reduced, and if the content of t-butylcatechol exceeds 10 μg / g, t-butylcatechol itself becomes a coloring component, and not only the long-term durability of the light diffusion plate is reduced, but also the strength is reduced.

[0013] <Styrene-based resin (A)> The styrene-based resin (A) is a resin obtained by polymerizing a styrene-based monomer as a main component, specifically, a resin containing more than 50% of a styrene-based monomer. The styrene-based monomer is an aromatic styrene-based monomer, such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, m-methylstyrene, ethylstyrene, pt-butylstyrene, etc., alone or in a mixture of two or more kinds, preferably styrene. The styrene-based resin (A) may also be a copolymer obtained by copolymerizing a monomer copolymerizable with a styrene-based monomer with a styrene-based monomer, and examples of the monomer copolymerizable with a styrene-based monomer include acrylic acid monomers such as acrylic acid and methacrylic acid, cyanide vinyl monomers such as acrylonitrile and methacrylonitrile, acrylic monomers such as butyl acrylate, ethyl acrylate, methyl acrylate, and methyl methacrylate, α,β-ethylenically unsaturated carboxylic acids such as maleic anhydride and fumaric acid, and imide monomers such as phenylmaleimide and cyclohexylmaleimide. These monomers can be used alone or in combination of two or more.

[0014] The weight average molecular weight (Mw) of the styrene resin (A) is preferably 200,000 to 400,000, more preferably 200,000 to 290,000. The ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the styrene resin (A) is preferably 1.0 to 3.0, more preferably 1.5 to 2.5. The ratio (Mz / Mw) of the Z average molecular weight (Mz) to the weight average molecular weight (Mw) of the styrene resin (A) is preferably 1.0 to 2.0. By setting it in such a range, both the moldability and the strength of the light diffusion plate can be achieved. When the weight average molecular weight (Mw) is 200,000 or more, the strength is good, and when it is 400,000 or less, the moldability is good. When the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 1.0 or more, the moldability is good, and when it is 3.0 or less, the strength is good. When the ratio (Mz / Mw) of the Z-average molecular weight (Mz) to the weight-average molecular weight (Mw) is 1.0 or more, the moldability is good, and when it is 2.0 or less, the strength is good.

[0015] <Antioxidant (B)> The antioxidant (B) includes at least one of a phosphorus-based antioxidant (B-1), a phenol-based antioxidant (B-2), and a phosphorus-phenol-based antioxidant (B-3).

[0016] The styrene resin composition contains a total of 0.001 to 0.5 parts by mass of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) relative to 100 parts by mass of the styrene resin, and preferably 0.05 to 0.3 parts by mass. By setting the content in such a range, the light diffusion plate has excellent long-term durability and strength. Specifically, the total content of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) is 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or 0.5 parts by mass relative to 100 parts by mass of the styrene resin, and may be within a range between any two of the numerical values ​​exemplified here.

[0017] The total content of the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) is 0.001 to 0.5 parts by mass, preferably 0.02 to 0.3 parts by mass, and more preferably 0.05 to 0.15 parts by mass. By setting the content in such a range, the light diffusion plate has excellent long-term durability and strength. Specifically, the total content of the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) is 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or 0.5 parts by mass relative to 100 parts by mass of the styrene-based resin, and may be within a range between any two of the numerical values ​​exemplified here.

[0018] The phosphorus-based antioxidant (B-1) is a phosphorous acid ester that does not have a phenolic hydroxyl group in its basic structure, and is preferably a phosphorous acid ester that is a trivalent phosphorus compound. The phosphorus-based antioxidant (B-1) is, for example, at least one selected from 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, tris(2,4-di-tert-butylphenyl)phosphite, 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane, tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4'diylbisphosphonite, and bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphorous acid ester.

[0019] The phenol-based antioxidant (B-2) is an antioxidant that has a phenolic hydroxyl group in the basic skeleton and is not a phosphorous ester. The phenol-based antioxidant (B-2) is, for example, at least one selected from octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylene bis(oxyethylene)bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane, etc.

[0020] The phosphorus-phenol-based antioxidant (B-3) is a phosphorous acid ester having a phenolic hydroxyl group in the basic skeleton, preferably a phosphorous acid ester which is a trivalent phosphorus compound having a phenolic hydroxyl group in the basic skeleton. The phosphorus-phenol-based antioxidant (B-3) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepin, etc.

[0021] <Light Diffuser (C)> The styrene resin composition contains 0.1 to 10 parts by mass of the light diffusing agent (C) relative to 100 parts by mass of the styrene resin, preferably 0.5 to 3 parts by mass, and more preferably 0.8 to 1.5 parts by mass. By setting the content in such a range, the light diffusion plate has excellent long-term durability and strength. The content of the light diffusing agent (C) is specifically 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, or 10 parts by mass relative to 100 parts by mass of the styrene resin, and may be within a range between any two of the numerical values ​​exemplified here.

[0022] The light diffusing agent (C) can be any particle having a different refractive index from that of the styrene resin composition and having the effect of diffusing incident light. For example, organic particles such as styrene polymer crosslinked particles, acrylic polymer crosslinked particles, and siloxane polymer crosslinked particles, and inorganic particles such as glass beads, silica particles, aluminum hydroxide particles, calcium carbonate particles, barium sulfate particles, titanium oxide particles, and talc can be used. Among them, at least one selected from acrylic polymer crosslinked particles, styrene polymer crosslinked particles, and siloxane polymer crosslinked particles is preferable. The average particle diameter of the light diffusing agent (C) is preferably 1 to 20 μm. If the average particle diameter of the light diffusing agent (C) is less than 1 μm, the hue may deteriorate, and if it exceeds 20 μm, the light diffusing property may not be sufficiently imparted.

[0023] <Other additives> Various additives can be blended into the styrene-based resin composition as necessary within the range that does not impair the characteristics of the present invention. The type of additive is not particularly limited as long as it is one that is generally used in plastics, and examples of the additive include flame retardants, lubricants, processing aids, antiblocking agents, antistatic agents, antifogging agents, light resistance improvers, softeners, plasticizers, inorganic reinforcing agents, crosslinking agents, pigments, dyes, and others, or mixtures thereof.

[0024] 2. Styrene-based resin composition and method for producing light diffusion plate The styrene-based resin composition of the present invention can be produced by adding an antioxidant (B) and a light diffusing agent (C) to a styrene-based resin (A). The antioxidant (B) and the light diffusing agent (C) may be added during polymerization of the styrene-based resin (A), or the styrene-based resin (A) may be polymerized, and then the antioxidant (B) and the light diffusing agent (C) may be dry-blended and melt-kneaded in an extruder to produce the composition. Alternatively, the antioxidant (B) and the light diffusing agent (C) may be melt-kneaded in advance with a small amount of a styrene-based resin to produce a pellet-shaped master batch, which may then be dry-blended with the styrene-based resin (A) and the master batch, and then melt-kneaded to adjust the composition.

[0025] The polymerization method of the styrene resin includes known styrene polymerization methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. In terms of quality and productivity, bulk polymerization and solution polymerization are preferred, and continuous polymerization is preferred. As the solvent, for example, alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methyl ethyl ketone, and aliphatic hydrocarbons such as hexane and cyclohexane can be used.

[0026] During polymerization of the styrene-based resin (A), polymerization aids such as polymerization initiators, chain transfer agents, crosslinking agents, and other polymerization aids may be used as necessary. As the polymerization initiator, radical polymerization initiators are preferred, and examples of the polymerization initiator include peroxyketals such as 1,1-di(t-butylperoxy)cyclohexane, 2,2-di(t-butylperoxy)butane, 2,2-di(4,4-di-t-butylperoxycyclohexyl)propane, and 1,1-di(t-amylperoxy)cyclohexane, hydroperoxides such as cumene hydroperoxide and t-butyl hydroperoxide, alkyl peroxides such as t-butyl peroxyacetate and t-amyl peroxyisononanoate, t-butyl cumyl peroxide, di-t-butyl peroxide, dicumyl peroxide, and di-t-hexyl peroxide. Examples of the dialkyl peroxide include dialkyl peroxides such as t-butyl peroxyacetate, t-butyl peroxybenzoate, and t-butyl peroxyisopropyl monocarbonate; peroxyesters such as t-butyl peroxyisopropyl carbonate and polyether tetrakis(t-butyl peroxycarbonate); peroxycarbonates such as N,N'-azobis(cyclohexane-1-carbonitrile), N,N'-azobis(2-methylbutyronitrile), N,N'-azobis(2,4-dimethylvaleronitrile), and N,N'-azobis[2-(hydroxymethyl)propionitrile], and these may be used alone or in combination of two or more. Examples of the chain transfer agent include aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer, and terpinolene.

[0027] In the case of continuous polymerization, the styrene-based resin can be produced by a method including a polymerization step, a devolatilization step, and a granulation step.

[0028] First, in the polymerization step, a known complete mixing tank type stirring tank or tower type reactor or the like is used, and the polymerization reaction is controlled by adjusting the polymerization temperature or the like so as to achieve the target molecular weight, molecular weight distribution, and reaction conversion rate.

[0029] The polymer solution containing the polymer that has left the polymerization process is transferred to the devolatilization process, where unreacted monomers and polymerization solvent are removed. The devolatilization process is comprised of a vacuum devolatilization tank with a heater and a devolatilization extruder with a vent. The molten polymer that has left the devolatilization process is transferred to the granulation process. In the granulation process, the molten resin is extruded in the form of strands from a multi-hole die, and processed into pellets by the cold cut method, the air hot cut method, or the underwater hot cut method.

[0030] The content of t-butylcatechol in the styrene-based resin composition can be adjusted by adjusting the content at the start of polymerization of the styrene-based resin (A) and the content in the subsequent devolatilization step, etc. Also, it may be adjusted by adding it before or after adding the styrene-based resin (A), the antioxidant (B), the light diffusing agent (C), etc. to the styrene-based resin (A).

[0031] The light diffusion plate can be manufactured by molding the styrene resin composition by various methods such as extrusion molding and injection molding, but is preferably manufactured by extrusion molding. An example of extrusion molding is a method in which the styrene resin composition is melt-kneaded using a single-screw extruder or a twin-screw extruder, continuously extruded from a T-die, and then cooled and solidified by a cooling roll unit. When forming an uneven shape on the surface of the light diffusion plate, a transfer mold is provided on the surface of the cooling roll, and any uneven shape can be formed by changing the shape of the transfer mold. In addition, when laminating a surface layer on both sides or one side of the light diffusion plate, methods such as co-extrusion, pasting, heat adhesion, solvent adhesion, casting, and surface coating can be adopted.

[0032] 3.Other The light diffusion plate according to one embodiment of the present invention can be used as a light diffusion plate for an LED light source. Moreover, a direct type surface light source unit according to one embodiment of the present invention is a direct type surface light source unit having the above-mentioned light diffusion plate. EXAMPLES

[0033] The present invention will be described in more detail with reference to the following examples. These are all illustrative and do not limit the content of the present invention.

[0034] 1. Measurement method <Molecular weight> The number average molecular weight (Mn), weight average molecular weight (Mw), and Z average molecular weight (Mz) were measured under the following conditions using gel permeation chromatography (GPC). GPC model: Shodex GPC-101 manufactured by Showa Denko K.K. Column: PLgel 10μm MIXED-B manufactured by Polymer Laboratories Mobile phase: Tetrahydrofuran Sample concentration: 0.2 mass% Temperature: Oven 40°C, injection port 35°C, detector 35°C Detector: Differential refractometer The molecular weight was calculated from the elution curve of monodisperse polystyrene at each elution time and was calculated as the molecular weight in terms of polystyrene.

[0035] <Melt mass flow rate (MFR)> It was measured according to JIS K 7210 under the conditions of a temperature of 200°C and a load of 49 N.

[0036] <Vicat softening temperature> It was measured according to JIS K 7206 at a heating rate of 50°C / hr and a test load of 50 N.

[0037] <Content of t-butylcatechol (TBC)> After dissolving 0.2 g of the styrene resin in a small amount of THF, 200 μL of BSTFA (M,O-bis(trimethylsilyl)trifluoroacetamide) was added, and trimethylsilyl derivatization treatment was carried out. After making up to 10 mL with THF and separating by centrifugation, the supernatant was measured by gas chromatography-mass spectrometry (GC / MS) under the following conditions. For the determination of the concentration, a calibration curve prepared in advance was used. GC device: 7890A manufactured by Agilent Column: Agilent DB-5ms (0.25mm id x 30m) Liquid phase film thickness 0.25μm Column temperature: 50℃ (1min) → (20℃ / min temperature increase) → 320℃(6.5min) total 20min Injection port: 300℃, 1.5mL / min, (split ratio 1:5) Injection volume: 1μL MS device: Agilent 5975C Interface temperature: 320℃ MS detection conditions: SIM measurement TBC (m / z 295 for quantification, m / z 310 for confirmation)

[0038] <Total light transmittance, YI> The total light transmittance in the thickness direction of the light diffusion plate was measured using a turbidity meter NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd. in accordance with JIS-K7105.

[0039] <yi> The YI of the light diffuser plate was measured by a reflection method using a colorimeter NDJ4000 manufactured by Nippon Denshoku Industries Co., Ltd. in accordance with JIS-K7105.

[0040] <Average luminance difference> After removing the liquid crystal panel, various optical films, and light diffusion plate from a commercially available 20-inch direct-illumination type LED liquid crystal television, the center of the light diffusion plate was hollowed out into a square shape of 115 mm x 115 mm, and an evaluation test piece obtained by cutting the light diffusion plate obtained in the examples and comparative examples to 115 mm x 115 mm or an evaluation test piece of 115 mm x 115 mm after a durability test was attached to the hollowed out portion, and this was attached to the frame of the liquid crystal television after the above removal. Thereafter, two commercially available light diffusion films were superimposed on the front side of the light diffusion plate, and one brightness enhancement film (3M's "DBFF") was further superimposed on the front side of the above. With the LED turned on, the brightness of each spot (51 x 51 = 2601 spots) was measured over the entire area of ​​the front of the liquid crystal television using a multi-point luminance meter, and the average value of these brightness values ​​was recorded as the average brightness (cd / m 2 Furthermore, the average brightness difference (%) was calculated using the following formula. Average luminance difference (%)={(average luminance of Example)-(average luminance of Comparative Example 1)} / (average luminance of Comparative Example 1)×100

[0041] <Diffusion rate> Using a goniophotometer manufactured by Optec, white light was incident on the surface of the light diffuser plate perpendicular to the light source, and the luminance of the transmitted light was measured at angles of 5 degrees, 20 degrees, and 70 degrees, with the perpendicular direction to the surface of the light diffuser plate being 0 degrees, and the diffusivity was calculated using the following formula. Light diffusion rate (%) = (20 degree brightness + 70 degree brightness) / (5 degree brightness x 2) x 100

[0042] <Strength> The light diffusion plate was cut into a square shape of 200 mm x 200 mm to prepare an evaluation test piece. Using a ball weighing 16.6 g, the 50% breaking height was measured according to JIS K-7211, and the evaluation was based on the following criteria. ○: 50% destruction height is over 40cm △: 50% destruction height is 20~40cm ×: 50% destruction height is less than 20cm

[0043] 2. Durability test After cutting the light diffusion plate to a size of 115 mm x 115 mm, the evaluation test piece was exposed for 1,000 hours in a tank set at a temperature of 80°C using a Kusumoto Chemicals Gear Oven HISPEC HT310.

[0044] The results of long-term durability are shown as the results of each measurement after the above durability test.

[0045] 3. Synthesis of styrene-based resin Styrene-based resins PS-1 to PS-5 were synthesized according to the conditions shown in Table 1.

[0046] (Synthesis of PS-1) The polymerization process was performed by connecting the first reactor and the second reactor, which were complete mixing type stirring tanks, and the third reactor, which was a plug flow type reactor equipped with a static mixer, in series, and producing a styrene-based resin under the conditions shown in Table 1. The capacity of each reactor was 39 liters for the first reactor, 39 liters for the second reactor, and 16 liters for the third reactor. A raw material solution was prepared with the raw material composition shown in Table 1, and the raw material solution was continuously supplied to the first reactor at the flow rate shown in Table 1. The polymerization initiator was added to the raw material solution at the inlet of the first reactor so as to have the addition concentration (concentration based on mass relative to the raw material styrene) shown in Table 1, and was mixed uniformly. The polymerization initiators shown in Table 1 are as follows. Polymerization initiator-1: 2,2-di(4,4-t-butylperoxycyclohexyl)propane (Pertetra A manufactured by NOF Corporation was used) Polymerization initiator-2: 1,1-di(t-butylperoxy)cyclohexane (Perhexa C manufactured by NOF Corporation was used) In the third reactor, a temperature gradient was applied along the direction of flow, and the temperatures in the middle and outlet parts were adjusted to the temperatures shown in Table 1. Next, the polymer-containing solution continuously removed from the third reactor was introduced into a vacuum devolatilizer tank equipped with a preheater consisting of two stages in series, and the temperature of the preheater was adjusted so as to obtain the resin temperature shown in Table 1, and the pressure was adjusted to the pressure shown in Table 1 to separate unreacted styrene and ethylbenzene. The polymer was then extruded into strands through a multi-hole die, and the strands were cooled and cut by a cold cut method to form pellets.

[0047] PS-2 to PS-5 were also synthesized in the same manner as PS-1, except that the conditions were changed according to Table 1.

[0048] [Table 1]

[0049] 4. Examples and Comparative Examples Examples and comparative examples were carried out in the following manner.

[0050] <Example 1> 100 parts by mass of styrene resin (PS-1) containing the amount of t-butylcatechol (TBC) shown in Table 1 was mixed with 0.2 parts by mass of phosphorus-based antioxidant (HP-10), 0.1 parts by mass of phosphorus-phenol-based antioxidant (GP), and 1.0 part by mass of light diffusing agent (MBX-8) in a blender, and the mixture was mixed in a single-screw extruder with a screw diameter of 40 mm at a cylinder temperature of 230°C and a screw rotation speed of 100 rpm to obtain a styrene resin composition.

[0051] Next, the styrene resin composition was fed to a single-screw vented extruder with a screw diameter of 90 mm and L / D=32, melt-kneaded at 200-235°C, and then discharged at a T-die temperature of 245-250°C through a T-die with a lip width of 1000 mm and a lip opening of 3.0 mm, cooled and solidified with a vertical three-roll cooling system, and the end face was trimmed to obtain a light diffusion plate with a width of 800 mm and a thickness of 2.0 mm. The initial optical properties of the light diffusion plate were measured based on each measurement method, and the initial strength was also measured. Next, the above durability test was performed, and the optical properties thereafter were measured based on each measurement method. These results are shown in Table 2.

[0052] <Examples 2 to 15 and Comparative Examples 1 to 7> For Examples 2 to 15 and Comparative Examples 1 to 7, resin compositions and light diffusion plates were obtained and measured in the same manner as in Example 1, except that the conditions were changed according to those shown in Tables 2 and 3. The results are shown in Tables 2 and 3.

[0053] [Table 2]

[0054] [Table 3]

[0055] The abbreviations in the table are as follows. HP-10: 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus (ADEKA; HP-10) 168: Tris(2,4-di-tert-butylphenyl)phosphite (BASF Japan; Irgafos168) PEP-36: 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (ADEKA; PEP-36) P-EPQ: Tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4'-diylbisphosphonite (BASF Japan; IrgafosP-EPQ) 38: Bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite (BASF Japan; Irgafos38) GP: 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine (Sumitomo Chemical Co., Ltd.; Sumilizer GP) 1076: Octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (BASF Japan; Irganox 1076) 245: Ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate] (BASF Japan; Irganox 245) 1010: Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (BASF Japan; Irganox 1010) AO80: 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (ADEKA; AO-80) MBX-8: Acrylic crosslinked particles (average particle size 8 μm, manufactured by Sekisui Plastics Co., Ltd.; MBX-8) SBX-8: Polystyrene crosslinked particles (average particle size 8 μm, manufactured by Sekisui Plastics Co., Ltd.; SBX-8) KMP-590: Silicone-based crosslinked particles (average particle size 2 μm, Shin-Etsu Chemical Co., Ltd.; KMP-590)< / yi>

Claims

1. A light diffusion plate comprising a styrene-based resin composition, The styrene-based resin composition contains a styrene-based resin (A), an antioxidant (B), and a light diffusing agent (C), The antioxidant (B) includes at least one of a phosphorus-based antioxidant (B-1), a phenol-based antioxidant (B-2), and a phosphorus-phenol-based antioxidant (B-3), Relative to 100 parts by mass of the styrene-based resin, the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) are present in a total amount of 0.01 to 0.5 parts by mass, the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) are 0.01 to 0.5 parts by mass in total, 0.1 to 10 parts by mass of the light diffusing agent (C), Contains the styrene-based resin composition has a t-butylcatechol content of 0.1 to 10 μg / g; The ratio (Mz / Mw) of the Z-average molecular weight (Mz) to the weight-average molecular weight (Mw) of the styrene-based resin (A) is 1.0 to 2.0; The styrene-based resin (A) is a homopolymer of a styrene-based monomer, A light diffusion plate, wherein the styrene-based resin (A) has a weight average molecular weight (Mw) of 290,000 or less (excluding those in which the ratio (Mz / Mw) is 2.0).

2. 2. The light diffusion plate according to claim 1, wherein the styrene-based resin (A) has a weight average molecular weight (Mw) of 200,000 to 290,000.

3. The phosphorus-based antioxidant (B-1), 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, Tris(2,4-di-tert-butylphenyl)phosphite, 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane, Tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4′-diylbisphosphonite, 3. The light diffusion plate according to claim 1, wherein the phosphorus compound is at least one selected from the group consisting of bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite.

4. The phenolic antioxidant (B-2), octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, Ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], The light diffusion plate according to any one of claims 1 to 3, wherein the compound is at least one selected from the group consisting of 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane.

5. The light diffuser plate described in any one of claims 1 to 3, wherein the phosphorus-phenol-based antioxidant (B-3) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine.

6. A light diffusion plate described in any one of claims 1 to 4, wherein the light diffusion agent (C) is at least one selected from the group consisting of acrylic polymer crosslinked particles, styrene polymer crosslinked particles, and siloxane polymer crosslinked particles.

7. The light diffusion plate according to any one of claims 1 to 6, which is for an LED light source.

8. A direct type surface light source unit comprising the light diffusion plate according to any one of claims 1 to 7.

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