JP7629868B2 - Light diffusion plate and direct type surface light source unit - Google Patents

Description

The present invention relates to a light diffusion plate and a direct-type surface light source unit.

The light diffusion plate is used in direct type liquid crystal display devices. A direct type liquid crystal display device is a display device that includes 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 the resin that constitutes the light diffusion plate (Patent Document 1).

JP 2010-134461 A

However, LEDs used as light sources have become more powerful in order to meet the demand for higher brightness, and the long-term durability of conventional light diffusion plates made of styrene-based resin compositions has become insufficient due to the increased heat generated. In addition, there have been cases where the strength did not meet the requirements.

The present invention has been made in consideration of such problems, and provides a light diffusion plate containing a styrene-based resin composition, which has excellent long-term durability and strength.

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

After extensive research, the inventors discovered that when the contents of t-butylcatechol, phosphorus-based antioxidant, phenol-based antioxidant, phosphorus-phenol-based antioxidant, and light diffusing agent in the styrene-based resin composition are within specified ranges, the composition exhibits excellent long-term durability and strength, which led to the completion of the present invention.

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, ethylene bis(oxyethylene) bis[3-(5-tert-butyl- a light diffusing plate which is at least one 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 crosslinked acrylic polymer particles, crosslinked styrene polymer particles, and crosslinked siloxane polymer 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.

The following describes an embodiment of the present invention. The various features shown in the following embodiment can be combined with each other. In addition, each feature can be an invention independently.

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.

The light diffusion plate according to one embodiment may have an antistatic agent applied to its surface. By applying an antistatic agent, adhesion of dust and the like due to static electricity is suppressed after the light diffusion plate is attached to a backlight device, so that the light diffusion plate can be used for a long period of time without a decrease in brightness. 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.

<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.

<Styrene-based resin (A)>
The styrene-based resin (A) is a resin obtained by polymerizing a styrene-based monomer as a main component, and 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, p-t-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.

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 the ratio in such a range, it is possible to achieve both moldability and strength of the light diffusion plate. 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.

<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).

The styrene resin composition contains a total of 0.001 to 0.5 parts by mass of phosphorus-based antioxidant (B-1) and phosphorus-phenol-based antioxidant (B-3) per 100 parts by mass of styrene resin, preferably 0.05 to 0.3 parts by mass. By setting the range in this way, the light diffusion plate has excellent long-term durability and strength. The total content of phosphorus-based antioxidant (B-1) and phosphorus-phenol-based antioxidant (B-3) is specifically 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 per 100 parts by mass of styrene resin, and may be within a range between any two of the numerical values exemplified here.

The phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) are contained in a total amount of 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 per 100 parts by mass of the styrene-based resin, and may be within a range between any two of the numerical values exemplified here.

The phosphorus-based antioxidant (B-1) is a phosphorous acid ester that does not have a phenolic hydroxyl group in the basic skeleton, 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, bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphorous acid ester, etc.

Phenol-based antioxidant (B-2) is an antioxidant that has a phenolic hydroxyl group in the basic skeleton and is not a phosphate 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.

The phosphorus-phenol-based antioxidant (B-3) is a phosphorous acid ester having a phenolic hydroxyl group in the basic structure, preferably a phosphorous acid ester which is a trivalent phosphorus compound having a phenolic hydroxyl group in the basic structure. 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, etc.

<Light Diffusing Agent (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.

The light diffusing agent (C) can be any particle having a different refractive index from 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 sufficient.

<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.

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

Polymerization methods for styrene-based resins include 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 preferable. Solvents that can be used include, 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.

During polymerization of the styrene-based resin (A), polymerization aids such as polymerization initiators, chain transfer agents, crosslinking agents, and other polymerization aids can be used as necessary. As the polymerization initiator, a radical polymerization initiator is preferred, and examples of the commonly used peroxyketals include 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 peroxy acetate and t-amyl peroxy isononanoate, t-butyl cumyl peroxide, di-t-butyl peroxide, dicumyl peroxide, and di-t-hexyl peroxide. Examples of the peroxycarbonylating agent include dialkyl peroxides such as t-butyl peroxyacetate, t-butyl peroxybenzoate, t-butyl peroxyisopropyl monocarbonate, and other peroxyesters, t-butyl peroxyisopropyl carbonate, t-butyl peroxyisopropyl carbonate, polyether tetrakis(t-butyl peroxycarbonate), and other peroxycarbonates, N,N'-azobis(cyclohexane-1-carbonitrile), N,N'-azobis(2-methylbutyronitrile), N,N'-azobis(2,4-dimethylvaleronitrile), N,N'-azobis[2-(hydroxymethyl)propionitrile], and the like. 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.

In the case of continuous polymerization, styrene-based resins can be produced by a method including a polymerization process, a devolatilization process, and a granulation process.

First, in the polymerization process, a known complete mixing tank type stirred tank or tower type reactor is used, and the polymerization reaction is controlled by adjusting the polymerization temperature, etc., to achieve the target molecular weight, molecular weight distribution, and reaction conversion rate.

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

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 process, 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).

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 bonding, solvent bonding, casting, and surface coating can be used.

3. Others 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.

The present invention will be described in more detail below with reference to examples. Note that these examples are merely illustrative and do not limit the scope of the present invention.

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

<Melt Mass Flow Rate (MFR)>
The measurement was carried out in accordance with JIS K 7210 under conditions of a temperature of 200° C. and a load of 49 N.

<Vicat softening temperature>
The measurement was carried out in accordance with JIS K 7206, at a temperature rise rate of 50° C./hr and a test load of 50 N.

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

<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.

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

<Average luminance difference>
After removing the liquid crystal panel, various optical films, and light diffusion plate from a commercially available 20-inch direct-light 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 the evaluation test piece obtained by cutting the light diffusion plate obtained in the examples and comparative examples into 115 mm x 115 mm, or the evaluation test piece after the durability test of 115 mm x 115 mm was attached to the hollowed out part, and this was attached to the frame of the liquid crystal television after the above removal. Then, two commercially available light diffusion films were superimposed on the front side of the light diffusion plate, and one brightness improvement film (3M's "DBFF") was further superimposed on the front side of the film. With the LED turned on, the brightness of each spot (51 x 51 = 2601 spots) was measured over the entire range of the front of the liquid crystal television using a multi-point brightness meter, and the average value of these brightnesses was taken as the average brightness (cd/ ^m2 ). 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

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

<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% breaking height is more than 40 cm △: 50% breaking height is 20 to 40 cm
×: 50% breaking height is less than 20 cm

2. Durability Test After cutting the light diffusion plate into 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 gear oven HISSPEC HT310 manufactured by Kusumoto Chemicals.

The long-term durability results show the results of each measurement after the durability tests mentioned above.

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

(Synthesis of PS-1)
The polymerization process was performed by connecting the first reactor and the second reactor, which are complete mixing type stirring tanks, and the third reactor, which is a plug flow type reactor 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 intermediate portion and outlet portion 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 connected 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 be converted into pellets.

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

4. Examples and Comparative Examples Examples and Comparative Examples were carried out in the following manner.

Example 1
100 parts by mass of a styrene-based resin (PS-1) containing the amount of t-butylcatechol (TBC) shown in Table 1 was mixed with 0.2 parts by mass of a phosphorus-based antioxidant (HP-10), 0.1 parts by mass of a phosphorus-phenol-based antioxidant (GP), and 1.0 part by mass of a light diffusing agent (MBX-8) in a blender, and the mixture was mixed in a single-screw extruder having 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-based resin composition.

The styrene resin composition was then 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 extruded through a T-die with a lip width of 1000 mm and lip opening of 3.0 mm at a T-die temperature of 245-250°C. After cooling and solidifying with three vertical cooling rolls, the end faces were 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 using each measurement method, and the initial strength was also measured. Next, the durability test was conducted, and the optical properties were measured after that using each measurement method. The results are shown in Table 2.

<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.

The abbreviations in the table are as follows.
HP-10: 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus (manufactured by ADEKA Corporation; HP-10)
168: Tris(2,4-di-tert-butylphenyl)phosphite (manufactured by BASF Japan; Irgafos 168)
PEP-36: 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (manufactured by ADEKA Corporation; PEP-36)
P-EPQ: tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4′diylbisphosphonite (manufactured by BASF Japan; IrgafosP-EPQ)
38: Bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite (manufactured by BASF Japan; Irgafos 38)
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 (manufactured by BASF Japan; Irganox 1076)
245: Ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate] (manufactured by BASF Japan; Irganox 245)
1010: Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (manufactured by 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 (manufactured by ADEKA Corporation; 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)

Claims (8)

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. 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. The phosphorus-based antioxidant (B-1) is
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. The phenol-based antioxidant (B-2) is
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. The light diffusion plate according to 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. The light diffusion plate according to any one of claims 1 to 4, wherein 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. The light diffusion plate according to any one of claims 1 to 6, which is for use with an LED light source. A direct type surface light source unit comprising the light diffusion plate according to any one of claims 1 to 7 .