Styrene-based resin composition, light guide plate, and edge-lit type surface light source unit

Abstract

Provided is a styrene-based resin composition having excellent transparency, hue, and dimensional stability, and also having excellent long-term durability to LED light sources.　The present invention provides a styrene-based resin composition containing: a styrene-based resin (A) that is a copolymer containing a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit; an antioxidant (B); and an anthraquinone-based colorant (C). The antioxidant (B) contains one or both of a phosphorus-based antioxidant (B-1) and a phosphorus-phenol-based antioxidant (B-2). The styrene-based resin composition contains, with respect to 100 parts by mass of the styrene-based resin (A), a total of 0.001-0.5 parts by mass of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2), and contains 0.1-150 ppb of the anthraquinone-based colorant (C) with respect to the styrene-based resin (A).

Description

Styrenic resin composition, light guide plate, and edge-light type surface light source unit

[0001] The present invention relates to a styrene-based resin composition, a light guide plate, and an edge-light type surface light source unit.

[0002] There are two types of backlights for LCD displays: direct-type, in which the light source is placed in front of the display device, and edge-lit, in which the light source is placed on the side. Edge-lit backlights use a component called a light guide plate that guides light from a light source placed on the side to the front of the display device. They are used in a wide range of applications, including televisions, desktop personal computer monitors, notebook personal computers, mobile phones, and car navigation monitors. Backlights that use light guide plates are also used in lighting devices, signs, etc.

[0003] Light guide plates are required to have particularly high light transmittance because the light transmission distance is relatively long and the light loss over the optical path length is large. For this reason, acrylic resins such as polymethyl methacrylate (PMMA) are used as the material for light guide plates. However, PMMA has high water absorption, which can cause warping and dimensional changes in the light guide plate. Furthermore, PMMA is prone to thermal decomposition during molding, which can lead to poor appearance of the molded product when molded at high temperatures. To address these issues, Patent Document 1, for example, proposes using a styrene-methyl (meth)acrylate copolymer as the material for the light guide plate.

[0004] On the other hand, molded articles of styrene-methyl (meth)acrylate copolymer have poorer hue than PMMA, and may cause color unevenness when used as a backlight. To solve this problem, Patent Document 2 proposes blending a specific amount of blue dye into styrene-methyl (meth)acrylate.

[0005] JP 2003-075648 A JP 2013-082800 A

[0006] In recent years, with the trend toward energy-saving backlight units for televisions, monitors, lighting, and other devices, it has become necessary to increase brightness with fewer LEDs, and high-brightness LEDs are sometimes used as edge light sources.However, conventional light guide plates made of styrene-methyl (meth)acrylate copolymers can sometimes turn yellow after being turned on for long periods of time, depending on the usage environment.When yellowing occurs, this is visible as color unevenness on televisions and monitors, and in lighting, the color temperature changes, making it impossible to maintain the original lighting design.

[0007] The present invention has been made in view of the above problems, and provides a styrene-based resin composition that is excellent in transparency, color, and dimensional stability, and also has excellent long-term durability against an LED light source.

[0008] According to the present invention, there is provided a styrene-based resin composition containing a styrene-based resin (A) which is a copolymer containing a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit, an antioxidant (B), and an anthraquinone-based colorant (C), wherein the antioxidant (B) contains either or both of a phosphorus-based antioxidant (B-1) and a phosphorus-phenol-based antioxidant (B-2), the total amount of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2) being 0.001 to 0.5 parts by mass per 100 parts by mass of the styrene-based resin (A), and the anthraquinone-based colorant (C) being 0.1 to 150 ppb relative to the styrene-based resin (A).

[0009] As a result of extensive investigations, the present inventors have found that a styrene-based resin composition containing a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit, an antioxidant in a predetermined range, and an anthraquinone-based colorant simultaneously satisfies the requirements for transparency, hue, dimensional stability, and long-term durability against an LED light source, thereby completing the present invention.

[0010] Various embodiments of the present invention are exemplified below. The embodiments shown below can be combined with each other. [1] A styrene-based resin composition containing a styrene-based resin (A) that is a copolymer containing a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit, an antioxidant (B), and an anthraquinone-based colorant (C), wherein the antioxidant (B) contains either or both of a phosphorus-based antioxidant (B-1) and a phosphorus-phenol-based antioxidant (B-2), the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2) are contained in a total amount of 0.001 to 0.5 parts by mass per 100 parts by mass of the styrene-based resin (A), and the anthraquinone-based colorant (C) is contained in an amount of 0.1 to 150 ppb relative to the styrene-based resin (A). [2] The styrene-based resin composition according to [1], wherein the styrene-based resin (A) contains 20 to 95 mass% of the styrene-based monomer units and 5 to 80 mass% of the (meth)acrylic acid ester-based monomer units, based on 100 mass% of the styrene-based resin (A). [3] The styrene-based resin composition according to either [1] or [2], wherein 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, bis-(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, 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. [4] The styrene-based resin composition according to any one of [1] to [3], wherein the phosphorus-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.[5] The styrene-based resin composition according to any one of [1] to [4], wherein an average transmittance at wavelengths of 380 to 780 nm at an optical path length of 115 mm, measured using a test piece prepared by molding the styrene-based resin composition into a size of 115 mm x 80 mm x 3 mm (thickness), is 85% or more. [6] The styrene-based resin composition according to any one of [1] to [5], wherein a YI value at an optical path length of 115 mm, measured using a test piece prepared by molding the styrene-based resin composition into a size of 115 mm x 80 mm x 3 mm (thickness), is 4.0 or less. [7] An edge-lit light guide plate that is a molded article of the styrene-based resin composition according to any one of [1] to [6]. [8] An edge-lit surface light source unit having the edge-lit light guide plate according to [7] and a light source that supplies light to an end surface of the light guide plate. [9] The edge-lit surface light source unit for illumination according to [7].

[0011] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently. Furthermore, in the following embodiments, elements not specified in the claims are optional elements and can be omitted. Any number of "0"s (for example, one or two) may be added to the end of numerical values ​​disclosed in the following description. For example, one or two "0"s may be added after "1.4" to make it "1.40" or "1.400".

[0012] 1. Styrenic Resin Composition A styrenic resin composition according to one embodiment of the present invention is a styrenic resin composition containing a styrenic resin (A), an antioxidant (B), and an anthraquinone colorant (C).

[0013] The styrene-based resin composition contains, for example, 95.000 to 99.999 mass%, preferably 98.000 to 99.999 mass%, and more preferably 99.000 to 99.999 mass% of the styrene-based resin (A) relative to 100 mass% of the styrene-based resin composition.

[0014] <Styrene-based Resin (A)> The styrene-based resin (A) is a resin obtained by copolymerizing a monomer including a styrene-based monomer and a (meth)acrylic acid ester-based monomer, and is a copolymer including a styrene-based monomer unit, which is a structural unit derived from the styrene-based monomer, and a (meth)acrylic acid ester-based monomer unit, which is a structural unit derived from the (meth)acrylic acid ester-based monomer. The styrene-based resin (A) is preferably a copolymer containing 20 to 95% by mass of styrene-based monomer units and 5 to 80% by mass of (meth)acrylic acid ester-based monomer units, more preferably a copolymer containing 25 to 90% by mass of styrene-based monomer units and 10 to 75% by mass of (meth)acrylic acid ester-based monomer units, and even more preferably a copolymer containing 30 to 85% by mass of styrene-based monomer units and 15 to 70% by mass of (meth)acrylic acid ester-based monomer units. By setting the content within these ranges, transparency, hue, and dimensional stability can be simultaneously satisfied. By setting the styrene-based monomer content to 95% by mass or less, a light guide plate excellent in transparency and hue can be obtained, and by setting the styrene-based monomer unit (styrene monomer unit) content to 20% by mass or more, a light guide plate excellent in dimensional stability can be obtained. Specifically, the content of the (meth)acrylic acid ester-based monomer unit in the styrene-based resin (A) is, for example, 5, 6, 7, 8, 9, 10, 15, 18, 20, 25, 30, 35, 40, 45, 47, 50, 55, 60, 65, 70, 75, or 80% by mass, and may be within a range between any two of the values ​​exemplified here.

[0015] The styrene-based resin (A) contains, for example, 80 to 100 mass%, preferably 90 to 100 mass%, more preferably 99 to 100 mass%, and particularly preferably (substantially) 100 mass% of styrene-based monomer units and (meth)acrylic acid ester-based monomer units in 100 mass% of the styrene-based resin (A). The total content of the styrene-based monomer units and (meth)acrylic acid ester-based monomer units in 100 mass% of the styrene-based resin (A) is, for example, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100 mass%, and may be within a range between any two of the values ​​exemplified here.

[0016] Examples of styrene-based monomers include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, m-methylstyrene, ethylstyrene, and p-t-butylstyrene. These can be used alone or in combination of two or more. The preferred styrene-based monomer is styrene.

[0017] Examples of (meth)acrylic acid ester monomers (acrylic acid ester monomers or methacrylic acid ester monomers) include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, isoamyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate; (meth)acrylic acid aryl esters such as phenyl (meth)acrylate and benzyl (meth)acrylate; (meth)acrylic acid cycloalkyl esters such as cyclohexyl (meth)acrylate, 4-t-butylcyclohexyl (meth)acrylate, tricyclodecanyl (meth)acrylate, and adamantyl (meth)acrylate; glycidyl (meth)acrylate; and dicyclopentadienyl (meth)acrylate. These may be used alone or in combination of two or more. The (meth)acrylic acid ester monomer is preferably a (meth)acrylic acid alkyl ester, more preferably a methacrylic acid alkyl ester, and even more preferably methyl methacrylate.

[0018] The styrene-based resin (A) may also be a copolymer obtained by copolymerizing a monomer copolymerizable with a styrene-based monomer and a (meth)acrylic acid ester-based monomer. Examples of the copolymerizable monomer include (meth)acrylic acids such as acrylic acid and methacrylic acid; vinyl cyanides such as acrylonitrile and methacrylonitrile; α,β-ethylenically unsaturated carboxylic acids such as maleic anhydride and fumaric acid; and imides such as phenylmaleimide and cyclohexylmaleimide. These may be used alone or in combination of two or more.

[0019] The weight average molecular weight (Mw) of the styrene-based resin (A) is preferably 50,000 to 400,000, more preferably 100,000 to 350,000. Specific examples of the weight average molecular weight (Mw) of the styrene-based resin (A) include 50,000, 100,000, 150,000, 160,000, 170,000, 180,000, 190,000, 200,000, 210,000, 220,000, 230,000, 240,000, 250,000, 300,000, 350,000, and 400,000, and may be within a range between any two of the values ​​exemplified here. Furthermore, the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the styrene-based resin (A) is preferably 1.0 to 3.5, more preferably 1.5 to 3.0. Specifically, the ratio (Mw / Mn) may be, for example, 1.0, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 3.0, or 3.5, and may be within a range between any two of the values ​​exemplified here. By setting the ratio within such a range, both moldability and strength of the light guide plate can be achieved. If the weight-average molecular weight (Mw) is less than 50,000, the strength of the molded product may be insufficient, and if it exceeds 400,000, moldability may be reduced. Furthermore, if the ratio (Mw / Mn) of the number-average molecular weight (Mn) is less than 1.0, moldability may be reduced, and if it exceeds 3.5, the strength of the molded product may be reduced.

[0020] <Antioxidant (B)> The antioxidant (B) includes either or both of a phosphorus-based antioxidant (B-1) and a phosphorus-phenol-based antioxidant (B-2).

[0021] The styrene-based resin composition contains a total of 0.001 to 0.5 parts by mass, preferably 0.002 to 0.4 parts by mass, and more preferably 0.005 to 0.3 parts by mass of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2) per 100 parts by mass of the styrene-based resin (A). By adjusting the content within such a range, transparency and hue can be improved. The total content of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2) per 100 parts by mass of the styrene-based resin (A) is, for example, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.025, 0.05, 0.075, 0.1, 0.15, 0.2, 0.3, 0.4, or 0.5 parts by mass, and may be within a range between any two of the values ​​exemplified herein.

[0022] The phosphorus-based antioxidant (B-1) is a phosphite ester having no phenolic hydroxyl group in the basic skeleton, and is preferably a phosphite ester which is a trivalent phosphorus compound. Specific examples of the phosphorus-based antioxidant (B-1) include 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, bis-(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, 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, and the like. These may be used alone or in combination of two or more.

[0023] The phosphorus-phenol-based antioxidant (B-2) is a phosphite ester having a phenolic hydroxyl group in its basic skeleton, preferably a phosphite ester, which is a trivalent phosphorus compound having a phenolic hydroxyl group in its basic skeleton. Examples of the phosphorus-phenol-based antioxidant (B-2) include 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine.

[0024] The styrene-based resin composition may also contain 0 to 0.5 parts by mass of a phenolic antioxidant (B-3) relative to 100 parts by mass of the styrene-based resin (A). A content of the phenolic antioxidant (C-3) exceeding 0.5 parts by mass is undesirable because it deteriorates the color. Specifically, the content of the phenolic antioxidant (C-3) relative to 100 parts by mass of the styrene-based resin (A) may be, for example, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, or 0.5 parts by mass, or may be within a range between any two of the values ​​exemplified herein.

[0025] The phenolic antioxidant (B-3) is an antioxidant that has a phenolic hydroxyl group in its basic skeleton and is not a phosphate ester. Examples of the phenolic antioxidant (B-2) include 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], 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. These antioxidants may be used alone or in combination of two or more.

[0026] <Anthraquinone-Based Colorant (C)> The styrene-based resin composition contains an anthraquinone-based colorant (C) in an amount of 0.1 to 150 ppb, preferably 0.5 to 100 ppb, and more preferably 1 ppb or more and less than 75 ppb, relative to the styrene-based resin (A). By adjusting the amount within this range, a styrene-based resin composition having good average transmittance and hue and excellent long-term stability against LED light sources can be obtained. The content of the anthraquinone-based colorant (B) relative to the styrene-based resin (A) is, for example, 0.1, 0.5, 1, 5, 10, 15, 20, 35, 30, 35, 40, 45, 47, 50, 55, 60, 65, 70, 74, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, or 150 ppb, or may be within a range between any two of the values ​​exemplified here. The anthraquinone colorant (C) may be used alone or in combination of two or more.

[0027] The anthraquinone colorant (C) is a compound having a structural unit represented by the following general formula (1).

[0028]

[0029] In general formula (1), R １ is a group selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, an aryl group, and an alkoxy group. ２ ~R ８ are each independently a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, an aryl group, an alkoxy group, a hydroxyl group, a halogen group, an amino group, a sulfo group, a carboxyl group, a cyano group, a nitro group, or —COR ９ , -COOR ９ , -NR ９ R １０ , -NR ９ COR １０ , -NR ９ SO ２ R １０ , -CONR ９ R １０ , -SO ３ R ９ , -CONHSO ２ R９ , -SO ２ NR ９ R １０ , and -SO ２ NHCOR ９ is a group selected from the group consisting of ９ and R １０ are independently a hydrogen atom or a group selected from the group consisting of an aliphatic carbon, an aromatic group, and a heterocyclic group. ３ H and / or —CO ２ When H is contained, it may form a salt such as a sodium salt or a potassium salt.

[0030] R １ ~R ８ Examples of the alkyl group having 1 to 10 carbon atoms in the formula (I) include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an isopropyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an isopentyl group, a neopentyl group, and a 2-ethylhexyl group, and these alkyl groups may have a substituent such as a hydroxyl group, a halogen group, an amino group, a sulfo group, a carboxyl group, a cyano group, a nitro group, or a nitrile group.

[0031] R １ ~R ８ Examples of the cycloalkyl group having 3 to 10 carbon atoms in the formula (I) include a cyclopropyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, and a tricyclodecyl group, and these cycloalkyl groups may have a substituent such as a hydroxyl group, a halogen group, an amino group, a sulfo group, a carboxyl group, a cyano group, a nitro group, or a nitrile group.

[0032] R １ ~R ８ Examples of the aryl group in include a phenyl group, an o-tolyl group, an m-tolyl group, a p-tolyl group, a cumyl group, a xylyl group, a propylphenyl group, an n-butylphenyl group, and a 4-tert-butylphenyl group, and these aryl groups may have a substituent such as a hydroxyl group, a halogen group, an amino group, a sulfo group, a carboxyl group, a cyano group, a nitro group, or a nitrile group.

[0033] R １ ~R ８ Examples of the alkoxy group in include a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a tert-butoxy group, a phenoxy group, and a naphthoxy group, and these alkoxy groups may have a substituent such as a hydroxyl group, a halogen group, an amino group, a sulfo group, a carboxyl group, a cyano group, a nitro group, or a nitrile group.

[0034] Specific examples of anthraquinone colorants include the following: The names listed below are color index names. Disperse Violet4, Disperse Violet8, Disperse Violet17, Disperse Violet26, Disperse Violet28, Disperse Violet31, Disperse Blue3, Disperse Blue14, Disperse Blue60, Disperse Blue72, Disperse Blue134, Disperse Blue181, Disperse Blue197, Solvent Violet11, Solvent Violet12, Solvent Violet13, Solvent Violet26, Solvent Violet31, Solvent Violet33, Solvent Violet34, Solvent Violet36, Solvent Violet37, Solvent Violet38, Solvent Violet48, Solvent Violet51, Solvent Violet59, Solvent Violet60, Solvent Blue11, Solvent Blue12, Solvent Blue13, Solvent Blue14, Solvent Blue16, Solvent Blue18, Solvent Blue35, Solvent Blue36, Solvent Blue45, Solvent Blue58, Solvent Blue59, Solvent Blue59:1, Solvent Blue63, Solvent Blue67, Solvent Blue68, Solvent Blue74, Solvent Blue76, Solvent Blue78, Solvent Blue79, Solvent Blue83, Solvent Blue90, Solvent Blue94, Solvent Blue95, Solvent Blue97, Solvent Blue98, Solvent Blue101, Solvent Blue102, Solvent Blue104, Solvent Blue105, Solvent Blue111, Solvent Blue112,Solvent Blue122, Solvent Blue128, Solvent Blue132, Solvent Blue136, Solvent Blue146, Acid Blue27, Acid Blue43, Acid Blue47, Acid Blue49, Acid Blue51, Acid Blue55, Acid Blue145, Mordant Blue23, Mordant Blue27, Disperse Green6:1, Solvent Green3, Solvent Green20, Solvent Green28, Solvent Green33, Acid Green25. ,

[0035] <Hindered amine light stabilizer (D)> The styrene-based resin composition may contain a hindered amine light stabilizer (D). The styrene-based resin composition preferably contains 0.001 to 1.0 part by mass, more preferably 0.001 to 0.5 part by mass, and even more preferably 0.05 to 0.3 part by mass of the hindered amine light stabilizer (D) per 100 parts by mass of the styrene-based resin (A). By containing the hindered amine light stabilizer (D) in such a range, light stability can be improved. The content of the hindered amine light stabilizer (D) relative to the styrene-based resin (A) is, for example, 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 parts by mass, and may be within a range between any two of the values ​​exemplified here. Furthermore, the hindered amine light stabilizer (D) may be used alone or in combination of two or more.

[0036] The hindered amine light stabilizer (D) is a compound having a structural unit represented by the following general formula (2).

[0037]

[0038] In general formula (2), X represents an organic group bonded to the 4-position of the piperidyl group via a carbon atom, an oxygen atom, or a nitrogen atom, and R represents a hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, a methylene group, or an alkoxy group. Here, when R represents a hydrogen atom, the stabilizer is referred to as an N-H hindered amine light stabilizer, when R represents a linear or branched alkyl group having 1 to 10 carbon atoms or a methylene group, the stabilizer is referred to as an N-R hindered amine light stabilizer, and when R represents an alkoxy group, the stabilizer is referred to as an N-OR hindered amine light stabilizer.

[0039] Specific examples of the N—H type hindered amine light stabilizer include bis(2,2,6,6-tetramethylpiperidin-4-yl)sebacate (TINUVIN770DF manufactured by BASF), 2,2,6,6-tetramethyl-4-piperidyl hexadecanoate, 2,2,6,6-tetramethyl-4-piperidyl octadecanoate (SABOSTAB UV91 manufactured by SONGWON), and tetrakis(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate (ADK STAB manufactured by ADEKA). LA-57), polycondensate of N,N'-bis(2,2,6,6-tetramethylpiperidin-4-yl)hexamethylenediamine and 4-morpholino 2,6-dichloro-1,3,5-triazine (SABOSTAB UV79, manufactured by SONGWON), polycondensate of 2,4-dichloro-6-(1,1,3,3-tetramethylbutylamino) and 1,3,5-triazine.N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)hexamethylenediamine (manufactured by BASF), Chimassorb 944FDL), polycondensate of N,N'-bis(2,2,6,6-tetramethylpiperidin-4-yl)hexamethylenediamine, 2,4,6-trichloro-1,3,5-triazine, N-butyl-1-butanamine, and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine (manufactured by SONGWON SABOSTAB UV40), 1,6,11-tris[2,4-bis(N-butyl-N-(2,2,6,6-tetramethyl-4-piperidyl)amino)-s-triazin-6-yl]aminoundecane (manufactured by BASF) Chimassorb2020FDL), polycondensate of 1,2,3,4-butanetetracarboxylic acid, 2,2,6,6-tetramethyl-4-piperidinol, and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5,5]undencane-3,9-diethanol (ADEKA ADKSTAB LA-68), dodecyl 3-(2,2,4,4-tetramethyl-21-oxo-7-oxa-3,20-diazadispiro(5.1.11.2)henicosan-20-yl)propionate, tetradecyl 3-(2,2,4,4-tetramethyl-21-oxo-7-oxa-3,20-diazadispiro(5.1.11.2)henicosan-20-yl)propionate (HOSTAVIN 3030, manufactured by CLARIANT), and polycondensate of 2,2,4,4-tetramethyl-7-oxa-3,20-diazadispiro-(5.1.11.2)henicosan-21-one and epichlorohydrin (HOSTAVIN N30P, manufactured by CLARIANT).

[0040] Specific examples of the N-R type hindered amine light stabilizer include methyl(1,2,2,6,6-pentamethylpiperidin-4-yl)sebacate, bis(1,2,2,6,6-pentamethylpiperidin-4-yl)sebacate (TINUVIN 292, TINUVIN 765, manufactured by BASF), bis(1,2,2,6,6-pentamethylpiperidin-4-yl)n-butyl 3,5-di-tert-butyl 4-hydroxybenzyl malonate (TINUVIN 144, manufactured by BASF), and polycondensate of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol and succinic acid dimethyl ester (manufactured by BASF). TINUVIN622SF), 1,5,8,12-tetrakis[4,6-bis(N-butyl-N-(1,2,2,6,6-pentamethyl-4-piperidyl)amino)-1,3,5-triazin-2-yl]-1,5,8,12-tetraazadodecane (CHIMASSORB119 manufactured by BASF), polycondensate of 1,2,3,4-butanetetracarboxylic acid, 1,2,2,6,6-pentamethyl-4-piperidinol, and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5,5]undecane-3,9-diethanol (ADKSTAB manufactured by ADEKA Corporation). LA-63P), a polycondensate of succinic acid and (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)ethanol, and a mixture of N,N',N'',N'''-tetrakis-(4,6-bis-(butyl-(N-methyl-2,2,6,6-tetramethylpiperidin-4-yl)amino)-triazin-2-yl)-4,7-diazadecane-1,10-diamine (TINUVIN111FDL, manufactured by BASF).

[0041] Specific examples of the N-OR type hindered amine light stabilizer include bis(1-octyloxy-2,2,6,6-tetramethylpiperidyl)sebacate (TINUVIN123 manufactured by BASF) and bis(1-undecanoxy-2,2,6,6-tetramethylpiperidin-4-yl)carbonate (ADKSTAB LA-81 manufactured by ADEKA Corporation).

[0042] <Other Components> The t-butylcatechol (TBC) content in the styrene-based resin composition is preferably 10 ppm or less, more preferably 5 ppm or less. By setting the content in this range, a light guide plate with excellent hue and transmittance can be obtained. Specific examples of the TBC content are 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 ppm, and may be within a range between any two of the values ​​exemplified here.

[0043] The 6-tert-butyl-2,4-xylenol (TBX) content in the styrene-based resin composition is preferably 10 ppm or less, more preferably 5 ppm or less. By setting the content in this range, a light guide plate with excellent hue and transmittance can be obtained. Specific examples of the TBX content include 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 ppm, and may be within a range between any two of the values ​​exemplified here.

[0044] The styrene-based resin composition may contain, within the scope not impairing the properties of the present invention, release agents such as sulfur-based antioxidants, lactone-based antioxidants, ultraviolet absorbers, antistatic agents, hydrophilic additives, liquid paraffin (mineral oil), polyethylene wax, microcrystalline wax, bluing agents, higher fatty acids such as lauric acid, myristic acid, palmitic acid, and stearic acid, higher fatty acid amides such as stearic acid amide, erucic acid amide, and ethylene bisstearic acid amide, higher fatty acid glycerides such as lauric acid monoglyceride, palmitic acid monoglyceride, stearic acid monoglyceride, and behenic acid monoglyceride, and higher alcohols such as myristyl alcohol, cetyl alcohol, and stearyl alcohol.

[0045] The melt mass flow rate (MFR) of the styrene resin composition at a temperature of 200°C and a load of 49 N is preferably 0.5 to 5.0 g / 10 min, more preferably 1.0 to 4.5 g / 10 min, and even more preferably 1.2 to 4.0 g / 10 min. Specific examples of the MFR include 0.5, 1.0, 1.2, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.5, 4.0, 4.5, and 5.0, and may be within a range between any two of the values ​​exemplified here. If the MFR is less than 0.5 g / 10 min, molding (extrusion) stability decreases, and if the MFR exceeds 5.0 g / 10 min, strength becomes insufficient.

[0046] The Vicat softening temperature of the styrene-based resin composition (measured at a heating rate of 50°C / hr and a test load of 50 N) is preferably 95°C or higher, and more preferably 98°C or higher. If the Vicat softening temperature is lower than 95°C, heat resistance is insufficient, and the light guide plate may be deformed depending on the usage environment. The upper limit of the Vicat softening temperature may be, for example, 130°C or 104°C.

[0047] The average transmittance at wavelengths of 380 to 780 nm at an optical path length of 115 mm measured using a test piece prepared by molding the styrene-based resin composition to a size of 115 mm x 80 mm x 3 mm is preferably 85% or higher, more preferably 86% or higher. The average transmittance (before a long-term durability test) is, for example, 85, 86, 87, 88, 89, 90, or 95%, and may be within a range between any two of the values ​​exemplified herein. Furthermore, the average transmittance at wavelengths of 380 to 780 nm at an optical path length of 115 mm measured using the test piece after a long-term durability test (storage in an oven at 80°C for 1,000 hours) is preferably 85% or higher. The average transmittance (after a long-term durability test) is, for example, 85, 86, 87, 88, 89, 90, or 95%, and may be within a range between any two of the values ​​exemplified herein.

[0048] The YI value (YI1) at an optical path length of 115 mm measured using a test piece prepared by molding the styrene-based resin composition to a size of 115 mm x 80 mm x 3 mm (thickness) is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. Specific examples of the YI value (YI1) include 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0, and may be within a range between any two of the values ​​exemplified here. Furthermore, the YI value (YI2) at an optical path length of 115 mm measured using the test piece after a long-term durability test (storage in an oven at 80°C for 1,000 hours) is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.5 or less. The YI value (YI2) is, for example, 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0, and may be within a range between any two of the values ​​exemplified here. ΔYI (YI2-YI1) is preferably 2.0 or less, more preferably 1.5 or less, and even more preferably 1.0 or less.

[0049] <Method for producing styrene-based resin composition> Examples of the polymerization method for the styrene-based resin (A) 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 preferred. Examples of solvents that can be used include alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methyl ethyl ketone, and aliphatic hydrocarbons such as hexane and cyclohexane.

[0050] During polymerization of the styrene-based resin (A), polymerization aids such as a polymerization initiator, a chain transfer agent, a crosslinking agent, and other polymerization aids may be used as necessary. The polymerization initiator is preferably a radical polymerization initiator, and examples of commonly used initiators 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-amylperoxyisononanoate, t-butylcumyl peroxide, di-t-butyl peroxide, dicumyl peroxide, and di-t-hexyl peroxide. peroxyesters such as t-butylperoxyacetate, t-butylperoxybenzoate, and t-butylperoxyisopropyl monocarbonate; peroxycarbonates such as t-butylperoxyisopropyl carbonate and polyethertetrakis(t-butylperoxycarbonate); 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 the like, and these can be used alone or in combination of two or more. Examples of the chain transfer agent include aliphatic mercaptans such as n-dodecyl mercaptan and tert-dodecyl mercaptan, aromatic mercaptans, thiocarboxylic acids such as thioglycolic acid and mercaptopropionic acid, polyfunctional mercaptans in which the hydroxyl group of a polyhydric alcohol such as ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, and sorbitol is esterified with thioglycolic acid or mercaptopropionic acid, pentaphenylethane, α-methylstyrene dimer, and terpinolene.Among these, aliphatic mercaptans, aromatic mercaptans, thiocarboxylic acids, and polyfunctional mercaptans are preferred because they allow easy molecular weight adjustment.

[0051] In the case of continuous polymerization, the styrene-based resin (A) can be produced by a method including a polymerization step, a devolatilization step, and a granulation step.

[0052] First, in the polymerization step, a known complete mixing tank type agitation tank or tower type reactor is used, and the polymerization reaction is controlled by adjusting the polymerization temperature etc. so as to achieve the target molecular weight, molecular weight distribution and reaction conversion rate.

[0053] 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 composed of a vacuum devolatilization tank equipped with a heater or a devolatilization extruder equipped 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 through a multi-hole die and processed into pellets using the cold cut method, the in-air hot cut method, or the underwater hot cut method.

[0054] The styrene-based resin composition can be produced by adding an antioxidant (B) and an anthraquinone-based colorant (C) to a styrene-based resin (A). The antioxidant (B) and the anthraquinone-based colorant (C) may be added to the raw material solution before polymerization of the styrene-based resin (A), or they may be mixed in an extruder or static mixer installed after polymerization of the styrene-based resin (A) and before granulation. Alternatively, the styrene-based resin composition may be produced by dry-blending pellets of the styrene-based resin (A) with the antioxidant (B) and the anthraquinone-based colorant (C), followed by melt-kneading. Alternatively, the antioxidant (B) and the anthraquinone-based colorant (C) may be melt-kneaded in advance with a small amount of styrene-based resin to prepare a pellet-shaped masterbatch, which may then be dry-blended with the styrene-based resin (A), followed by melt-kneading and preparation.

[0055] The content of t-butylcatechol or 6-tert-butyl-2,4-xylenol in the styrene resin composition can be adjusted at the start of polymerization of the styrene resin (A) and in the subsequent devolatilization step or the like.

[0056] 2. Light Guide Plate A light guide plate according to one embodiment of the present invention is a molded article obtained by molding the styrene-based resin composition. The light guide plate can be used for a variety of purposes, but is particularly suitable for use in an edge-light type surface light source unit. The edge-light type surface light source unit may be, for example, a unit for lighting, and the light guide plate may be a lighting light guide plate used in the unit.

[0057] <Shape of Light Guide Plate> The light guide plate may have an uneven surface. More specifically, the light guide plate may have a plurality of lenticular and / or prism-shaped convex portions on the surface. The convex portions are preferably provided on at least one surface of the light guide plate, and are particularly provided on one surface that is the front surface (light-emitting surface) of the light guide plate. They may also be provided on other surfaces if necessary, but it is more preferable that they are provided only on the front surface (light-emitting surface) of the light guide plate.

[0058] Here, the lenticular-shaped convex portions are arc-shaped convex portions, and are protrusions with an arc-shaped cross-sectional edge. Furthermore, the prism-shaped convex portions are arc-shaped convex portions, and are protrusions with a triangular mountain-shaped cross-sectional edge. Furthermore, multiple convex portions can be formed in parallel to each other. Furthermore, the convex portions can be formed integrally with the light guide plate.

[0059] The thickness of the light guide plate is 0.2 to 3.0 mm, preferably 0.3 to 2.5 mm, and more preferably 0.4 to 2.4 mm. Within such a range, it is easy to produce a light guide plate that is excellent in moldability, such as excellent extrusion stability, and strength, when molding the styrene-based resin composition.

[0060] <Optical Properties> The light guide plate has an average transmittance of 85% or more, more preferably 86% or more, for wavelengths of 380 to 780 nm when the light guide plate has an optical path length of 115 mm. After a long-term durability test (storage in an oven at 80°C for 1000 hours), the average transmittance of the light guide plate is preferably 85% or more for wavelengths of 380 to 780 nm when the light guide plate has an optical path length of 115 mm.

[0061] The YI value (YI1) of the light guide plate at an optical path length of 115 mm is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. Specifically, the YI value (YI1) may be, for example, 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0, and may be within a range between any two of the values ​​exemplified here. Furthermore, the YI value (YI2) of the light guide plate at an optical path length of 115 mm after a long-term durability test (storage in an oven at 80°C for 1,000 hours) is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.5 or less. The YI value (YI2) is, for example, 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0, and may be within a range between any two of the values ​​exemplified here. ΔYI (YI2-YI1) is preferably 2.0 or less, more preferably 1.5 or less, and even more preferably 1.0 or less.

[0062] <Method for Manufacturing Light Guide Plate> A light guide plate according to one embodiment of the present invention is obtained by molding the above-mentioned styrene-based resin composition. Although known methods such as sheet extrusion, injection molding, and compression molding can be used as the molding method, continuous sheet extrusion molding equipped with a surface shape transfer mold is preferred in terms of productivity and ease of producing large-sized molded products. An example of such sheet extrusion molding is a continuous sheet extrusion molding method that includes an extrusion step in which a resin is supplied in a heated and molten state to a feed block and continuously extruded from a die to produce an extrusion sheet, a pressing step in which the resin sheet is sandwiched between a pressure roll and a cooling roll, and a conveying step in which, after the pressing step, the resin sheet is conveyed while being in close contact with the cooling roll, and the cooling roll is equipped with a transfer mold on its surface. By changing the shape of the transfer mold, any desired uneven shape can be transferred to the sheet surface.

[0063] The light guide plate may have an uneven surface on its front surface (light-emitting surface), and the rear surface may be subjected to a reflective treatment for diffusely reflecting light. Examples of reflective treatments include silk printing, inkjet printing, and a method for imparting dot-shaped unevenness by laser irradiation. For printing the dot pattern, ink containing fine particles that diffuse light can be used.

[0064] 3. Edge-lit Type Surface Light Source Unit An edge-lit type surface light source unit according to one embodiment of the present invention is an edge-lit type surface light source unit having the above-described light guide plate and a light source that supplies light to an end face of the light guide plate. The edge-lit type surface light source unit is suitably used as a surface light source device for illumination or a liquid crystal display device, etc.

[0065] The present invention will be described in more detail below with reference to examples, but these examples are merely illustrative and are not intended to limit the scope of the present invention.

[0066] 1. Production of a styrene-based resin composition [Example 1] A polymerization process was performed by connecting a first reactor, which was a complete mixing type agitation tank, and a second reactor, which was a plug flow reactor equipped with a static mixer, in series, to produce a styrene-based resin. The volumes of the first reactor and the second reactor were 30 liters and 12 liters, respectively. The raw material composition was 65% by mass of styrene (TBC concentration 11 μg / g), 25% by mass of methyl methacrylate (TBX concentration 7 μg / g), and 10% by mass of ethylbenzene. The raw material solution was then adjusted at the inlet of the first reactor to contain 100 ppm of t-butylperoxyisopropyl monocarbonate (Perbutyl I, manufactured by NOF Corporation) as a polymerization initiator and 150 ppm of n-dodecyl mercaptan (manufactured by Arkema Inc.) as a chain transfer agent (all concentrations based on the mass of the raw material styrene). The raw material solution was then continuously supplied to the first reactor, which was set at 135°C, at a rate of 8.0 kg / h. The obtained polymerization solution was then continuously fed to a second reactor, where the polymerization was completed. The polymerization rate of the monomers was 70%. A temperature gradient was applied to the second reactor along the flow direction, adjusting the temperature to 135°C at the middle and 145°C at the outlet. The polymer-containing solution continuously removed from the second reactor was then introduced into a vacuum devolatilizer tank equipped with a preheater, which was configured in series with two stages. The temperature of the preheater was adjusted so that the resin temperature was 240°C, and unreacted styrene and ethylbenzene were separated at a pressure of 0.8 kPa. The obtained molten polymer was continuously fed to an extruder, and 0.2 parts by mass of a phosphorus-based antioxidant (168) and 0.1 parts by weight of a phosphorus-phenol-based antioxidant (GP) were added through an additive feed port relative to 100 parts by mass of the polymer, and an anthraquinone-based colorant (SV13) was added so that the concentration was 47 ppb relative to the polymer. After mixing at a set temperature of 220°C, the mixture was extruded into strands through a multi-hole die, and the strands were cooled and cut by a cold cut method to be pelletized.

[0067] Examples 2 to 18 and Comparative Examples 1 to 8 Styrenic resin compositions and light guide plates were produced in the same manner as in Example 1, except that the composition of the raw material solution and polymerization conditions were changed as shown in Table 1, and the blending ratios of the phosphorus-based antioxidant (B-1), phosphorus-phenol-based antioxidant (B-2), phenol-based antioxidant (B-3), anthraquinone-based colorant (C), and hindered amine-based light stabilizer (D) were changed as shown in Tables 2 to 4. The contents of the phosphorus-based antioxidant (B-1), phosphorus-phenol-based antioxidant (B-2), phenol-based antioxidant (B-3), and hindered amine-based light stabilizer (D) are expressed as the content relative to 100 parts by mass of the polymer (styrene-based resin), and the content of the anthraquinone-based colorant (C) is expressed as a ratio relative to the polymer (styrene-based resin). The results of various measurements and evaluations are shown in Tables 2 to 4.

[0068]

[0069]

[0070]

[0071]

[0072] The phosphorus-based antioxidant (B-1), phosphorus-phenol-based antioxidant (B-2), anthraquinone-based colorant (C), and hindered amine-based light stabilizer (D) in Tables 2 and 3 are as follows:

[0073] (Phosphorus-based antioxidants (B-1)) 168: Tris(2,4-di-tert-butylphenyl)phosphite (Irgafos 168, manufactured by BASF) 6260: Bis-(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (Songnox 6260, manufactured by SONGWON) HP-10: 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus (ADKSTAB HP-10, manufactured by ADEKA)

[0074] (Phosphorus-phenol-based antioxidant (B-2)) GP: 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine (Sumilizer GP, manufactured by Sumitomo Chemical Co., Ltd.)

[0075] (Phenol-based antioxidants (B-3)) 245: Ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate] (Irganox 245, manufactured by BASF Japan Ltd.) 1010: Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 1010, manufactured by BASF Japan Ltd.) 1076: Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1076, manufactured by BASF Japan Ltd.)

[0076] (Anthraquinone-based colorants (C)) SV13 (Solvent Violet 13): 1-hydroxy-4-(4-methylphenylamino)anthracene-9,10-dione (Macrolex Violet B Gran, manufactured by LANXESS Corporation) SV33 (Solvent Violet 33): (Dia Resin Blue J, manufactured by Mitsubishi Chemical Corporation) SB45 (Solvent Blue 45): 3,3'-[(9,10-dihydro-9,10-dioxo-1,4-anthracene)diimino]bis[N-cyclohexyl-2,4,6-trimethylbenzenesulfonamide] (Transparent Blue S-RLS, manufactured by EPSILON Corporation)

[0077] (Hindered amine light stabilizer (D)) 292: a mixture of 25% methyl(1,2,2,6,6-pentamethylpiperidin-4-yl)sebacate and 75% bis(1,2,2,6,6-pentamethylpiperidin-4-yl)sebacate (TINUVIN 292, manufactured by BASF)

[0078] 2. Evaluation The resin properties in Table 1 and the properties of the styrene resin compositions shown in Tables 2 to 4 were measured and evaluated by the following methods.

[0079] <Melt Mass Flow Rate (MFR)> The melt mass flow rate was measured in accordance with JIS K 7210 under conditions of a temperature of 200°C and a load of 49N.

[0080] <Vicat Softening Temperature> The Vicat softening temperature was measured in accordance with JIS K 7206 at a temperature rise rate of 50° C. / hr and a test load of 50 N.

[0081] <Contents of TBC and TBX in Styrenic Resin (Copolymer)> 0.2 g of 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 perform 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 concentrations. The TBC and TBX contained in the styrene-based resin composition can be calculated from the blending ratio of the resins used. Similar measurements can also be performed on styrene-based resin compositions. GC device: 7890A manufactured by Agilent Co. Column: DB-5ms (0.25 mm i.d. x 30 m) manufactured by Agilent Co., Ltd. Liquid phase film thickness 0.25 μm Column temperature: 50°C (1 min) → (20°C / min temperature increase) → 320°C (6.5 min) Total 20 min Inlet : 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 quantitative, m / z 310 for confirmation)

[0082] <Contents of Phosphorus-Based Antioxidant (B-1) and Phosphorus-Phenol-Based Antioxidant (B-2) in Styrenic Resin Composition> 1.0 g of the styrene-based resin composition was completely dissolved in 20 mL of THF, and then 5 mL of methanol was added dropwise and stirred for 20 minutes. The mixture was centrifuged at 4000 rpm for 10 minutes, and the separated supernatant was measured by gas chromatography (GC) under the following conditions. A calibration curve prepared in advance was used to determine the concentrations. GC apparatus: Shimadzu GC2010 Plus Column: DB-1 (30 m x 0.25 mm i.d., df = 0.10 μm) Column temperature: 240 ° C (1 min) → (heating rate 10 ° C / min) → 320 ° C (15 min) Injection port: 320 ° C, 1.02 mL / min (split ratio 1:5) Injection volume: 1 μL

[0083] <Weight Average Molecular Weight (Mw)> The weight average molecular weight (Mw), Z average molecular weight (Mz), and number average molecular weight (Mn) were measured using gel permeation chromatography (GPC) under the following conditions: GPC model: Shodex GPC-101 manufactured by Showa Denko K.K. Column: PLgel 10 μm MIXED-B manufactured by Polymer Laboratories, Inc. 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 as a polystyrene-equivalent molecular weight by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene.

[0084] <Average Transmittance and YI Value of Styrenic Resin Composition> The average transmittance and YI value were measured according to the following procedure. Using pellets of the styrene-based resin composition, injection molding was performed at a cylinder temperature of 190°C and a mold temperature of 40°C to form a plate-shaped molded product measuring 115 mm x 80 mm x 3 mm thick. Next, for the obtained molded product, a UV-visible spectrophotometer V-670 manufactured by JASCO Corporation was used to measure the spectral transmittance at wavelengths of 350 nm to 800 nm at an optical path length of 115 mm, with incident light measuring 20 x 1.6 mm and having a spread angle of 0°. The YI value at a field of view of 2° under C light source was calculated in accordance with JIS K7105. The average transmittance (total light transmittance) was calculated as the average of the spectral transmittance at wavelengths of 380 to 780 nm. In addition, to evaluate long-term thermal stability, the obtained molded product was stored in an oven at 80°C for 1,000 hours. The test light guide plate before storage was the initial test light guide plate, and the test light guide plate after storage was the test light guide plate after the long-term durability test. Test pieces were similarly cut out from the test light guide plate after the long-term durability test, and the YI value and average transmittance were calculated.

[0085] 3. Production and Evaluation of Light Guide Plate <Production of Light Guide Plate> A styrene-based resin composition was supplied to a single-screw vented extruder having a screw diameter of 90 mm and an L / D ratio of 32, and melt-kneaded at 200 to 235°C. The composition was then extruded through a T-die having a lip width of 800 mm and a lip opening of 3.0 mm at a T-die temperature of 245 to 250°C and a screw rotation speed of 75 rpm. The composition was cooled and solidified with three vertical cooling rolls, and the edge surfaces were trimmed to obtain a test light guide plate having a width of 600 mm and a thickness of 2.0 mm.

[0086] <Evaluation of Light Guide Plates> The test light guide plates in Tables 2 to 4 were evaluated for extrusion stability (surging), color temperature difference, and dimensional stability (deformation due to moisture absorption) as follows.

[0087] <Extrusion stability (surging)> In the production of test light guide plates, the screw rotation speed of the extruder was gradually increased to confirm the screw rotation speed at which surging (poor discharge such as fluctuations in plate thickness) occurred, and the extrusion stability was evaluated based on the following criteria: ◎: Stable production possible at a screw rotation speed of over 150 rpm. ○: Stable production possible at a screw rotation speed in the range of 100 to 150 rpm. △: Stable production possible at a screw rotation speed of less than 100 rpm.

[0088] <Color Temperature Change (Long-Term Durability Against LED Light Source)> The test light guide plate was incorporated into a 600 mm x 600 mm edge-lit lighting unit, and a white LED with a color temperature of 5700 K was turned on. The color temperature at the center of the surface was measured at a measurement distance of 1 m using a Konica Minolta CL-200A colorimeter (initial color temperature). Next, the lighting was turned on continuously for 5,000 hours, and the color temperature was measured in the same manner (color temperature after 5,000 hours). The color temperature change was evaluated according to the following criteria: Color Temperature Change (%) = [Initial Color Temperature (K) - Color Temperature after 5,000 hours (K)] / Initial Color Temperature (K) x 100. ◯: Color temperature change is less than 1%. △: Color temperature change is in the range of 1 to 5%. ×: Color temperature change is more than 5%.

[0089] <Dimensional Stability (Moisture Absorption Deformation)> A 200 mm × 300 mm test piece was cut out from the test light guide plate, and the test piece was stored for 500 hours under conditions of a temperature of 60°C and a relative humidity of 90%. The dimensional change in the long side before and after storage was measured, and the deformation rate was calculated using the following formula: Deformation rate = ((Length of long side after storage) - (Length of long side before storage)) ÷ (Length of long side before storage) × 100 (%) The dimensional stability (moisture absorption change) of the light guide plate was evaluated with a change rate of less than 0.10% being ◯, a change rate of 0.10 to 0.15% being △, and a change rate of more than 0.15% being ×.

[0090] Examples 1 to 18 were good in transparency, hue, extrusion stability, long-term hue stability, and dimensional stability, and Examples 1 to 16 were particularly excellent in both extrusion stability and dimensional stability. Furthermore, Example 3, in which the hindered amine light stabilizer (D) was added, showed a 0.2% smaller color temperature change than Example 2, in which the hindered amine light stabilizer (D) was not added, and was superior in long-term durability against an LED light source.

Claims

1. A styrene-based resin composition containing a styrene-based resin (A) which is a copolymer containing a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit, an antioxidant (B), and an anthraquinone-based colorant (C), wherein the antioxidant (B) contains either or both of a phosphorus-based antioxidant (B-1) and a phosphorus-phenol-based antioxidant (B-2), the total amount of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-2) is 0.001 to 0.5 parts by mass per 100 parts by mass of the styrene-based resin (A), and the anthraquinone-based colorant (C) is 0.1 to 150 ppb relative to the styrene-based resin (A).

2. The styrene-based resin composition according to claim 1, wherein the styrene-based resin (A) contains 20 to 95 mass% of the styrene-based monomer units and 5 to 80 mass% of the (meth)acrylic acid ester-based monomer units per 100 mass% of the styrene-based resin (A).

3. The styrene-based resin composition according to claim 1, wherein 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, bis-(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, 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.

4. The styrene-based resin composition according to claim 1, wherein the phosphorus-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.

5. The styrene-based resin composition according to claim 1, wherein the average transmittance of a wavelength of 380 to 780 nm at an optical path length of 115 mm measured using a test piece prepared by molding the styrene-based resin composition into a size of 115 mm x 80 mm x 3 mm (thickness) is 85% or more.

6. The styrene-based resin composition according to claim 1, wherein the YI value at an optical path length of 115 mm measured using a test piece prepared by molding the styrene-based resin composition into a size of 115 mm x 80 mm x 3 mm (thickness) is 4.0 or less.

7. An edge-light type light guide plate which is a molded article of the styrene resin composition according to any one of claims 1 to 6.

8. An edge-light type surface light source unit comprising the edge-light type light guide plate according to claim 7 and a light source for supplying light to the end face of the light guide plate.

9. The edge-light type surface light source unit for lighting according to claim 7.

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