JP7590412B2 - Optical styrene resin composition, light guide plate and edge-light type surface light source unit - Google Patents

Description

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

There are two types of backlights for LCD displays: direct-light types, in which the light source is placed on the back of the display device, and edge-lit types, 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 entire surface of the display device, and are used in a wide range of applications, including televisions, monitors for desktop personal computers, notebook personal computers, mobile phones, and monitors for car navigation systems. Backlights that use light guide plates are also used as lighting devices and signs.

Light guide plates are required to have a particularly high light transmittance because the light transmission distance is relatively long and the light loss in 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, and water absorption may cause warping or dimensional changes in the light guide plate. In addition, PMMA is prone to thermal decomposition during molding, so there is a problem that molding at high temperatures is likely to cause poor appearance of the molded product. In order to improve these problems, it has been proposed to use a styrene-methyl (meth)acrylate copolymer as the material for the light guide plate (see, for example, Patent Document 1).
On the other hand, styrene-based resins made from styrene-based monomers have excellent low water absorption, but have a problem of the molded product becoming cloudy due to environmental changes such as temperature and humidity. Specifically, when a molded product is exposed from a high-temperature and high-humidity environment to a room temperature environment or from a room temperature environment to a low temperature environment, the moisture uniformly present in the molded product becomes unstable and undergoes phase separation, causing minute defects in the molded product, resulting in cloudiness. In order to solve this problem, a plate-shaped molded product made of a styrene-based resin copolymerized with a styrene-based monomer and a small amount of a (meth)acrylic acid ester monomer has been proposed (see Patent Document 2).

JP 2003-075648 A JP2013-170186Publication

However, it was difficult for the optical styrene-based resin composition that constitutes the light guide plate to simultaneously satisfy the requirements for transparency, hue, hue stability, extrusion stability, dimensional stability, resistance to moist and heat whitening, and strength.

The present invention has been made in consideration of such problems, and provides a styrene-based resin composition for light guide plates that simultaneously satisfies hue, hue stability, extrusion stability, dimensional stability, resistance to moist and heat whitening, and strength.

According to the present invention, there is provided an optical styrene-based resin composition comprising a styrene-based resin (A) which is a copolymer containing 51 to 99% by mass of a styrene-based monomer unit and 1 to 49% by mass of a (meth)acrylic acid ester-based monomer unit, and an antioxidant (B), wherein the antioxidant (B) comprises 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 optical styrene-based resin composition contains, relative to 100 parts by mass of the styrene-based resin (A), 0.1 to 0.5 parts by mass in total of the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3), and 0.01 to 0.5 parts by mass in total of the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3).

After extensive research, the inventors discovered that when the content of styrene-based monomer units and (meth)acrylic acid ester-based monomer units in the styrene-based resin, and the content of antioxidant are within specified ranges, an optical styrene-based resin composition simultaneously satisfies the desired hue, hue stability, extrusion stability, dimensional stability, resistance to moist heat whitening, and strength, and thus completed 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 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, and is an optical styrene-based resin composition.
Preferably, the phenol-based antioxidant (B-2) is 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, which is an optical styrene-based resin composition.
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, which is an optical styrene-based resin composition.
Preferably, the optical styrene-based resin composition contains 0.1 to 20 ppm of 6-tert-butyl-2,4-xylenol.
Preferably, the optical styrene-based resin composition contains 0.1 to 500 ppm of sulfur.
Preferably, in the optical styrene-based resin composition, the weight average molecular weight (Mw) of the styrene-based resin (A) is 50,000 to 400,000, and 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 1.0 to 3.0.
Preferably, the optical styrene-based resin composition has an average transmittance of 85% or more at a wavelength of 380 to 780 nm when the initial optical path length is 115 mm.
According to another aspect of the present invention, there is provided a light guide plate obtained by molding the above-mentioned optical styrene-based resin composition.
Preferably, the light guide plate has a thickness of 1.0 to 3.0 mm.
According to another aspect of the present invention, there is provided an edge-light type surface light source unit having the above light guide plate and a light source that supplies light to an end face of the light guide 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. Styrene-Based Resin Composition The styrene-based resin composition according to one embodiment of the present invention is an optical styrene-based resin composition containing a styrene-based resin (A) and an antioxidant (B).

<Styrene-based resin (A)>
The styrene-based resin (A) is a resin obtained by copolymerizing a monomer containing a styrene-based monomer and a (meth)acrylic acid ester-based monomer. The styrene-based resin (A) is a copolymer containing 51 to 99% by mass of a styrene-based monomer unit and 1 to 49% by mass of a (meth)acrylic acid ester-based monomer unit, preferably a copolymer containing 51 to 85% by mass of a styrene-based monomer unit and 15 to 49% by mass of a (meth)acrylic acid ester-based monomer unit, more preferably a copolymer containing 80 to 85% by mass of a styrene-based monomer unit and 15 to 20% by mass of a (meth)acrylic acid ester-based monomer unit. By setting the content in such a range, transparency, hue, hue stability, extrusion stability, dimensional stability, resistance to wet heat whitening, and strength can be simultaneously satisfied. In particular, by setting the content of the styrene-based monomer to 99% by mass or less, a light guide plate with little wet heat whitening can be obtained, and by setting the content of the styrene-based monomer to 51% or more, a light guide plate with excellent dimensional stability can be obtained. The content of the (meth)acrylic acid ester monomer unit in the styrene-based resin (A) is specifically, for example, 1, 5, 10, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, or 50 mass%, and may be within a range between any two of the numerical values exemplified here.

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 styrene-based monomer is preferably styrene.

Examples of (meth)acrylic 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 an alkyl (meth)acrylate, and more preferably methyl methacrylate.

The styrene-based resin (A) may also be a copolymer obtained by copolymerizing a styrene-based monomer and a monomer copolymerizable with 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 can be used alone or in combination of two or more.

The weight average molecular weight (Mw) of the styrene resin (A) is preferably 50,000 to 400,000, more preferably 100,000 to 140,000. The ratio (Mw/Mn) of the weight average molecular weight (Mw) and 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. By setting it in 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, the moldability may decrease. If the ratio (Mw/Mn) of the number average molecular weight (Mn) is less than 1.0, the moldability may decrease, and if it exceeds 3.0, the strength of the molded product may decrease.

<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 optical styrene resin composition contains a total of 0.1 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-based resin (A). By setting the content in this range, transparency, hue, hue stability, extrusion stability, dimensional stability, resistance to wet heat whitening, and strength can be simultaneously satisfied. The total content of phosphorus-based antioxidant (B-1) and phosphorus-phenol-based antioxidant (B-3) is specifically, for example, 0.1, 0.2, 0.3, 0.4, or 0.5 parts by mass per 100 parts by mass of styrene-based resin (A), and may be within a range between any two of the numerical values exemplified here.

The optical styrene-based resin composition contains a total of 0.01 to 0.5 parts by mass, preferably 0.05 to 0.3 parts by mass, of the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) per 100 parts by mass of the styrene-based resin (A). By setting the content in such a range, transparency, hue, hue stability, extrusion stability, dimensional stability, resistance to wet heat whitening, and strength can be simultaneously satisfied. The total content of the phenol-based antioxidant (B-2) and the phosphorus-phenol-based antioxidant (B-3) is, for example, 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, or 0.5 parts by mass per 100 parts by mass of the styrene-based resin (A), 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. These may be used alone or in combination of two or more.

The phenolic antioxidant (B-2) is an antioxidant that has a phenolic hydroxyl group in the basic skeleton and is not a phosphorous ester. The phenolic 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. These can be used alone or in combination of two or more.

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. An example of 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.

<Other ingredients>
The optical styrene resin composition contains 6-tert-butyl-2,4-xylenol (TBX) in an amount of preferably 0.1 to 20 ppm, more preferably 1 to 5 ppm. By setting the content in this range, a light guide plate having excellent hue and transmittance can be obtained. The content of TBX is specifically, for example, 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, or 20 ppm, and may be within a range between any two of the values exemplified here.

The optical styrene resin composition preferably contains 0.1 to 100 ppm of t-butylcatechol (TBC), more preferably 1 to 20 ppm. By setting the content in this range, a light guide plate with excellent hue and transmittance can be obtained. The TBC content is specifically, for example, 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, 20, 30, 40, 50, 60, 70, 80, 90, or 100 ppm, and may be within a range between any two of the values exemplified here.

The optical styrene resin composition preferably contains 0.1 to 500 ppm of sulfur, more preferably 1 to 400 ppm, and even more preferably 5 to 350 ppm. By setting the sulfur content in this range, a light guide plate with particularly excellent hue can be obtained. The sulfur content is specifically, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 200, 300, 400, or 500 ppm, and may be within a range between any two of the numerical values exemplified here.

The optical styrene-based resin composition may contain release agents such as sulfur-based antioxidants, lactone-based antioxidants, UV absorbers, hindered amine-based stabilizers, 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 stearamide, erucic acid amide, and ethylene bisstearamide, and higher alcohols such as myristyl alcohol, cetyl alcohol, and stearyl alcohol, to the extent that the properties of the present invention are not impaired.

<Optical properties>

The styrene-based resin composition preferably has an average transmittance of 85% or more, and more preferably 86% or more, for wavelengths of 380 to 780 nm at an initial optical path length of 115 mm.

The styrene-based resin composition preferably has an average transmittance of 80% or more, more preferably 83% or more, and even more preferably 85% or more at wavelengths of 380 to 780 nm at an optical path length of 115 mm after a long-term durability test.

The styrene-based resin composition preferably has a YI value of 6.0 or less, and more preferably 4.0 or less, at an initial optical path length of 115 mm.

The styrene-based resin composition preferably has a YI value of 7.0 or less, and more preferably 5.0 or less, at an optical path length of 115 mm after a long-term durability test.

The styrene-based resin composition preferably has a difference in YI value (ΔYI) between the initial YI value at an optical path length of 115 mm and the YI value at an optical path length of 115 mm after a long-term durability test of 3.0 or less, more preferably 1.5 or less, and even more preferably 1.0 or less.

＜Other＞
The Vicat softening temperature of the styrene-based resin composition is preferably 95 to 104° C., and more preferably 100 to 104° C. If the Vicat softening temperature is less than 95° C., the heat resistance is insufficient, and the light guide plate may be deformed depending on the usage environment.

<Method of producing styrene-based resin composition>
The polymerization method of the styrene-based resin (A) may be a known styrene polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, or emulsion polymerization. In terms of quality and productivity, bulk polymerization and solution polymerization are preferred, and continuous polymerization is preferred. As the solvent, for example, alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methyl ethyl ketone, and aliphatic hydrocarbons such as hexane and cyclohexane can be used.

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. 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 these may 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 obtained by esterifying the hydroxyl groups of polyhydric alcohols such as ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, and sorbitol 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 from the viewpoint of being able to adjust the sulfur content.

In the case of continuous polymerization, styrene-based resin (A) 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 styrene-based resin composition can be produced by adding an antioxidant (B) to a styrene-based resin (A). The antioxidant (B) may be added during polymerization of the styrene-based resin (A), or the styrene-based resin (A) may be polymerized, and then the antioxidant (B) may be dry-blended and melt-kneaded to produce the composition. Alternatively, the antioxidant (B) may be melt-kneaded in advance with a small amount of styrene-based resin to produce a pellet-shaped master batch, which may then be dry-blended with the styrene-based resin (A) and then melt-kneaded to adjust the composition.

The content of t-butylcatechol or 6-tert-butyl-2,4-xylenol and the sulfur content in the styrene-based resin composition can be adjusted at the start of polymerization of the styrene-based resin (A) and during the subsequent devolatilization process, etc. They may also be added and adjusted at any time.

2. Light Guide Plate A light guide plate according to one embodiment of the present invention is a molded product obtained by molding the optical styrene-based resin composition. The light guide plate is a light guide plate that can be used in an edge light type surface light source unit.

<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 be provided on other surfaces as well if necessary, but it is more preferable that they are provided only on the front surface (light-emitting surface) of the light guide plate.

Here, the lenticular-shaped convex portion refers to an arc-shaped convex portion, which is a protrusion whose cross-sectional edge shape is an arc-shaped cross section. Also, the prism-shaped convex portion refers to an arc-shaped convex portion, which is a protrusion whose cross-sectional edge shape is a triangular mountain shape. Also, multiple convex portions can be formed in a parallel relationship with each other. Also, the convex portions can be formed integrally with the light guide plate.

The thickness of the light guide plate is 1.0 to 3.0 mm, preferably 1.5 to 2.5 mm, and more preferably 1.6 to 2.4 mm. Within this range, it is easy to manufacture a light guide plate that has excellent moldability, such as excellent extrusion stability, and strength, when molding a styrene-based resin composition to which an antioxidant has been added.

<Method of manufacturing light guide plate>
The light guide plate according to one embodiment of the present invention is obtained by molding the above-mentioned styrene-based resin composition, and the molding method can be a known method such as sheet extrusion molding, injection molding, compression molding, etc., but in terms of productivity and ease of making large-sized molded products, continuous sheet extrusion molding equipped with a surface shape transfer mold is preferable. An example of the sheet extrusion molding is a continuous sheet extrusion molding method having an extrusion process in which a resin is supplied to a feed block in a heated and molten state and an extrusion sheet is continuously produced from a die, a pressing process in which the resin sheet is sandwiched between a pressure roll and a cooling roll, and a conveying process in which the resin sheet is conveyed while being in close contact with the cooling roll after the pressing process, and a transfer mold is provided on the surface of the cooling roll, and by changing the shape of the transfer mold, any uneven shape can be transferred to the sheet surface.

The light guide plate may have an uneven surface on the front surface (light-emitting surface), and the back surface may be given a reflective treatment that diffuses light. Examples of reflective treatments include silk screen printing and inkjet printing, as well as a method of applying dot-shaped unevenness by laser irradiation. For printing the dot pattern, an ink containing fine particles that diffuse light can be used.

3. Edge-light type surface light source unit The edge-light type surface light source unit according to one embodiment of the present invention is an edge-light type surface light source unit having the above-mentioned light guide plate and a light source that supplies light to an end face of the light guide plate. The edge-light type surface light source unit is suitably used as a surface light source device for a liquid crystal display device.

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. Production of styrene-based resins A-1 to A-11 A polymerization process was performed by connecting the first reactor, which was a complete mixing type stirring tank, and the second reactor, which was a plug-flow type reactor equipped with a static mixer, in series, and the production of styrene-based resins was performed under the conditions shown in Table 1. The capacity of each reactor was 30 liters for the first reactor and 12 liters for the second 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 raw materials shown in Table 1 are as follows.
Polymerization initiator: t-butylperoxyisopropyl monocarbonate (Perbutyl I, manufactured by NOF Corporation)
Chain transfer agent: n-dodecyl mercaptan (manufactured by Arkema Co., Ltd.)
In the second reactor, a temperature gradient was applied along the flow direction of the reaction liquid, and the temperatures at the intermediate portion and outlet portion were adjusted to the temperatures shown in Table 1.
Next, the polymer-containing solution continuously removed from the second 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 into pellets by a cold cut method.

<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 49 N.

<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.

<Sulfur content>
The measurement was performed by combustion chromatography using an ion chromatograph (DX-120 manufactured by DIONEX Corporation) under the measurement conditions described below.
Combustion pretreatment equipment: AQF-100, WS100, GA-100 (manufactured by Mitsubishi Chemical Corporation)
Sample amount: 100 mg
Combustion tube temperature: inlet 900°C, outlet 1000°C
Absorption solution: 600 mg/L _H2O2 + 10 mg/L _{PO43- (} internal standard ⁾
Amount of absorbed liquid: 5 mL
Detector: Electrical conductivity detector Column: AS12A
Flow rate: 1.5mL/min
Mobile phase: _{2.7mM Na2CO3} + 0.3mM _NaHCO3
Sample introduction volume: 20 μL

<Content of TBC and TBX in styrene-based resin>
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 id x 30 m) 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
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)

<Molecular weight>
The weight average molecular weight (Mw), Z average molecular weight (Mz), and number average molecular weight (Mn) 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.

2. Production of styrene-based resin composition [Example 1]
100 parts by mass of the pelletized styrene-based resin (A-1) was mixed with 0.2 parts by mass of a phosphorus-based antioxidant (HP-10) and 0.1 parts by mass of a phosphorus-phenol-based antioxidant (GP) 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.

[Examples 2 to 18 and Comparative Examples 1 to 8]
A styrene-based resin composition and a light guide plate were produced in the same manner as in Example 1, except that the formulation was changed as shown in Tables 2 to 4. The results of various measurements and evaluations are shown in Tables 2 to 4.

The phosphorus-based antioxidant (B-1), phenol-based antioxidant (B-2), and phosphorus-phenol-based antioxidant (B-3) in Tables 2 to 4 are as follows:

(Phosphorus-based antioxidant (B-1))
HP-10: 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus (ADEKA STAB HP-10 manufactured by ADEKA CORPORATION)
168: Tris(2,4-di-tert-butylphenyl)phosphite (Irgafos 168, manufactured by BASF Japan Ltd.)
PEP-36: 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (ADEKA STAB PEP-36 manufactured by ADEKA Corporation)
P-EPQ: tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4-diylbiphosphonite (Hostanox P-EPQ, manufactured by Clariant Co., Ltd.)
38: Bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite (Irgafos 38, manufactured by BASF Japan Ltd.)

(Phenol-based antioxidant (B-2))
1076: Octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (Irganox 1076, manufactured by BASF Japan Ltd.)
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.)
AO80: 3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (ADEKA STAB AO-80, manufactured by ADEKA Corporation)

(Phosphorus-phenol-based antioxidant (B-3))
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)

3. Evaluation The contents of TBC, TBX and sulfur in Tables 2 to 4 are values calculated based on the contents in the styrene-based resin used.

<Average transmittance and YI value of styrene-based resin composition>
The average transmittance and the YI value were measured by the following procedure: Pellets of the styrene-based resin composition were used and injection molding was performed at a cylinder temperature of 230° C. and a mold temperature of 50° C. to mold a plate-shaped molded product (initial sample) having dimensions of 127×127×3 mm.
The samples for evaluating the long-term durability (samples after the long-term durability test) were stored in an oven at 80° C. for 1000 hours.
Next, a test piece having a thickness of 115×85×3 mm was cut out from the plate-shaped molded product of the initial sample and the sample after the long-term durability test, and the end surface was polished by buffing to prepare a plate-shaped molded product having a mirror surface on the end surface. For the plate-shaped molded product after polishing, 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 of a size of 20×1.6 mm and a spread angle of 0°, and the YI value at a visual field of 2° with a C light source was calculated according to JIS K7105. The average transmittance (total light transmittance) was calculated as the average of the spectral transmittance at wavelengths of 380 to 780 nm.

<Manufacture of light guide plate>
The styrene-based resin composition was supplied to a single-screw vented extruder having a screw diameter of 90 mm and L/D = 32, melt-kneaded at 200 to 235°C, and then extruded through a T-die having a lip width of 1000 mm and a lip opening of 3.0 mm at a T-die temperature of 245 to 250°C. The composition was cooled and solidified with three vertical cooling rolls, and the end faces were trimmed to obtain a light guide plate having a width of 800 mm and a thickness of 2.0 mm.

<Evaluation of formability and strength of light guide plate>
The light guide plates in Tables 2 to 4 were evaluated for extrusion stability (no resin), dimensional stability (deformation due to moisture absorption), resistance to wet heat whitening, and strength as follows.

(Extrusion stability (eye resin))
The resin temperature in the die was adjusted to 300° C., and the occurrence of die resin near the die was evaluated based on the following criteria.
◯: No eye boogers were observed even after 30 minutes had passed since the start of sheet extrusion.
Δ: Eye boogers were observed 10 to 30 minutes after the start of sheet extrusion.
x: Eye boogers were observed within 10 minutes after the start of sheet extrusion.
It should be noted that "jet ooze" refers to brown or black resin deterioration that occurs around the die exit nozzle. It usually increases with the extrusion amount, and when it exceeds a certain amount, it detaches from the nozzle and adheres to the sheet surface.

(Dimensional stability (deformation due to moisture absorption))
A test piece of 200 mm × 300 mm was cut out from the light guide plate obtained above, and the test piece was stored under conditions of a temperature of 60°C and a relative humidity of 90% for 500 hours. The dimensional change of the long side before and after storage was measured, and the deformation rate was calculated by the following formula.
Deformation rate = ((long side length after storage) - (long side length before storage)) ÷ (long side length before storage) x 100 (%)
The dimensional stability (change due to moisture absorption) of the light guide plate was evaluated based on the following criteria: a deformation rate of 0.10% or less was rated as ◯, a deformation rate of 0.10 to 0.15% was rated as △, and a deformation rate of 0.15% or more was rated as x.

(Wet and heat whitening resistance)
A test piece of 200 mm x 200 mm was cut out from the light guide plate obtained above, and the test piece was exposed to an environment of 60°C and 90% relative humidity for 150 hours, and then the test piece was taken out and placed in an environment of 23°C and 50% relative humidity, rapidly cooled, and then left for another hour. After that, the whitening phenomenon occurring inside the test piece was observed, and the resistance to moist heat whitening was evaluated based on the following criteria.
◯: No whitening occurs at all △: Slight whitening occurs, but almost disappears after 24 hours ×: Significant whitening occurs, and does not disappear even after 24 hours

(Strength)
A test piece of 200 mm x 200 mm was cut out from the light guide plate obtained above, and the 50% breaking height was measured using a ball weighing 16.6 g according to JIS K-7211. The strength of the light guide plate was measured, with a 50% breaking height of 50 cm or more being ◯, a 50% breaking height of 30 to 50 cm being △, and a 50 cm or less being ×.

In the examples, the transparency, hue, hue stability, extrusion stability, dimensional stability, resistance to wet heat whitening and strength were good, and in Examples 1 to 12 and Examples 15 to 18, the resistance to wet heat whitening was particularly excellent.
Moreover, in Examples 1 to 16 and 18, in which the sulfur content was 0.1 to 500 ppm, the color was particularly excellent.

Claims (12)

An optical styrene-based resin composition comprising: (A) a styrene-based resin which is a copolymer containing 51 to 85% by mass of a styrene-based monomer unit and 15 to 49% by mass of a (meth)acrylic acid ester-based monomer unit; and (B) an antioxidant,
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 (A),
the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) are 0.1 to 0.5 parts by mass in total,
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,
Contains
An optical styrene-based resin composition (excluding those in which the styrene-based resin (A) contains 10% by mass or more of unsaturated dicarboxylic anhydride monomer units , and those in which an N-phenyl-substituted maleimide-olefin copolymer consisting of units represented by the following general formula (I) and units represented by the following general formula (II) and having a number average molecular weight of 1 x 103 ^or more and 5 x 106 ^or less is contained in an amount of 30 to 95 parts by mass, when the total amount of the styrene-based resin (A) and the N-phenyl-substituted maleimide-olefin copolymer is taken as 100 parts by mass ):
(wherein R1 is a phenyl group, a naphthyl group, a halogen element, a carboxylic acid, a carboxylic acid ester,
Steryl, hydroxyl, cyano, nitro, linear alkyl group with 1 to 8 carbon atoms,
It represents a phenyl group or naphthyl group substituted with a branched alkyl group of the formula:
(wherein R2 and R3 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.) An optical styrene-based resin composition comprising: (A) a styrene-based resin which is a copolymer containing 51 to 85% by mass of a styrene-based monomer unit and 15 to 49% by mass of a (meth)acrylic acid ester-based monomer unit; and (B) an antioxidant,
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 (A),
the phosphorus-based antioxidant (B-1) and the phosphorus-phenol-based antioxidant (B-3) are 0.1 to 0.5 parts by mass in total,
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,
Contains
An optical styrene-based resin composition having a Vicat softening temperature of 95 to 104°C, measured in accordance with JIS K 7206 at a heating rate of 50°C/hr and a test load of 50 N (excluding those in which the styrene-based resin (A) contains 10% by mass or more of unsaturated dicarboxylic anhydride-based monomer units) . The optical styrene-based resin composition according to claim 1 or 2, wherein the phosphorus-based antioxidant (B-1) is at least one selected from 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, tris(2,4-di-tert-butylphenyl)phosphite, 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane, tetrakis(2,4-di-tert-butylphenyl)[1,1biphenyl]-4,4 diylbisphosphonite, and bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphite. The optical styrene-based resin composition according to any one of claims 1 to 3, wherein the phenol-based antioxidant (B-2) is 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. The optical styrene-based resin composition according to any one of claims 1 to 4, 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 optical styrene-based resin composition according to any one of claims 1 to 5, wherein the optical styrene-based resin composition contains 0.1 to 20 ppm of 6-tert-butyl-2,4-xylenol. The optical styrene-based resin composition according to any one of claims 1 to 6, wherein the optical styrene-based resin composition contains 0.1 to 500 ppm of sulfur. The weight average molecular weight (Mw) of the styrene-based resin (A) is 50,000 to 400,000,
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 1.0 to 3.0.
The optical styrene-based resin composition according to any one of claims 1 to 7. The optical styrene-based resin composition according to any one of claims 1 to 8, in which the average transmittance at wavelengths of 380 to 780 nm at an initial optical path length of 115 mm is 85% or more. A light guide plate formed by molding the optical styrene-based resin composition according to any one of claims 1 to 9. The light guide plate according to claim 10, having a thickness of 1.0 to 3.0 mm. An edge-light type surface light source unit having the light guide plate according to claim 10 or 11 and a light source that supplies light to an end surface of the light guide plate.