JP7519348B2 - Styrenic resin composition, molded article and light guide plate - Google Patents

Description

The present invention relates to a styrene-based resin composition and a molded article and light guide plate made therefrom.

There are two types of backlights for LCD devices: direct-light type, in which the light source is placed in front of the display device, and edge-light type, in which the light source is placed on the side. Edge-light type 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.
The light guide plate is made of acrylic resins such as polymethyl methacrylate (PMMA). However, PMMA is highly water absorbing, which can cause the light guide plate to warp or change in size. PMMA is also prone to thermal decomposition during molding, which can lead to poor appearance of the molded product when molded at high temperatures.
In order to solve these problems, it has been proposed to use a styrene-methyl (meth)acrylate copolymer as a material for the light guide plate (see, for example, Patent Document 1).

JP 2003-075648 A

The objective of the present invention is to provide a styrene-based resin composition having excellent resistance to moist heat, and a molded article and a light guide plate made from the composition.

That is, the present invention is as follows.
(1) A styrene-based resin composition in which t1 is an oxidation induction time measured in an oxygen atmosphere at 200° C. and t2 is an oxidation induction time measured in an oxygen atmosphere at 200° C. after 500 hours of wet heat treatment in an air atmosphere at 80° C. and 90% humidity, and t1-t2 is 20 minutes or less.
(2) The styrene-based resin composition according to (1), wherein the t1 is 50 minutes or more.
(3) The styrene-based resin composition according to (1) or (2), which contains a hindered phenol-based antioxidant (B) and a phosphorus-based antioxidant (C2) having no phenolic hydroxyl group.
(4) The styrene-based resin composition according to any one of (1) to (3), which contains a styrene-based resin (A) having a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit.
(5) The styrene-based resin composition according to any one of (1) to (4), which contains a phosphorus-based antioxidant (C1) having a phenolic hydroxyl group.
(6) The styrene-based resin composition according to (3), wherein the hindered phenol-based antioxidant (B) is at least one compound selected from the group consisting of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].
(7) The styrene-based resin composition according to (3), wherein the phosphorus-based antioxidant (C2) is at least one compound selected from the group consisting of 2,2'-methylenebis(4,6-di-tert-butylphenyl)2-ethylhexyl phosphite and tris(2,4-di-tert-butylphenyl)phosphite.
(8) The styrene-based resin composition according to (5), wherein the phosphorus-based antioxidant (C1) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine.
(9) The styrene-based resin composition according to any one of (1) to (8), having a YI value of 2.5 or less at an initial optical path length of 115 mm.
(10) A molded article using the styrene-based resin composition according to any one of (1) to (9).
(11) A light guide plate using the molded article according to (10).

The present invention provides a styrene-based resin composition having excellent resistance to moist heat, as well as a molded article and a light guide plate made from the composition.

FIG. 1 is a conceptual diagram of a method for measuring the oxidation induction time by a chemiluminescence method.

In this specification, the term "A to B" means greater than or equal to A and less than or equal to B.

In the styrene-based resin composition of the present invention, when the oxidation induction time measured in an oxygen atmosphere at 200°C is t1 and the oxidation induction time measured in an oxygen atmosphere at 200°C after 500 hours of wet heat treatment in an air atmosphere at 80°C and 90% humidity is t2, t1-t2 is 20 minutes or less, preferably 15 minutes or less, and more preferably 10 minutes or less. If t1-t2 is in such a range, deterioration of the hue when stored in a high-temperature and high-humidity environment can be suppressed. In addition, an increase in the amount of antioxidant added contributes to maintaining the oxidation induction time, but on the other hand may have an adverse effect on the hue, so when a balance is taken into consideration, t1-t2 is preferably 1 minute or more, more preferably 2 minutes or more, and even more preferably 4 minutes or more. Specifically, t1-t2 may be, for example, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 minutes, or may be within a range between any two of the values exemplified here.

t1 is preferably 50 minutes or more, more preferably 70 minutes or more, and even more preferably 90 minutes or more. If t1 is in this range, yellowing in a high-temperature environment can be suppressed. In addition, while increasing the amount of antioxidant added contributes to extending the oxidation induction time, it may also have a negative effect on the hue. Therefore, when a balance is taken into consideration, t1 is preferably 500 minutes or less, and more preferably 200 minutes or less.

The oxidation induction time is a value measured by a chemiluminescence method. The change over time in the amount of light emitted by the styrene resin composition was measured under the following conditions, and the time at which the line before the amount of light emitted changed and the line after the amount of light emitted increased were calculated from the relationship between the measurement time and the amount of light emitted, as shown in FIG. 1. The styrene resin composition is gradually oxidized when heated in the presence of oxygen. When an antioxidant is present in the sample, the antioxidant is gradually consumed by oxidation, but a constant amount of light emitted is observed while the antioxidant is present. When the antioxidant is gone, the resin itself is oxidized, and a sudden increase in light emission is observed. In other words, the oxidation induction time represents the time at which the antioxidant is consumed and the oxidation of the resin composition progresses rapidly.
Chemiluminescence measuring device: CLA-FS4 (Tohoku Electronics Industries Co., Ltd.)
Measurement temperature: 200°C
Oxygen flow rate: 100 mL/min

The styrene-based resin composition of the present invention preferably contains a styrene-based resin (A) which is a copolymer of a styrene-based monomer and a (meth)acrylic acid ester-based monomer.

Styrene-based monomers are aromatic vinyl monomers, and include styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, ethylstyrene, p-tert-butylstyrene, etc., either alone or in mixtures of two or more. Of these monomers, it is preferable to use styrene because of its good hue.

(Meth)acrylic acid ester monomers include methacrylic acid esters of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-methylhexyl acrylate, 2-ethylhexyl acrylate, and decyl acrylate, either alone or in mixtures of two or more. From the viewpoint of excellent hue and heat resistance, it is preferable to use methyl (meth)acrylate.

The styrene-based resin (A) preferably has a styrene-based monomer unit content of 20 to 80% by mass and a (meth)acrylic acid ester-based monomer unit content of 80 to 20% by mass, and preferably has a styrene-based monomer unit content of 30 to 60% by mass and a (meth)acrylic acid ester-based monomer unit content of 70 to 40% by mass. When the monomer unit content is within such a range, the water absorption rate and deformation rate (water absorption) are low, and it is possible to suppress warping and dimensional changes caused by moisture absorption, and to suppress scratches due to deterioration of color and reduction in surface hardness.

Styrene-based resin (A) may be copolymerized with a monomer other than a (meth)acrylic acid ester-based monomer in an amount of 5% by mass or less. The monomer to be copolymerized may be a vinyl monomer that can be copolymerized with a styrene-based monomer and a (meth)acrylic acid ester-based monomer, such as acrylonitrile, methacrylic acid, acrylic acid, or maleic anhydride.

The contents of the styrene-based monomer units and the (meth)acrylic acid ester-based monomer units in the styrene-based resin can be measured by pyrolysis gas chromatography under the following conditions.
Pyrolysis furnace: PYR-2A (manufactured by Shimadzu Corporation)
Pyrolysis furnace temperature setting: 525℃
Gas chromatograph: GC-14A (manufactured by Shimadzu Corporation)
Column: Glass, 3 mm diameter x 3 m
Filler: FFAP Chromsorb WAW
Column temperature: 120 ° C.
Carrier gas: Nitrogen

Styrene-based resins can be produced by known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. The reactor can be operated in any of the following ways: continuous, batch (batchwise), and semi-batch. In terms of quality such as transparency and productivity, bulk or solution polymerization is preferred, and continuous polymerization is preferable. Solvents for bulk or solution polymerization 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.

A known method can be used to polymerize styrene-based resins. Radical polymerization is preferred because it is a simple process and has excellent productivity.

In bulk or solution polymerization of styrene-based resins, a polymerization initiator and a chain transfer agent can be used, and the polymerization temperature is preferably in the range of 110 to 170°C. When performing bulk or solution polymerization in a continuous manner, from the viewpoint of productivity, it is preferable to carry out the polymerization so that the conversion rate of the styrene-based monomer and the (meth)acrylic acid ester-based monomer is 60% or more at the outlet of the polymerization process.

Polymerization initiators include organic peroxides such as benzoyl peroxide, tert-butyl peroxybenzoate, 1,1-di(t-butylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-bis(4,4-di-tert-butylperoxycyclohexyl)propane, tert-butylperoxyisopropylcarbonate, dicumyl peroxide, tert-butylcumyl peroxide, tert-butylperoxyacetate, tert-butylperoxy-2-ethylhexanoate, polyether tetrakis(tert-butylperoxycarbonate), ethyl-3,3-di(tert-butylperoxy)butyrate, and tert-butylperoxyisobutyrate.

The amount of polymerization initiator added is preferably 0.001 to 0.2% by mass, and more preferably 0.001 to 0.05% by mass, based on 100% by mass of the total monomers. If too much polymerization initiator is added, the color may deteriorate.

Chain transfer agents include aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer and terpinolene.

The amount of chain transfer agent added is preferably 0.001 to 0.5% by mass, and more preferably 0.005 to 0.2% by mass, based on 100% by mass of the total monomers. When the amount of chain transfer agent added is 0.001 to 0.5% by mass, the thermal stability is good.

A known method can be used to remove volatile components such as unreacted monomers and the solvent used in the solution polymerization from the solution after polymerization of the styrene resin. For example, a vacuum devolatilization tank with a preheater or a devolatilization extruder with a vent can be used. The temperature of the styrene resin in the devolatilization process is preferably 200°C to 300°C, more preferably 220°C to 260°C. If the temperature of the styrene resin in the devolatilization process is too high, the color may deteriorate. The devolatilized molten styrene resin is transferred to the granulation process, extruded into strands through a multi-hole die, and processed into pellets by the cold cut method, the air hot cut method, or the underwater hot cut method.

The unreacted monomers removed in the devolatilization process and the solvent used in the solution polymerization are preferably recovered and purified to remove impurities such as polymerization inhibitors, and then mixed with fresh raw materials to be used as recovered raw materials. By mixing the recovered raw materials with fresh raw materials that do not contain polymerization inhibitors, it is possible to reduce the content of polymerization inhibitors in the raw materials supplied to the polymerization process. The content of polymerization inhibitors in the raw materials supplied to the polymerization process is preferably less than 12 ppm, more preferably less than 9 ppm, even more preferably less than 6 ppm, and most preferably less than 4 ppm. If the content of polymerization inhibitors in the raw materials supplied to the polymerization process is less than 12 ppm, the transmittance and transparency are good. It is difficult to remove all the polymerization inhibitors, and they often contain 0.01 ppm or more. Here, the fresh raw materials are raw materials that are newly supplied to the production process of styrene-(meth)acrylic acid ester copolymers, and are called so in order to distinguish them from recovered raw materials.

The unreacted monomers removed in the devolatilization step and the solvent used in the solution polymerization can be recovered and purified by known methods. For example, the unreacted monomers and solvent gases removed in the devolatilization step are condensed and liquefied in a condenser, and then purified in a flash distillation tower to separate and remove high boiling components. Another method is to first condense and separate only the high boiling components from the unreacted monomers and solvent gases removed in the devolatilization step using a condenser or spray tower, and then condense the remaining gas in a condenser. The boiling points of the polymerization inhibitors 4-tert-butylcatechol and 6-tert-butyl-2,4-xylenol are 285°C and 249°C, respectively, and can be separated and removed from the monomers and solvent as high boiling components (the boiling point of styrene is 145°C, the boiling point of methyl (meth)acrylate is 101°C, and the boiling point of ethylbenzene is 136°C).

The weight average molecular weight (Mw) of the styrene resin is preferably 50,000 to 450,000, more preferably 70,000 to 300,000, and even more preferably 70,000 to 200,000. If the weight average molecular weight (Mw) is less than 50,000, the strength of the light guide plate may decrease. If the weight average molecular weight (Mw) is more than 200,000, the fluidity may decrease and the moldability may deteriorate. The weight average molecular weight (Mw) can be controlled by the reaction temperature and residence time of the polymerization process, the type and amount of polymerization initiator, the type and amount of chain transfer agent, the type and amount of solvent used during polymerization, etc.

The weight average molecular weight (Mw) can be measured by gel permeation chromatography (GPC) under the following conditions.
GPC model: Showa Denko Co., Ltd. Shodex GPC-101
Column: Polymer Laboratories PLgel 10 μm MIXED-B
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 in the present invention is calculated as a polystyrene-equivalent molecular weight by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene.

The total amount of residual monomer and polymerization solvent in the styrene-based resin is preferably 0.5% by mass or less, more preferably 0.2% by mass. If the total amount of residual monomer and polymerization solvent exceeds 0.5% by mass, the heat resistance may be insufficient.

The residual monomer and polymerization solvent are the amounts of monomer and polymerization solvent remaining in the styrene-based resin, such as styrene, methyl (meth)acrylate, and ethylbenzene. The amounts of the residual monomer and polymerization solvent can be adjusted by the configuration of the devolatilization process and the conditions of the devolatilization process.

The amounts of the remaining monomer and polymerization solvent were measured by precisely weighing 0.2 g of the styrene-based resin, dissolving it in 10 mL of tetrahydrofuran containing p-diethylbenzene as an internal standard substance, and using a capillary gas chromatograph under the following conditions.
Capillary gas chromatograph: GC-4000 (GL Sciences, Inc.)
Column: InertCap WAX manufactured by GL Sciences, inner diameter 0.25 mm, length 30 m, film thickness 50 μm
Injection temperature: 180°C
Column temperature: 60°C to 170°C
Detector temperature: 210°C
Split ratio: 5/1

The total amount of the styrene monomer and the dimer or trimer (hereinafter referred to as oligomer) of the (meth)acrylic acid ester monomer in the styrene resin is preferably 2% by mass or less. More preferably, it is 1% by mass or less. If the total amount of the oligomer exceeds 1% by mass, the heat resistance of the light guide plate may be insufficient.

The oligomer content was measured by dissolving 200 mg of the styrene resin in 2 mL of 1,2-dichloromethane, adding 2 mL of methanol to precipitate the polymer, and allowing the mixture to stand. The supernatant was then measured using a gas chromatograph under the following conditions.
Gas chromatograph: HP-5890 (manufactured by Hewlett-Packard)
Column: DB-1 (ht) 0.25 mm x 30 m, film thickness 0.1 μm
Injection temperature: 250°C
Column temperature: 100-300°C
Detector temperature: 300°C
Split ratio: 50/1
Internal standard: n-eicosane Carrier gas: nitrogen

The styrene-based resin composition of the present invention preferably contains a hindered phenol-based antioxidant (B). The hindered phenol-based antioxidant (B) contained in the styrene-based resin composition is an antioxidant having a phenolic hydroxyl group in the basic skeleton. Examples of the hindered phenol antioxidant include octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylene bis(oxyethylene)bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)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, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 4,6-bis(octylthiomethyl)-o-cresol, 4,6-bis[(dodecylthio)methyl]-o-cresol, and 2,4-dimethyl-6-(1-methylpentadecyl)phenol. tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane, 4,4'-thiobis(6-tert-butyl-3-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-butylidenebis(3-methyl-6-tert-butylphenol), bis-[3,3-bis-(4'-hydroxy-3'-tert-butylphenyl)butanoic acid]-glycol ester, 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, and the like. The hindered phenol-based antioxidants may be used alone or in combination of two or more kinds.

The content of the hindered phenol-based antioxidant (B) is preferably 0.001 to 0.3 parts by mass, and more preferably 0.03 to 0.09 parts by mass, per 100 parts by mass of the styrene-based resin (A). By adjusting the content of the hindered phenol-based antioxidant (B) within this range, a styrene-based resin composition with excellent hue can be obtained.

The styrene resin composition of the present invention preferably contains a phosphorus-based antioxidant (C1) having a phenolic hydroxyl group. The phosphorus-based antioxidant (C1) contained in the styrene resin composition is a trivalent phosphorus compound having a phenolic hydroxyl group in the basic skeleton. The phosphorus-based antioxidant (C1) has the property of being more easily hydrolyzed than other phosphorus-based antioxidants, and has a high effect of improving the hue of the resulting styrene resin composition. Examples of the phosphorus-based antioxidant (C1) include 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepine.

The content of the phosphorus-based antioxidant (C1) is preferably 0.001 to 0.3 parts by mass, more preferably 0.03 to 0.09 parts by mass, and even more preferably 0.05 to 0.08 parts by mass, per 100 parts by mass of the styrene-based resin (A). By adjusting the content of the phosphorus-based antioxidant (C1) within this range, a styrene-based resin composition with excellent hue can be obtained.

The styrene-based resin composition of the present invention preferably contains a phosphorus-based antioxidant (C2) other than the phosphorus-based antioxidant (C1) having a phenolic hydroxyl group. The phosphorus-based antioxidant (C2) contained in the styrene-based resin composition is a trivalent phosphorus compound that does not have a phenolic hydroxyl group in its basic skeleton. The phosphorus-based antioxidant (C2) is less susceptible to hydrolysis than the phosphorus-based antioxidant (C1), but the hue improvement effect of the styrene-based resin composition is sustained for a long period of time. In other words, it can particularly contribute to moist heat resistance. Examples of the phosphorus-based antioxidant (C2) include 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, 2,2'-methylenebis(4,6-di-tert-butylphenyl)2-ethylhexyl phosphite, tris(2,4-di-tert-butylphenyl)phosphite, bis[2,4-bis(1,1-dimethylethyl)-6-methylphenyl]ethyl ester phosphorous acid, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, and cyclic neopentane tetraphosphate. Examples of suitable antioxidants include bis(octadecyl phosphite), bis(nonylphenyl)pentaerythritol diphosphite, tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenylenediphosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and tetrakis(2,4-di-tert-butyl-5-methylphenyl)-4,4'-biphenylenediphosphonite. From the viewpoint of sustained effect of improving hue, it is preferable to use 2,2'-methylenebis(4,6-di-tert-butylphenyl)2-ethylhexyl phosphite or tris(2,4-di-tert-butylphenyl)phosphite. The phosphorus-based antioxidant (C2) may be used alone or in combination of two or more.

The content of the phosphorus-based antioxidant (C2) is preferably 0.001 to 0.3 parts by mass, more preferably 0.03 to 0.09 parts by mass, and even more preferably 0.05 to 0.08 parts by mass, per 100 parts by mass of the styrene-based resin (A). By adjusting the content of the phosphorus-based antioxidant (C2) within this range, a styrene-based resin composition that has excellent initial and long-term hue can be obtained.

The blending ratio of the phosphorus-based antioxidant (C1) to the phosphorus-based antioxidant (C2) is preferably in the range of 3:1 to 1:3 by mass, and more preferably 2:1 to 1:2. By adjusting the blending ratio to this range, the hue of the obtained styrene-based resin composition and its durability are well balanced.

A known method can be used to produce the styrene resin composition. There is a method of adding the hindered phenol-based antioxidant (B), the phosphorus-based antioxidant (C1), and the phosphorus-based antioxidant (C2) in the production process such as the polymerization process, the devolatilization process, and the granulation process of the styrene resin, and it is preferable to add them after the unreacted monomer and the solvent are removed in the devolatilization process. When using a vacuum devolatilization tank, the molten hindered phenol-based antioxidant (B), the phosphorus-based antioxidant (C1), and the phosphorus-based antioxidant (C2) can be added to the styrene resin extracted from the devolatilization tank and mixed with a static mixer, or when using a vented devolatilization extruder, the hindered phenol-based antioxidant (B), the phosphorus-based antioxidant (C1), and the phosphorus-based antioxidant (C2) can be added and mixed after the vent zone. The hindered phenol-based antioxidant (B), the phosphorus-based antioxidant (C) and the phosphorus-based antioxidant (C2) can also be melt-kneaded with the granulated styrene-based resin (A) using an extruder.

The styrene resin composition may contain mineral oil as long as the transparency is not impaired. Internal lubricants such as stearic acid and ethylene bisstearylamide, and external lubricants such as sulfur-based antioxidants, lactone-based antioxidants, ultraviolet absorbers, hindered amine-based stabilizers, antistatic agents, and ethylene bisstearylamide may also be added.

UV absorbers have the function of suppressing deterioration and discoloration caused by ultraviolet rays, and include benzophenone-based, benzotriazole-based, triazine-based, benzoate-based, salicylate-based, cyanoacrylate-based, malonic acid ester-based, and formamidine-based UV absorbers. These can be used alone or in combination of two or more types, and may be used in combination with a light stabilizer such as a hindered amine.

The Vicat softening point of the styrene resin is preferably 95°C or higher, and more preferably 98°C or higher. If the Vicat softening point is less than 95°C, the heat resistance is insufficient, and depending on the usage environment, the molded product may deform. (Vicat softening temperature was tested in accordance with JIS K 7206, at a heating rate of 50°C/hr and a test load of 50N.)

The styrene-based resin composition can be used to produce plate-shaped molded products by known methods such as extrusion molding, injection molding, compression molding, and blow molding, and can be processed into light guide panels and other products for use.

Because the styrene-based resin composition of the present invention has excellent thermal stability, unproductized parts such as sheet scraps from extrusion molding and spools and runners from injection molding can be recovered and crushed, and mixed with virgin raw materials for use.

A light guide plate is a component that has the function of guiding light incident from the end face of a plate-shaped molded product to the face side of the plate-shaped molded product, causing it to emit light, by using a reflective pattern formed on one side of the plate-shaped molded product. The reflective pattern can be formed by methods such as screen printing, laser processing, and inkjet printing. In addition, a prism pattern or the like can be provided on the opposite side (light-emitting surface) to the side on which the reflective pattern is formed. The reflective pattern and prism pattern of a plate-shaped molded product can be formed when the plate-shaped molded product is molded; in injection molding, they can be formed by the mold shape, and in extrusion molding, they can be formed by roll transfer, etc.

The YI value of the styrene resin composition measured at an initial optical path length of 115 mm is preferably 2.5 or less, more preferably 2.0 or less. Here, "initial" means before 500 hours of wet heat treatment in an air atmosphere at 80°C and 90% humidity. The measurement is a value calculated by measuring the spectral transmittance at wavelengths of 350 nm to 800 nm at an optical path length of 115 mm, and calculating the YI value at a field of view of 2° with a C light source according to JIS K 7373. If the YI value measured at an optical path length of 115 mm is higher than 2.5, the color changes with the optical path length, so that when used as a backlight, color unevenness may occur on the surface of a liquid crystal display device. In addition, the average value of the spectral transmittance at wavelengths of 350 nm to 800 nm measured at an optical path length of 115 mm is preferably 87% or more, more preferably 88% or more, and even more preferably 89% or more.

The present invention will be explained in detail below with reference to examples, but the present invention is not limited to these examples.

<Production Example of Styrene-Based Resin A-1>
The styrene resin was produced by continuous solution polymerization using a radical polymerization method. A complete mixing tank type stirring tank was used as the first reactor, and a plug flow type reactor with a static mixer was used as the second reactor, and they were connected in series to form a polymerization process. The capacity of the first reactor was 30 L, and the capacity of the second reactor was 12 L. When industrially used styrene (hereinafter referred to as fresh Sty) was prepared as the styrene monomer, the concentration of 4-tert-butylcatechol (hereinafter referred to as TBC) was 10.2 ppm. When industrially used methyl (meth)acrylate (hereinafter referred to as fresh MMA) was prepared as the (meth)acrylic acid ester monomer, the concentration of 6-tert-butyl-2,4-xylenol (hereinafter referred to as TBX) was 4.9 ppm. As the polymerization solvent, industrially used ethylbenzene (hereinafter referred to as fresh EB) was prepared. In addition, gases such as monomers and polymerization solvents separated from a vacuum devolatilizer tank described later were condensed in a condenser, and purified in a flash distillation column and used as the recovered raw material. The concentrations of TBX and TBC in the recovered raw material were below the detection limit. Fresh Sty, fresh MMA, and the recovered raw material were used to prepare a raw material solution with a composition of Sty: 49 mass%, MMA: 41 mass%, and EB: 10 mass%, and the raw material solution was continuously supplied to the polymerization step at a flow rate of 8.0 kg/h. The usage ratio of the recovered raw material in the raw material solution was 33 mass%. In addition, t-butylperoxyisopropyl monocarbonate as a polymerization initiator was continuously added to the raw material solution supply line so that the concentrations of the raw material solution were 150 ppm and n-dodecyl mercaptan as a chain transfer agent were 500 ppm. The temperature of the first reactor was adjusted to 135°C, and the temperature of the second reactor was adjusted to 130°C in the middle and 145°C at the outlet along the direction of flow. The polymer concentration at the outlet of the polymerization step was 65%, and the conversion of styrene and methyl (meth)acrylate was 72%. The polymer solution continuously taken out from the reactor was supplied to a vacuum devolatilizer tank equipped with a preheater, and unreacted styrene, methyl (meth)acrylate, ethylbenzene, and the like were separated. The temperature of the preheater was adjusted so that the polymer temperature in the devolatilizer tank was 240°C, and the pressure in the devolatilizer tank was 1 kPa. The polymer was extracted from the vacuum devolatilizer tank by a gear pump, extruded into a strand shape, cooled with cooling water, and then cut to obtain pellet-shaped styrene-based resin A-1. The composition of A-1 was Sty: 50% by mass, MMA: 50% by mass. The weight average molecular weight of A-1 was 145,000, the total amount of the remaining monomers and polymerization solvent was 0.07% by mass, and the total amount of the remaining oligomers was 0.35% by mass.

<Production Example of Styrene-Based Resin A-2>
The same procedure as in A-1 was carried out except that the raw material composition was changed to Sty: 77% by mass, MMA: 13% by mass, and EB: 10% by mass, the addition of n-dodecyl mercaptan was stopped, the temperature of the first reactor was set to 140°C, the temperature of the middle part of the second reactor was set to 140°C, and the temperature of the outlet part was set to 160°C. The proportion of the recovered raw material in the raw material solution was 33% by mass. The composition of A-2 was Sty: 82% by mass, MMA: 18% by mass. The weight average molecular weight of A-2 was 240,000, the total amount of the residual monomer and polymerization solvent was 0.06% by mass, and the total amount of the residual oligomer was 0.33% by mass.

<Production Example of Styrene-Based Resin A-3>
The raw material composition was changed to Sty: 8% by mass, MMA: 79% by mass, and EB: 13% by mass, the feed flow rate was 5.7 kg/h, the concentration of tert-butylperoxyisopropyl monocarbonate was 100 ppm, the concentration of n-dodecyl mercaptan was 3000 ppm, the temperature of the first reactor was 122 ° C, the temperature of the middle part of the second reactor was 140 ° C, and the temperature of the outlet part was 150 ° C., but the same procedure as in A-1 was carried out. The proportion of the recovered raw material in the raw material solution was 34% by mass. The composition of A-3 was Sty: 10% by mass, MMA: 90% by mass. The weight average molecular weight of A-3 was 80,000, the total amount of the residual monomer and polymerization solvent was 0.06% by mass, and the total amount of the residual oligomer was 0.34% by mass.

<Examples 1 to 10, Comparative Example 1, Reference Examples 1 to 2>
The styrene-based resins A-1 to A-3 obtained in the production examples were mixed with the hindered phenol-based antioxidant (B), phosphorus-based antioxidant (C1), and phosphorus-based antioxidants (C2-1) and (C2-2) shown below in the amounts shown in Table 1, and the antioxidants were melt-kneaded using a sheet extruder manufactured by LEADER to obtain a sheet molded product of 450 mm x 500 mm x 2 mm. The sheet extruder was composed of a 50 mmφ single-screw extruder, a T-die, and three mirror-finished rolls, and the sheet extrusion was performed at a cylinder temperature of 225 ° C. and a screw rotation speed of 120 rpm. The width of the T-die was 450 mm, and the opening was 3 mm.
(B) Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1076, manufactured by BASF Japan Ltd.)
(C1) 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)
(C2-1) 2,2'-methylenebis(4,6-di-tert-butylphenyl)2-ethylhexyl phosphite (ADEKA STAB HP-10, manufactured by ADEKA CORPORATION)
(C2-2) Tris(2,4-di-tert-butylphenyl)phosphite (Irgafos 168, manufactured by BASF Japan Ltd.)

(Oxidation induction time)
A test piece measuring 30 mm x 30 mm x 2 mm was cut out from the obtained sheet molded product, and the amount of light emitted was measured at 200°C and an oxygen flow rate of 100 mL/min using a chemiluminescence analyzer CLA-FS4 manufactured by Tohoku Electronics Industries Co., Ltd., and the oxidation induction time was determined from the relationship between the measurement time and the amount of light emitted by the method shown in Figure 1. The measurement results of the oxidation induction time (t1) of the sheet molded product immediately after extrusion and the oxidation induction time (t2) of the sheet molded product subjected to a wet heat treatment for 500 hours in an air atmosphere at 80°C and 90% humidity are shown in Table 1. In Reference Example 1 and Reference Example 2, the amount of light emitted continued to increase immediately after the start of the measurement, and the oxidation induction time could not be measured.

(Optical characteristics at optical path length of 115 mm)
A test piece measuring 115 mm x 85 mm x 2 mm thick was cut out from the obtained sheet molded product, and the end surface was polished 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 of a wavelength of 350 nm to 800 nm at an optical path length of 115 mm with incident light of a size of 20 x 1.6 mm and a spread angle of 0°, and the YI value at a field of view of 2° with a C light source was calculated according to JIS K7373. The transmittance shown in Table 1 indicates the average transmittance of a wavelength of 380 nm to 780 nm. The results of measurements of the sheet molded product (initial) immediately after extrusion, the sheet molded product subjected to moist heat treatment for 500 hours in an air atmosphere at 80 ° C. and 90% humidity, and the sheet molded product stored in an environment of 80 ° C. for 1000 hours are shown in Table 1.

(Water Absorbency)
The obtained sheet molded product was cut to obtain a molded product of 200 mm × 300 mm size. This molded product was stored for 500 hours under conditions of a temperature of 60 ° C. and a humidity of 90%, and the mass and dimensional changes of the long side before and after storage were measured, and the water absorption rate and deformation rate were calculated as indicators of water absorption by the following formula.
(Water absorption rate) = ((mass after storage) - (mass before storage)) ÷ (mass before storage) x 100 (%)
(Deformation rate) = ((Long side length after storage) - (Long side length before storage)) ÷ (Long side length before storage) x 100 (%)
The evaluation results are shown in Table 1. A water absorption rate of 1.0 or less was judged to be good, and a deformation rate of 0.30 or less was judged to be good.

The styrene-based resin composition of the present invention and its molded products have excellent moist heat resistance, and therefore are less susceptible to deterioration in hue and transparency, making them suitable for use in light guide panels for televisions, desktop personal computers, notebook personal computers, mobile phones, car navigation systems, indoor lighting, and other applications.

Claims (8)

The oxidation induction time measured under an oxygen atmosphere at 200 ° C. is t1,
When the oxidation induction time measured in an oxygen atmosphere at 200° C. after 500 hours of wet heat treatment in an air atmosphere at 80° C. and 90% humidity is t2,
t1-t2 is 20 minutes or less;
A styrene-based resin composition comprising:
The styrene-based resin composition contains a styrene-based resin (A), a hindered phenol-based antioxidant (B), a phosphorus-based antioxidant having a phenolic hydroxyl group (C1), and a phosphorus-based antioxidant not having a phenolic hydroxyl group (C2),
The styrene-based resin (A) has a styrene-based monomer unit and a (meth)acrylic acid ester-based monomer unit,
The styrene-based resin (A) has a styrene-based monomer unit content of 20 to 80 mass% and a (meth)acrylic acid ester-based monomer unit content of 80 to 20 mass%,
The phosphorus-based antioxidant (C1) having a phenolic hydroxyl group is a trivalent phosphorus compound having a phenolic hydroxyl group,
The hindered phenol-based antioxidant (B) is an antioxidant having a phenolic hydroxyl group and does not contain a trivalent phosphorus compound having a phenolic hydroxyl group,
The content of the hindered phenol-based antioxidant (B) is 0.001 to 0.3 parts by mass relative to the styrene-based resin (A),
The content of the phosphorus-based antioxidant (C1) having a phenolic hydroxyl group is 0.001 to 0.3 parts by mass based on the styrene-based resin (A), and the content of the phosphorus-based antioxidant (C2) not having a phenolic hydroxyl group is 0.001 to 0.3 parts by mass based on the styrene-based resin (A). The styrene-based resin composition according to claim 1, wherein t1 is 50 minutes or more. The styrene-based resin composition according to claim 1 or 2, wherein the hindered phenol-based antioxidant (B) is at least one compound selected from the group consisting of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylene bis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. The styrene-based resin composition according to any one of claims 1 to 3, wherein the phosphorus-based antioxidant (C2) is at least one compound selected from the group consisting of 2,2'-methylenebis(4,6-di-tert-butylphenyl) 2 -ethylhexyl phosphite and tris(2,4-di-tert-butylphenyl)phosphite. The styrene-based resin composition according to any one of claims 1 to 4, wherein the phosphorus-based antioxidant (C1) 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 YI value at an initial optical path length of 115 mm is 2.5 or less.
The styrene-based resin composition according to any one of claims 1 to 5 . A molded article using the styrene-based resin composition according to any one of claims 1 to 6 . A light guide plate using the molded product according to claim 7 .