JP7129430B2 - Styrene resin composition, molded article and light guide plate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a styrene-based resin composition excellent in hue, transparency and heat resistance, a molded article thereof, and a light guide plate.

Backlights for liquid crystal display devices include a direct type in which the light source is arranged in front of the display device and an edge light type in which the light source is arranged on the side. An edge-light backlight uses a component called a light guide plate that guides light from a light source arranged on the side to the front of the display device.
As a material of the light guide plate, acrylic resin represented by polymethyl methacrylate (PMMA) is used. However, since PMMA has high water absorption, the light guide plate may warp or change dimensions due to water absorption. In addition, since it is likely to be thermally decomposed during molding, there is a problem that the appearance of the molded product tends to be poor when molded at a high temperature.
In order to solve these problems, it has been proposed to use a styrene-(meth)methyl acrylate copolymer as a material for light guide plates (see, for example, Patent Document 1). In addition, techniques for improving the hue of styrene-(meth)methyl acrylate copolymer include techniques for blending blue dyes (see, for example, Patent Document 2), techniques for limiting the content of residual polymerization inhibitors (for example, , see Patent Document 3).

JP 2003-075648 A Japanese Unexamined Patent Application Publication No. 2013-082800 WO2016/129675

An object of the present invention is to provide a styrene-based resin composition excellent in hue, transparency and heat resistance, a molded article thereof, and a light guide plate.

That is, the present invention is as follows.
(1) a styrene resin (A) having 20 to 80% by mass of styrene monomer units and 80 to 20% by mass of (meth)acrylic acid ester monomer units, and a hindered phenolic antioxidant (B) ), a phosphorus antioxidant (C1) having a phenolic hydroxyl group, and a phosphorus antioxidant (C2) other than C1.
(2) 100 parts by mass of a styrene resin (A), 0.001 to 0.3 parts by mass of a hindered phenolic antioxidant (B), and 0.001 to 0.3 parts by mass of a phosphorus antioxidant (C1) The styrenic resin composition according to (1), containing parts by mass and 0.001 to 0.3 parts by mass of the phosphorus antioxidant (C2).
(3) The styrene resin according to (1) or (2), wherein the content of the phosphorus antioxidant (C1) and the phosphorus antioxidant (C2) is 3:1 to 1:3 in mass ratio. Composition.
(4) Hindered phenolic antioxidant (B) is octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylenebis(oxyethylene)bis[3-(5- (1 ) to (3) The styrene resin composition according to any one of the above.
(5) the phosphorus antioxidant (C1) is 6-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-tert-butyl The styrenic resin composition according to any one of (1) to (4), which is dibenzo[d,f][1,3,2]dioxaphosphepine.
(6) the phosphorus antioxidant (C2) is 2,2′-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, tris(2,4-di -tert-butylphenyl)phosphite, the styrenic resin composition according to any one of (1) to (5).
(7) The YI value at an optical path length of 115 mm is 2.5 or less, and the oxidation induction time t1 measured in an oxygen atmosphere at 200 ° C., 80 ° C., and after moist heat treatment in an air atmosphere with 90% humidity for 500 hours. The styrenic resin composition according to any one of (1) to (6), wherein t1 is 50 minutes or more and t1-t2 is less than 20 minutes, where t2 is the oxidation induction time measured in an oxygen atmosphere at 200°C. thing.
(8) A molded article using the styrene resin composition according to any one of (1) to (7).
(9) A light guide plate using the molded product described in (8).

INDUSTRIAL APPLICABILITY According to the present invention, a styrene-based resin composition excellent in hue, transparency and heat resistance, a molded article thereof, and a light guide plate can be obtained.

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

In the specification of the present application, the description "A to B" means A or more and B or less.

The styrene resin composition of the present invention comprises a styrene resin (A), a hindered phenol antioxidant (B), a phosphorus antioxidant having a phenolic hydroxyl group (C1), and a phosphorus antioxidant other than C1. and an antioxidant (C2).

The styrene-based resin (A) is a copolymer of a styrene-based monomer and a (meth)acrylic acid ester-based monomer.

Styrenic monomers are aromatic vinyl monomers, styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, ethylstyrene, p-tert-butylstyrene, etc., alone or in combination of two or more. there is a mixture. Among these monomers, styrene is preferably used from the viewpoint of good hue.

(Meth) acrylic ester-based monomers include methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, methacrylic acid esters of 2-ethylhexyl (meth) acrylate, methyl acrylate, ethyl acrylate, n -Butyl acrylate, 2-methylhexyl acrylate, 2-ethylhexyl acrylate, decyl acrylate 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 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, preferably a styrene-based monomer The content of the body unit is 30 to 60% by mass, and the content of the (meth)acrylic acid ester monomer unit is 70 to 40% by mass. When the content of the styrene-based monomer units is small, the water absorption rate and deformation rate (water absorption) increase, so that moisture absorption may increase warpage and dimensional deformation. If the content of the styrene-based monomer units is too high, the hue may deteriorate and the surface hardness may decrease, resulting in susceptibility to scratches.

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

The content of styrene-based monomer units and (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°C
Gas chromatograph: GC-14A (manufactured by Shimadzu Corporation)
Column: glass 3 mm diameter x 3 m
Filler: FFAP Chromsorb WAW 10%
Column temperature: 120°C
Carrier gas: Nitrogen

Styrenic resins can be produced by known bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and the like. As for the operating method of the reactor, any of a continuous system, a batch system (batch system), and a semi-batch system can be applied. In terms of quality such as transparency and productivity, bulk polymerization or solution polymerization is preferred, and continuous polymerization is preferred. Solvents for bulk polymerization 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 employed for the polymerization method of the styrene-based resin. A radical polymerization method is preferred because it is a simple process and has excellent productivity.

In bulk polymerization or solution polymerization of styrene resin, 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 polymerization or solution polymerization in a continuous system, from the viewpoint of productivity, the conversion rate of styrene-based monomers and (meth)acrylic acid ester-based monomers is 60% or more at the exit of the polymerization process. It is preferred to carry out such polymerization.

Polymerization initiators include benzoyl peroxide, tert-butylperoxybenzoate, 1,1-di(t-butylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethyl Cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-bis(4,4-di-tert-butylperoxycyclohexyl)propane, tert-butylperoxy Isopropyl carbonate, dicumyl peroxide, tert-butyl cumyl peroxide, tert-butyl peroxyacetate, tert-butyl peroxy-2-ethylhexanoate, polyether tetrakis(tert-butyl peroxycarbonate), ethyl-3 , 3-di(tert-butylperoxy)butyrate, tert-butylperoxyisobutyrate, and other organic peroxides.

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

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

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

As a devolatilization method for removing volatile components such as unreacted monomers and the solvent used for solution polymerization from the solution after polymerization of the styrene resin, a known method can be adopted. A volatilization tank or vented devolatilization extruder can be used. The temperature of the styrene-based resin in the devolatilization step is preferably 200°C to 300°C, more preferably 220°C to 260°C. If the temperature of the styrene-based resin in the devolatilization step is too high, the hue may deteriorate. The devolatilized molten styrene resin is transferred to the granulation process, extruded from a multi-hole die in a strand shape, and processed into pellets by a cold cut method, an air hot cut method, or an underwater hot cut method. .

The unreacted monomers removed in the devolatilization step and the solvent used in the solution polymerization are recovered, purified to remove impurities such as polymerization inhibitors, and then mixed with fresh raw materials as recovered raw materials for use. is preferred. By mixing the recovered raw material with a fresh raw material containing no polymerization inhibitor, it is possible to reduce the content of the polymerization inhibitor in the raw material supplied to the polymerization step. The content of the polymerization inhibitor in the raw material supplied to the polymerization step is preferably less than 12 ppm, more preferably less than 9 ppm, still more preferably less than 6 ppm, and most preferably less than 4 ppm. When the content of the polymerization inhibitor in the raw material supplied to the polymerization step is less than 12 ppm, good transmittance and transparency can be obtained. In addition, it is difficult to completely remove the polymerization inhibitor, and in many cases, the content is 0.01 ppm or more. Here, the fresh raw material is a raw material newly supplied to the production process of the styrene-(meth)acrylic acid ester copolymer, and is so called to distinguish it from the recovered raw material.

As a method for recovering and purifying the unreacted monomers removed in the devolatilization step and the solvent used in the solution polymerization, known methods can be employed. There is a method in which the solvent gas is condensed in a condenser to be liquefied and refined in a flash distillation column to separate and remove high boiling point components. In addition, from the unreacted monomer and solvent gas removed in the devolatilization process, only the high boiling point components are first condensed and separated using a condenser or spray tower, and the remaining gas is There is a way to condense. The polymerization inhibitor 4-tert-butylcatechol has a boiling point of 285°C, and 6-tert-butyl-2,4-xylenol has a boiling point of 249°C. (Styrene boiling point 145°C, methyl (meth)acrylate boiling point 101°C, ethylbenzene boiling point 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. When the weight-average molecular weight (Mw) exceeds 200,000, the fluidity may be lowered and the moldability may be deteriorated. The weight average molecular weight (Mw) can be controlled by the reaction temperature and residence time in the polymerization process, the type and amount of polymerization initiator added, the type and amount of chain transfer agent used, the type and amount of solvent used during polymerization, and the like. can.

The weight average molecular weight (Mw) can be 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
Mobile phase: Tetrahydrofuran Sample concentration: 0.2% by mass
Temperature: oven 40°C, inlet 35°C, detector 35°C
Detector: Differential refractometer The molecular weight of the present invention is calculated as the polystyrene equivalent molecular weight by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene.

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

The residual monomer and polymerization solvent are the amounts of the monomer and polymerization solvent remaining in the styrene resin, and include styrene, methyl (meth)acrylate, ethylbenzene, and the like. The amount of the residual monomer and the polymerization solvent can be adjusted by the structure of the devolatilization process and the conditions of the devolatilization process.

The amounts of residual monomers and polymerization solvent were measured by accurately weighing 0.2 g of a styrene 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 (manufactured by GL Sciences Co., Ltd.)
Column: InertCap WAX manufactured by GL Sciences Co., Ltd., 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 dimers or trimers (hereinafter referred to as oligomers) of styrene-based monomers and (meth)acrylic acid ester-based monomers in the styrene-based resin is preferably 2% by mass or less. More preferably, it is 1% by mass or less. If the total amount of oligomers exceeds 1% by mass, the heat resistance of the light guide plate may be insufficient.

Oligomer measurement was performed by dissolving 200 mg of styrene resin in 2 mL of 1,2-dichloromethane, adding 2 mL of methanol to precipitate the polymer, allowing the polymer to stand, and using a gas chromatograph on the supernatant under the following conditions. measured in
Gas chromatograph: HP-5890 (manufactured by Hewlett-Packard)
Column: DB-1 (ht) 0.25 mm × 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 substance: n-eicosane Carrier gas: Nitrogen

The hindered phenol-based antioxidant (B) contained in the styrene-based resin composition is an antioxidant having a phenolic hydroxyl group in its basic skeleton. Hindered phenolic antioxidants include octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylenebis(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, 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. The hindered phenol-based antioxidants may be used alone or in combination of two or more.

The content of the hindered phenolic antioxidant (B) is preferably 0.001 to 0.3 parts by mass, more preferably 0.03 to 0.15 parts by mass, relative to 100 parts by mass of the styrene resin (A). part is more preferred. By adjusting the content of the hindered phenol-based antioxidant (B) within this range, a styrene-based resin composition having excellent hue can be obtained.

The phosphorus antioxidant (C1) contained in the styrene resin composition is a trivalent phosphorus compound having a phenolic hydroxyl group in its basic skeleton. The phosphorus-based antioxidant (C1) has a characteristic of being easily hydrolyzed compared to other phosphorus-based antioxidants, and is highly effective in improving the hue of the resulting styrene-based resin composition. The phosphorus 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]dioxaphosphepin and the like.

The content of the phosphorus antioxidant (C1) is preferably 0.001 to 0.3 parts by mass, more preferably 0.03 to 0.15 parts by mass with respect to 100 parts by mass of the styrene resin (A). parts by mass, more preferably 0.05 to 0.1 parts by mass. By adjusting the content of the phosphorus antioxidant (C1) within this range, a styrene-based resin composition having excellent hue can be obtained.

The phosphorus antioxidant (C2) contained in the styrenic resin composition is a trivalent phosphorus compound having no phenolic hydroxyl group in its basic skeleton. The phosphorus-based antioxidant (C2) is less hydrolyzed than the phosphorus-based antioxidant (C1), but maintains the effect of improving the hue of the styrene-based resin composition over a long period of time. The phosphorus antioxidant (C2) is 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-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus, 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, cyclic neopentanetetrayl bis(octadecylphosphite), bis(nonylphenyl)pentaerythritol diphosphite, 4,4'-biphenylenediphosphinic acid tetrakis(2,4- di-tert-butylphenyl), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, tetrakis(2,4-di-tert-butyl-5-methylphenyl)-4,4' -Biphenylene diphosphonite and the like, and from the viewpoint of maintaining the effect of improving hue, preferably 2,2'-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy) ) phosphorous or tris(2,4-di-tert-butylphenyl)phosphite. The phosphorus antioxidant (C2) may be used alone or in combination of two or more.

The content of the phosphorus antioxidant (C2) is preferably 0.001 to 0.3 parts by mass, more preferably 0.03 to 0.15 parts by mass with respect to 100 parts by mass of the styrene resin (A). parts by mass, more preferably 0.05 to 0.1 parts by mass. By adjusting the content of the phosphorus-based antioxidant (C2) within this range, a styrene-based resin composition having excellent hue over a long period of time can be obtained.

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

A known method can be adopted as a method for producing a styrene-based resin composition. Hindered phenol-based antioxidant (B), phosphorus-based antioxidant (C1), and phosphorus-based antioxidant (C2) are added in manufacturing processes such as the polymerization process, devolatilization process, and granulation process of styrene resin. It is preferable to add after removing unreacted monomers and solvent in the devolatilization step. When a vacuum devolatilization tank is used, the hindered phenol antioxidant (B), the phosphorus antioxidant (C1) and the phosphorus antioxidant (C2) in a molten state are added to the styrene resin extracted from the devolatilization tank. In the method of adding and mixing with a static mixer, or when using a vented devolatilizing extruder, after the vent zone hindered phenol antioxidant (B), phosphorus antioxidant (C1) and phosphorus oxidation An inhibitor (C2) can be added and mixed. Melt-kneading the hindered phenol-based antioxidant (B), the phosphorus-based antioxidant (C), and the phosphorus-based antioxidant (C2) into the styrenic resin (A) after granulation using an extruder. can also

Mineral oil may be added to the styrenic resin composition as long as the transparency is not impaired. In addition, internal lubricants such as stearic acid and ethylenebisstearylamide, and external lubricants such as sulfur antioxidants, lactone antioxidants, ultraviolet absorbers, hindered amine stabilizers, antistatic agents, and ethylenebisstearylamide. may be added.

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

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

The styrene resin composition had an oxidation induction time t1 measured in an oxygen atmosphere at 200°C, and was subjected to a wet heat treatment in an air atmosphere of 80°C and 90% humidity for 500 hours. When the induction time is t2, it is preferable that t1 is 50 minutes or more and t1-t2 is less than 20 minutes. t1 is preferably 70 minutes or longer, more preferably 90 minutes or longer. Also, t1-t2 is preferably less than 10 minutes. If t1 is less than 50 minutes, yellowing may occur in a high-temperature environment, and the hue and transmittance of the long optical path may deteriorate. If t1-t2 is 20 minutes or longer, the hue and transmittance of the long optical path after molding may deteriorate when the styrene resin composition is stored in a high-temperature and high-humidity environment. The oxidation induction time of the styrenic resin composition can be lengthened by increasing the amounts of the hindered phenolic antioxidant and the phosphorus antioxidant. However, if these antioxidants are increased too much, the hue of the styrenic resin composition may deteriorate.
The oxidation induction time is a value measured by a chemiluminescence method (chemiluminescence method). The change in the luminescence intensity of the styrene resin composition over time was measured under the following conditions, and from the relationship between the measurement time and the luminescence intensity obtained, the straight line before the luminescence intensity changed and the luminescence intensity increased as shown in FIG. After that, the time of intersection with the straight line was calculated. A styrenic 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 luminescence is exhibited while the antioxidant is present. When the antioxidant disappears, the resin itself is oxidized, and a rapid increase in light emission is observed. That is, the oxidation induction time represents the time during which the antioxidant is consumed and the oxidation of the resin composition rapidly progresses.
Chemiluminescence measuring instrument: CLA-FS4 (manufactured by Tohoku Electronics Industry Co., Ltd.)
Measurement temperature: 200°C
Oxygen flow rate: 100 mL/min

The styrene-based resin composition can be used by producing a plate-like molded product by a known method such as extrusion molding, injection molding, compression molding, blow molding, etc., and processing it into a light guide plate or the like.

Since the styrenic resin composition of the present invention has excellent thermal stability, unmanufactured parts such as sheet scraps during extrusion molding and spools and runners during injection molding are collected and pulverized, and mixed with virgin raw materials. can be used

A light guide plate is a member that has a function of guiding light incident from an end surface of a plate-shaped molded product to the surface side of the plate-shaped molded product by means of a reflection pattern formed on one surface of the plate-shaped molded product and emitting light. The reflective pattern can be formed by methods such as screen printing, laser processing, and inkjet. Also, a prism pattern or the like can be provided on the opposite surface (light emitting surface) of the surface on which the reflection pattern is formed. The reflection pattern and the prism pattern of the plate-shaped molded product can be formed during molding of the plate-shaped molded product, and can be formed by mold shape in injection molding, roll transfer in extrusion molding, and the like.

The YI value of the styrene-based resin composition measured at an optical path length of 115 mm is 2.5 or less, preferably 2.0 or less. The measurement is a value obtained by measuring the spectral transmittance at a wavelength 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° under C light source following JIS K7105. If the YI value measured at an optical path length of 115 mm is higher than 2.5, the color changes along with the optical path length, and when used as a backlight, color unevenness may occur on the surface of a liquid crystal display device. The average 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 still more preferably 89% or more.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples.

<Production example of styrene resin A-1>
The styrenic resin was produced by continuous solution polymerization using a radical polymerization method. A complete mixing tank type stirred tank was used as the first reactor, and a plug flow type reactor with a static mixer was used as the second reactor, which were connected in series to form a polymerization process. The capacity of the first reactor was 30L and the capacity of the second reactor was 12L. When styrene for industrial use (hereinafter referred to as fresh Sty) was prepared as a styrene monomer, the concentration of 4-tert-butylcatechol (hereinafter referred to as TBC) was 10.2 ppm. Industrially used methyl (meth) acrylate (hereinafter referred to as fresh MMA) was prepared as a (meth) acrylic acid ester-based monomer, and 6-tert-butyl-2,4-xylenol (hereinafter referred to as TBX) concentration was 4.9 ppm. Industrially used ethylbenzene (hereinafter referred to as fresh EB) was prepared as a polymerization solvent. Gases such as a monomer and a polymerization solvent separated from a vacuum devolatilization tank, which will be described later, were condensed in a condenser and purified in a flash distillation column to be used as recovered raw materials. Concentrations of TBX and TBC in the recovered raw material were below the detection limit. Fresh Sty, fresh MMA, and recovered raw materials were used to prepare a raw material solution having a composition of Sty: 49% by mass, MMA: 41% by mass, and EB: 10% by mass. supplied to The usage ratio of the recovered raw material in the raw material solution was 33% by mass. In addition, t-butyl peroxyisopropyl monocarbonate as a polymerization initiator and n-dodecyl mercaptan as a chain transfer agent were continuously added to the supply line of the raw material solution so as to have concentrations of 150 ppm and 500 ppm, respectively, with respect to the raw material solution. . The temperature of the first reactor was adjusted to 135°C and the second reactor was adjusted to a temperature gradient along the direction of flow of 130°C in the middle section and 145°C in the outlet section. The polymer concentration at the outlet of the polymerization process was 65%, and the conversion of styrene and methyl (meth)acrylate was 72%. A polymer solution continuously taken out from the reactor was supplied to a vacuum devolatilization tank equipped with a preheater to separate unreacted styrene, methyl (meth)acrylate, ethylbenzene, and the like. The temperature of the preheater was adjusted so that the polymer temperature in the devolatilization tank was 240° C., and the pressure in the devolatilization tank was 1 kPa. The polymer was extracted from the vacuum devolatilization tank by a gear pump, extruded in the form of a strand, cooled with cooling water, and cut to obtain a styrene-based resin A-1 in the form of pellets. The composition of A-1 was Sty: 50% by mass and MMA: 50% by mass. A-1 had a weight average molecular weight of 145,000, a total amount of residual monomers and polymerization solvent of 0.07% by mass, and a total amount of residual oligomers of 0.35% by mass.

<Production example of styrene resin A-2>
The raw material composition was changed to Sty: 77% by mass, MMA: 13% by mass, and EB: 10% by mass, the addition of n-dodecylmercaptan was stopped, the temperature of the first reactor was 140 ° C., and the temperature of the second reactor was The procedure was carried out in the same manner as in A-1, except that the temperature of the intermediate portion was 140°C and the temperature of the outlet portion was 160°C. The usage ratio of the recovered raw material in the raw material solution was 33% by mass. The composition of A-2 was Sty: 82% by mass and MMA: 18% by mass. A-2 had a weight average molecular weight of 240,000, a total amount of residual monomers and polymerization solvent of 0.06% by mass, and a total amount of residual oligomers of 0.33% by mass.

<Production example of styrene resin A-3>
The raw material composition was changed to Sty: 8% by mass, MMA: 79% by mass, EB: 13% by mass, the feed flow rate was 5.7 kg/h, the concentration of tert-butylperoxyisopropyl monocarbonate was 100 ppm, and n-dodecyl Conducted in the same manner as A-1 except that the concentration of 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. did. The usage ratio of the recovered raw material in the raw material solution was 34% by mass. The composition of A-3 was Sty: 10% by mass and MMA: 90% by mass. A-3 had a weight average molecular weight of 80,000, a total amount of residual monomers and polymerization solvent of 0.06% by mass, and a total amount of residual oligomers of 0.34% by mass.

<Examples 1 to 5, Comparative Examples 1 to 8>
To the styrene resins A-1 to A-3 obtained in Production Examples, the following hindered phenol antioxidant (B), phosphorus antioxidant (C1) and phosphorus antioxidant (C2-1 ), (C2-2) are mixed at the contents shown in Table 1 or Table 2, and the antioxidant is melted and kneaded using a sheet extruder manufactured by LEADER, and a sheet molded product of 450 mm × 500 mm × 2 mm got The sheet extruder consisted of a single screw extruder with a diameter of 50 mm, a T-die, and three mirror-finished rolls. 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 (Sumilizer GP manufactured by Sumitomo Chemical Co., Ltd.)
(C2-1) 2,2′-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy)phosphorus (ADEKA STAB HP-10 manufactured by ADEKA Co., Ltd.)
(C2-2) tris (2,4-di-tert-butylphenyl) phosphite (Irgafos 168 manufactured by BASF Japan Ltd.)

(oxidation induction time)
A test piece with a thickness of 30 mm × 30 mm × 2 mm was cut out from the obtained sheet molded product, and the amount of light emitted was measured using a chemiluminescence analyzer CLA-FS4 manufactured by Tohoku Denshi Sangyo Co., Ltd. under the conditions of 200 ° C. and an oxygen flow rate of 100 mL / min. The oxidation induction time was determined by the method shown in FIG. Table 1 or Table 2 shows 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 wet heat treatment in an air atmosphere of 80 ° C. and 90% humidity for 500 hours. shown in In Comparative Examples 1 and 5, the luminescence amount continued to increase immediately after the start of measurement, and the oxidation induction time could not be measured.

(Optical characteristics at optical path length of 115 mm)
A test piece having a thickness of 115 mm×85 mm×2 mm was cut out from the obtained sheet molding, and the end face was polished to prepare a plate-shaped molding having a mirror surface on the end face. For the plate-shaped molded article after polishing, using an ultraviolet-visible spectrophotometer V-670 manufactured by JASCO Corporation, the size of 20 × 1.6 mm, the incident light with a spread angle of 0 °, the wavelength at the optical path length of 115 mm The spectral transmittance of 350 nm to 800 nm was measured, and the YI value at a field of view of 2° under C light source was calculated according to JIS K7105. The transmittance shown in Tables 1 and 2 indicates the average transmittance at wavelengths of 380 nm to 780 nm. Table 1 shows the measurement results of the sheet molded product immediately after extrusion (initial), the sheet molded product subjected to wet heat treatment in an air atmosphere of 80 ° C. and 90% humidity for 500 hours, and the sheet molded product stored in an environment of 80 ° C. for 1000 hours. Or shown in Table 2.

(water absorption)
The obtained sheet molded article was cut to obtain a molded article having a size of 200 mm×300 mm. This molded article was stored for 500 hours under conditions of a temperature of 60°C and a humidity of 90%, and the change in the mass and length of the long side before and after storage was measured. did.
(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 (%)
Table 1 or Table 2 shows the evaluation results. A water absorption of 1.0 or less was judged to be good, and a deformation ratio of 0.30 or less was judged to be good.

As shown in Tables 1 and 2, in Examples in which a hindered phenol antioxidant (B), a phosphorus antioxidant having a phenolic hydroxyl group (C1) and a phosphorus antioxidant other than C1 (C2) were added , superior in hue and transparency immediately after molding as compared with the comparative examples, less change in hue under high temperature and high humidity, and superior in heat resistance.

The styrenic resin composition of the present invention and its molded article have low water absorption, excellent transparency and hue in a long light path, and excellent durability. It can be suitably used for light guide plate applications such as mobile phones, car navigation systems, and interior lighting.

Claims (5)

A styrene resin (A) having 20 to 80% by mass of styrene monomer units and 80 to 20% by mass of (meth)acrylic acid ester monomer units, a hindered phenolic antioxidant (B), A styrenic resin composition containing a phosphorus antioxidant (C1) having a phenolic hydroxyl group and a phosphorus antioxidant (C2) other than C1. 100 parts by mass of a styrene resin (A), 0.001 to 0.3 parts by mass of a hindered phenol antioxidant (B), and 0.001 to 0.3 parts by mass of a phosphorus antioxidant (C1) , containing 0.001 to 0.3 parts by mass of a phosphorus antioxidant (C2) ,
The content of the phosphorus antioxidant (C1) and the phosphorus antioxidant (C2) is 3:1 to 1:3 in mass ratio,
Hindered phenolic antioxidant (B) is octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, ethylenebis(oxyethylene)bis[3-(5-tert-butyl -4-hydroxy-m-tolyl)propionate], pentaerythritol tetrakis [3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],
The phosphorus 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]dioxaphosphepin,
The phosphorus antioxidant (C2) is 2,2′-methylenebis(4,6-di-tert-butyl-1-phenyloxy)(2-ethylhexyloxy) phosphorous, tris(2,4-di-tert- The styrenic resin composition according to claim 1 , which is at least one compound selected from butylphenyl)phosphite . YI value at an optical path length of 115 mm is 2.5 or less,
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 moist heat treatment for 500 hours in an air atmosphere of 80°C and 90% humidity is t2. , t1 is 50 minutes or more and t1-t2 is less than 20 minutes. A molded article using the styrene resin composition according to any one of claims 1 to 3 . A light guide plate using the molded article according to claim 4 .

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