JP7833926B2 - Rubber-modified styrene resin composition, sheet, and molded article - Google Patents

Description

This invention relates to rubber-modified styrene resin compositions, sheets, and molded articles.

Rubber-modified styrene resin sheets offer an excellent balance of rigidity and impact resistance, and are inexpensive. Therefore, they are secondarily molded into various containers such as food trays, lunch boxes, and cups through thermoforming methods such as vacuum forming and vacuum pressure forming. In recent years, however, there has been a growing trend towards more complex shapes for aesthetic reasons, and furthermore, a demand for thinner and lighter designs to reduce costs.

Patent Document 1 discloses a rubber-modified styrene resin composition that exhibits an excellent balance of rigidity and impact resistance, minimizes wall thickness variation due to molding, and reduces the decrease in buckling strength and drop strength due to complex container shapes and weight reduction.

Japanese Patent Publication No. 2016-222751

However, while conventional rubber-modified styrene resins exhibit excellent Charpy impact strength (a property related to impact resistance), they sometimes lack sufficient flexural modulus (a property related to rigidity). Therefore, there is a need for a rubber-modified styrene resin with improved flexural modulus, resulting in superior Charpy impact strength and flexural modulus.

This invention was made in view of the above problems, and provides a rubber-modified styrene resin with excellent Charpy impact strength and flexural modulus, and sheets and molded articles using the rubber-modified styrene resin.

According to the present invention, a rubber-modified styrene-based resin composition is provided, comprising a styrene-based resin forming a matrix phase and rubbery polymer particles dispersed in the matrix phase, wherein the gel content is 8.0 to 12.0% by mass, and the difference between the 85th percentile diameter and the 15th percentile diameter of the integrated volume distribution curve of the rubbery polymer particles is 5.1 to 6.9 μm.

Through diligent research, the inventors discovered that by setting the difference between the 85% and 15% volume integrated diameters of the gel content and rubber particle diameters within a specific range, a rubber-modified styrene-based resin composition exhibiting superior Charpy impact strength and flexural modulus can be obtained, leading to the completion of the present invention.

The following are examples of various embodiments of the present invention. The embodiments shown below can be combined with each other.
According to another aspect of the present invention, a sheet is provided comprising a layer made of the above-mentioned rubber-modified styrene resin composition.
According to another aspect of the present invention, a molded article is provided which is formed from the above-mentioned sheet.

Figure 1 shows an example of a volume integrated distribution curve for rubbery polymer particles. The figure shows the median particle diameter of the rubbery polymer particles, and the difference between the 85th percentile diameter and the 15th percentile diameter of the integrated value of the volume integrated distribution curve.

The embodiments of the present invention will be described below. The various features shown in the embodiments below are interchangeable. Furthermore, each feature constitutes an independent invention. In this specification, A to B refer to a range from A to B.

1. Rubber-Modified Styrene Resin Composition The rubber-modified styrene resin composition according to one embodiment of the present invention is a composition comprising a styrene resin forming a matrix phase and rubbery polymer particles dispersed in the matrix phase.

The styrene-based resin forming the matrix phase contains styrene-based monomer units as monomer units constituting the copolymer. Examples of styrene-based monomers from which the styrene-based monomer units originate include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, etc., either alone or in mixtures, with styrene being particularly preferred. Furthermore, other vinyl monomers copolymerizable with styrene-based monomers include acrylic acid monomers such as acrylic acid and methacrylic acid, vinyl cyanide monomers such as acrylonitrile and methacrylonitrile, acrylic monomers such as butyl acrylate and methyl methacrylate, α,β-ethylene unsaturated carboxylic acids such as maleic anhydride and fumaric acid, and imide monomers such as phenylmaleimide and cyclohexylmaleimide. These can be copolymerized within a range that does not impair the effects of the present invention.

The rubbery polymer particles dispersed in the matrix phase are formed by graft polymerization of styrene monomers onto the rubbery polymer. The styrene monomers graft-polymerized onto the rubbery polymer may be the same as or different from the styrene monomers from which the styrene monomer units constituting the styrene resin forming the matrix phase originate. Examples of rubbery polymers here include homopolymers of diene monomers such as polybutadiene, low-cis polybutadiene, high-cis polybutadiene, and high-cis high-vinyl polybutadiene, as well as styrene-butadiene copolymers, styrene-butadiene block copolymers, hydrogenated (partially hydrogenated) polybutadiene, hydrogenated (partially hydrogenated) styrene-butadiene copolymers, hydrogenated (partially hydrogenated) styrene-butadiene block copolymers, ethylene-propylene copolymers, ethylene-propylene-unconjugated diene ternary copolymers, isoprene polymers, and styrene-isoprene copolymers. Among these, the use of homopolymers of diene monomers is preferred. Furthermore, the rubbery polymer can be used alone or in combination of two or more types. The rubbery polymer particles preferably contain styrene-based polymer particles and have a sea-island structure.

The gel content of the rubber-modified styrene resin composition is 8.0 to 12.0% by mass, preferably 9.0 to 12.0% by mass. This range results in excellent Charpy impact strength and flexural modulus. Methods for adjusting the gel content include adjusting the rubber content during the polymerization process, adjusting the amount of initiator, and adjusting it by blending with a styrene homopolymer after polymerization. Specifically, the gel content may be, for example, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, or 12.0% by mass, and may also be within the range of any two of the values exemplified here.

The gel content is the proportion of rubbery polymer particles in the rubber-modified styrene resin composition. 1.00 g of the rubber-modified styrene resin composition is accurately weighed (W), 35 ml of a 50% methyl ethyl ketone/50% acetone mixed solution is added and dissolved, and the solution is centrifuged at 10,000 rpm for 30 minutes using a centrifuge (H-2000B (rotor: H) manufactured by Kokusan Co., Ltd.) to settle the insoluble matter. The supernatant is removed by decantation to obtain the insoluble matter, pre-dried in a safety oven at 90°C for 2 hours, then dried under reduced pressure in a vacuum dryer at 125°C for 1 hour, and cooled in a desiccator for 20 minutes. The mass G of the dried insoluble matter is then measured and determined as follows.
Gel content (amount of rubbery dispersed particles) (mass%) = (G/W) × 100

The difference between the 85th percentile diameter and the 15th percentile diameter of the integrated volume distribution curve of rubbery polymer particles (rubber particle diameter difference) is 5.1 to 6.9 μm, preferably 5.5 to 6.5 μm. Being within this numerical range results in excellent Charpy impact strength and flexural modulus. Specifically, the difference between the 85th percentile diameter and the 15th percentile diameter is, for example, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, and 6.9 μm, and may also be within the range of any two of the values exemplified here.

The median volume particle size of the rubbery polymer particles contained in the rubber-modified styrene resin composition is preferably 4.5 to 8.0 μm, and more preferably 6.0 to 7.0 μm. Specifically, this median volume particle size may be, for example, 4.5, 5.0, 5.5, 6.0, 6.1, 6.3, 6.5, 6.7, 7.0, 7.5, or 8.0 μm, and may be within the range of any two of the values exemplified here.

The median particle size, the 85th percentile diameter, and the 15th percentile diameter of the integrated volume distribution curve of rubber-like polymer particles can be calculated based on the 50th percentile, 85th percentile, and 15th percentile diameters of the particle size integrated volume distribution curve (volume-based particle size distribution curve) shown in Figure 1. The integrated volume distribution curve can be obtained as a graph as shown in Figure 1 by dissolving the rubber-modified styrene resin composition in dimethylformamide and measuring it using a laser diffraction/scattering particle size distribution analyzer (LA-960, manufactured by Horiba, Ltd.: relative refractive index 120A000I). Methods for adjusting particle size include adjusting the stirring speed in the phase transition region of the rubber particles during the polymerization process, and adjusting the amount of chain transfer agent in the raw material liquid.

The melt mass flow rate (MFR) of the rubber-modified styrene resin composition, measured under conditions of 200°C and a 49N load, is preferably 1.5 to 8.0 g/10 min, more preferably 2.5 to 7.0 g/10 min, and even more preferably 4.5 to 6.5 g/10 min. This numerical range allows for optimal fluidity during molding and practical strength. The MFR can be measured according to JIS K-7210. Specifically, the MFR may be, for example, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 5.7, 6.0, 6.5, 7.0, 7.5, or 8.0 g/10 min, and may also be within the range of any two of the values exemplified here.

The Vicat softening temperature of the rubber-modified styrene resin composition, measured under a 50N load, is preferably 90°C or higher, and more preferably 90-93°C. This numerical range provides excellent heat resistance and moldability. The Vicat softening temperature was determined according to JIS K-7206, using a heating rate of 50°C/hr and a test load of 50N. Specifically, the Vicat softening temperature may be, for example, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100°C, and may also be within the range of any two of the values exemplified here.

The Charpy impact strength of the rubber-modified styrene resin composition is preferably 8.5 kJ/ ^m² or higher, and more preferably 9.0 kJ/ ^m² or higher. The Charpy impact strength was determined by preparing test specimens using an injection molding machine and measuring them in accordance with JIS K-7111-1.

The flexural modulus of the rubber-modified styrene resin composition is preferably 2500 MPa or higher, and more preferably 2650 MPa or higher. The flexural modulus was determined by preparing test specimens using an injection molding machine and measuring them according to JIS K-7171.

The rubber-modified styrene resin composition may contain additives such as phosphorus-based, phenol-based, and amine-based antioxidants; higher fatty acids such as stearic acid, zinc stearate, calcium stearate, and magnesium stearate, and their salts; lubricants such as ethylene bisstearylamide; plasticizers such as liquid paraffin; inorganic fillers such as talc and calcium carbonate; ultraviolet absorbers; antistatic agents; silicone oil; flame retardants; colorants; pigments; deodorants; and antibacterial agents, to the extent that they do not impair the effects of the present invention.

<Method for producing rubber-modified styrene resin composition>
A method for producing a rubber-modified styrene resin composition according to one embodiment of the present invention includes a step of mixing a styrene resin (a) and rubber-modified polystyrene (b). The rubber-modified styrene resin composition may contain other resins such as thermoplastic resins or rubber reinforcing resins other than the styrene resin (a) and rubber-modified polystyrene (b), as long as their physical properties are not impaired. Furthermore, there are no particular restrictions on the blending method, and methods include melt-kneading multiple resins and re-granulating them with an extruder, or dry-blending a mixture of multiple resins using a Henschel mixer, ribbon blender, super mixer, V-blender, etc.

Furthermore, the method for producing the rubber-modified styrene-based resin composition may include a step of polymerizing styrene monomers to synthesize styrene-based resin (a). The step of synthesizing styrene-based resin (a) may further include a step of synthesizing a polyfunctional vinyl copolymer that serves as a crosslinking agent. The step of synthesizing styrene-based resin (a) may also include a step of polymerizing the styrene monomers in the presence of a polymerization initiator, a solvent, and, if necessary, a required crosslinking agent.

Furthermore, the method for producing the rubber-modified styrene resin composition may include a step of synthesizing rubber-modified polystyrene (b) by graft polymerization of styrene monomers in the presence of a rubbery polymer. Graft polymerization can be carried out by known methods, such as bulk polymerization, suspension polymerization, bulk-suspension two-stage polymerization, or solution polymerization. As solvents, 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. Examples of reactor types include fully mixed reactors, plug-flow reactors, and loop reactors that remove a portion of the polymerization solution while polymerization is progressing. A so-called continuous polymerization method, combining these with a volatile matter removal step to remove unreacted monomers, may also be used.

In the process of synthesizing rubber-modified polystyrene (b), polymerization solvents, polymerization initiators such as organic peroxides, and chain transfer agents such as aliphatic mercaptans can be used as needed to control the polymerization reaction.

As polymerization initiators, radical polymerization initiators are preferred, such as peroxyketals like 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 like cumene hydroperoxide and t-butyl hydroperoxide; alkyl peroxides like t-butylperoxyacetate and t-amylperoxyisononanoate; t-butylcumylperoxide, di-t-butylperoxide, dicumylperoxide, and di-t-hexylperoxide. Examples include dialkyl peroxides, peroxyesters such as t-butyl peroxyacetate, t-butyl peroxybenzoate, and t-butyl peroxyisopropyl monocarbonate, peroxycarbonates such as t-butyl peroxyisopropyl carbonate and polyethertetrakis(t-butyl peroxycarbonate), and N,N'-azobis(cyclohexane-1-carbonitride), N,N'-azobis(2-methylbutyronitrile), N,N'-azobis(2,4-dimethylvaleronitrile), and N,N'-azobis[2-(hydroxymethyl)propionitrile]. One or more of these can be used in combination.

As the chain transfer agent, either a monofunctional chain transfer agent having one chain transfer group or a polyfunctional chain transfer agent having multiple chain transfer groups may be used. Examples of monofunctional chain transfer agents include aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer, and terpinolene. Examples of polyfunctional chain transfer agents include polyfunctional mercaptans obtained by esterifying the hydroxyl group of polyhydric alcohols such as ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, and sorbitol with thioglycolic acid or mercaptopropionic acid. One or more of these can be used in combination.

2. Sheets and molded products <Sheets>
A sheet according to one embodiment of the present invention is a sheet comprising a layer made of the above-mentioned rubber-modified styrene resin composition. The sheet may be a single-layer sheet comprising only a layer made of the above-mentioned rubber-modified styrene resin composition, or it may be a multilayer sheet comprising a layer made of the above-mentioned rubber-modified styrene resin composition and a layer made of a composition different from the above-mentioned rubber-modified styrene resin composition. The multilayer sheet may have a structure in which, for example, a layer made of the rubber-modified styrene resin composition is used as a core layer, and surface layers are provided directly on both sides of the core layer or via other layers. In one embodiment, the multilayer sheet is a two-type, three-layer multilayer sheet with surface layers directly on both sides of the core layer. The thickness of the layer made of the above-mentioned rubber-modified styrene resin composition is, for example, 0.20 to 0.45 mm, preferably 0.30 to 0.40 mm. The overall thickness of the multilayer sheet is, for example, 0.50 to 0.90 mm.

<Method of manufacturing the sheet>
The sheets can be manufactured by molding the above-mentioned rubber-modified styrene resin composition using known molding methods. Examples of molding methods include extruding molten resin from a T-die, calendering, and inflation molding, but using a T-die is preferable in terms of productivity and film thickness accuracy. Methods for manufacturing multilayer sheets include co-extrusion using a feed block die or a multi-manifold die, and a method in which the surface layer is prepared separately in advance and then heat-laminated with a base sheet (which will become the core layer, etc.).

<Molded product>
A molded article according to one embodiment of the present invention is a molded article obtained by molding the above-mentioned sheet. Examples of molded articles include containers. More specifically, containers include various shapes of food containers such as yogurt containers, dessert containers, prepared food containers and bento boxes, trays, beverage containers, frozen dessert containers, food containers such as milk portion containers, and food container lids such as coffee cup lids.

<Method of manufacturing molded products>
Molded products can be manufactured by molding the above-mentioned sheet using known molding methods. Examples of methods for molding the sheet into a container include thermoforming, which involves applying heat during molding. More specifically, these include vacuum forming, pressure forming, vacuum pressure forming, matched molding, reverse draw, air stripping, ridge forming, plug-and-ridge forming, and hot plate forming.

Furthermore, the molded product may be manufactured from the above-mentioned rubber-modified styrene resin composition by molding methods such as extrusion molding, injection molding, foam molding, or blow molding.

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

[Styrene resin (a)]
The manufacturing of styrene-based resin (a) will be described using resins A and B.

<Resin A>
The polymerization process was constructed by connecting a first reactor and a second reactor, both fully mixed stirred tanks, and a third reactor, a plug-flow type reactor with a static mixer, in series. The capacities of each reactor were 39 liters for the first reactor, 39 liters for the second reactor, and 16 liters for the third reactor. A raw material solution consisting of 95 parts by mass of styrene monomer, 5 parts by mass of ethylbenzene, and 3.5 parts by mass of liquid paraffin was continuously supplied to the first reactor at a supply rate of 18 L/hr, and polymerization was carried out at temperatures of 135°C for the first reactor, 135°C for the second reactor, and 135-135°C for the third reactor. The obtained polymerization solution was introduced into a vacuum defloration tank with a preheater, consisting of two stages in series, to separate unreacted styrene and ethylbenzene. After being extruded into strands and cooled, it was cut to form pellets. The resin temperature in the first defoliation chamber was set to 180°C and the pressure in the vacuum defoliation chamber was set to 67 kPa. The resin temperature in the second defoliation chamber was set to 230°C and the pressure in the vacuum defoliation chamber was set to 0.7 kPa. The properties of the obtained resin A are shown in Table 1.

<Resin B>
3.1 moles (399.4 g) of divinylbenzene, 0.7 moles (95.1 g) of ethylvinylbenzene, 0.3 moles (31.6 g) of styrene, 2.3 moles (463.5 g) of 2-phenoxyethyl methacrylate, and 974.3 g of toluene were placed in a 3.0 L reactor, and 42.6 g of boron trifluoride diethyl ether complex was added at 50°C, and the reaction was allowed to proceed for 6.5 hours. After stopping the polymerization reaction with a sodium bicarbonate solution, the oil layer was washed three times with pure water, and the reaction mixture was added to a large amount of methanol at room temperature to precipitate the polymer. The obtained polymer was washed with methanol, filtered, dried, and weighed to obtain 372.5 g of polyfunctional vinyl aromatic copolymer (crosslinking agent A).

The polymerization process was constructed by connecting a first reactor and a second reactor, both fully mixed stirred tanks, and a third reactor, a plug-flow type reactor with a static mixer, in series. The capacities of each reactor were 39 liters for the first reactor, 39 liters for the second reactor, and 16 liters for the third reactor. A raw material solution consisting of 90 parts by mass of styrene monomer, 10 parts by mass of ethylbenzene, 0.020 parts by mass of 2,2-di(4,4-t-butylperoxycyclohexyl)propane, and 0.030 parts by mass of crosslinking agent A was continuously supplied to the first reactor at a supply rate of 14 L/hr, and polymerization was carried out at temperatures of 118°C for the first reactor, 125°C for the second reactor, and 120-135°C for the third reactor. The obtained polymerization solution was introduced into a vacuum defloration tank with a preheater, consisting of two stages in series, to separate unreacted styrene and ethylbenzene. After being extruded into strands and cooled, it was cut to form pellets. The resin temperature in the first defoliation chamber was set to 185°C, and the pressure in the vacuum defoliation chamber was set to 67 kPa. The resin temperature in the second defoliation chamber was set to 240°C, and the pressure in the vacuum defoliation chamber was set to 0.4 kPa. The properties of the obtained resin B are shown in Table 1.

<Meltmass Flowrate>
The melt mass flow rate of styrene resin (a) was measured according to JIS K-7210 under conditions of 200°C and a load of 49N.

<Vicat softening temperature>
The Vicat softening temperature of styrene resin (a) was measured according to JIS K-7206 under conditions of a 50N load and a heating rate of 50°C/hr.

<Methanol-soluble components>
The methanol-soluble portion of styrene resin (a) was calculated by accurately weighing 1.00 g of rubber-modified styrene resin (P), dissolving it in 40 ml of methyl ethyl ketone, and rapidly adding 400 ml of methanol to precipitate and settle the methanol-insoluble portion (resin component). After standing for about 10 minutes, the methanol-soluble portion was separated by gradually filtering through a glass filter, and the mixture was dried under reduced pressure at 120°C for 2 hours in a vacuum dryer. After cooling in a desiccator for 25 minutes, the mass N of the dried methanol-insoluble portion was measured and calculated based on the following formula.
Methanol-soluble content (mass%) = (P - N) / P × 100

<Molecular weight>
The Mn (number-average molecular weight), Mw (weight-average molecular weight), and Mz (Z-average molecular weight) of styrene resin (a) were measured using gel permeation chromatography (GPC) under the following conditions.
GPC model: Showa Denko Co., Ltd. Shodex GPC-101
Column: PLgel 10 μm MIXED-B, 300 × 7.5 mm, manufactured by Polymer Laboratories, Inc.
Mobile phase: Tetrahydrofuran 1.0 ml/min
Sample concentration: 0.2% by mass
Temperature: Oven 40°C, Inlet 35°C, Detector 35°C
Detector: Differential refractometer. Molecular weight was calculated from the elution curve of monodisperse polystyrene at each elution time, and then calculated as the molecular weight in polystyrene equivalent.

[Rubber-modified polystyrene (b)]
The manufacturing process for rubber-modified polystyrene (b) will be described using resins C to F.

<Resin C>
A fully mixed reactor with a stirring blade and a volume of 25 L was designated as the first reactor, a plug-flow reactor with a stirring blade and a volume of 40 L was designated as the second reactor, a plug-flow reactor with a stirring blade and a volume of 50 L was designated as the third reactor, and a static mixer-type plug-flow reactor with a volume of 50 L was designated as the fourth reactor. These reactors were connected in series to constitute the polymerization process. A raw material solution consisting of 77.3 parts by mass of styrene monomer, 13.6 parts by mass of ethylbenzene, 9.1 parts by mass of polybutadiene "Asaprene 730AX" manufactured by Nippon Elastomer Co., Ltd. as a rubbery polymer, 0.010 parts by mass of 1,1-di(t-butylperoxy)cyclohexane, and 0.020 parts by mass of t-dodecyl mercaptan was continuously supplied to the reactor at a supply rate of 22 L/hr. Polymerization was carried out at a temperature of 120°C and a stirring speed of 100 rpm in the first reactor, and at a temperature of 125-128°C and a stirring speed of 140 rpm in the second reactor. Then, t- After adding 0.025 parts by mass of dodecyl mercaptan and polymerizing in the third reactor at a temperature of 130-130°C and a stirring speed of 30 rpm, and in the fourth reactor at a temperature of 130-140°C, the mixture is introduced into a vacuum defloration tank with a preheater, consisting of two stages in series, to separate unreacted styrene and ethylbenzene. Then, liquid paraffin (Kaneda Co., Ltd. "Hycol K-350") is added to a concentration of 1.0% by mass relative to resin C, and silicone oil (Dow Chemical Corporation "DOWSIL SH200") is added to a concentration of 0.1% by mass. 100 cSt) was added/mixed, extruded into strands, cooled, and then cut to form pellets. The resin temperature in the first defoliation chamber was set to 190°C and the pressure in the vacuum defoliation chamber was set to 67 kPa. The resin temperature in the second defoliation chamber was set to 230°C and the pressure in the vacuum defoliation chamber was set to 1.3 kPa. The properties of the obtained resin C are shown in Table 2.

<Resin D>
A fully mixed reactor with a stirring blade and a volume of 25 L was designated as the first reactor, a plug-flow reactor with a stirring blade and a volume of 40 L was designated as the second reactor, a plug-flow reactor with a stirring blade and a volume of 50 L was designated as the third reactor, and a static mixer-type plug-flow reactor with a volume of 50 L was designated as the fourth reactor, and these were connected in series to constitute the polymerization process. 77.6 parts by mass of styrene monomer, 13.7 parts by mass of ethylbenzene, 8.7 parts by mass of polybutadiene "BR-15HB" manufactured by Ube Industries as a rubbery polymer, t- A raw material solution consisting of 0.025 parts by mass of dodecyl mercaptan was continuously supplied to the reactor at a supply rate of 22 L/hr. Polymerization was carried out at the following temperatures: 125°C and 100 rpm in the first reactor, 128-130°C and 40 rpm in the second reactor, 130-130°C and 30 rpm in the third reactor, and 135-160°C in the fourth reactor. The mixture was then introduced into a vacuum defloration tank with a preheater, consisting of two stages in series, to separate unreacted styrene and ethylbenzene. Liquid paraffin ("Highcol K-350" manufactured by Kaneda Co., Ltd.) was added and mixed to a concentration of 1.4% by mass relative to resin D. The mixture was extruded into strands, cooled, and then cut to form pellets. The resin temperature in the first defoliation chamber was set to 190°C and the pressure in the vacuum defoliation chamber was set to 60 kPa. The resin temperature in the second defoliation chamber was set to 230°C and the pressure in the vacuum defoliation chamber was set to 1.3 kPa. The properties of the obtained resin D are shown in Table 2.

<Resin E to F>
Resins E to F were obtained by adjusting the raw material composition and polymerization temperature, etc., to achieve the resin properties shown in Table 2, using the same method as for resin C. For resins E to F, Asahi Kasei's polybutadiene "Diene 55AE" was used as the rubbery polymer.

<Meltmass Flowrate>
The melt mass flow rate of rubber-modified polystyrene (b) was measured according to JIS K-7210 under conditions of 200°C and a load of 49N.

<Vicat softening temperature>
The Vicat softening temperature of rubber-modified polystyrene (b) was measured according to JIS K-7206 under conditions of a 50N load and a heating rate of 50°C/hr.

<Rubber content>
The rubber content of rubber-modified polystyrene (b) was calculated by dissolving rubber-modified polystyrene (b) in chloroform, adding a certain amount of iodine monochloride/carbon tetrachloride solution, leaving it in the dark for about 1 hour, then adding potassium iodide solution, and titrating the excess iodine monochloride with a 0.1 N sodium thiosulfate/ethanol aqueous solution.

<Rubber particle size (volume median diameter)>
The rubber particle size (volume median particle size) was measured in the same manner as in the rubber-modified styrene resin composition described later.

<Gel component>
The gel content was measured in the same manner as in the rubber-modified styrene resin composition described later.

<Graft rate>
The graft rate was calculated from the gel content (mass%) and rubber content (mass%) of rubber-modified polystyrene (b) based on the following formula.
Graft ratio = (Gel portion - Rubber portion) / Rubber portion

<Methanol-soluble components>
The methanol-soluble content was measured in the same manner as in the styrene-based resin (a) described above.

<Swelling degree>
The swelling ratio was calculated by accurately weighing 1 g of rubber-modified polystyrene (b), dissolving it in 30 mL of toluene, centrifuging the solution in a centrifuge (H-2000B (rotor: H) manufactured by Kokusan Co., Ltd.) at 14,000 rpm for 30 minutes to settle the insoluble matter, removing the supernatant by decantation, measuring the mass S of the insoluble matter swollen with toluene, pre-drying the toluene-swollen insoluble matter in a safety oven at 90°C for 2 hours, then vacuum drying in a vacuum dryer at 125°C for 1 hour, cooling in a desiccator for 20 minutes, measuring the dry mass D of the insoluble matter, and calculating the ratio based on the following formula.
Swelling ratio=S/D

[Examples 1-4, Comparative Examples 1-15]
The styrene-based resins (resins A to B) and rubber-modified polystyrene (resins C to F) produced by the above method were pre-mixed in the ratios shown in Tables 3 and 4, and then melt-kneaded in a single-screw extruder (Ikegai, PMS40-28V) set to 180 to 220°C. The resin properties of the obtained rubber-modified styrene-based resin compositions are shown in Tables 3 and 4.

<Meltmass Flowrate>
The melt mass flow rate of the rubber-modified styrene resin composition was measured according to JIS K-7210 under conditions of 200°C and a 49N load.

<Vicat softening temperature>
The Vicat softening temperature of the rubber-modified styrene resin composition was measured according to JIS K-7206 under conditions of a 50N load and a heating rate of 50°C/hr.

<Gel component>
The gel content of the rubber-modified styrene resin composition was calculated by accurately weighing 1.00 g of the rubber-modified styrene resin composition (W), adding 35 ml of a 50% methyl ethyl ketone/50% acetone mixed solution to dissolve it, and centrifuging the solution in a centrifuge (H-2000B (rotor: H) manufactured by Kokusan Co., Ltd.) at 10,000 rpm for 30 minutes to settle the insoluble matter. The supernatant was removed by decantation to obtain the insoluble matter, pre-drying in a safety oven at 90°C for 2 hours, then vacuum drying in a vacuum dryer at 125°C for 1 hour, and cooling in a desiccator for 20 minutes. The mass G of the dried insoluble matter was then measured and calculated based on the following formula.
Gel content (amount of rubbery dispersed particles) (mass%) = (G/W) × 100

<Rubber particle diameter (volume median particle diameter) and rubber particle diameter difference (85% volume integrated diameter - 15% volume integrated diameter)>
The median particle size, the 85th percentile diameter of the integrated volume distribution curve, and the 15th percentile diameter of the integrated volume distribution curve of the rubber-like polymer particles were calculated based on the 50th percentile, 85th percentile, and 15th percentile diameters of the integrated volume distribution curve. The integrated volume distribution curve was obtained by dissolving the rubber-modified styrene resin composition in dimethylformamide and measuring it using a laser diffraction/scattering particle size distribution analyzer (LA-960, manufactured by Horiba, Ltd.: relative refractive index 120A000I).

<Charpy impact strength>
Using an injection molding machine (manufactured by Japan Steel Works, Ltd., J100E-P), a Type A test specimen (dumbbell) as described in JIS K-7139 was molded at a cylinder temperature of 220°C and a mold temperature of 45°C. A test specimen was cut from the center of the dumbbell and a notch (Type A, r=0.25 mm) was made by cutting. Measurements were then performed in accordance with JIS K-7111-1.

<Flexural modulus>
Using an injection molding machine (manufactured by Japan Steel Works, Ltd., J100E-P), a Type A test specimen (dumbbell) as described in JIS K-7139 was molded at a cylinder temperature of 220°C and a mold temperature of 45°C. A test specimen cut from the center of the dumbbell was measured in accordance with JIS K-7171.

Claims (3)

It comprises a styrene-based resin that forms a matrix phase and rubbery polymer particles dispersed in the matrix phase,
The gel content is 8.0 to 12.0% by mass.
The difference between the 85th percentile diameter and the 15th percentile diameter in the volume integral distribution curve of the rubbery polymer particles is 5.1 to 6.9 μm.
Rubber-modified styrene resin composition. A sheet comprising a layer made of the rubber-modified styrene resin composition described in claim 1. A molded article obtained by molding the sheet described in claim 2.