JP2025153577A - Rubber-modified styrene-based resin composition and molded article - Google Patents

Abstract

The present invention provides a rubber-modified styrene-based resin composition from which molded articles having excellent flexural modulus and extremely excellent Charpy impact strength can be obtained, and a molded article using the rubber-modified styrene-based resin composition.
[Solution] According to the present invention, there is provided a rubber-modified styrene-based resin composition comprising a styrene-based resin forming a matrix phase and rubber-like polymer particles dispersed in the matrix phase, the composition having a gel content of 26.0 to 30.0 mass %, and the difference between the 85% diameter and the 15% diameter of the integrated value in a volume-integrated particle diameter distribution curve of the rubber-like polymer particles being 5.1 to 6.9 μm.
[Selection diagram] None

Description

The present invention relates to a rubber-modified styrene-based resin composition and a molded article.

Rubber-modified styrene-based resin compositions have excellent rigidity and impact resistance, and are used in a variety of applications, including food trays, lunch boxes, cups, and other food containers, packaging, office equipment, home appliance parts, and miscellaneous goods.

For example, Patent Document 1 discloses a rubber-modified styrene-based resin composition that has an excellent balance of rigidity and impact resistance, little thickness deviation due to molding, and little reduction in buckling strength or drop strength due to the complex shape or lightweight nature of the container.

JP 2016-222751 A

However, depending on the application and usage environment, further improvement may be required, particularly in Charpy impact strength, a physical property related to impact resistance, and previous rubber-modified styrene-based resins have sometimes not provided sufficient Charpy impact strength.

The present invention was made in consideration of these problems and provides a rubber-modified styrene-based resin composition that can produce molded articles with excellent flexural modulus and extremely excellent Charpy impact strength, as well as molded articles made from this rubber-modified styrene-based resin composition.

The present invention provides a rubber-modified styrene-based resin composition comprising a styrene-based resin that forms a matrix phase and rubber-like polymer particles dispersed in the matrix phase, with a gel content of 26.0 to 30.0 mass %, and a particle size distribution curve for the rubber-like polymer particles in which the difference between the 85% diameter and the 15% diameter is 5.1 to 6.9 μm.

After extensive research, the inventors discovered that by adjusting the difference between the 85% volume cumulative diameter and the 15% volume cumulative diameter of the rubber particle diameter and the gel content within specific ranges, it is possible to obtain a rubber-modified styrene-based resin composition that exhibits excellent flexural modulus and extremely excellent Charpy impact strength, 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.

[1] A rubber-modified styrene-based resin composition comprising a styrene-based resin forming a matrix phase and rubber-like polymer particles dispersed in the matrix phase, wherein the gel content is 26.0 to 30.0 mass %, and the difference between the 85% diameter and the 15% diameter of the integrated value in a volume-integrated particle size distribution curve of the rubber-like polymer particles is 5.1 to 6.9 μm.
[2] The rubber-modified styrene-based resin composition according to [1], having a melt mass flow rate of 3.5 g/10 min or less measured at 200°C and 49 N.
[3] The rubber-modified styrene-based resin composition according to [1] or [2], wherein the rubber content is more than 11% by mass.
[4] A molded article obtained by molding the rubber-modified styrene-based resin composition according to any one of [1] to [3].

Fig. 1 shows an example of a volume-integrated distribution curve of rubber-like polymer particles, which shows the volume median particle diameter of the rubber-like polymer particles and the difference between the 85% diameter and the 15% diameter of the integrated value of the volume-integrated distribution curve.

The following describes embodiments of the present invention. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature can be an independent invention. Note that in this specification, "A-B" means greater than or equal to A and less than or equal to B.

1. Rubber-Modified Styrenic Resin Composition A rubber-modified styrenic resin composition according to one embodiment of the present invention is a composition containing a styrenic resin that forms a matrix phase and rubbery polymer particles dispersed in the matrix phase.

The styrene-based resin that forms 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 are derived include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, and the like, either alone or in mixtures. Styrene is particularly preferred. Other vinyl-based monomers that can be copolymerized 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-ethylenically unsaturated carboxylic acids such as maleic anhydride and fumaric acid, and imide-based monomers such as phenylmaleimide and cyclohexylmaleimide, all of which can be copolymerized to the extent that the effects of the present invention are not impaired.

The rubbery polymer particles dispersed in the matrix phase are formed by graft polymerizing a styrene-based monomer onto a rubbery polymer. The styrene-based monomer graft polymerized onto the rubbery polymer may be the same as or different from the styrene-based monomer from which the styrene-based monomer units constituting the styrene-based resin forming the matrix phase are derived. Examples of rubbery polymers include homopolymers of diene monomers such as polybutadiene, low-cis polybutadiene, high-cis polybutadiene, and high-cis high-vinyl polybutadiene; 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-non-conjugated diene terpolymers; isoprene polymers; and styrene-isoprene copolymers. Among these, homopolymers of diene monomers are preferred. The rubbery polymer particles preferably contain styrene-based polymer particles and have an island-in-a-sea structure. The rubbery polymer and rubbery polymer particles can be used singly or in combination. A rubber-modified styrene-based resin composition according to one embodiment of the present invention preferably contains two or more rubbery polymers and rubbery polymer particles. A rubber-modified styrene-based resin composition according to one embodiment of the present invention can contain two types of rubbery polymers and rubbery polymer particles. The two or more rubbery polymers and rubbery polymer particles can differ in at least one of the monomer units constituting the rubbery polymer, their composition, molecular weight, and particle size (85% diameter of integrated value, 15% diameter of integrated value, volume median particle size) of the rubbery polymer particles. For example, the two types of rubbery polymers and rubbery polymer particles can differ in at least the particle size (85% diameter of integrated value, 15% diameter of integrated value, volume median particle size) of the rubbery polymer particles.

<Gel content>
The gel content of the rubber-modified styrene-based resin composition is 26.0 to 30.0% by mass, preferably 26.5 to 29.0% by mass. This range of values results in extremely excellent Charpy impact strength. Methods for adjusting the gel content include adjusting the rubber content during the polymerization process of the rubber-modified styrene-based resin composition, adjusting the amount of initiator, and blending with a styrene homopolymer after polymerization. Specific examples of the gel content are 26.0, 26.5, 27.0, 27.5, 28.0, 28.5, 29.0, 29.5, and 30.0% by mass, and may be within a range between any two of the values exemplified here.

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

<Rubber content>
The rubber content of the rubber-modified polystyrene and rubber-modified styrene-based resin composition according to one embodiment of the present invention can be more than 11% by mass, and preferably more than 12% by mass. By being in this range, the Charpy impact strength is further improved. The rubber content is, for example, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13.0, 13.1, 13.2, 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, 14.0, 14.1, 14.2, 14.3, 14.4, 14.5, 14.6, 14.7, 14.8, 14.9, or 15.0% by mass, and may be within a range between any two of the values exemplified here.

The rubber content can be calculated by dissolving rubber-modified polystyrene (or rubber-modified styrene-based resin composition) in chloroform, adding a certain amount of iodine monochloride/carbon tetrachloride solution, leaving the mixture in a dark place for approximately one hour, adding potassium iodide solution, and titrating the excess iodine monochloride with 0.1N sodium thiosulfate/ethanol aqueous solution.

<Swelling degree>
The swelling degree of the rubber-modified polystyrene and rubber-modified styrene-based resin composition according to one embodiment of the present invention may be 10.0 to 17.0, for example, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0, 15.5, 16.0, 16.5, or 17.0, and may be within a range between any two of the values exemplified here.

The swelling degree can be calculated based on the following formula: 1 g of rubber-modified polystyrene (or rubber-modified styrene-based resin composition) is weighed out, 30 mL of toluene is added to dissolve the resultant solution, and the solution is centrifuged at 14,000 rpm for 30 minutes in a centrifuge (H-2000B (rotor: H) manufactured by Kokusan Co., Ltd.) to precipitate the insoluble matter. The supernatant is removed by decantation, and the mass S of the insoluble matter swollen with toluene is measured. The insoluble matter swollen with toluene is then pre-dried in a safety oven at 90°C for 2 hours, and then vacuum-dried in a vacuum dryer at 125°C for 1 hour. The resultant is cooled in a desiccator for 20 minutes, and the dry mass D of the insoluble matter is measured.
Swelling degree = S/D

<Grafting rate>
The graft ratio of the rubber-modified polystyrene and rubber-modified styrene-based resin composition according to one embodiment of the present invention may be 1.0 to 2.0, for example, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0, and may be within a range between any two of the values exemplified here.

The graft ratio can be calculated from the gel content (mass%) and the rubber content (mass%) according to the following formula:
Graft ratio = (gel content - rubber content) / rubber content

<Methanol soluble matter>
The methanol-soluble content of the rubber-modified polystyrene and rubber-modified styrene-based resin composition according to one embodiment of the present invention may be 1.5 to 3.0% by mass, for example, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 3.0% by mass, and may be within a range between any two of the values exemplified here.

The methanol-soluble content of rubber-modified polystyrene and rubber-modified styrene-based resin compositions can be measured as follows. 1.00 g of rubber-modified polystyrene (or rubber-modified styrene-based resin composition) was precisely weighed (P), dissolved in 40 ml of methyl ethyl ketone, and 400 ml of methanol was added rapidly to separate and precipitate the methanol-insoluble content (resin component). After allowing to stand for about 10 minutes, the mixture was slowly filtered through a glass filter to separate the methanol-soluble content. The mixture was then dried under reduced pressure at 120°C for 2 hours in a vacuum dryer, and then allowed to cool in a desiccator for 25 minutes. The mass N of the dried methanol-insoluble content was measured, and the mass N was calculated based on the following formula:
Methanol soluble matter (mass%)=(P−N)/P×100

In addition, when the rubber-modified styrene-based resin composition is obtained by blending multiple rubber-modified polystyrenes, parameters such as the rubber content, gel content, and methanol-soluble content of the rubber-modified styrene-based resin composition can also be calculated from the parameters such as the rubber content, gel content, and methanol-soluble content of each of the multiple rubber-modified polystyrenes contained in the rubber-modified styrene-based resin composition and the blending mass ratio.

The difference between the 85% diameter and the 15% diameter of the integrated value in the volume cumulative particle diameter distribution curve of the rubber-like polymer particles (rubber particle diameter difference) is 5.1 to 6.9 μm, preferably 5.5 to 6.5 μm. Being in this range results in excellent flexural modulus and extremely excellent Charpy impact strength. Specific examples of the difference between the 85% diameter and the 15% diameter of the integrated value are 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 be within a range between any two of the values exemplified here.

The volume median particle diameter of the rubber-like polymer particles contained in the rubber-modified styrene-based resin composition is preferably 4.5 to 8.0 μm, and more preferably 6.0 to 7.0 μm. Being within this range results in a superior flexural modulus. Specific examples of this volume median particle diameter include 4.5, 5.0, 5.5, 6.0, 6.1, 6.3, 6.5, 6.7, 7.0, 7.5, and 8.0 μm, and may fall within a range between any two of the values exemplified here.

The volume median particle size, 85% diameter of the integrated value in the volume cumulative distribution curve, and 15% diameter of the integrated value in the volume cumulative distribution curve of rubbery polymer particles can be calculated based on the 50%, 85%, and 15% diameters of the volume cumulative particle size distribution curve (volume-based particle size distribution curve) shown in Figure 1. The volume cumulative distribution curve can be obtained as a graph like the one shown in Figure 1 by dissolving the rubber-modified styrene-based resin composition in dimethylformamide and measuring it with a laser diffraction/scattering particle size distribution analyzer (HORIBA, Ltd. LA-960: relative refractive index 120A000I). Methods for adjusting the particle size include adjusting the stirring speed in the phase inversion region of the rubber particles during the polymerization process, adjusting the amount of chain transfer agent in the raw material liquid, mixing two or more rubber-modified polystyrenes containing rubbery polymer particles with different particle sizes, and using two or more different rubbery polymers as raw materials for producing the rubber-modified styrene-based resin composition.

The melt mass flow rate (MFR) of the rubber-modified styrene-based resin composition measured at 200°C under a load of 49 N is preferably 3.5 g/10 min or less, and more preferably 3.3 g/10 min or less. Being in this range ensures fluidity during molding and practical strength. MFR can be measured in accordance with JIS K-7210. Specific examples of MFR are 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, and 3.0 g/10 min, and may be within a range between any two of the values exemplified here.

The Vicat softening temperature of the rubber-modified styrene-based resin composition, measured under a 50N load, is preferably 90°C or higher, and more preferably 90 to 94°C. Being in this range provides excellent heat resistance and moldability. The Vicat softening temperature was measured in accordance with JIS K-7206 at a temperature rise rate of 50°C/hr and a test load of 50N. Specific examples of the Vicat softening temperature include 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, and 100°C, and may be within a range between any two of the values exemplified here.

The Charpy impact strength of a molded article of the rubber-modified styrene-based resin composition is preferably 18.0 kJ/ ^m2 or more, more preferably 19.0 kJ/ ^m2 or more. The Charpy impact strength is, for example, 18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, or 25.0 kJ/ ^m2 , and may be within a range between any two of the values exemplified here. The Charpy impact strength was determined by preparing a test piece using an injection molding machine and measuring it in accordance with JIS K-7111-1.

The flexural modulus of a molded article of the rubber-modified styrene-based resin composition is preferably 1800 MPa or more, and more preferably 1850 MPa or more. The flexural modulus may be, for example, 1800, 1850, 1900, 1950, 2000, 2050, 2100, 2150, or 2200 MPa, or may be within a range between any two of the values exemplified here. The flexural modulus was determined by preparing test specimens using an injection molding machine and measuring them in accordance with JIS K-7171.

The rubber-modified styrene resin composition may contain additives such as phosphorus-based, phenolic-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, deodorizers, and antibacterial agents, to the extent that the effects of the present invention are not impaired.

<Method of producing rubber-modified styrene-based resin composition>
A method for producing a rubber-modified styrene-based resin composition according to one embodiment of the present invention may include a step of graft polymerizing a styrene-based monomer in the presence of a rubbery polymer to synthesize a rubber-modified polystyrene. The graft polymerization may be carried out by a known method, such as bulk polymerization, suspension polymerization, two-stage bulk/suspension polymerization, or solution polymerization. Solvents that can be used include alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene; ketones such as acetone and methyl ethyl ketone; and aliphatic hydrocarbons such as hexane and cyclohexane. Examples of reactor types include complete mixing reactors, plug flow reactors, and loop reactors in which a portion of the polymerization liquid is withdrawn as the polymerization proceeds. A so-called continuous polymerization method, which combines these with a devolatilization step to remove unreacted monomers, may also be used.

Examples of rubbery polymers include the rubbery polymers described above. In a graft polymerization process according to one embodiment of the present invention, a styrene-based monomer can be graft polymerized in the presence of two or more types of rubbery polymers to synthesize rubber-modified polystyrene. The two or more types of rubbery polymers can differ in at least one of the monomer units constituting the rubbery polymer, their composition, and molecular weight.

In the process of synthesizing rubber-modified polystyrene, 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.

The polymerization initiator is preferably a radical polymerization initiator, for example, peroxyketals such as 1,1-di(t-butylperoxy)cyclohexane, 2,2-di(t-butylperoxy)butane, 2,2-di(4,4-di-t-butylperoxycyclohexyl)propane, and 1,1-di(t-amylperoxy)cyclohexane; hydroperoxides such as cumene hydroperoxide and t-butyl hydroperoxide; alkyl peroxides such as t-butyl peroxyacetate and t-amylperoxyisononanoate; t-butylcumyl peroxide, di-t-butyl peroxide, dicumyl peroxide, and di-t-hexyl peroxide. Examples of suitable peroxides include dialkyl peroxides, peroxyesters such as t-butylperoxyacetate, t-butylperoxybenzoate, and t-butylperoxyisopropyl monocarbonate, peroxycarbonates such as t-butylperoxyisopropyl carbonate and polyether tetrakis(t-butylperoxycarbonate), N,N'-azobis(cyclohexane-1-carbonitrile), N,N'-azobis(2-methylbutyronitrile), N,N'-azobis(2,4-dimethylvaleronitrile), and N,N'-azobis[2-(hydroxymethyl)propionitrile], and these can be used alone or in combination of two or more.

The chain transfer agent may be a monofunctional chain transfer agent having one chain transfer group, or a polyfunctional chain transfer agent having multiple chain transfer groups. 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 groups of polyhydric alcohols such as ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, and sorbitol with thioglycolic acid or mercaptopropionic acid. These agents may be used alone or in combination of two or more.

A method for producing a rubber-modified styrene-based resin composition according to one embodiment of the present invention preferably includes a step of mixing two or more types of rubber-modified polystyrenes. The two or more types of rubber-modified polystyrenes may have rubber-like polymers and rubber-like polymer particles that differ in at least one of the following: the monomer units constituting the rubber-like polymers, their compositions, molecular weights, and rubber-like polymer particle sizes (85% diameter of integrated value, 15% diameter of integrated value, volume median particle size), and may have rubber-like polymers and rubber-like polymer particles that differ in at least particle size (85% diameter of integrated value, 15% diameter of integrated value, volume median particle size).

A method for producing a rubber-modified styrenic resin composition according to one embodiment of the present invention may include a step of mixing a styrenic resin and a rubber-modified polystyrene. The rubber-modified styrenic resin composition may contain other resins, such as thermoplastic resins other than the styrenic resin and rubber-modified polystyrene, or rubber reinforcing resins, to the extent that the physical properties of the rubber-modified styrenic resin composition are not impaired.

The method for producing a rubber-modified styrene-based resin composition may include a step of polymerizing a styrene-based monomer to synthesize a styrene-based resin. The step of synthesizing the styrene-based resin may further include a step of synthesizing a multifunctional vinyl copolymer that serves as a cross-linking agent. Furthermore, the step of synthesizing the styrene-based resin may include a step of polymerizing the styrene-based monomer in the presence of a polymerization initiator, a solvent, and, if necessary, a necessary cross-linking agent, etc.

There are no particular limitations on the method for mixing two or more types of rubber-modified polystyrene, or the method for mixing a styrene resin with rubber-modified polystyrene. Examples include melt-kneading multiple resins and re-granulating them in an extruder, or dry-blending the mixture of multiple resins in a Henschel mixer, ribbon blender, super mixer, V blender, or the like.

2. Molded Articles A molded article according to one embodiment of the present invention is a molded article characterized by comprising the above-described rubber-modified styrene-based resin composition of the present invention, and may be a molded article of any shape or application, but because the molded article according to one embodiment of the present invention has excellent impact resistance and rigidity, examples of the molded article include parts such as housings and chassis of large home appliances such as televisions and air conditioners, household electrical appliances such as refrigerator interiors, office automation equipment such as copiers, printers, facsimiles and personal computers, and business machines.

<Method of manufacturing molded products>
The method for obtaining a molded product is not particularly limited, and known molding methods such as extrusion molding, injection molding, injection blow molding, and foam molding can be applied, and molding methods combining various molding techniques are also acceptable. Furthermore, methods for molding into a sheet or film using a T-die sheet extruder, a biaxial stretching processing device, or an inflation processing device can also be applied, but injection molding is preferred as a method for obtaining a molded product.

The present invention will be explained in more detail below based on examples, but the present invention should not be construed as being limited to these examples.

[Rubber-modified polystyrene]
The methods for producing resins A to D, which are rubber-modified polystyrenes, will now be described.

<Resin A>
A 25 L volume completely mixed reactor equipped with an agitator was used as the first reactor, a 40 L volume plug flow reactor equipped with an agitator was used as the second reactor, a 50 L volume plug flow reactor equipped with an agitator was used as the third reactor, and a 50 L volume static mixer type plug flow reactor was used as the fourth reactor, and these reactors were connected in series to constitute a polymerization step. 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 Japan Elastomer Co., Ltd. as a rubber-like 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 reactors at a supply rate of 22 L/hr, and polymerization was carried out at a temperature of 120°C and a stirring rate of 100 rpm in the first reactor, and at a temperature of 125 to 128°C and a stirring rate of 140 rpm in the second reactor. To the polymerization liquid from the outlet of the third reactor, 0.025 parts by mass of t-dodecyl mercaptan was added, and polymerization was carried out at a temperature of 130 to 130°C, a stirring rate of 30 rpm, and a temperature of 130 to 140°C in the fourth reactor. Thereafter, the resulting mixture was introduced into a vacuum devolatilizer equipped with a preheater, which was composed of two stages connected in series, and unreacted styrene and ethylbenzene were separated. Thereafter, liquid paraffin ("Hicol K-350" manufactured by Kaneda Co., Ltd.) was added/mixed to a concentration of 1.0% by mass relative to Resin A, and silicone oil ("DOWSIL SH200 100 cSt" manufactured by The Dow Chemical Company) was added/mixed to a concentration of 0.1% by mass relative to Resin A, and the mixture was extruded into a strand shape, cooled, and then cut into pellets. The resin temperature in the first devolatilization tank was set to 190° C., the pressure in the vacuum devolatilization tank was 67 kPa, the resin temperature in the second devolatilization tank was set to 230° C., and the pressure in the vacuum devolatilization tank was 1.3 kPa. The properties of the obtained Resin A are shown in Table 1.

<Resin B>
The polymerization process was carried out by connecting in series a 25 L volume completely mixed reactor equipped with an agitator as the first reactor, a 40 L volume plug flow reactor equipped with an agitator as the second reactor, a 50 L volume plug flow reactor equipped with an agitator as the third reactor, and a 50 L volume static mixer type plug flow reactor as the fourth reactor. 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, Ltd. as a rubber-like polymer, t - A raw material solution consisting of 0.025 parts by mass of dodecyl mercaptan was continuously supplied to the reactors at a supply rate of 22 L/hr, and polymerization was carried out at a first reactor temperature of 125°C, a stirring rate of 100 rpm, a second reactor temperature of 128 to 130°C, a stirring rate of 40 rpm, a third reactor temperature of 130 to 130°C, a stirring rate of 30 rpm, and a fourth reactor temperature of 135 to 160°C. The resulting mixture was then introduced into a vacuum devolatilizer tank equipped with a preheater, which was configured in series with two stages, and unreacted styrene and ethylbenzene were separated. After that, liquid paraffin ("Hicol K-350" manufactured by Kaneda Co., Ltd.) was added/mixed to a concentration of 1.4% by mass relative to Resin B, extruded into a strand shape, cooled, and then cut into pellets. The resin temperature in the first devolatilization tank was set to 190° C., the pressure in the vacuum devolatilization tank was 60 kPa, the resin temperature in the second devolatilization tank was set to 230° C., and the pressure in the vacuum devolatilization tank was 1.3 kPa. The properties of the obtained Resin B are shown in Table 1.

<Resin C>
The polymerization process was carried out by connecting a 25 L volume completely mixed reactor equipped with an agitator as the first reactor, a 40 L volume plug flow reactor equipped with an agitator as the second reactor, a 50 L volume plug flow reactor equipped with an agitator as the third reactor, and a 50 L volume static mixer type plug flow reactor as the fourth reactor in series. A raw material solution consisting of 80.9 parts by mass of styrene monomer, 14.3 parts by mass of ethylbenzene, and 4.8 parts by mass of Asahi Kasei Corporation's polybutadiene "Diene 55AE" as a rubber polymer was continuously supplied to the reactors at a supply rate of 20 L/hr. The temperature of the first reactor was 125°C, the stirring rate was 100 rpm, and the temperature of the second reactor was 128 to 130°C, the stirring rate was 80 rpm. After that, 0.03% t-butylcumyl peroxide was added to the polymerization solution from the outlet of the second reactor. 8 parts by mass of styrene was added, and polymerization was carried out at a third reactor temperature of 128 to 128°C, a stirring rate of 30 rpm, and a fourth reactor temperature of 125 to 140°C. The resulting mixture was then introduced into a two-stage preheater-equipped vacuum devolatilizer tank in series. Unreacted styrene and ethylbenzene were separated, and then liquid paraffin ("Hicol K-350" manufactured by Kaneda Co., Ltd.) was added/mixed to a concentration of 2.0% by mass relative to Resin C. The resulting mixture was extruded into a strand, cooled, and then cut into pellets. The resin temperature in the first devolatilizer tank was set to 190°C, the pressure in the vacuum devolatilizer tank was 67 kPa, and the resin temperature in the second devolatilizer tank was set to 230°C, and the pressure in the vacuum devolatilizer tank was 1.3 kPa. The properties of the resulting Resin C are shown in Table 1.

<Resin D>
The polymerization process was carried out by connecting a 25 L volume of a complete mixing reactor equipped with an agitator as the first reactor, a 40 L volume of a plug flow reactor equipped with an agitator as the second reactor, a 50 L volume of a plug flow reactor equipped with an agitator as the third reactor, and a 50 L volume of a static mixer type plug flow reactor in series. A raw material solution consisting of 77.4 parts by mass of styrene monomer, 13.6 parts by mass of ethylbenzene, 9.0 parts by mass of polybutadiene "BR-15HB" manufactured by Ube Industries, Ltd. as a rubber polymer, and 0.025 parts by mass of t-dodecyl mercaptan was continuously supplied to the reactors at a supply rate of 20 L/hr. The temperature of the first reactor was 120°C, the stirring rate was 100 rpm, and the temperature of the second reactor was 122 to 123°C, the stirring rate was 100 rpm. After that, t-butylcumyl mercaptan was added to the polymerization solution from the outlet of the second reactor. 0.009 parts by mass of peroxide was added, and polymerization was carried out at a temperature of 126 to 127°C in the third reactor, a stirring speed of 30 rpm, and a temperature of 130 to 135°C in the fourth reactor. The resulting mixture was then introduced into a two-stage preheater-equipped vacuum devolatilizer tank in series, where unreacted styrene and ethylbenzene were separated. Liquid paraffin ("Hicol K-350" manufactured by Kaneda Co., Ltd.) was then added/mixed to a concentration of 1.0% by mass relative to Resin D. The resulting mixture was extruded into a strand, cooled, and then cut into pellets. The resin temperature in the first devolatilizer tank was set to 190°C, the pressure in the vacuum devolatilizer tank was 60 kPa, and the resin temperature in the second devolatilizer tank was set to 230°C, and the pressure in the vacuum devolatilizer tank was 1.3 kPa. The properties of the resulting Resin D are shown in Table 1.

<Melt Mass Flow Rate>
The melt mass flow rate of the rubber-modified polystyrene was measured at 200° C. under a load of 49 N in accordance with JIS K-7210.

<Vicat softening temperature>
The Vicat softening temperature of the rubber-modified polystyrene was measured under conditions of a load of 50 N and a temperature rise rate of 50° C./hr in accordance with JIS K-7206.

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

<Rubber particle size (volume median particle size)>
The measurement was carried out in the same manner as the rubber particle diameter (volume median particle diameter) in the rubber-modified styrene-based resin composition described below.

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

<Grafting rate>
The graft ratio was calculated from the gel content (mass%) of the rubber-modified polystyrene and the rubber content (mass%) of the rubber-modified polystyrene according to the following formula:
Graft ratio = (gel content - rubber content) / rubber content

<Methanol soluble matter>
The methanol-soluble content of the rubber-modified polystyrene was measured as follows. 1.00 g of rubber-modified polystyrene (P) was precisely weighed and dissolved in 40 mL of methyl ethyl ketone. 400 mL of methanol was added rapidly to separate and precipitate the methanol-insoluble content (resin component). After allowing to stand for approximately 10 minutes, the mixture was slowly filtered through a glass filter to separate the methanol-soluble content. The mixture was then dried under reduced pressure at 120°C for 2 hours in a vacuum dryer and allowed to cool in a desiccator for 25 minutes. The mass N of the dried methanol-insoluble content was measured and calculated based on the following formula:
Methanol soluble matter (mass%)=(P−N)/P×100

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

[Examples 1 and 2, Comparative Examples 1 to 6]
The rubber-modified polystyrenes (resins A to D) produced by the above methods were premixed in the ratios shown in Table 2 and melt-kneaded in a single-screw extruder (Ikegai Corporation, PMS40-28V) set at 180 to 220° C. The resin properties of the resulting rubber-modified styrene-based resin compositions are shown in Table 2.

<Melt Mass Flow Rate>
The melt mass flow rate of the rubber-modified styrene resin composition was measured at 200° C. under a load of 49 N in accordance with JIS K-7210.

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

<Gel content>
The gel content of the rubber-modified styrene-based resin composition was measured by the following method: 1.00 g of the rubber-modified styrene-based resin composition was precisely weighed (W), and 35 mL of a 50% methyl ethyl ketone/50% acetone mixed solution was added to dissolve it. The solution was centrifuged at 10,000 rpm for 30 minutes in a centrifuge (H-2000B (rotor: H) manufactured by Kokusan Co., Ltd.) to precipitate the insoluble matter. The supernatant was removed by decantation to obtain the insoluble matter. This was pre-dried in a safety oven at 90°C for 2 hours, further dried under reduced pressure at 125°C for 1 hour in a vacuum dryer, and cooled 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 dispersed rubber particles) (mass%) = (G/W) x 100

<Rubber particle diameter (volume median particle diameter) and rubber particle diameter difference (85% volume cumulative diameter - 15% volume cumulative diameter)>
The volume median particle diameter, 85% diameter of the integrated value in the volume cumulative distribution curve, and 15% diameter of the integrated value of the rubber-like polymer particles were calculated based on the 50% diameter, 85% diameter, and 15% diameter of the volume cumulative distribution curve. The volume cumulative distribution curve was obtained by dissolving the rubber-modified styrene-based resin composition in dimethylformamide and measuring it with 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 The Japan Steel Works, Ltd., J100E-P), A-type test pieces (dumbbells) according to JIS K-7139 were molded at a cylinder temperature of 220°C and a mold temperature of 45°C. Test pieces were cut out from the center of the dumbbells and notched (type A, r = 0.25 mm) by machining, and measurements were carried out in accordance with JIS K-7111-1.

<Flexural modulus>
Using an injection molding machine (manufactured by The Japan Steel Works, Ltd., J100E-P), A-type test pieces (dumbbells) according to JIS K-7139 were molded at a cylinder temperature of 220°C and a mold temperature of 45°C. Test pieces cut out from the center of the dumbbells were used for measurements in accordance with JIS K-7171.

Claims (4)

The composition comprises a styrene-based resin that forms a matrix phase and rubber-like polymer particles dispersed in the matrix phase,
The gel content is 26.0 to 30.0% by mass,
The rubber-modified styrene resin composition has a particle diameter of the rubber-like polymer particles, the difference between the 85% diameter of the integrated value and the 15% diameter of the integrated value in a volume-integrated distribution curve of the particle diameter of the rubber-like polymer particles being 5.1 to 6.9 μm. The rubber-modified styrene-based resin composition according to claim 1, having a melt mass flow rate measured at 200°C and 49 N of 3.5 g/10 min or less. The rubber-modified styrene-based resin composition according to claim 1 or claim 2, wherein the rubber content is greater than 11% by mass. A molded article obtained by molding the rubber-modified styrene-based resin composition described in claim 1 or claim 2.