Expanded polystyrene resin particles, method for producing the same, polystyrene foam particles, and polystyrene foam molded articles

Abstract

The present invention provides foamed polystyrene resin particles, a method for producing said resin particles, polystyrene foam particles using said resin particles, and a polystyrene foam molded article, which enable the formation of a foam molded article with practical strength even when using foamed polystyrene resin particles obtained through resin mixing or extrusion processes. [Solution] Expandable polystyrene resin particles are used, which contain zinc stearate, with the zinc stearate content being 0.025% by weight or more and 0.095% by weight or less per 100% by weight of the expandable polystyrene resin particles, and the average cell diameter of the expanded particles obtained by expanding the expandable polystyrene resin particles 50 times is 20 μm or more and 250 μm or less.

Description

[Technical Field]

[0001] The present invention relates to foamable polystyrene resin particles, a method for producing the same, polystyrene foam particles, and polystyrene foam molded articles. [Background technology]

[0002] Foamed molded products, made by foaming polystyrene resin particles, are lightweight and have excellent cushioning and heat insulation properties, making them widely used as packaging materials (trays) for food containers, various packing materials, building and civil engineering components, and automotive components.

[0003] As a method for producing foamed polystyrene resin particles used in such applications, for example, Patent Document 1 discloses a method that includes the steps of producing polystyrene resin particles by melting and kneading water and a heat stabilizer with a polystyrene resin in an extruder, extruding the mixture through a die having small holes, and then cutting it with a cutter, and suspending the obtained polystyrene resin particles in water and impregnating the polystyrene resin particles with a foaming agent. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-059843 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, as disclosed in Patent Document 1, when resin mixing or extrusion processes are involved, the molecular weight of the polystyrene resin constituting the molded article decreases, which tends to reduce the strength of the molded article. For this reason, it was necessary to use raw materials with a relatively high molecular weight to prevent a decrease in the strength of the molded article.

[0006] The present invention has been made in view of the above problems, and aims to provide foamed polystyrene resin particles, a method for producing said resin particles, polystyrene foam particles using said resin particles, and a polystyrene foam molded article, which can form a foam molded article with practical strength even when a polystyrene foam molded article is manufactured using foamed polystyrene resin particles that have undergone resin kneading or extrusion processes. [Means for solving the problem]

[0007] The inventors of the present invention have found that the above problems can be solved by producing foamed polystyrene resin particles with a resin composition containing a specific amount of zinc stearate during resin kneading or extrusion, and have completed the present invention.

[0008] Aspects of this disclosure relate to the following foamable polystyrene resin particles, a method for producing said resin particles, polystyrene foamed particles using said resin particles, and polystyrene foamed molded articles.

[0009] [1] Contains zinc stearate in foamed polystyrene resin particles, The zinc stearate content is 0.025% by weight or more and 0.095% by weight or less based on 100% by weight of the foamed polystyrene resin particles. Expandable polystyrene resin particles, wherein the average cell diameter of the expanded polystyrene resin particles obtained by expanding the aforementioned expanded polystyrene resin particles 50 times is 20 μm or more and 250 μm or less. [2] Expandable polystyrene resin particles as described in [1], wherein the average particle weight is 0.5 mg or more and 1.6 mg or less. [3] The foamed polystyrene resin particles are made using a polystyrene resin, The foamable polystyrene resin particles according to [1] or [2], wherein the polystyrene resin comprises recycled polystyrene resin. [4] Furthermore, it contains a foaming agent, The content of the foaming agent is 2.8% by weight or more and 10% by weight or less based on 100% by weight of foamed polystyrene resin particles. The foaming agent is a foamable polystyrene resin particle according to any one of [1] to [3], wherein the weight ratio of butane to pentane is butane / pentane = 0 / 100 to 80 / 20. [5] A method for producing foamable polystyrene resin particles according to any one of [1] to [4], comprising the steps of extruding a molten mixture containing a polystyrene resin, zinc stearate, and a foaming agent into pressurized circulating water and cutting it with a rotary cutter to form particles. [6] A method for producing foamable polystyrene resin particles according to any one of [1] to [4], comprising the steps of: dispersing resin particles, which are granulated by extruding a molten mixture containing a polystyrene resin and zinc stearate, in an aqueous medium containing an organic dispersant and impregnating them with a foaming agent. [7] A method for producing foamable polystyrene resin particles according to any one of [1] to [4], comprising the steps of: dispersing resin particles obtained by granulating a molten kneaded product containing a polystyrene resin and zinc stearate in an aqueous medium containing a dispersant; impregnating with a styrene monomer and polymerizing it; and then impregnating with a foaming agent. [8] Polystyrene foam particles, which are pre-foamed polystyrene resin particles as described in any of [1] to [4]. A polystyrene foam molded article, which is a molded article of polystyrene foam particles as described in [9] [8]. [Effects of the Invention]

[0010] According to the present invention, even when a polystyrene foam molded article is manufactured using foamable polystyrene resin particles obtained through resin kneading or extrusion processes, a foam molded article having practical strength can be formed. The present invention provides foamable polystyrene resin particles, a method for producing said resin particles, polystyrene foam particles using said resin particles, and a polystyrene foam molded article. [Modes for carrying out the invention]

[0011] <<Expandable polystyrene resin particles>> The foamed polystyrene resin particles of this embodiment contain zinc stearate (B) in the resin particles, The content of zinc stearate (B) is 0.025% by weight or more and 0.095% by weight or less based on 100% by weight of expandable polystyrene resin particles, The average cell diameter of expanded particles obtained by expanding the expandable polystyrene resin particles at 50-fold expansion is 20 µm or more and 250 µm or less.

[0012] According to the expandable polystyrene-based resin particles of the present embodiment, since they contain a specific amount of zinc stearate (B), it is possible to form a polystyrene-based expanded molded article having practical strength in which a decrease in compressive strength and bending strength is suppressed.

[0013] The expandable polystyrene-based resin particles are formed using a polystyrene-based resin (A), and contain zinc stearate (B) and a blowing agent (C) as essential components. Hereinafter, the essential or optional components contained in the expandable polystyrene-based resin particles will be described.

[0014] <Polystyrene-based resin (A)> The polystyrene-based resin (A) contains at least one selected from a virgin polystyrene-based resin (A1) and a recycled polystyrene-based resin (A2).

[0015] The weight average molecular weight of the polystyrene-based resin (A) is preferably 100,000 or more and 400,000 or less, more preferably 150,000 or more and 350,000 or less, and still more preferably 180,000 or more and 300,000 or less.

[0016] (Virgin polystyrene-based resin (A1)) The virgin polystyrene-based resin (A1) is a so-called virgin polystyrene-based resin, and means a new polystyrene-based resin excluding recycled polystyrene-based resins. As the virgin polystyrene-based resin (A1), a styrene homopolymer (polystyrene homopolymer) is preferably included because of excellent compatibility with the recycled polystyrene-based resin (A2). By using a polystyrene-based homopolymer, expandable particles excellent in expandability and moldability, and blocks can be obtained.

[0017] Further, the virgin polystyrene-based resin (A1) may be a copolymer of styrene with another monomer copolymerizable with styrene or a derivative thereof, as long as the effects of the present invention are not impaired. These may be used alone, or two or more of them may be used in combination.

[0018] Examples of other monomers copolymerizable with styrene or derivatives thereof include styrene derivatives such as methylstyrene, dimethylstyrene, ethylstyrene, diethylstyrene, isopropylstyrene, bromostyrene, dibromostyrene, tribromostyrene, chlorostyrene, dichlorostyrene, and trichlorostyrene; polyfunctional vinyl compounds such as divinylbenzene; unsaturated carboxylic acids such as acrylic acid and methacrylic acid; (meth)acrylic acid ester compounds such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, and butyl methacrylate; vinyl cyanide compounds such as (meth)acrylonitrile; diene compounds such as butadiene or derivatives thereof; unsaturated carboxylic acid anhydrides such as maleic anhydride and itaconic anhydride; and N-alkyl substituted maleimide compounds such as N-methylmaleimide, N-butylmaleimide, N-cyclohexylmaleimide, N-phenylmaleimide, N-(2)-chlorophenylmaleimide, N-(4)-bromophenylmaleimide, and N-(1)-naphthylmaleimide. These may be used alone, or two or more of them may be used in combination.

[0019] The virgin polystyrene-based resin (A1) preferably contains a styrene homopolymer, since it is relatively inexpensive, enables foam molding with low-pressure steam or the like without using a special method, and has an excellent balance of heat insulating properties and cushioning properties. The content of the styrene homopolymer is preferably 80% by weight or more, more preferably 90% by weight or more, based on 100% by weight of the virgin polystyrene-based resin (A1).

[0020] The weight-average molecular weight of the virgin polystyrene resin (A1) is preferably 100,000 to 400,000, more preferably 150,000 to 350,000, even more preferably 180,000 to 300,000, and even more preferably 180,000 to 250,000, from the viewpoint of forming a polystyrene foam molded article that does not require a high vapor pressure during molding and suppresses a decrease in compressive strength and flexural strength.

[0021] (Recycled polystyrene resin (A2)) Recycled polystyrene resin (A2) refers to polystyrene resin derived from recycled materials. Examples of polystyrene resin derived from recycled materials include: recycled raw materials obtained by recovering used polystyrene foam molded products such as fish boxes, home appliance cushioning materials, or food packaging trays and regenerating them using limonene dissolution or heat volume reduction methods; recycled raw materials obtained by crushing, melting and kneading non-foamed polystyrene molded products separated and recovered from home appliances, etc., and repelling them; and recycled raw materials obtained by crushing, melting and kneading scraps generated after punching out food packaging trays, etc., from polystyrene foam sheets and repelling them.

[0022] From the viewpoint of forming a polystyrene foam molded article with suppressed reduction in compressive strength and flexural strength, the weight-average molecular weight of the recycled polystyrene resin (A2) is preferably 100,000 to 400,000, more preferably 150,000 to 350,000, and even more preferably 180,000 to 300,000.

[0023] <Zinc Stearate (B)> Zinc stearate (B) is used to form polystyrene foam molded articles in which the reduction in compressive strength and flexural strength is suppressed.

[0024] From the viewpoint of forming a polystyrene foam molded article in which the reduction in compressive strength and flexural strength is suppressed by suppressing the decrease in molecular weight during extrusion processing, the zinc stearate (B) content is preferably 0.025% by weight or more and 0.095% by weight or less, more preferably 0.025% by weight or more and 0.09% by weight or less, more preferably 0.025% by weight or more and 0.085% by weight or less, and even more preferably 0.025% by weight or more and 0.08% by weight or less, based on 100% by weight of foamed polystyrene resin particles.

[0025] Here, the recycled polystyrene resin (A2) mentioned above may contain a small amount of zinc stearate (B) as a lubricant, etc. Therefore, when using recycled polystyrene resin (A2) as a raw material, the zinc stearate (B) content is determined in advance, and the amount of zinc stearate (B) added during the manufacturing process is adjusted so that the zinc stearate (B) content in the resulting foamed polystyrene resin particles matches the preferred configuration described above. In other words, when using recycled polystyrene resin (A2) as a raw material, the zinc stearate (B) content in the foamed polystyrene resin particles refers to the sum of the zinc stearate (B) content in the recycled polystyrene resin (A2) and the amount of zinc stearate (B) added during the manufacturing process.

[0026] Furthermore, when manufacturing pre-foamed foams of expandable polystyrene resin particles, zinc stearate is sometimes dry-blended with the expandable polystyrene resin particles as a pre-treatment to prevent the particles from sticking together. Although zinc stearate is present on the surface of these pre-treated expandable polystyrene resin particles, it is not present inside the expandable polystyrene resin particles, and is therefore excluded from the zinc stearate (B) content when determining the zinc stearate (B) content as described above.

[0027] <Foaming agent (C)> While there are no particular limitations on the foaming agent (C), hydrocarbons with 3 to 6 carbon atoms are preferred, and hydrocarbons with 4 to 5 carbon atoms are even more preferred, from the viewpoint of a good balance between foaming and product life, and ease of achieving high foaming ratios in actual use. The reason for using hydrocarbons with 3 or more carbon atoms is that the foaming agent has low volatility, so the foaming agent is less likely to escape from the resulting foamed styrene resin particles. As a result, when the foamed styrene resin particles are actually used in the foaming process, sufficient foaming agent remains within the foamed styrene resin particles, making it possible to obtain sufficient foaming power and facilitating high foaming ratios. The reason for using hydrocarbons with 6 or fewer carbon atoms is that the boiling point of the foaming agent is not too high, making it easy to obtain sufficient foaming power by heating during pre-foaming, and thus facilitating high foaming. Examples of hydrocarbons with 3 to 6 carbon atoms include propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclopentane, n-hexane, or cyclohexane. These may be used individually or in combination of two or more. Other blowing agents that may be used in this embodiment include hydrofluoroolefins, hydrochlorofluoroolefins, hydrofluorocarbons, nitrogen, and carbon dioxide. Among these, it is preferable to include butane and / or pentane because they are easy to handle in industrial production and provide sufficient foaming power. Examples of butane include isobutane and n-butane. Examples of pentane include isopentane and n-pentane.

[0028] The content of the foaming agent (C) is not particularly limited, but is preferably 2.8% by weight or more and 10% by weight or less based on 100% by weight of foamable polystyrene resin particles. The lower limit is more preferably 3.5% by weight or more, even more preferably 4.0% by weight or more, and still more preferably 4.5% by weight or more. The upper limit is more preferably 9.0% by weight or less, even more preferably 8.0% by weight or less, and still more preferably 7.0% by weight or less. When the content of the foaming agent (C) is 2.8% by weight or more, sufficient foaming power is achieved, sufficient bead life can be ensured, and the production of polystyrene foam molded articles becomes easier. Furthermore, when the content of the foaming agent (C) is 10.0% by weight or less, the production time (molding cycle) when producing polystyrene foam molded articles does not become too long, and production costs can be reduced.

[0029] When using butane and pentane as blowing agents, from the viewpoint of obtaining sufficient blowing power and not making the cell diameter too small, the weight ratio of butane to pentane is preferably butane / pentane = 0 / 100 to 80 / 20, more preferably 0 / 100 to 70 / 30, and even more preferably 0 / 100 to 60 / 40. Furthermore, butane / pentane = 10 / 90 to 80 / 20 is even more preferable.

[0030] (Other ingredients) The foamed polystyrene resin particles may contain components other than the polystyrene resin (A), zinc stearate (B), and foaming agent (C) (hereinafter also referred to as "other components"), as long as they do not impair the effects of the present invention. Furthermore, other resins may be used in combination with the polystyrene resin (A) as the main component, as long as the effects of the present invention are not impaired. Examples of other resins include polyolefin resins, polyester resins, polycarbonate resins, and acrylic resins. The content of other resins is not particularly limited, but it is preferably 20% by weight or less, and more preferably 10% by weight or less, relative to 100% by weight of the polystyrene resin (A).

[0031] Other components include flame retardants, heat stabilizers, flame retardant additives, nucleating agents, radical generators, fiber reinforcing agents, processing aids, foaming aids, antistatic agents, colorants such as pigments, light stabilizers, radiant heat transfer inhibitors, and surfactants. These other components may be used individually or in combination of two or more.

[0032] <Flame retardant> Examples of flame retardants include brominated butadiene polymers. A brominated butadiene polymer is a copolymer containing structural units (p1) derived from butadiene and structural units (p2) derived from aromatic hydrocarbons, wherein the structural units (p1) and / or (p2) are brominated. Examples of brominated butadiene polymers include brominated styrene-butadiene block copolymers, brominated styrene-butadiene random copolymers, and brominated styrene-butadiene graft copolymers. These brominated butadiene polymers may be used individually or in combination of two or more types.

[0033] The flame retardant may contain brominated flame retardants other than the above-mentioned brominated butadiene polymer (hereinafter also referred to as "other flame retardants"), as long as the effects of the present invention are not impaired. Other flame retardants include, for example, brominated bisphenol compounds such as 2,2-bis[4-(2,3-dibromo-2-methylpropoxy)-3,5-dibromophenyl]propane (also known as tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether)) and 2,2-bis[4-(2,3-dibromopropoxy)-3,5-dibromophenyl]propane (also known as tetrabromobisphenol A-bis(2,3-dibromopropyl ether)), and tetrabromocyclooctane.

[0034] The flame retardant content is preferably 3.0 parts by weight or more and 10.0 parts by weight or less, more preferably 3.2 parts by weight or more and 9.0 parts by weight or less, and even more preferably 3.4 parts by weight or more and 8.0 parts by weight or less, per 100 parts by weight of polystyrene resin (A).

[0035] <Heat stabilizer> Examples of heat stabilizers include hindered amine compounds, phosphorus compounds, phenolic stabilizers, and epoxy compounds. Heat stabilizers can be used individually or in combination of two or more. These heat stabilizers can also be used as light-resistant stabilizers.

[0036] The amount of heat stabilizer is not particularly limited, but is preferably 1 to 25 parts by weight, more preferably 2 to 20 parts by weight, and even more preferably 3 to 18 parts by weight, per 100 parts by weight of flame retardant.

[0037] <Flame retardant additive> As a flame retardant additive, compounds with cumene as the basic constituent unit can be used. Preferably, the flame retardant additive has a 10% thermal decomposition temperature of 150°C to 250°C. The 10% decomposition temperature refers to the temperature at which the weight loss rate exceeds 10% when the sample is heated from 35°C at a heating rate of 10°C / min in an atmosphere with 100 mL / min of nitrogen flowing in. Such compounds are also known as CC initiators. Examples of flame retardant additives include 1,1'-(1,1,2,2-tetramethyl)dibenzene, poly-1,4-diisopropylbenzene, 2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane, 3,4-diethyl-3,4-diphenylhexane, 2,7-dimethyl-4,5-diethyl-4,5-diphenyloctane, 3,4-dibenzyl-3,4-ditolylhexane, and 1,1,2,2-tetraphenyl-1,2-ethanediol. Flame retardant additives may be used individually or in combination of two or more.

[0038] The amount of flame retardant additive is not particularly limited, but is preferably 0.1 parts by weight or more and 2.0 parts by weight or less, and more preferably 0.3 parts by weight or more and 1.0 part by weight or less, per 100 parts by weight of polystyrene resin (A).

[0039] <Nucleating agent> Examples of nucleating agents include inorganic compounds such as calcium silicate, wollastonite, kaolin, clay, mica, zinc oxide, sodium bicarbonate-citric acid mixture, monosodium citrate, talc, and calcium carbonate; polymer compounds such as methyl methacrylate copolymers or ethylene-vinyl acetate copolymer resins; olefin waxes such as polyethylene wax; or fatty acid bisamides such as methylene bisstearyl amide, ethylene bisstearyl amide, hexamethylene bispalmitate amide, or ethylene bisoleate amide. Nucleating agents may be used individually or in combination of two or more.

[0040] The content of the nucleating agent is not particularly limited, but is preferably 0.5 parts by weight or more and 5.0 parts by weight or less, and more preferably 1.0 part by weight or more and 3.0 parts by weight or less, per 100 parts by weight of polystyrene resin (A).

[0041] <Radical Generating Agent> The radical generator can be used in appropriate combinations depending on the type of polystyrene resin (A), the type and content of the blowing agent (C), and the type and content of the brominated flame retardant. Examples of radical generators include cumene hydroperoxide, dicumyl peroxide, and t-butyl hydroperoxide. Radical generators can be used individually or in combination of two or more.

[0042] <Physical properties of foamed polystyrene resin particles> (Weight average molecular weight) From the viewpoint of forming a polystyrene foam molded article with suppressed reduction in compressive strength and flexural strength, the weight-average molecular weight of the foamed polystyrene resin particles is preferably 100,000 to 400,000, more preferably 150,000 to 350,000, and even more preferably 180,000 to 300,000.

[0043] (Average grain weight) The average particle weight of the foamed polystyrene resin particles is preferably 0.5 mg to 1.6 mg, more preferably 0.5 mg to 1.5 mg, and even more preferably 0.5 mg to 1.4 mg, from the viewpoint of improving the surface appearance of the foamed polystyrene molded article and shortening the molding cycle.

[0044] ≪Method for producing foamed polystyrene resin particles≫ The method for producing the recycled foamable polystyrene resin particles of this embodiment is not particularly limited, but examples include the production methods I to III described below.

[0045] <Manufacturing Method I> The method for producing foamed polystyrene resin particles by this manufacturing method includes the step of extruding a molten mixture containing polystyrene resin (A), zinc stearate (B), and a foaming agent (C) into pressurized circulating water, and cutting it with a rotary cutter to form particles.

[0046] According to this manufacturing method, the entire process from kneading the components such as polystyrene resin (A), zinc stearate (B), and blowing agent (C) to producing foamed polystyrene resin particles can be carried out using a single piece of equipment. Therefore, by continuing continuous production, manufacturing costs can be reduced.

[0047] The components used in this manufacturing method are the same as those described in the embodiment described in the section on "Expandable Polystyrene Resin Particles" above.

[0048] In a preferred embodiment of this manufacturing method, a polystyrene resin (A), zinc stearate (B), a foaming agent (C), and other components as needed are melted and kneaded in an extruder. The resulting molten mixture is extruded through a die with multiple small holes attached to the tip of the extruder into a cutter chamber filled with pressurized circulating water. Immediately after extrusion, it is cut into particles by a rotary cutter and then cooled and solidified by pressurized circulating water. In this process, the melting and kneading in the extruder may be carried out using a single extruder, multiple extruders connected together, or the extruder in combination with a second kneading device such as a static mixer or a stirrer without a screw, and these can be selected as appropriate.

[0049] The preferred temperature setting for the melt-mixing section of the extruder is 100°C to 250°C. Furthermore, the residence time within the extruder from the supply of various components to the end of melt-mixing is preferably 10 minutes or less.

[0050] Here, the melting and mixing section of the extruder refers to the section from the feed section to the tip of the final extruder downstream, in the case of an extruder having a single-screw or twin-screw extruder. If a second mixing device, such as a static mixer or a stirrer without a screw, is used in conjunction with the first extruder, it refers to the section from the feed section of the first extruder to the tip of the second mixing device.

[0051] The water pressure of the pressurized circulating water is preferably between 0.9 MPa and 1.5 MPa, and more preferably between 0.95 MPa and 1.4 MPa. If the water pressure is 0.9 MPa or higher, foaming can be suppressed, the apparent density of the foamed polystyrene resin particles will be higher, and a decrease in the foaming ratio and transport efficiency will be less likely to occur. On the other hand, if the water pressure is 1.5 MPa or lower, the rotary cutter will not be pushed back by the water pressure, the extruded molten resin will not wrap around the rotary cutter, and foamed polystyrene resin particles can be produced stably. In addition, the strain applied to the molten resin will not be large, the shape of the foamed polystyrene resin particles will be good, and the foaming properties and moldability will be excellent. The temperature of the pressurized circulating water can be adjusted as needed to avoid clumping of the foamed polystyrene resin particles, achieve the desired particle shape, suppress foaming, and prevent die clogging, and can be set to 40°C to 80°C.

[0052] The die described above is not particularly limited, but examples include one having a small hole with a diameter of 0.3 mm to 2.0 mm, preferably 0.4 mm to 1.0 mm.

[0053] The cutting device for cutting the molten resin extruded into pressurized circulating water is not particularly limited, but examples include a device in which the resin is cut into small spheres by a rotary cutter that contacts the die lip, and then transported to a centrifugal dewatering machine for dewatering and aggregation.

[0054] <Manufacturing Method II> The method for producing foamed polystyrene resin particles by this manufacturing method includes the steps of dispersing resin particles, which are granulated by extruding a molten mixture containing polystyrene resin (A) and zinc stearate (B), in an aqueous medium containing an organic dispersant, and impregnating them with a foaming agent (C).

[0055] The components used in this manufacturing method are the same as those described in the embodiment described in the section on "Expandable Polystyrene Resin Particles" above.

[0056] In this manufacturing method, a preferred embodiment for obtaining the above-mentioned resin particles involves mixing a polystyrene resin (A) and zinc stearate (B), along with other components as needed, in a blender, feeding the resulting resin mixture into an extruder, heating and melting it, extruding it in strand form through a die with small holes, rapidly cooling it in water, and then cutting it with a pelletizer to obtain resin particles. In this case, the melt-kneading in the extruder may involve using a single extruder, connecting multiple extruders, or using the extruder in combination with a second kneading device such as a static mixer or a stirrer without a screw, and these can be selected as appropriate.

[0057] In this manufacturing method, the foaming agent (C) can be added while the sealed container is preferably heated to 95°C to 130°C, and then the cylindrical resin particles that have been strand-cut can be made spherical by heating the sealed container to 100°C to 130°C. In this manufacturing method, the addition temperature of the foaming agent (C) can preferably be 95°C to 130°C, and more preferably 100°C to 110°C. If the addition temperature is below 95°C, aggregates tend to increase and the yield tends to deteriorate. In this manufacturing method, the temperature at which the resin particles are sphericalized and impregnated with the foaming agent (C) is preferably 100°C to 130°C, and more preferably 110°C to 120°C. If the impregnation temperature is below 100°C, the recycled foamable polystyrene resin particles are less likely to become spherical and tend to become more cylindrical.

[0058] In this manufacturing method, an organic dispersant and a metal salt can be included in the aqueous medium. The organic dispersant disperses the resin particles in the aqueous medium, and the metal salt acts as a moisture regulator to adjust the internal moisture content of the foamed polystyrene resin particles in order to uniformly stabilize the cells.

[0059] Examples of organic dispersants include anionic surfactants such as sodium α-olefin sulfonate and sodium dodecylbenzenesulfonate, and polymeric dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylamide.

[0060] In addition to the organic dispersant mentioned above, the aqueous medium may also contain water, inorganic salts, calcium phosphate, hydroxyapatite, magnesium pyrophosphate, kaolin, and the like.

[0061] <Manufacturing Method III> The present method for producing foamed polystyrene resin particles includes the steps of: dispersing resin particles (SR) obtained by granulating a molten mixture containing polystyrene resin (A) and zinc stearate (B) in an aqueous medium containing a dispersant; impregnating with styrene monomer (a) and polymerizing it; and then impregnating with a foaming agent (C).

[0062] The components used in this manufacturing method are the same as those described in the embodiment described in the section on "Expandable Polystyrene Resin Particles" above.

[0063] This manufacturing method includes a polymerization step in which styrene monomers are polymerized to obtain polystyrene resin particles using resin particles (SR) obtained by granulating a molten kneaded product containing polystyrene resin (A) and zinc stearate (B) as a seed resin, and an impregnation step in which a foaming agent (C) is impregnated into the obtained polystyrene resin particles.

[0064] The method for preparing the resin particles (SR) used as seed resin, and the method for impregnating the polystyrene resin particles with the foaming agent (C), are the same as those described in Manufacturing Method II. The polymerization process will now be described.

[0065] (Polymerization process) Examples of styrene monomers include styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, chlorostyrene, and other styrene derivatives. Furthermore, components copolymerizable with styrene, such as esters of acrylic and methacrylic acids including methyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, and cetyl methacrylate, as well as various monomers such as acrylonitrile, dimethyl fumarate, and ethyl fumarate, and difunctional monomers such as divinylbenzene and alkylene glycol dimethacrylate, are also included. Styrene monomers may be used individually or in combination of two or more types.

[0066] Examples of dispersants include poorly water-soluble inorganic salts such as calcium phosphate, hydroxyapatite, and magnesium pyrophosphate. When using these poorly water-soluble inorganic salts, it is effective to use them in combination with anionic surfactants such as sodium α-olefin sulfonate or sodium dodecylbenzenesulfonate to increase dispersion stability.

[0067] In the polymerization process, it is preferable to use a polymerization initiator. Examples of polymerization initiators include organic oxides such as benzoyl peroxide, t-butyl peroxybenzoate, isopropyl-t-butyl peroxycarbonate, and butyl perbenzoate, as well as azo compounds such as azobisisobutyronitrile.

[0068] In the polymerization process, the ratio of seed resin (SR) to styrene monomer (a) (SR / a) is not particularly limited and can be set appropriately within the range of, for example, 70 / 30 to 30 / 70.

[0069] In the polymerization process, the polymerization temperature is not particularly limited, but it is preferably 90°C to 110°C. In the polymerization process, from the viewpoint of stably advancing the polymerization of styrene monomer (a), it is preferable to add styrene monomer (a) over time from the start of polymerization. The polymerization time is also not particularly limited and can be set appropriately within the range of 1 to 10 hours.

[0070] <Polystyrene foam particles and polystyrene foam molded articles> Polystyrene foam particles are pre-foamed polystyrene resin particles as described above. Polystyrene foam molded articles are molded articles made from these polystyrene foam particles. The above-mentioned polystyrene foam molded article can be manufactured, for example, by a pre-foaming method in which molding is performed using the above-mentioned polystyrene foam particles, which are pre-foamed polystyrene resin particles obtained by foaming the aforementioned foamable polystyrene resin particles to a predetermined foaming ratio.

[0071] Expandable polystyrene resin particles are expanded to 10 to 110 times their original volume using a known pre-foaming process, for example, by expanding them with steam (pre-foaming process). After curing for a certain period of time as needed, the resulting polystyrene foam particles are molded using a known molding machine with steam to produce a polystyrene foam molded body. Depending on the shape of the mold used, molded bodies with complex shapes or block-shaped molded bodies can be obtained.

[0072] (Pre-foaming process) The pre-foaming process can be carried out using a pre-foaming machine in the same manner as the pre-foaming of conventional foamable polystyrene resin particles.

[0073] Known pre-foaming machines can be used, for example, a pre-foaming machine equipped with a stirring device and comprising a can containing expandable polystyrene resin particles, a steam chamber installed below the can to supply steam to the can, and a pre-foaming particle discharge port can be used.

[0074] The internal pressure (cage pressure) of the can when steam is introduced is not particularly limited, but is preferably 0.001 to 0.15 MPa, more preferably 0.01 to 0.10 MPa, and even more preferably 0.03 to 0.08 MPa. When the internal pressure is 0.001 MPa or higher, the steam introduction time in pre-foaming can be reduced to 500 seconds or less when obtaining a high foaming ratio. When the internal pressure is 0.15 MPa or lower, it becomes unnecessary to increase the steam pressure, the number of blocking phenomena decreases, and the pre-foaming yield increases.

[0075] Furthermore, the pre-foaming process can be carried out using either a continuous or batch method.

[0076] The continuous method involves continuously supplying recycled expandable polystyrene resin particles into the can and discharging pre-foamed particles from an outlet located at the top of the can. The foaming ratio of the pre-foamed particles can be adjusted, for example, by appropriately selecting the amount (weight) of recycled expandable polystyrene resin particles supplied into the can per unit time. In the continuous method, the residence time within the pre-foaming machine can, from the time the recycled expandable polystyrene resin particles are supplied to the can until the pre-foamed particles are discharged, is defined as the water vapor injection time.

[0077] The batch method involves placing a predetermined amount of expandable polystyrene resin particles into a can, pre-foaming them to a predetermined foaming ratio, then stopping the supply of water vapor, and subsequently blowing air into the can as needed to cool and dry the pre-foamed particles (polystyrene foam particles), before removing them from the can. The foaming ratio of the pre-foamed particles can be adjusted by appropriately selecting the amount (weight) of expandable polystyrene resin particles to be placed into the can per batch. Since the batch method pre-foams the introduced expandable polystyrene resin particles to a predetermined volume, the smaller the amount introduced per batch, the higher the foaming ratio of the resulting pre-foamed particles.

[0078] (Average cell diameter) As mentioned above, the average cell diameter of the foamed polystyrene resin particles obtained by foaming the foamed polystyrene resin particles of this embodiment 50 times is 20 μm or more and 250 μm or less, preferably 50 μm or more and 200 μm or less, and more preferably 70 μm or more and 150 μm or less. If the average cell diameter of the foamed polystyrene resin particles obtained by foaming the foamed polystyrene resin particles 50 times is less than 20 μm, the surface of the polystyrene foam molded article obtained by molding the foamed particles tends to melt and its aesthetic appeal is impaired, and if the average cell diameter exceeds 250 μm, the strength of the polystyrene foam molded article obtained by molding the foamed particles tends to be inferior.

[0079] Polystyrene foam molded articles can be molded using the aforementioned pre-foamed particles (polystyrene foam particles) at a molding vapor pressure of 0.05 MPa to 0.11 MPa. The allowable molding vapor pressure is preferably 0.05 MPa to 0.10 MPa, and more preferably 0.05 MPa to 0.09 MPa.

[0080] Polystyrene foam molded products can be used in a variety of applications, such as building insulation materials used for floors, walls, and roofs; agricultural and marine product boxes such as boxes for transporting fish and other marine products, boxes for transporting vegetables and other agricultural products; reusable shipping containers (shippers) used by consumer cooperatives; bathroom insulation materials; and hot water storage tank insulation materials.

[0081] <Physical properties of molded products> (Compressive strength) The 5% compressive strength of a polystyrene foam molded article obtained by foaming polystyrene resin particles 40 times is preferably 0.18 MPa or higher, and more preferably 0.19 MPa or higher.

[0082] (Bending strength) The flexural strength of a polystyrene foam molded article obtained by foaming polystyrene resin particles 40 times is preferably 0.39 MPa or higher, and more preferably 0.41 MPa or higher. [Examples]

[0083] The present invention will be described more specifically below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0084] <Ingredients> In the examples and comparative examples, virgin polystyrene resins (A1) were A1-1 and A1-2 below, and recycled polystyrene resins (A2) were A2-1 to A2-6 below, which were made by crushing and melt-extruding ingots produced by reducing the volume of used polystyrene foam molded products, and had different weight-average molecular weights (Mw) and zinc stearate content.

[0085] Virgin polystyrene resin A1-1: Styrene homopolymer (PS Japan Co., Ltd. "679") A1-2: Styrene homopolymer (PS Japan Co., Ltd. "680")

[0086] Recycled polystyrene resin A2-1: Mw: 258891, Zinc stearate content: 0.06% by weight A2-2: Mw: 265994, Zinc stearate content: 0.04% by weight A2-3: Mw: 196512, Zinc stearate content: 0.06% by weight A2-4: Mw: 202649, Zinc stearate content: 0.04% by weight A2-5: Mw: 188210, Zinc stearate content: 0.04% by weight A2-6:Mw:208251, Zinc stearate content: 0.06% by weight Note that the zinc stearate content is expressed as a percentage of the total amount of recycled polystyrene resin. Zinc stearate (B) contained in recycled polystyrene resin (A2) is denoted as B1, and the amount of B1 in A2 (parts) is shown in Tables 1 and 2.

[0087] <Molecular weight measurement of polystyrene resins and foamed polystyrene resin particles> The weight-average molecular weight (Mw) of polystyrene resin (A) and foamed polystyrene resin particles was determined by gel permeation chromatography under the following conditions. a) Sample concentration: 2.5 mg / mL (solvent: tetrahydrofuran) b) Equipment used: Waters e2695 c) Columns used: Two Shodex GPC-K-806M columns directly coupled. d) Measurement conditions: Temperature; 40°C, Solvent; Tetrahydrofuran, Sample injection volume; 50 μL, Flow rate; 1.0 mL / min, Detection method; UV (254 nm), Standard polystyrene; Showa Denko K.K. "Shodex STANDARD SM-105"

[0088] In the examples and comparative examples, B2 was used as zinc stearate (B). B2: Zinc stearate GF-200 (manufactured by NOF Corporation)

[0089] In the examples and comparative examples, the following C1 to C4 were used as the foaming agent (C). C1: Isobutane (manufactured by Mitsui Chemicals, Inc.) C2: Mixed Butane: A mixture of normal butane and isobutane, with a mixing ratio of normal butane / isobutane = 7 / 3 (by weight) (manufactured by Iwatani Gas Co., Ltd.) C3: Isopentane (manufactured by Wako Pure Chemical Industries, Ltd.) C4: n-pentane (manufactured by Wako Pure Chemical Industries, Ltd.)

[0090] <Amount of Zinc Stearate in Recycled Polystyrene Resin> The content of zinc stearate (B1) contained in the recycled polystyrene resin (A2) was determined under the following conditions. After 1 g of the recycled polystyrene resin (A2) was dissolved in 100 mL of tetrahydrofuran, the solvent-insoluble content and the solution were separated by centrifugation. 50 mL of ethanol (5 operations of 10 mL each) was added to the insoluble content, and the mixture was heated to approximately 75°C to extract fatty acid metal salts including zinc stearate. The zinc content was determined by ICP-MS for the extract, and converted to the amount of zinc stearate to obtain the content. The ICP-MS measurement conditions were as follows.

[0091] <Pretreatment Method> 0.1 g of the extract was accurately weighed into a TFM decomposition vessel, sulfuric acid and nitric acid were added, and pressurized acid decomposition was performed with a microwave decomposition apparatus. Apparatus: ETHOS One manufactured by Milestone General Reagent: Sulfuric acid, Ultrapur-100 ultra-high purity sulfuric acid manufactured by Kanto Chemical Co., Inc. Nitric acid, EL grade ultra-high purity nitric acid manufactured by Kanto Chemical Co., Inc.

[0092] The obtained decomposition liquid was adjusted to a constant volume of 50 mL and subjected to ICP-MS measurement. <ICP-MS Measurement Conditions> Apparatus: 7900 manufactured by Agilent Technologies RF power: 1.5 kw Plasma gas: 15 L / min Auxiliary gas: 0.90 L / min Make-up gas: 0.50 L / min Carrier gas: 0.70 L / min Reaction gas: H2 5.0 mL / min

[0093] (Quantification mode) Calibration curve condition: multi-point calibration curve method Element to be measured: Zn (m / z 68) Internal standard element: Y (m / z 89)

[0094] <Preparation of foamed polystyrene resin particles> (Example 1) 100 parts by weight of virgin polystyrene resin (A1-1), 2 parts by weight of nucleating agent (Tamura Talc TP-20, average particle size 12.1 μm), and 0.05 parts by weight of zinc stearate (B2) were supplied to a tandem two-stage extruder consisting of a 40 mm diameter coaxial twin-screw extruder (first extruder) and a 90 mm diameter single-screw extruder (second extruder) connected in series. In the first extruder, the mixture was melt-kneaded at a set temperature of 190°C and a rotation speed of 167 rpm. 2.5 parts by weight of isobutane (C1) were injected into the first extruder midway through the process, and 2.0 parts by weight of mixed pentane [a mixture of normal pentane (C4) and isopentane (C3) = 4 / 1 (weight ratio)] were injected, adding a total of 4.5 parts by weight of foaming agent (C). Subsequently, the mixture was supplied to the second extruder through a continuation tube set to 200°C.

[0095] In the second extruder, the obtained polystyrene resin molten material was cooled to 160°C. Then, the resin molten material was extruded at a discharge rate of 58.2 kg / hour into pressurized circulating water at 64°C and 1.1 MPa through a die attached to the tip of the second extruder, which had 60 small holes with a diameter of 0.65 mm and a land length of 3.0 mm and was set to 250°C. The extruded resin molten material was cut and granulated using a rotary cutter with 8 blades in contact with the die, and then transferred to a centrifugal dewatering machine to obtain foamed polystyrene resin particles. At this time, the residence time in the first extruder was 2 minutes, and the residence time in the second extruder was 5 minutes.

[0096] (Example 2) Foamable polystyrene resin particles were obtained in the same manner as in Example 1, except that instead of virgin polystyrene resin (A1-1), virgin polystyrene resin and recycled polystyrene resin according to the formulations in Table 1 were used, and zinc stearate (B2) as described in Table 1 was added and melt-kneaded.

[0097] (Comparative Example 1) Expandable polystyrene resin particles were obtained by the same method as in Example 1, except that zinc stearate (B2) was not used and the mixture was melt-kneaded.

[0098] (Example 3, Comparative Example 3) Foamable polystyrene resin particles were obtained in the same manner as in Example 1, except that instead of virgin polystyrene resin (A1-1), virgin polystyrene resin and recycled polystyrene resin according to the formulations in Table 1 were used, and zinc stearate (B2) as described in Table 1 was added and melt-kneaded.

[0099] (Comparative Example 2) Foamable polystyrene resin particles were obtained in the same manner as in Example 1, except that virgin polystyrene resins of the formulations listed in Table 1 and recycled polystyrene resins were used instead of virgin polystyrene resin (A1-1), and zinc stearate (B2) was not added during extrusion before melt kneading.

[0100] (Example 4) Recycled polystyrene resin (A2-3) and virgin polystyrene resin (A1-2) were placed in a blender in a 50:50 weight ratio and blended for 10 minutes to obtain a resin mixture. The obtained resin mixture was placed in a 50 mmφ single-screw extruder, heated and melted, then extruded in strand form through a die with small holes, rapidly cooled in water, and then cut into pieces of 0.5 mg / particle, 0.8 mm in length and 0.9 mm in diameter using a pelletizer to obtain polystyrene resin particles. The extrusion conditions are as follows:

[0101] [Extruder conditions] · Cylinder temperature [W / 160 / 160 / 140 / 140 / 160 / 160 / 210 / 215 / 220 / 210 / 210 / 210 / 21 5(F) / 220(S / C) / 225(D)] ·Discharge amount: 300kg / Hr • Screw rotation speed: 450 rpm [Pelletizer conditions] • Pickup speed: 52.5 m / min • Cutter rotation speed: 1280 rpm.

[0102] In a reactor equipped with a stirrer, 200 parts by weight of pure water, 0.3 parts by weight of tricalcium phosphate, 0.008 parts by weight of sodium α-olefin sulfonate, 2 parts by weight of salt, and 100 parts by weight of the resulting polystyrene resin particles were added under stirring. Then, 0.05 parts by weight of ethylenebis-stearamide (average particle size 0.24 mm, manufactured by NOF Corporation, Alflo H50-S) as an amide compound was added under stirring, after being pre-mixed with 0.002 parts by weight of sodium α-olefin sulfonate and water in a homogenizer and stirred at 2600 rpm for 30 minutes to form an emulsion.

[0103] Subsequently, after raising the temperature to 100°C, 6 parts by weight of mixed pentane (a mixture of n-pentane (C4) and isopentane (C3) = 4 / 1 (by weight ratio)) was added as a blowing agent (C), and the temperature was further increased to 118°C for 5 hours. After cooling to room temperature, the material was removed from the reactor, washed, dehydrated, and dried to obtain foamable polystyrene resin particles.

[0104] (Examples 5 and 6, Comparative Example 5) Except for impregnating each of the above components and their respective particle weights with the formulations described in Tables 1 and 2, using the same method as in Example 4, foamed polystyrene resin particles of Examples 5, 6, and Comparative Example 5 were obtained.

[0105] (Example 7) (Seed resin production process) 100 parts by weight of recycled polystyrene resin (A2-4) and 0.03 parts by weight of zinc stearate (B2) were fed into a 50 mmφ single-screw extruder. After heating and melting, the mixture was extruded in strand form through a die with small holes. After rapid cooling in water, the mixture was cut into 0.45 mg / particle, 0.9 mm in length and 0.8 mm in diameter using a pelletizer to obtain polystyrene resin particles. The obtained polystyrene resin particles were used as seed resin in the following polymerization process.

[0106] (Polymerization process) In a 6L autoclave equipped with a stirrer, 93.5 parts by weight of pure water, 0.38 parts by weight of tricalcium phosphate, 0.0104 parts by weight of sodium α-olefin sulfonate, 0.1 parts by weight of sodium chloride, and 50 parts by weight of seed resin were charged, and stirring was started. Subsequently, the temperature was raised to 92°C. Upon reaching 92°C, styrene monomer, polymerization initiator, and other additives were added stepwise according to the following formulation, and polymerization was started.

[0107] (1) From the start of polymerization (0 hours) to less than 1 hour: Styrene monomer was added at a rate of 0.243 parts / min (total 14.6 parts). A polymerization initiator with a 10-hour half-life temperature of 50°C or higher and less than 90°C (benzoyl peroxide, 10-hour half-life temperature of 73.6°C) was added at 0 minutes, 15 minutes, 30 minutes, and 45 minutes from the start of polymerization, at a rate of 0.0175 parts each (total 0.070 parts).

[0108] (2) 1 hour to less than 2 hours after the start of polymerization: Styrene monomer was added at a rate of 0.295 parts / min (total 17.7 parts). A polymerization initiator (benzoyl peroxide) with a 10-hour half-life temperature of 50°C or higher and less than 90°C was added at 0, 15, 30, and 45 minutes after 1 hour after the start of polymerization (total 0.047 parts).

[0109] (3) Two hours after the start of polymerization: 0.18 parts of dispersant (tricalcium phosphate) were added.

[0110] (4) 2 hours to less than 3 hours after the start of polymerization: Styrene monomer was added at a rate of 0.295 parts / min (total 17.7 parts). A polymerization initiator (benzoyl peroxide) with a 10-hour half-life temperature of 50°C or higher and less than 90°C was added at 0, 15, 30, and 45 minutes after 2 hours of polymerization (total 0.023 parts).

[0111] (5) 2 hours and 30 minutes after the start of polymerization: 0.064 parts of a polymerization initiator (1,1-di(t-butylperoxy)cyclohexane, with a 10-hour half-life temperature of 90.7°C or higher) was added.

[0112] Three hours after the start of polymerization: The addition of styrene monomer, polymerization initiator, and other additives was completed, and the polymerization process was finished. The time required for the polymerization process was 3.0 hours. Subsequently, the system was maintained at 92°C for a further 30 minutes (holding step) to obtain polystyrene resin particles.

[0113] (Impregnation process) After the above process, 1.0 part by weight of cyclohexane was added to the system containing the obtained polystyrene resin particles, then 2.0 parts by weight of mixed pentane (a mixture of n-pentane (C4) and isopentane (C3) = 4 / 1 (by weight ratio)) was added over 10 minutes, and then 5.0 parts by weight of mixed butane (C2) (a mixture of n-butane and isobutane, with a mixing ratio of n-butane / isobutane = 7 / 3) was added over 20 minutes, and the temperature was raised to 120°C. The system was held at 120°C for 1 hour and 45 minutes, then cooled to room temperature, and the polymerization slurry was removed from the autoclave. The removed polymerization slurry was washed, dehydrated and dried, and the material that passed through a 2.0 mm sieve was sieved again, and the material remaining on the sieve (that did not pass through) was collected to obtain foamed polystyrene resin particles with an average particle weight of 0.9 mg.

[0114] (Comparative Example 4) Except for using the type and amount of foaming agent as described in Table 2, foamed polystyrene resin particles were obtained in the same manner as in Example 7.

[0115] <Preparation of pre-foamed particles> (Examples 1A to 7A, Comparative Examples 1A to 3A, 5A) The expandable polystyrene resin particles of Examples 1-7 and Comparative Examples 1-3 and 5 were stored at 15°C for at least two weeks. Then, 0.04 parts by weight of zinc stearate was dry-blended as an additive to 100 parts by weight of each expandable polystyrene resin particle. 512 g of each expandable polystyrene resin particle containing the above additive was placed in a pre-foaming machine (manufactured by Daikai Kogyo Co., Ltd., 0.5 MPa pressurized pre-foaming machine), and the internal pressure setting was 0.05 kg / cm². 2 ~0.15 kg / cm 2Then, 0.10 MPa of steam was introduced into the pre-foaming machine to foam the mixture to a volume ratio of 40 times, obtaining pre-foamed particles for Examples 1A to 7A and Comparative Examples 1A to 3A and 5A.

[0116] (Comparative example 4A) Pre-foamed particles were obtained in the same manner as in Examples 1A to 7A, Comparative Examples 1A to 3A, and 5A, except that the foamed polystyrene resin particles of Comparative Example 4 were stored at 10°C for more than 3 months.

[0117] <Fabrication of polystyrene foam molded articles> (Examples 1B-7B, Comparative Examples 1B-5B) After curing the pre-foamed particles of Examples 1A to 7A and Comparative Examples 1A to 5A at 30°C for 24 hours, each of the pre-foamed particles was filled into an in-mold molding die (400 mm long x 400 mm wide x 25 mm thick) attached to a polystyrene foam molding machine (Daisen Kogyo Co., Ltd., KR-57). After introducing 0.06 MPa of steam to cause in-mold foaming, the die was cooled by spraying water onto it. The polystyrene foam molded body was held in the die until the pressure it exerted on the die reached 0.01 MPa (gauge pressure). The polystyrene foam molded body was then removed to obtain the polystyrene foam molded bodies of Examples 1B to 7B and Comparative Examples 1B to 5B.

[0118] <Rating> The obtained polystyrene foam molded articles were evaluated for foaming ratio, flexural strength, 5% compressive strength, and surface elongation according to the following method. The results are shown in Tables 1 and 2.

[0119] (Method for measuring the bulk ratio of pre-foamed particles) W (g) of pre-foamed particles are collected as a sample for measurement. This sample is allowed to fall naturally into a graduated cylinder, and then the graduated cylinder is tapped to obtain the apparent volume V (cm³) of the sample. 3 The weight W (g) and volume V (cm³) were kept constant. 3 The bulk ratio was calculated based on the following formula. Volume ratio (cm 3 ( / g) = Volume V (cm³) of the sample being measured 3) / Weight W (g) of measurement sample

[0120] (Average cell diameter of polystyrene-based expanded particles) (1) Observation conditions Equipment: DIGITAL MICROSCOPE VHX-900 manufactured by Keyence Corporation (2) Measurement conditions Polystyrene-based expanded particles expanded to a bulk magnification of 50 times in the same manner as in <Preparation of pre-expanded particles> were cut into equal parts (passing through the center) with a razor, and the cut surface was observed with a microscope. At that time, a photograph of the cut surface was taken at a magnification of 100 times. A 60 mm straight line was drawn near the center of the cut surface in the photograph, and the average chord length was determined based on the following formula from the number of bubbles intersecting the straight line, which was taken as the average cell diameter. Average chord length t (μm) = 60 (mm) × 1000 / (number of bubbles × 100 (magnification))

[0121] (Expansion ratio) The weight (g) of a sample of a polystyrene-based expanded molded article was measured, and the vertical dimension, horizontal dimension, and thickness dimension were measured using a vernier caliper. The volume (cm 3 ) of the sample was calculated from the measured respective dimensions, and the expansion ratio was calculated according to the following calculation formula. Expansion ratio (cm 3 / g) = Sample volume (cm 3 ) / Sample weight (g) It should be noted that the expansion ratio of polystyrene-based expanded molded articles is conventionally expressed in units of "cm 3 / g".

[0122] (Bending strength) After allowing the polystyrene-based expanded molded articles of Examples 1B to 7B and Comparative Examples 1B to 5B to stand at a temperature of 23°C for 48 hours, three test pieces each measuring 300 mm long × 100 mm wide × 25 mm thick were cut out. For each test piece, in accordance with JIS K 7221:2006, the maximum point stress when deformed at a speed of 20 mm / min was measured, and the average value of the maximum point stresses of the three test pieces was obtained.

[0123] (5% compressive strength) Polystyrene foam molded bodies of Examples 1B to 7B and Comparative Examples 1B to 5B were left to stand at 23°C for 48 hours, after which three test specimens measuring 50 mm (length) x 50 mm (width) x 25 mm (thickness) were cut out. For each test specimen, the zero deformation point and deformation amount were calculated from the elastic deformation linear portion of the force-deformation curve when compressed at a speed of 2.5 mm / min in accordance with JIS K7220:2006, and the 5% deformation compressive stress was determined. The average value of the 5% deformation compressive stress of the three test specimens was calculated.

[0124] (Surface stretch) The surface condition of four sections of the polystyrene foam molded material was visually observed and evaluated on a five-point scale as follows. The average value of these four sections was used as the surface elongation score. A higher number indicates fewer gaps between foam particles and a smoother, cleaner surface. A score of "4" or higher indicates good marketability and a desirable appearance, while a score of "3" indicates an acceptable appearance. 5: No gaps found. 4: There are some gaps, but they are hardly noticeable. 3: Although there are gaps in places, the overall result is acceptable. 2: The gaps are noticeable 1: There are many gaps

[0125] (comprehensive evaluation) The polystyrene foam molded articles were comprehensively evaluated according to the following evaluation criteria. Excellent (◎): Bending strength of 0.41 MPa or higher, compressive strength of 0.19 MPa or higher, and surface elongation of "4" or higher. Good (○): Bending strength is 0.39 MPa or higher and less than 0.41 MPa, compressive strength is 0.18 MPa or higher and less than 0.19 MPa, and surface elongation is "4" or higher. Allowable (△): Bending strength of 0.39 MPa or higher and less than 0.41 MPa, compressive strength of 0.18 MPa or higher and less than 0.19 MPa, and surface elongation of "3" or higher and less than "4". Defective (×): Bending strength less than 0.39 MPa, or compressive strength less than 0.18 MPa, or surface elongation of "2" or less.

[0126] [Table 1]

[0127] [Table 2]

[0128] From Tables 1 and 2, it can be seen that when the zinc stearate (B) content is 0.025% by weight or more and 0.095% by weight or less relative to 100% by weight of the foamed polystyrene resin particles, a foamed molded article with practical strength can be formed.

Claims

1. The foamed polystyrene resin particles contain zinc stearate. The zinc stearate content is 0.025% by weight or more and 0.095% by weight or less based on 100% by weight of the foamed polystyrene resin particles. Expanded polystyrene resin particles, wherein the average cell diameter of the expanded polystyrene resin particles obtained by expanding the aforementioned expanded polystyrene resin particles 50 times is 20 μm or more and 250 μm or less.

2. The foamed polystyrene resin particles according to claim 1, wherein the average particle weight is 0.5 mg or more and 1.6 mg or less.

3. The foamed polystyrene resin particles are made using polystyrene resin, The foamable polystyrene resin particles according to claim 1 or 2, wherein the polystyrene resin includes recycled polystyrene resin.

4. Furthermore, it contains a foaming agent, The content of the foaming agent is 2.8% by weight or more and 10% by weight or less based on 100% by weight of foamed polystyrene resin particles. The foamable polystyrene resin particles according to claim 1 or 2, wherein the weight ratio of butane to pentane in the foaming agent is butane / pentane = 0 / 100 to 80 / 20.

5. A method for producing foamed polystyrene resin particles according to claim 1 or 2, comprising the steps of extruding a molten mixture containing a polystyrene resin, zinc stearate, and a foaming agent into pressurized circulating water, and cutting it with a rotary cutter to form particles.

6. A method for producing foamed polystyrene resin particles according to claim 1 or 2, comprising the steps of: dispersing resin particles, which are granulated by extruding a molten kneaded mixture containing a polystyrene resin and zinc stearate, in an aqueous medium containing an organic dispersant and impregnating them with a foaming agent.

7. A method for producing foamed polystyrene resin particles according to claim 1 or 2, comprising the steps of: dispersing resin particles, obtained by granulating a molten kneaded product containing polystyrene resin and zinc stearate, in an aqueous medium containing a dispersant; impregnating with a styrene monomer and polymerizing it; and then impregnating with a foaming agent.

8. Polystyrene foam particles, which are a pre-foamed material for the foamable polystyrene resin particles described in claim 1 or 2.

9. A polystyrene foam molded article, which is a molded article of polystyrene foam particles as described in claim 8.

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