Method for manufacturing expandable polystyrene resin particle

Abstract

To provide a novel method for manufacturing an expandable polystyrene resin particle by which it is possible to provide the expandable polystyrene resin particle excellent in uniformity of particle diameter which is intended to reduce generation of cell spots with the passage of time.SOLUTION: A method for manufacturing an expandable polystyrene resin particle includes a polymerization step of polymerizing a styrene monomer in an aqueous suspension including water, a seed resin, a nucleating agent B, a polymerization initiator, and the styrene monomer. The seed resin includes a base resin and a nucleating agent A.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing expandable polystyrene-based resin particles. [Background technology]

[0002] Polystyrene foam beads are used for cushion beads, lightweight aggregates, and moldings that require aesthetic appeal.

[0003] Various techniques have been developed for expandable polystyrene resin particles, which are the raw material for polystyrene expanded particles, or for methods for producing the same (for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 03-166239 [Patent Document 2] Japanese Patent Publication No. 2022-144234 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the conventional techniques described above, cell irregularities (cell roughness) may occur in polystyrene foam molded articles obtained by expanding and molding expandable polystyrene resin particles after a certain period of time has passed since their production, and there is room for further improvement. Also, there is a need for expandable polystyrene resin particles with excellent particle size uniformity.

[0006] One embodiment of the present invention has been made in consideration of the above-mentioned problems. An object of one embodiment of the present invention is to provide a novel method for producing expandable polystyrene-based resin particles that can provide expandable polystyrene-based resin particles having excellent particle size uniformity and capable of providing a polystyrene-based foamed molded article in which the occurrence of cell spots is suppressed even after a period of time has passed since production. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems and have completed the present invention.

[0008] That is, one embodiment of the present invention includes the following configuration.

[0009] [1] A polymerization step of polymerizing a styrene-based monomer in an aqueous suspension containing water, a seed resin, a nucleating agent B, a polymerization initiator, and a styrene-based monomer, The seed resin comprises a base resin and a nucleating agent A different from the nucleating agent B, in accordance with a method for producing expandable polystyrene-based resin particles.

[0010] [2] The method for producing expandable polystyrene-based resin particles according to [1], wherein the content of the nucleating agent A in the seed resin is 0.05 to 0.33 parts by weight per 100 parts by weight of the base resin.

[0011] [3] The method for producing expandable polystyrene-based resin particles according to [1] or [2], wherein the amount of the nucleating agent B used is 0.05 parts by weight to 0.50 parts by weight per 100 parts by weight of the total amount of the seed resin used and the styrene-based monomer used.

[0012] [4] The method for producing expandable polystyrene-based resin particles according to any one of [1] to [3], wherein the nucleating agent A contains an acrylic resin.

[0013] [5] The method for producing expandable polystyrene-based resin particles according to any one of [1] to [4], wherein the nucleating agent B contains ethylene bisstearic acid amide and / or polyethylene wax. [Effects of the Invention]

[0014] According to one embodiment of the present invention, it is possible to provide expandable polystyrene-based resin particles that have excellent particle size uniformity and that can provide polystyrene-based foamed molded articles in which the occurrence of cell spots is suppressed even after a period of time has passed since production. DETAILED DESCRIPTION OF THE INVENTION

[0015] One embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the respective configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. All academic literature and patent documents described in this specification are incorporated herein by reference. Furthermore, unless otherwise specified in this specification, the term "A to B" representing a numerical range means "greater than or equal to A (including and greater than A) and less than or equal to B (including and less than B)."

[0016] In this specification, the term "X unit" contained in a polymer, copolymer, or resin refers to a "structural unit derived from an X monomer." For example, the term "styrene-based unit" refers to a "structural unit derived from a styrene-based monomer."

[0017] In this specification, the terms "X-based resin" and "X-based polymer" refer to a resin and a polymer, respectively, in which the content of X-based units is the highest among all the structural units constituting the resin and polymer.

[0018] Unless otherwise specified in this specification, the structural unit is X 1 Units, X 2 Units, ... and X n A copolymer containing units (n is an integer of 2 or more) is called "X 1 / X 2 / ··· / X n Also called "copolymer". X 1 / X 2 / ··· / X n Unless otherwise specified, the polymerization mode of the copolymer is not particularly limited, and the copolymer may be a random copolymer, an alternating copolymer, a block copolymer, or a graft copolymer.

[0019] 1. Method for producing expandable polystyrene resin particles A method for producing expandable polystyrene-based resin particles according to one embodiment of the present invention includes a polymerization step of polymerizing a styrene-based monomer in an aqueous suspension containing water, a seed resin, a nucleating agent B, a polymerization initiator, and a styrene-based monomer, wherein the seed resin includes a base resin and a nucleating agent A different from the nucleating agent B.

[0020] The expandable polystyrene resin particles obtained by this production method can be expanded to obtain expanded polystyrene particles. The expanded polystyrene particles can be molded (for example, by in-mold expansion molding using a mold) to obtain a polystyrene foam molded article.

[0021] In this specification, "a method for producing expandable polystyrene-based resin particles" may be simply referred to as "the production method," and "a method for producing expandable polystyrene-based resin particles according to one embodiment of the present invention" may be referred to as "the present production method." Furthermore, in this specification, "expandable polystyrene-based resin particles" may be referred to as "expandable resin particles," "polystyrene-based expanded particles" may be referred to as "expanded particles," and "polystyrene-based expanded molded articles" may be referred to as "expanded molded articles."

[0022] The present manufacturing method, having the above-described configuration, has the advantage of being able to provide (i) expandable polystyrene-based resin particles having excellent particle size uniformity, and (ii) expandable polystyrene-based resin particles that can provide a polystyrene-based foam molded article in which the occurrence of cell spots is suppressed even after a period of time has passed since production. In this specification, "a polystyrene-based foam molded article in which the occurrence of cell spots is suppressed" specifically refers to a "polystyrene-based foam molded article in which there are no cell spots or in which there are very few cell spots." The method for evaluating the particle size uniformity of expandable polystyrene-based resin particles will be described in detail in the Examples below. The method for evaluating the cell spots of polystyrene-based foam molded articles will be described in detail in the Examples below.

[0023] In this production method, a seed resin is used. Therefore, this production method can also be called a seed polymerization method. Below, the seed resin will be explained, and then the production method will be explained.

[0024] (1-1. Seed resin) <Base resin> The seed resin includes a base resin. The term "base resin" in the seed resin refers to a resin component that substantially constitutes the seed resin.

[0025] The base resin is not particularly limited, and examples of the base resin include (i) any one homopolymer selected from the group consisting of a styrene-based monomer, an unsaturated fatty acid, an unsaturated fatty acid ester, and acrylonitrile, (ii) two or more copolymers selected from the group consisting of a styrene-based monomer, an unsaturated fatty acid, an unsaturated fatty acid ester, acrylonitrile, and a bifunctional monomer, and (iii) a mixture of two or more selected from the group consisting of the above-mentioned homopolymers and copolymers.

[0026] Examples of styrene-based monomers include styrene and styrene derivatives, such as α-methylstyrene, p-methylstyrene, m-methylstyrene, o-methylstyrene, t-butylstyrene, and chlorostyrene.

[0027] Examples of unsaturated fatty acids include (meth)acrylic acid and maleic anhydride.

[0028] Examples of the unsaturated fatty acid ester include alkyl (meth)acrylates (for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate).

[0029] As used herein, "(meth)acrylic" refers to "acrylic and / or methacrylic." For example, "(meth)acrylic acid" refers to "acrylic acid and / or methacrylic acid," and "alkyl (meth)acrylate" refers to "alkyl acrylate and / or alkyl methacrylate."

[0030] Examples of the difunctional monomer include divinylbenzene and alkylene glycol dimethacrylate.

[0031] Since a foamed molded article having high rigidity can be obtained, the base resin preferably contains a polystyrene-based resin, and more preferably is composed only of a polystyrene-based resin (is a polystyrene-based resin). In this specification, the term "polystyrene-based resin" refers to a resin having the highest content of styrene-based units among all the constituent units constituting the resin.

[0032] The polystyrene resin contained in the base resin preferably contains 50 mol% or more of styrene units, more preferably 60 mol% or more, more preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, particularly preferably 95 mol% or more, and most preferably 100 mol% of all structural units (100 mol%). In other words, the polystyrene resin contained in the base resin is most preferably composed only of styrene units.

[0033] The polystyrene resin contained in the base resin preferably contains 50 mol% or more of styrene units, more preferably 60 mol% or more, more preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, particularly preferably 95 mol% or more, and most preferably 100 mol% of all structural units (100 mol%). In other words, the polystyrene resin contained in the base resin is most preferably composed of only styrene units.

[0034] <Nucleating agent A> In the present production method, the nucleating agent A contained in the seed resin is not particularly limited, as long as it is different from the nucleating agent B used in the polymerization step, separate from the seed resin.

[0035] Nucleating agent A is preferably a nucleating agent that can be finely dispersed in a polystyrene resin matrix. Examples of nucleating agent A include acrylic resins, polystyrene resins, olefin waxes, fatty acid esters, hydrogenated vegetable oils, amides, and higher fatty acids. In this specification, the term "acrylic resin" refers to a resin having the highest content of acrylic units among all the structural units constituting the resin.

[0036] Examples of acrylic resins that can be suitably used as the nucleating agent A include acrylic ester resins, methacrylic ester resins, and methacrylic ester / acrylic ester copolymers (such as methyl methacrylate / butyl acrylate copolymers).

[0037] Examples of acrylic ester resins that can be suitably used as the nucleating agent A include homopolymers of acrylic esters and copolymers of acrylic esters and monomers other than acrylic esters.

[0038] Methacrylate resins that can be suitably used as nucleating agent A include homopolymers of methacrylate esters and copolymers of methacrylate esters and monomers other than methacrylate esters (e.g., methyl methacrylate / butadiene / styrene copolymers).

[0039] Commercially available acrylic resins may be used as the nucleating agent A. Examples of such commercially available products include Kane Ace (registered trademark) PA-20 (methyl methacrylate / butyl acrylate copolymer) and Kane Ace (registered trademark) PA-210 (methyl methacrylate / butyl acrylate copolymer), both manufactured by Kaneka Corporation.

[0040] Examples of polystyrene resins that can be suitably used as nucleating agent A include styrene / butadiene copolymers, high-impact polystyrene, styrene / butadiene / styrene block copolymers, hydrogenated styrene / butadiene / styrene block copolymers, and acrylonitrile / styrene copolymers.

[0041] Examples of olefin waxes that can be suitably used as the nucleating agent A include polyethylene wax and ethylene / vinyl acetate copolymer wax.

[0042] Fatty acid esters that can be suitably used as nucleating agent A include glycerin monostearate, glycerin monobehenate, glycerin mono-12-hydroxystearate, glycerin monolaurate, glycerin tri-12-hydroxystearate, glycerin tristearate, glycerin tripalmitate, glycerin trilaurate, glycerin tribehenate, and pentaerythol tetrastearate.

[0043] Examples of hydrogenated vegetable oils that can be suitably used as nucleating agent A include hydrogenated castor oil, hydrogenated soybean oil, and hydrogenated rapeseed oil.

[0044] Examples of amides that can be suitably used as the nucleating agent A include fatty acid amides (for example, ethylene bisstearic acid amide), fatty acid dicarboxylic acid diamides, aromatic bisamides, and aromatic dicarboxylic acid diamides.

[0045] Examples of higher fatty acids that can be suitably used as nucleating agent A include stearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, and lactic acid.

[0046] As the nucleating agent A, one of the above-mentioned compounds may be used alone, or two or more of them may be used in combination.

[0047] The nucleating agent A contained in the seed resin preferably contains one or more selected from the group consisting of acrylic resins, polystyrene resins, olefin waxes, fatty acid esters, hydrogenated vegetable oils, amides, and higher fatty acids, and more preferably contains one or more selected from this group; (ii) more preferably contains one or more selected from the group consisting of acrylic resins, polystyrene resins, olefin waxes, and fatty acid esters, and more preferably contains one or more selected from this group; (iii) more preferably contains one or more selected from the group consisting of acrylic resins, polystyrene resins, and olefin waxes, and more preferably contains one or more selected from this group; (iv) more preferably contains one or more selected from the group consisting of acrylic resins (including an acrylic resin), and more preferably contains one or more selected from this group (is an acrylic resin); (v) more preferably contains a methacrylic acid ester / acrylic acid ester copolymer, and even more preferably is a methacrylic acid ester / acrylic acid ester copolymer; and (vi) particularly preferably contains a methyl methacrylate / butyl acrylate copolymer, and most preferably is a methyl methacrylate / butyl acrylate copolymer. According to this configuration, even when expandable resin particles that have been produced for a long time are used, it is possible to provide a foamed molded article in which the occurrence of cell irregularities is suppressed, that is, the cells are advantageously stable over a long period of time. In this specification, "X is one or more species selected from the group" means "X is composed only of one or more species selected from the group."

[0048] There are no particular limitations on the content of one or more selected from the group consisting of acrylic resins, polystyrene resins, olefin waxes, fatty acid esters, hydrogenated vegetable oils, amides, and higher fatty acids in nucleating agent A. The content of one or more selected from the group consisting of acrylic resins, polystyrene resins, olefin waxes, fatty acid esters, hydrogenated vegetable oils, amides, and higher fatty acids in nucleating agent A (the total content when two or more types are included) is preferably 60 parts by weight or more, more preferably 70 parts by weight or more, more preferably 80 parts by weight or more, even more preferably 90 parts by weight or more, particularly preferably 95 parts by weight or more, and may be 100 parts by weight, per 100 parts by weight of nucleating agent A.

[0049] The content of fatty acid metal salt in nucleating agent A may be 10 parts by weight or less, 5 parts by weight or less, 3 parts by weight or less, 1 part by weight or less, or 0 part by weight, per 100 parts by weight of nucleating agent A. In other words, nucleating agent A may not contain fatty acid metal salt.

[0050] The weight average molecular weight of the acrylic resin used as nucleating agent A is preferably 500,000 to 1,500,000, more preferably 600,000 to 1,400,000, even more preferably 700,000 to 1,300,000, and particularly preferably 750,000 to 1,200,000. When the weight-average molecular weight of the acrylic resin used as nucleating agent A is (a) 500,000 or more, an effective nucleating effect is easily obtained, and (b) when it is 1,500,000 or less, the acrylic resin (nucleating agent A) tends to be present in a uniformly dispersed state in the resulting seed resin. The method for measuring the weight-average molecular weight of the acrylic resin will be explained in detail in the Examples below.

[0051] The content of nucleating agent A in the seed resin is not particularly limited. The content of nucleating agent A in the seed resin is preferably 0.05 to 0.33 parts by weight, more preferably 0.06 to 0.30 parts by weight, even more preferably 0.07 to 0.25 parts by weight, and particularly preferably 0.07 to 0.20 parts by weight, relative to 100 parts by weight of the base resin. This configuration has the advantage that it is possible to provide expandable resin beads and foamed molded articles having excellent cell uniformity, and the advantage that it is easy to obtain expandable resin beads having (approximately) uniform particle diameters.

[0052] The shape of the seed resin is not particularly limited. The seed resin may be in the form of particles. A seed resin having a particulate shape may also be called a seed resin particle.

[0053] (1-2. Method of manufacturing seed resin) The method for producing the seed resin is not particularly limited. The seed resin can be produced by methods such as suspension polymerization, bulk polymerization, emulsion polymerization, and solution polymerization. Suspension polymerization, bulk polymerization, emulsion polymerization, and solution polymerization can all produce particulate seed resins (seed resin particles). Because seed resin particles with a highly uniform particle size can be obtained, suspension polymerization is preferred as the method for producing the seed resin. Among suspension polymerization methods, droplet polymerization is more preferred. The term "droplet polymerization" refers to a method in which a monomer is dispersed in an aqueous medium as droplets by passing it through a nozzle under regular vibration, and the monomer is polymerized without causing droplet coalescence or additional dispersion (or with the occurrence of droplet coalescence and additional dispersion significantly reduced). The seed resin produced by a conventional suspension polymerization method may be classified using a sieve or the like before use.

[0054] The liquid polymerization method, which is a preferred method for producing the seed resin, will be described below.

[0055] <Droplet generation polymerization method> The droplet formation polymerization method for obtaining the seed resin has, for example, the following configuration: (1) a dispersing step in which a monomer mixture containing at least a monomer component, a nucleating agent A, a polymerization initiator, and optionally other additives is passed through a droplet-generating nozzle under regular vibration to form droplets, thereby dispersing the droplets in an aqueous medium containing water and, optionally, a dispersant, etc., to prepare an aqueous suspension; (2) a temperature-raising step of raising the temperature of the aqueous suspension to a predetermined polymerization temperature; (3) Next, a seed resin polymerization step in which the aqueous suspension is reacted at a predetermined polymerization temperature for a predetermined polymerization time to carry out a polymerization reaction, thereby obtaining a copolymer. The steps (1) to (3) are usually carried out under gentle stirring (for example, at 200 rpm or less).

[0056] In the droplet generation polymerization method, droplets are dispersed in an aqueous medium by passing them through a droplet generation nozzle under regular vibration, which reduces or eliminates coalescence and additional dispersion of the droplets. Therefore, the seed resin particles produced by the droplet generation polymerization method tend to have a narrow particle size distribution and excellent particle size uniformity.

[0057] <Device> The apparatus used in the droplet generation polymerization method preferably comprises a polymerization reactor and a droplet generation section.

[0058] The polymerization reactor is not particularly limited as long as it is equipped with a droplet inlet for introducing droplets into the polymerization reactor, and any reactor generally used for polymerization reactions can be used. The polymerization reactor is preferably equipped with a stirrer.

[0059] The droplet generating section is equipped with a droplet generating nozzle and a vibrator, and is in communication with the droplet inlet of the polymerization reactor via a droplet introducing pipe.

[0060] The droplet generating section may further include an aqueous dispersant solution inlet for introducing the aqueous dispersant solution into the droplet generating section.

[0061] The droplet generating nozzle is a nozzle that ejects the monomer mixture as droplets. There may be one or more droplet generating nozzles. The droplet generating nozzle is composed of, for example, a nozzle plate having an ejection port for ejecting the monomer mixture, and a nozzle box having a monomer mixture inlet for introducing the monomer mixture. There may be one or more ejection ports.

[0062] By adjusting the number of discharge ports, the particle size and particle size distribution (UT) of the seed resin particles can be controlled. In addition, by adjusting the diameter of the discharge ports, the droplet size can be controlled, and thus the particle size and particle size distribution (UT) of the seed resin particles can be controlled.

[0063] The vibrator includes a vibrating unit that applies regular vibrations to the droplet generating nozzle.

[0064] <Dispersion process> The dispersion step in the droplet generation polymerization method is a step of passing a monomer mixture containing a monomer component, a nucleating agent A, a polymerization initiator, and optionally other additives through a nozzle under regular vibration to form droplets and disperse them in an aqueous medium containing water and optionally a dispersant, etc., to prepare an aqueous suspension.

[0065] <Monomer mixture> <Monomer component> The term "monomer component" in the droplet generation polymerization method is described as a general term for the monomers used in the droplet generation polymerization method.

[0066] The monomer used in the droplet generation polymerization method is not particularly limited. The monomer (monomer component) used in the droplet generation polymerization method may contain, for example, one or more monomers selected from the group consisting of styrene-based monomers, unsaturated fatty acids, unsaturated fatty acid esters, acrylonitrile, and bifunctional monomers, or may be composed solely of one or more monomers selected from the group. This configuration allows for the production of a seed resin containing the base resin.

[0067] <Nucleating agent A> Nucleating agent A has already been explained in the section <Nucleating agent A> above, so that explanation is incorporated herein by reference and will not be repeated here.

[0068] The amount of nucleating agent A used in the droplet generation polymerization method is not particularly limited, but is preferably an amount such that the content of nucleating agent A in the seed resin falls within the preferred range described above in the section on <Nucleating Agent A>. The monomer components used in the droplet generation polymerization method may form the base resin in the seed resin. Therefore, the amount of nucleating agent A used in the droplet generation polymerization method is, for example, preferably 0.05 to 0.33 parts by weight, more preferably 0.06 to 0.30 parts by weight, even more preferably 0.07 to 0.25 parts by weight, and particularly preferably 0.07 to 0.20 parts by weight, per 100 parts by weight of the total amount of the monomer components.

[0069] <Polymerization initiator> Examples of the polymerization initiator used in the droplet generation polymerization method include organic peroxides and azo compounds.

[0070] Examples of organic peroxides used in droplet generation polymerization include benzoyl peroxide, lauroyl peroxide, ditoluyl peroxide, toluylbenzoyl peroxide, t-butyl peroxybenzoate, isopropyl-t-butyl peroxycarbonate, butyl perbenzoate, t-butylperoxy-2-ethylhexanoate, t-butyl perpivalate, t-butylperoxyisopropyl carbonate, di-t-butylperoxyhexahydroterephthalate, 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di-t-butylperoxy-2,4-di-t-butylcyclohexane, t-butylperoxy-2-ethylhexyl carbonate, and t-amylperoxy-2-ethylhexyl carbonate. "t-Butylperoxy-2-ethylhexyl carbonate" and "t-amylperoxy-2-ethylhexyl carbonate" are also called "t-butylperoxy-2-ethylhexyl monocarbonate" and "t-amylperoxy-2-ethylhexyl monocarbonate," respectively.

[0071] Azo compounds used in droplet generation polymerization include azobisisobutyronitrile and azobisdimethylvaleronitrile.

[0072] These polymerization initiators may be used alone or in combination of two or more.

[0073] Among the above-mentioned polymerization initiators, (a) benzoyl peroxide, lauroyl peroxide, ditoluyl peroxide, toluylbenzoyl peroxide, t-butyl perpivalate, di-t-butylperoxyhexahydroterephthalate, azobisisobutyronitrile, and azobisdimethylvaleronitrile are low-temperature decomposition type polymerization initiators, and (b) t-butyl peroxybenzoate, isopropyl-t-butylperoxycarbonate, butyl perbenzoate, t-butylperoxy Di-2-ethylhexanoate, t-butylperoxyisopropyl carbonate, 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di-t-butylperoxy-2,4-di-t-butylcyclohexane, and t-butylperoxy-2-ethylhexyl carbonate are polymerization initiators that decompose at high temperatures.

[0074] In the droplet generation polymerization method, it is preferable to use a combination of at least one low-temperature decomposition type polymerization initiator and at least one high-temperature decomposition type polymerization initiator as the polymerization initiator, which has the advantage of being able to control the weight-average molecular weight and / or the amount of residual monomer of the seed resin while controlling the reaction rate.

[0075] The amount of the polymerization initiator used in the droplet generation polymerization method is preferably 0.1 to 0.5 parts by weight, more preferably 0.15 to 0.25 parts by weight, per 100 parts by weight of the monomer components contained in the monomer mixture. This method has the advantage of preventing abnormal heat generation and enabling stable production of the seed resin.

[0076] The monomer mixture may optionally contain other additives in addition to the monomer components, nucleating agent A, and polymerization initiator. Examples of other additives include a chain transfer agent, a plasticizer, and a solvent.

[0077] When a chain transfer agent is used in the droplet formation polymerization method, the weight average molecular weight of the resulting seed resin can be adjusted to fall within a desired range.

[0078] Mercaptan compounds used as chain transfer agents in droplet-forming polymerization include n-octyl mercaptan, n-dodecyl mercaptan, and t-dodecyl mercaptan.

[0079] When a plasticizer is used in the droplet generation polymerization method, there are advantages in that expandable resin particles with excellent expandability can be obtained due to the plasticizing effect, and that the expandable resin particles can provide expanded particles and foamed molded articles with excellent cell uniformity.

[0080] Examples of plasticizers include (a) fatty acid glycerides such as stearic acid triglyceride, palmitic acid triglyceride, lauric acid triglyceride, stearate diglyceride, and stearate monoglyceride, (b) vegetable oils such as coconut oil, palm oil, and palm kernel oil, (c) aliphatic esters such as dioctyl adipate and dibutyl sebacate, and (d) organic hydrocarbons such as liquid paraffin and cyclohexane. These plasticizers may be used alone or in combination of two or more.

[0081] The amount of plasticizer used in the droplet generation polymerization method is not particularly limited. The amount of plasticizer used in the droplet generation polymerization method is preferably 0.5 to 5.0 parts by weight, more preferably 0.5 to 3.0 parts by weight, and even more preferably 0.5 to 2.0 parts by weight, per 100 parts by weight of the total amount of the monomer components. When the amount of plasticizer used in the droplet generation polymerization method is 0.5 parts by weight or more per 100 parts by weight of the total amount of the monomer components, (i) expandable resin particles with excellent expandability are obtained due to the plasticizing effect, and (ii) the cells of the expanded beads and foamed molded articles obtained from the expandable resin particles are uniform and fine. As a result, there is an advantage that the expandable resin particles can be used to obtain expanded beads and foamed molded articles with a high expansion ratio (low density) without a decrease in the expandability of the expandable resin particles. When the amount of plasticizer used in the droplet generation polymerization method is 5.0 parts by weight or less per 100 parts by weight of the total amount of the monomer components, shrinkage of the expandable resin particles during expansion is reduced. As a result, there is an advantage that expanded beads and foamed molded articles with a high expansion ratio (low density) can be obtained using the expandable resin beads.

[0082] The method for preparing the monomer mixture is not particularly limited. For example, (i) the monomer components, nucleating agent A, and polymerization initiator may be mixed simultaneously to prepare the monomer mixture, (ii) several mixed liquids may be prepared in advance, and then the resulting mixed liquids may be mixed before being supplied to the droplet generating nozzle to prepare the monomer mixture, or (iii) several mixed liquids may be prepared in advance, and then the resulting mixed liquids may be supplied to the droplet generating nozzle via separate routes, and then the respective mixed liquids may be mixed in the droplet generating nozzle (particularly, in the nozzle box) to prepare the monomer mixture.

[0083] In a preferred embodiment of the present invention, mixed liquid 1 containing the monomer component and nucleating agent A and mixed liquid 2 containing the monomer component and nucleating agent A are prepared separately, and then mixed liquid 1 and mixed liquid 2 are mixed to prepare a monomer mixture. After preparing the monomer mixture by mixing mixed liquid 1 and mixed liquid 2, it is preferable to supply the monomer mixture to a droplet generating nozzle. Mixing of mixed liquid 1 and mixed liquid 2 may be performed, for example, in a transport line connected to a monomer mixture inlet provided in the droplet generating nozzle.

[0084] <Aqueous medium> <Water> The water used in the droplet generation polymerization method is not particularly limited, and examples of the water used in the droplet generation polymerization method include pure water such as RO water (water purified by reverse osmosis membrane), deionized water (water purified by ion exchange resin), and distilled water.

[0085] The amount of chain transfer agent used in the droplet formation polymerization method is not particularly limited.

[0086] The aqueous medium preferably further contains a dispersant to enhance the dispersibility of the seed resin and / or the monomer mixture in water. In other words, it is preferable to further use a dispersant in the droplet generation polymerization method. The dispersant is not particularly limited, and examples thereof include organic dispersants (e.g., water-soluble polymers) and inorganic dispersants (e.g., poorly water-soluble inorganic salts and water-soluble inorganic salts). An aqueous medium containing a dispersant is sometimes referred to as an "aqueous dispersion medium."

[0087] Examples of water-soluble polymers that can be used as organic dispersants include polyvinyl alcohol, partially saponified polyvinyl alcohol, polyacrylates, polyacrylamide, polyvinylpyrrolidone, carboxymethyl cellulose, and methyl cellulose.

[0088] Examples of poorly water-soluble inorganic salts used as inorganic dispersants include kaolin, calcium pyrophosphate, calcium phosphate (for example, tricalcium phosphate), calcium carbonate, magnesium pyrophosphate, magnesium phosphate, magnesium carbonate, and magnesium oxide.

[0089] Examples of water-soluble inorganic salts that can be used as inorganic dispersants include sodium nitrite, sodium chloride, potassium chloride, sodium sulfate, sodium hydrogen sulfite, potassium hydrogen sulfite, and ammonium hydrogen sulfite.

[0090] These dispersants may be used alone or in combination of two or more.

[0091] In order to make the dispersibility of the seed resin and / or monomer mixture during polymerization more stable, the dispersant used in the polymerization step preferably (i) contains a poorly water-soluble inorganic salt, and more preferably is a poorly water-soluble inorganic salt, more preferably contains tricalcium phosphate, and more preferably is tricalcium phosphate, and further preferably contains (ii) hydroxyapatite having the structural formula 3[Ca3(PO4)2·Ca(OH)2] of CAS number 1306-06-05, and is particularly preferably hydroxyapatite.

[0092] The form of the tertiary calcium phosphate is not particularly limited, but examples thereof include powder and slurry (for example, aqueous slurry).

[0093] In the droplet generation polymerization method, a surfactant (e.g., an anionic surfactant) may be further used. When a poorly water-soluble inorganic salt is used as a dispersant in the droplet generation polymerization method, it is particularly preferable to use a surfactant (e.g., an anionic surfactant) in combination with the dispersant.

[0094] Examples of anionic surfactants include sodium alkyldiphenyl ether sulfonate, sodium α-olefin sulfonate, sodium dodecylbenzene sulfonate, etc. These surfactants may be used alone or in combination of two or more.

[0095] The amounts of the dispersant and surfactant used in the droplet-forming polymerization method are not particularly limited.

[0096] In addition to the dispersant and surfactant, the aqueous medium may further contain other additives as desired.

[0097] In the dispersion step, the monomer mixture introduced into the nozzle box through the monomer mixture inlet is discharged from the outlet of the nozzle plate. At this time, the vibration unit of the vibrator applies regular vibrations to the droplet generating nozzle, so that the monomer mixture discharged from the outlet is dispersed in the aqueous medium as droplets. The aqueous medium containing the droplets is introduced from the droplet inlet through the droplet introduction pipe into a polymerization reactor containing the aqueous medium.

[0098] The particle size and particle size distribution (UT) of the seed resin particles can be controlled by adjusting the supply rate (flow rate) of the monomer mixture to the nozzle box of the droplet generating nozzle.

[0099] The particle size and particle size distribution (UT) of the seed resin particles can be controlled by adjusting the frequency of vibration applied to the droplet generating nozzle through which the monomer mixture passes. The frequency is, for example, preferably 200 Hz to 3000 Hz, more preferably 500 Hz to 2500 Hz, still more preferably 500 Hz to 2000 Hz, even more preferably 500 Hz to 1500 Hz, and particularly preferably 500 Hz to 1000 Hz.

[0100] As described above, the particle size and particle size distribution (UT) of the seed resin particles can be controlled by appropriately adjusting the diameter and number of the outlets of the droplet generating nozzle, the frequency of the vibration applied to the droplet generating nozzle, the flow rate (the supply rate of the monomer mixture), etc. The diameter and number of the outlets of the droplet generating nozzle, the frequency of the vibration applied to the droplet generating nozzle, and the flow rate (the supply rate of the monomer mixture) may each be adjusted appropriately depending on the viscosity of the monomer mixture.

[0101] In the dispersion step, the mixing ratio of the monomer mixture to the aqueous medium (weight of the monomer mixture / weight of the aqueous medium) is preferably 1.0 / 1.0 to 1.0 / 2.0.

[0102] In the dispersion step, an aqueous dispersant solution may be prepared separately from the monomer mixture and introduced into the droplet generating section. The aqueous dispersant solution is, for example, an aqueous solution obtained by mixing a dispersant such as a water-soluble polymer and a water-soluble inorganic salt with an aqueous medium (e.g., water).

[0103] The aqueous suspension in the polymerization reactor may be gently stirred. The stirring speed is preferably 200 rpm or less, more preferably 150 rpm or less, to prevent droplets from coalescing and additional dispersion. The lower limit of the stirring speed is not particularly limited, but is preferably 50 rpm or more, more preferably 60 rpm or more, to prevent droplets from coalescing.

[0104] (heating process) The temperature-raising step is a step of raising the temperature of the aqueous suspension to a predetermined polymerization temperature in order to initiate polymerization. In the temperature-raising step, the temperature of the aqueous suspension is raised to the polymerization temperature of the polymerization step described below. The temperature-raising rate is not particularly limited.

[0105] (Seed resin polymerization process) The seed resin polymerization step is a step of obtaining a seed resin (seed resin particles) by carrying out a polymerization reaction in an aqueous suspension at a predetermined polymerization temperature for a predetermined polymerization time. The polymerization reaction proceeds in the aqueous suspension by maintaining the aqueous suspension at the predetermined polymerization temperature for a predetermined polymerization time.

[0106] The polymerization temperature in the seed resin polymerization step is not particularly limited and may be appropriately set depending on the types and amounts of the monomer components, polymerization initiator, and other additives used, etc. The polymerization temperature in the seed resin polymerization step is, for example, preferably 60°C to 100°C, more preferably 70°C to 90°C.

[0107] The polymerization time of the seed resin polymerization step is the time required to reach a desired polymerization conversion rate. The polymerization time may be appropriately set depending on the types and amounts of the monomer components, polymerization initiator, and other additives used, and is not particularly limited. The polymerization time of the seed resin polymerization step is, for example, preferably 1 hour to 8 hours, more preferably 3 hours to 5 hours.

[0108] When the desired polymerization conversion rate is reached, the polymerization can be completed (the polymerization reaction can be terminated) by cooling the aqueous suspension in the polymerization reactor. Then, the seed resin particles can be recovered from the cooled aqueous suspension by solid-liquid separation or the like.

[0109] The present production method may include the droplet-forming polymerization method described above as a seed resin production step prior to the polymerization step. More specifically, the present production method may include the dispersion step, the temperature increase step, and the seed resin polymerization step described above prior to the polymerization step.

[0110] (1-3. Polymerization process) The polymerization step is a step of polymerizing a styrene-based monomer in an aqueous suspension containing water, a seed resin, a nucleating agent B, a polymerization initiator, and a styrene-based monomer.

[0111] In the polymerization step, the method for preparing an aqueous suspension containing water, a seed resin, a nucleating agent B, a polymerization initiator, and a styrene-based monomer is not particularly limited. Examples of methods for preparing an aqueous suspension include a method in which the polymerization initiator and the styrene-based monomer are added simultaneously or separately to a mixed solution containing water, a seed resin, and a nucleating agent B. It should be noted that a mixed solution containing water, a seed resin, and a nucleating agent B can also be called an "aqueous suspension."

[0112] The polymerization step may be, for example, a step of polymerizing the styrene-based monomer by adding a polymerization initiator and a styrene-based monomer to an aqueous suspension containing water, a seed resin, and a nucleating agent B. More specifically, the polymerization step may be the following step: (1) A step of simultaneously adding a polymerization initiator and a styrene-based monomer in a total amount or in a fixed amount over a fixed period of time to an aqueous suspension containing water, a seed resin, and a nucleating agent B, and maintaining the temperature of the aqueous suspension at a fixed temperature (e.g., the decomposition temperature of the polymerization initiator) or higher, thereby allowing the polymerization of the styrene-based monomer to proceed in the aqueous suspension in parallel with the addition of the polymerization initiator and the styrene-based monomer; (2) A step of simultaneously and completely adding either the styrene-based monomer alone or the polymerization initiator and the styrene-based monomer together to an aqueous suspension containing water, a seed resin, and a nucleating agent B and having a temperature below a certain temperature (for example, the decomposition temperature of the polymerization initiator). Then, by maintaining the temperature of the aqueous suspension at a temperature below the certain temperature (for example, the decomposition temperature of the polymerization initiator) for a certain period of time, the seed resin is impregnated with either the styrene-based monomer alone or the polymerization initiator and the styrene-based monomer together. Subsequently, a step of raising the temperature of the aqueous suspension to a temperature above the certain temperature (for example, the decomposition temperature of the polymerization initiator), and adding a polymerization initiator to the aqueous suspension as needed, thereby allowing the polymerization of the styrene-based monomer to proceed in the aqueous suspension; (3) To an aqueous suspension containing water, seed resin, and nucleating agent B and maintained at a temperature below a certain temperature (e.g., the decomposition temperature of the polymerization initiator), either the styrene-based monomer alone or the polymerization initiator and the styrene-based monomer are added simultaneously and in fixed amounts over a fixed period of time. This allows the seed resin to be impregnated with the styrene-based monomer alone or the polymerization initiator and the styrene-based monomer. Subsequently, the temperature of the aqueous suspension is raised to a certain temperature (e.g., the decomposition temperature of the polymerization initiator) or higher, and a polymerization initiator is added to the aqueous suspension as needed, thereby allowing the polymerization of the styrene-based monomer to proceed in the aqueous suspension.

[0113] The polymerization step may include, in order, (i) step 1 of preparing an aqueous suspension containing water, a seed resin, and a nucleating agent B, but not containing a polymerization initiator or a styrene-based monomer, and (ii) step 2 of adding a polymerization initiator and a styrene-based monomer to the obtained aqueous suspension to polymerize the styrene-based monomer.

[0114] <Container> The container used in this production method is not particularly limited. The container used in this production method is preferably a sealable container that is pressure-resistant and heat-resistant. When the container is pressure-resistant, the polymerization step and the blowing agent impregnation step described below can be carried out continuously in one container.

[0115] In order to efficiently disperse the seed resin, styrene-based monomer, nucleating agent B, polymerization initiator, obtained polystyrene-based resin particles, obtained expandable polystyrene-based resin particles, etc. in the aqueous suspension, the container is more preferably equipped with a stirrer. Suitable examples of the container include an autoclave equipped with a stirrer.

[0116] <Water> The water used in this production method is not particularly limited, and examples of the water used in this production method include pure water such as RO water (water purified by reverse osmosis membrane), deionized water (water purified by ion exchange resin), and distilled water.

[0117] The amount of water used in the polymerization step (particularly step 1), in other words, the content of water in the aqueous suspension, may be appropriately set depending on the amount of seed resin, and is not particularly limited. In order to stabilize the dispersibility of the seed resin and / or the monomer mixture during polymerization, the ratio of the amount of seed resin used to the amount of water used in the polymerization step (particularly step 1) (amount of seed resin used / amount of water used) is preferably 1.0 / 0.6 to 1.0 / 3.0.

[0118] <Seed resin> The seed resin itself has already been explained in the above section (1-1. Seed Resin), so the explanation therefor is incorporated by reference and will not be repeated here.

[0119] The amount of seed resin used in this production method, in other words, the content of seed resin in the aqueous suspension, is not particularly limited. The amount of seed resin used in this production method is preferably 5 to 60 parts by weight, more preferably 5 to 40 parts by weight, even more preferably 5 to 30 parts by weight, and particularly preferably 10 to 20 parts by weight, relative to 100 parts by weight of the total amount of seed resin used and styrene-based monomer used in the polymerization step. When the amount of seed resin used in this production method is (a) 5 parts by weight or more relative to 100 parts by weight of the total amount of seed resin used and styrene-based monomer used, the proportion of styrene-based monomer used in the polymerization step that polymerizes with the seed resin within the seed resin without polymerizing with itself tends to increase. When the amount of seed resin used is (b) 60 parts by weight or less, it becomes possible to polymerize a larger amount of styrene-based monomer in one production run, which is economically advantageous.

[0120] <Nucleating agent B> In this production method, the nucleating agent B used in the polymerization step, separate from the seed resin, is not particularly limited as long as it is different from the nucleating agent A contained in the seed resin. The compounds listed in the above section on <Nucleating agent A> can be used as nucleating agent B. For example, the compounds that can be suitably used as nucleating agent A can also be suitably used as nucleating agent B. Therefore, the description in the above section on <Nucleating agent A> is incorporated by reference for specific examples of nucleating agent B, and a detailed description will be omitted here. As nucleating agent B, one of the compounds listed in the above section on <Nucleating agent A> may be used alone, or two or more of them may be used in combination.

[0121] Nucleating agent B preferably contains one or more selected from the group consisting of amides, olefin waxes, polystyrene resins, fatty acid esters, and hydrogenated vegetable oils, more preferably one or more selected from this group; (ii) more preferably contains one or more selected from the group consisting of amides, olefin waxes, fatty acid esters, and hydrogenated vegetable oils, more preferably one or more selected from this group; (iii) more preferably contains one or more selected from the group consisting of amides, olefin waxes, and fatty acid esters, more preferably one or more selected from this group; (iv) more preferably contains amides and / or olefin waxes, more preferably amides and / or olefin waxes; (v) even more preferably contains fatty acid amides and / or olefin waxes, even more preferably fatty acid amides and / or olefin waxes; and (vi) particularly preferably contains ethylene bisstearamide and / or polyethylene wax, most preferably ethylene bisstearamide and / or polyethylene wax. This configuration has the advantage that it is possible to provide expanded resin beads and foamed molded articles with excellent cell uniformity, whether the expandable resin beads are immediately after production or are expanded resin beads produced long periods of time ago.

[0122] Commercially available polyethylene waxes can also be used as nucleating agent B. Commercially available polyethylene waxes that can be used as nucleating agent B include, for example, (i) PEW-S manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd., and (ii) Polywax 1000-80M manufactured by Toyo Adre Co., Ltd.

[0123] The number-average molecular weight of the polyethylene wax used as nucleating agent B is preferably 300 g / mol to 1000 g / mol. A number-average molecular weight of 300 g / mol or more has the advantage of easily achieving the nucleating effect of the polyethylene wax. A polyethylene wax having a number-average molecular weight of 1000 g / mol or less is easily soluble in the styrene-based monomer added in the polymerization process. Therefore, a number-average molecular weight of 1000 g / mol or less has the advantage of allowing the polyethylene wax to be (almost) uniformly impregnated into the seed resin. When a commercially available polyethylene wax is used as nucleating agent B, information on the number-average molecular weight provided by the manufacturer of the commercially available polyethylene wax (e.g., a data sheet, etc.) can be used as the number-average molecular weight of the polyethylene wax.

[0124] The volume average particle diameter of the polyethylene wax used as nucleating agent B is preferably less than 300 μm, more preferably less than 200 μm, even more preferably less than 100 μm, even more preferably less than 50 μm, and particularly preferably 20 μm or less. Polyethylene waxes having a volume average particle diameter within the above-mentioned range are easily soluble in the styrene-based monomer added in the polymerization process. Therefore, when the volume average particle diameter of the polyethylene wax is within the above-mentioned range, it has the advantage of being able to (almost) uniformly impregnate the seed resin with the polyethylene wax. The lower limit of the volume average particle diameter of the polyethylene wax is not particularly limited, but is, for example, 1 μm or more. The method for measuring the volume average particle diameter of the polyethylene wax will be described in detail in the Examples below. When a commercially available polyethylene wax is used as nucleating agent B, information regarding the volume average particle diameter provided by the manufacturer of the commercially available polyethylene wax (e.g., a data sheet, etc.) can be used as the volume average particle diameter of the polyethylene wax.

[0125] There are no particular limitations on the content of one or more selected from the group consisting of amides, olefin waxes, polystyrene resins, fatty acid esters, and hydrogenated vegetable oils in nucleating agent B. The content of one or more selected from the group consisting of amides, olefin waxes, polystyrene resins, fatty acid esters, and hydrogenated vegetable oils in nucleating agent B (the total content when two or more types are included) is preferably 60 parts by weight or more, more preferably 70 parts by weight or more, more preferably 80 parts by weight or more, even more preferably 90 parts by weight or more, particularly preferably 95 parts by weight or more, and may be 100 parts by weight, per 100 parts by weight of nucleating agent B.

[0126] The amount of nucleating agent B used in this production method is not particularly limited. The amount of nucleating agent B used in this production method is preferably 0.05 to 0.50 parts by weight, more preferably 0.07 to 0.45 parts by weight, even more preferably 0.11 to 0.40 parts by weight, and particularly preferably 0.15 to 0.30 parts by weight, relative to 100 parts by weight of the total amount of the seed resin and the styrene-based monomer. This configuration has the advantage of being able to provide expanded beads and foamed molded articles with excellent cell uniformity, whether the expandable resin beads are immediately after production or have been produced for a long period of time.

[0127] The amount of nucleating agent B used in this production method is preferably the same as or greater than the content of nucleating agent A in the seed resin. This configuration has the advantage of providing expanded beads and foamed molded articles with superior cell uniformity, whether the expandable resin beads are produced immediately after production or after a long period of time has passed since production. The ratio of the content of nucleating agent A in the seed resin to the amount of nucleating agent B used in this production method (content of nucleating agent A in the seed resin / amount of nucleating agent B used in this production method) is preferably 0.1 to 1.0, more preferably 0.2 to 0.9, even more preferably 0.25 to 0.8, and particularly preferably 0.3 to 0.7. This configuration has the advantage of providing expanded beads and foamed molded articles with superior cell uniformity, whether the expandable resin beads are produced immediately after production or after a long period of time has passed since production.

[0128] The present inventors independently obtained the following novel findings during the course of intensive research: It has been discovered that by selecting an acrylic resin rather than ethylene bisstearic acid amide and polyethylene wax as the nucleating agent A, and selecting ethylene bisstearic acid amide and / or polyethylene wax rather than an acrylic resin as the nucleating agent B, it is surprisingly possible to provide expandable polystyrene-based resin particles that are particularly excellent in particle size uniformity and that can provide polystyrene-based foamed molded articles in which the generation of cell spots is particularly suppressed even some time has passed since production. That is, an embodiment in which an acrylic resin is selected as nucleating agent A and ethylene bisstearic acid amide and / or polyethylene wax is selected as nucleating agent B is a particularly preferred embodiment of the present invention.

[0129] <Dispersant> In the polymerization step (particularly step 1), it is preferable to further use a dispersant to improve the dispersibility in water of the seed resin, the obtained polystyrene-based resin particles, and the obtained expandable polystyrene-based resin particles in the aqueous suspension.

[0130] As the dispersant used in the polymerization step (particularly step 1), the compounds listed as dispersants in the <Dispersion step> section of the <Droplet generation polymerization method> section can be used. Therefore, for specific examples of dispersants used in the polymerization step, the description in the <Dispersion step> section is incorporated herein by reference, and a detailed description will be omitted here. In the polymerization step, the compounds listed in the <Dispersion step> section may be used alone or in combination of two or more.

[0131] The amount of dispersant used in the polymerization step (particularly step 1) may be appropriately determined depending on the types and amounts of water, seed resin, and styrene-based monomer used, and is not particularly limited.

[0132] <Surfactant> A surfactant (e.g., an anionic surfactant) may be further used in the polymerization step (particularly, step 1). When an inorganic dispersant (e.g., a poorly water-soluble inorganic salt) is used as the dispersant in the polymerization step, it is particularly preferable to use a surfactant (e.g., an anionic surfactant) in combination with the dispersant.

[0133] Examples of anionic surfactants include sodium alkyldiphenyl ether sulfonate, sodium α-olefin sulfonate, and sodium dodecylbenzene sulfonate.

[0134] The amount of surfactant used in the polymerization step (particularly step 1) may be appropriately determined depending on the types and amounts of water, seed resin, styrene monomer, and dispersant used, and is not particularly limited.

[0135] <Other additives> In the polymerization step (particularly step 1), in addition to water, seed resin, nucleating agent B, dispersant, and surfactant, any additives (also referred to as "other additives") may be further used within a range that does not impair the desired physical properties of the resulting expandable polystyrene-based resin particles.

[0136] Examples of other additives include fillers, plasticizers, cell regulators, flame retardants, flame retardant assistants, lubricants, colorants, and ultraviolet absorbers. Known substances may be used as these other additives in any amount.

[0137] <Styrene-based monomers> The styrene-based monomer used in the polymerization step (particularly step 2) is the same as the styrene-based monomer listed in the section <Base resin> above, so the description therein is incorporated by reference and a detailed description is omitted here.

[0138] The amount of styrene-based monomer used in this production method may be appropriately determined depending on the type and amount of seed resin used, and is not particularly limited. The amount of styrene-based monomer used in this production method is preferably 40 to 95 parts by weight, more preferably 60 to 95 parts by weight, even more preferably 70 to 95 parts by weight, and particularly preferably 80 to 90 parts by weight, relative to 100 parts by weight of the total amount of seed resin used and styrene-based monomer used. When the amount of styrene-based monomer used in this production method is (a) 40 parts by weight or more relative to 100 parts by weight of the total amount of seed resin used and styrene-based monomer used, it becomes possible to polymerize a larger amount of styrene-based monomer in one production run, which is economically advantageous. When the amount is (b) 95 parts by weight or less, the styrene-based monomer used in the polymerization step tends to increase in proportion to the seed resin polymerized without polymerizing with itself.

[0139] <Polymerization initiator> As the polymerization initiator used in the polymerization step (particularly step 2), the compounds listed in the <Dispersion step> section of the <Droplet generation polymerization method> section can be used. Therefore, for specific examples of the polymerization initiator used in the polymerization step, the description in the <Dispersion step> section is incorporated herein by reference, and a description thereof will be omitted here. In the polymerization step, the compounds listed in the <Dispersion step> section may be used alone or in combination of two or more.

[0140] The polymerization initiator used in the polymerization step preferably contains one or more selected from the group consisting of benzoyl peroxide, di-t-butylperoxyhexahydroterephthalate, lauroyl peroxide, t-butylperoxy-2-ethylhexyl carbonate, and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, and more preferably is one or more selected from the group consisting of benzoyl peroxide, di-t-butylperoxyhexahydroterephthalate, t-butylperoxy-2-ethylhexyl carbonate, and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, because of its excellent reactivity. (iii) more preferably includes one or more selected from the group consisting of benzoyl peroxide, t-butylperoxy-2-ethylhexyl carbonate, and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, more preferably includes one or more selected from the group consisting of benzoyl peroxide, t-butylperoxy-2-ethylhexyl carbonate, and 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, and more preferably includes one or more selected from the group; (iv) further preferably includes benzoyl peroxide and / or t-butylperoxy-2-ethylhexyl carbonate, and particularly preferably includes benzoyl peroxide and / or t-butylperoxy-2-ethylhexyl carbonate.

[0141] The amount of the polymerization initiator used in the present production method is not particularly limited and may be appropriately set depending on the types and amounts of the seed resin and styrene-based monomer used, etc. Due to its excellent reactivity, the amount of the polymerization initiator used in the present production method is preferably 0.050 to 5,000 parts by weight, more preferably 0.050 to 3,000 parts by weight, more preferably 0.050 to 1,000 parts by weight, and even more preferably 0.100 to 0.500 parts by weight, relative to 100 parts by weight of the total amount of the seed resin and the styrene-based monomer used.

[0142] The polymerization step (particularly step 2) is preferably carried out in at least two stages by changing the polymerization temperature. Of the two polymerization steps with different polymerization temperatures, the step carried out first may be referred to as the first polymerization step, and the step carried out later may be referred to as the second polymerization step. In other words, it can be said that the polymerization step (particularly step 2) preferably includes a first polymerization step and a second polymerization step that are successively carried out at different polymerization temperatures. This configuration has the advantage of being able to provide expandable polystyrene-based resin particles with a low content (emission amount) of volatile organic compounds (hereinafter sometimes abbreviated as "VOCs," an acronym for Volatile Organic Compounds), such as unreacted styrene-based monomers.

[0143] <First polymerization step> In the first polymerization step, it is preferable to use a polymerization initiator having a 10-hour half-life temperature of 74° C. or higher and lower than 90° C. (hereinafter also referred to as polymerization initiator (X)).

[0144] Examples of polymerization initiators (X) having a 10-hour half-life temperature of 74°C or higher but lower than 90°C include (a) organic peroxides such as benzoyl peroxide (also known as dibenzoyl peroxide), lauroyl peroxide, ditoluyl peroxide, toluylbenzoyl peroxide, di-t-butylperoxyhexahydroterephthalate, and t-butyl perpivalate, and (b) azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile. Benzoyl peroxide is particularly preferred as the polymerization initiator (X) due to its excellent reactivity. These polymerization initiators (X) may be used alone or in combination of two or more. The polymerization initiator (X) is suitable for use in the first polymerization step.

[0145] The polymerization temperature in the first polymerization step is not particularly limited. The polymerization temperature in the first polymerization step is preferably 85°C to 95°C, more preferably 87°C to 93°C, even more preferably 88°C to 92°C, and particularly preferably 89°C to 91°C. This configuration allows the amount of decomposition of the polymerization initiator (X) to be adjusted within an optimal range. This (a) makes it easy to adjust the polymerization rate (reaction rate), thereby improving polymerization stability, and (b) makes it easy to obtain expandable polystyrene-based resin particles in an appropriate molecular weight range.

[0146] The polymerization time of the first polymerization step is not particularly limited. The polymerization time of the first polymerization step is preferably 4 to 9 hours, more preferably 5 to 8 hours, and particularly preferably 6 to 7 hours. This configuration has the advantage of being able to achieve both productivity and polymerization stability.

[0147] <Second polymerization step> The second polymerization step is preferably carried out consecutively to the first polymerization at any time point after the polymerization conversion rate reaches 90%.

[0148] The polymerization temperature of the second polymerization step is not particularly limited as long as it is different from the polymerization temperature of the first polymerization step. The polymerization temperature of the second polymerization step is preferably 110 to 120°C, more preferably 110 to 119°C, more preferably 110 to 118°C, more preferably 111 to 117°C, even more preferably 112 to 116°C, and particularly preferably 113 to 115°C. When the polymerization temperature of the second polymerization step is (a) 110°C or higher, the VOC content (particularly the styrene content) in the resulting expandable polystyrene resin particles can be reduced. When the polymerization temperature is (b) 120°C or lower, the internal pressure of the polymerization machine used in the polymerization step does not become too high, and high pressure resistance is not required, thereby eliminating the need for a heavy-duty polymerization machine and reducing production costs. In other words, when the polymerization temperature of the second polymerization step is within the above-mentioned range, there is the advantage that VOCs can be efficiently reduced at or below the upper limit of the internal pressure of a typical polymerization machine. The polymerization temperature of the second polymerization step is preferably higher than that of the first polymerization step. According to this configuration, the VOC content in the resulting expandable polystyrene resin particles can be further reduced.

[0149] The polymerization time of the second polymerization step is not particularly limited. The polymerization time of the second polymerization step is preferably 0.5 to 4 hours, more preferably 0.7 to 3 hours, and particularly preferably 0.9 to 2 hours. If the polymerization time of the second polymerization step is within the above-mentioned preferred range, there is an advantage that the VOC content in the expandable polystyrene-based resin particles can be reduced.

[0150] In the second polymerization step, it is preferable to use a polymerization initiator having a 10-hour half-life temperature of 90° C. or more and 100° C. or less (hereinafter also referred to as polymerization initiator (Y)).

[0151] Examples of polymerization initiators (Y) having a 10-hour half-life temperature of 90°C or higher and 100°C or lower include t-butylperoxyisopropyl monocarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, t-amylperoxyisopropyl monocarbonate, t-amylperoxy-2-ethylhexyl monocarbonate, 1,1-bis(t-butylperoxy)cyclohexane, and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. These polymerization initiators (Y) may be used alone or in combination of two or more. The polymerization initiator (Y) is suitable for use in the second polymerization step.

[0152] 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane cleaves in two stages. The 10-hour half-life temperature at which 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane before cleavage undergoes the first stage of cleavage is 86°C. The 10-hour half-life temperature at which the intermediate product produced after the first stage of cleavage undergoes the second stage of cleavage is higher than the 10-hour half-life temperature of the first stage, at approximately 94°C. Regarding 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, the final product produced after the second stage of cleavage primarily acts in the second polymerization step. Therefore, in this specification, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane is considered to be polymerization initiator (Y) rather than polymerization initiator (X).

[0153] As the polymerization initiator (Y), t-butylperoxy-2-ethylhexyl monocarbonate and t-amylperoxy-2-ethylhexyl monocarbonate are particularly preferred from the viewpoint of reducing the amount of residual monomer in the expandable resin particles.

[0154] The second polymerization step may be carried out in combination with the blowing agent impregnation step described below, that is, it may be carried out in the presence of a blowing agent.

[0155] <Crosslinking agent> A crosslinking agent may be further used in the polymerization step (particularly step 2). When a crosslinking agent is used in the polymerization step, there is an advantage that the resulting expandable resin particles and expanded particles are less susceptible to heat damage by steam during expansion and molding.

[0156] Crosslinking agents include divinylbenzene (DVB), m-diisopropenylbenzene, and ethylene glycol diacrylate.

[0157] The amount of crosslinking agent used in the polymerization step (particularly step 2) is not particularly limited and may be appropriately determined depending on the types and amounts of water, seed resin, styrene-based monomer, and polymerization initiator. The crosslinking agent is preferably used in step 2 of the polymerization step, and more preferably in the latter half of the first polymerization step.

[0158] <Chain transfer agent> A chain transfer agent (e.g., a mercaptan-based compound) may be further used in the polymerization step (particularly, step 2). When a chain transfer agent is used in the polymerization step, the weight average molecular weight of the resulting expandable polystyrene-based resin particles can be adjusted to fall within a desired range.

[0159] Examples of mercaptan compounds used as chain transfer agents include n-octyl mercaptan, n-dodecyl mercaptan, and t-dodecyl mercaptan.

[0160] The amount of chain transfer agent used in the polymerization step (particularly step 2) may be appropriately determined depending on the types and amounts of water, seed resin, styrene-based monomer, and polymerization initiator used, and is not particularly limited.

[0161] <Polymerization modifier> A polymerization regulator may also be used in the polymerization step (particularly step 2). When a polymerization regulator is used in the polymerization step, the polymerization rate can be adjusted within a desired range.

[0162] Examples of polymerization modifiers include α-methylstyrene dimer, which is commonly used in the polymerization of acrylonitrile / styrene copolymers. α-Methylstyrene dimer can also contribute to adjusting the weight-average molecular weight of the resulting expandable polystyrene resin particles. In other words, α-methylstyrene dimer can also be considered a chain transfer agent.

[0163] The amount of polymerization regulator used in the polymerization step (particularly step 2) may be appropriately determined depending on the types and amounts of water, seed resin, styrene-based monomer, polymerization initiator, and chain transfer agent, and is not particularly limited.

[0164] (1-4. Foaming agent impregnation process) In addition to the polymerization step, the present production method preferably further includes, after the polymerization step, a blowing agent impregnation step of impregnating the polystyrene-based resin particles obtained in the polymerization step with a blowing agent.

[0165] The blowing agent used in the blowing agent impregnation step is preferably a volatile blowing agent. Examples of volatile blowing agents include propane, isobutane, normal butane, isopentane, normal pentane, and neopentane. Fluorinated hydrocarbons with an ozone depletion potential of zero, such as difluoroethane and tetrafluoroethane, can also be used as the blowing agent. One of these compounds may be used alone, or two or more may be used in combination. It is more preferred that the blowing agent be (i) a mixture of isobutane and normal butane, and / or (ii) a mixture of isopentane and normal pentane, optionally with neopentane, etc.

[0166] The amount of the foaming agent used in the present production method is preferably 4.0 parts by weight or more and 10.0 parts by weight or less, more preferably 5.0 parts by weight or more and 9.0 parts by weight or less, and even more preferably 6.0 parts by weight or more and 8.0 parts by weight or less, relative to 100 parts by weight of the total amount of the seed resin used and the styrene-based monomer used.

[0167] In the foaming agent impregnation step, an organic solvent (solvent) may be further used to adjust the foaming rate. Examples of the organic solvent include cyclohexane and aromatic hydrocarbons (e.g., toluene, xylene, and ethylbenzene).

[0168] The temperature in the blowing agent impregnation step is preferably 110° C. or higher, since this allows the blowing agent to be efficiently impregnated into the polystyrene resin particles.

[0169] [2. Expandable polystyrene resin particles] One embodiment of the present invention also provides expandable polystyrene-based resin particles obtained by this production method.

[0170] The expandable polystyrene-based resin particles according to one embodiment of the present invention are produced by the present production method, and therefore have the advantages of being able to provide a polystyrene-based foamed molded article that (i) has excellent uniformity of particle size and (ii) suppresses the occurrence of cell spots even after a period of time has passed since production.

[0171] [3. Polystyrene foam particles] One embodiment of the present invention also provides expanded polystyrene resin particles obtained by expanding the expandable polystyrene resin particles according to one embodiment of the present invention described in the section [2. Expandable polystyrene resin particles] above.

[0172] The polystyrene-based expanded beads according to one embodiment of the present invention have the above-described configuration, and therefore have the advantages of being able to provide a polystyrene-based expanded molded article that (i) has excellent particle size uniformity and (ii) suppresses the occurrence of cell spots even after a period of time has passed since production.

[0173] The method for expanding expandable polystyrene-based resin particles, i.e., the method for producing expanded polystyrene-based beads according to one embodiment of the present invention, is not particularly limited, and known methods can be used. Examples of the expansion method include a method in which the following steps (1) to (3) are carried out in sequence: (1) expanding expandable polystyrene-based resin particles are placed in a container equipped with a stirrer, (2) the expandable polystyrene-based resin particles are heated with a heat source such as steam, and (3) expansion is continued until a desired expansion ratio is reached, thereby obtaining expanded polystyrene-based beads. Expanded polystyrene-based beads are sometimes referred to as pre-expanded polystyrene-based resin particles, and therefore the expansion method for obtaining pre-expanded polystyrene-based resin particles is sometimes referred to as a pre-expansion method.

[0174] [4. Polystyrene foam molding] One embodiment of the present invention also provides a polystyrene-based foamed molded article obtained by molding (e.g., in-mold foam molding) the polystyrene-based foamed beads according to one embodiment of the present invention described in the section [3. Polystyrene-based foamed beads] above.

[0175] The polystyrene foam molded article according to one embodiment of the present invention has the above-described structure, and therefore has the advantage that the occurrence of cell spots is suppressed, that is, there are no cell spots at all, or only a small number of cell spots.

[0176] The method for molding polystyrene-based expanded beads, i.e., the method for producing a polystyrene-based expanded molded article according to one embodiment of the present invention, is not particularly limited, and known methods can be used. For example, the molding method involves filling a mold that can be closed but cannot be sealed with expanded beads, and then heating and fusing the expanded beads with steam to produce an expanded molded article. Another molding method involves filling a closed mold having a desired shape and having a large number of small holes drilled in the wall, injecting a heating medium such as steam through the small holes in the mold to heat the expanded beads to a temperature above the softening point of the expanded beads, fusing them together, and then cooling the expanded beads before removing them from the mold to produce a polystyrene-based expanded molded article of the desired shape. [Example]

[0177] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following examples, comparative examples, tables, etc., "parts" and "%" are by weight (parts by weight and % by weight) unless otherwise specified.

[0178] [Method of manufacturing polystyrene foam beads] The expandable polystyrene resin particles were placed in a pressurized pre-expansion machine (BHP-110, manufactured by Daikai Kogyo Co., Ltd.) and heated using steam as a heating medium at a blown-in steam pressure of 0.1 MPa (gauge pressure). By this operation, the expandable polystyrene resin particles were expanded to an expansion ratio (bulk ratio) of 60 times, to obtain expanded polystyrene particles.

[0179] [Method of manufacturing polystyrene foam molded products] The produced polystyrene foam beads were left at room temperature for 24 hours, and then used to produce a polystyrene foam molded article. A mold measuring 450 mm long x 300 mm wide x 25 mm deep and a molding machine (KR-57, manufactured by Daisen Co., Ltd.) were used to produce a polystyrene foam molded article according to the following procedure: (1) Polystyrene foam particles were filled into a mold; (2) In-mold foam molding was performed using steam as the heating medium, with the steam injection time set to 22 seconds and the steam pressure set to 0.085 MPa (gauge pressure); (3) The mold was water-cooled for 5 seconds, and then vacuum-cooled for 100 seconds; (4) The product was removed from the mold to obtain a polystyrene foam molded article.

[0180] [Measurement and evaluation methods] (Method for measuring weight-average molecular weight of acrylic resin) The weight-average molecular weight of the acrylic resin (methyl methacrylate / butyl acrylate copolymer) used as nucleating agent A in the following examples was measured by the following method. First, the acrylic resin to be measured was dissolved in THF, and the resulting solution was used as a sample. The resulting sample was subjected to gel permeation chromatography (Tosoh Corporation, HLC-8320GPC) to measure the weight-average molecular weight. The measurement conditions were as follows: Sample solution: 0.02g / THF 20ml Measurement temperature: 40℃, Detector: RI, Injection volume; 10μl, Column: TSKgel superHZM-H, Mobile phase; THF. Separately, polystyrene with a known weight-average molecular weight was subjected to gel permeation chromatography under the same conditions as the sample, and a calibration curve was prepared. Using the resulting calibration curve, the weight-average molecular weight of the acrylic resin was calculated in terms of polystyrene.

[0181] (Method for evaluating particle size uniformity of expandable polystyrene resin particles) The particle size of the expandable polystyrene resin particles was measured on a volume basis using an image processing Millitrac JPA particle size analyzer. The results were displayed as a cumulative distribution, and a particle size distribution table was created. Using the resulting distribution table, the particle size distribution (UT) of the expandable polystyrene resin particles was calculated according to the following formula: Particle size distribution (UT)=D90 / D40+D60 / D10. Here, D90, D60, D40 and D10 refer to particle sizes at which the cumulative volume percentages in the distribution table are 90%, 60%, 40% and 10%, respectively.

[0182] Next, the obtained UT was used to evaluate the particle size uniformity of the expandable polystyrene resin particles based on the following index. The smaller the UT (the larger the evaluation number), the better the uniformity, and a rating of "1" or higher was considered to be acceptable: 2: UT is 2.10 or less; 1: UT is greater than 2.10 and less than or equal to 2.15; 0:UT exceeds 2.15.

[0183] (Method for evaluating bulk ratio of expanded polystyrene beads) 10g of polystyrene foam particles are placed on a 1000cm 3 and calculate the volume (cm) of 10 g of polystyrene foam particles from the measuring cylinder. 3 The bulk ratio (cm) was measured using the following formula: 3 / g) was calculated: Bulk magnification (cm 3 / g) = Volume of polystyrene foam particles (cm 3 ) / 10g.

[0184] In this specification, the bulk ratio of the expanded beads can also be called the expansion ratio. The unit of the bulk ratio is actually "cm" based on the above formula. 3 / g", but in this specification, for convenience, the unit of bulk magnification may also be expressed as "times".

[0185] (Method for evaluating cell spots in polystyrene foam beads) A polystyrene foam molded article was cut into a size of 80 mm × 80 mm × 25 mm using a vertical slicer, and then a sample of 80 mm × 80 mm × 0.1 mm was prepared using a bread slicer. A photograph of the 80 mm × 80 mm × 0.1 mm cross section (80 mm × 80 mm) was taken. The total number of expanded beads contained in the 80 mm × 80 mm cross section of the obtained photograph was counted. The expanded beads constituting the foam molded articles obtained in the examples mainly contained cell diameters of 30 μm to 70 μm (centered at 50 μm, ±20 μm). Therefore, the number of expanded beads having cells with a cell diameter of less than 30 μm or more than 70 μm (hereinafter also referred to as "expanded beads having cells with cell diameters outside this range") was counted among the expanded beads contained in the 80 mm × 80 mm cross section of the obtained photograph. The cell diameter (average chord length) on the cross section was measured by cutting out cells at each location and observing the cells under SEM. Next, the number of expanded beads having cells with a diameter outside the range was divided by the total number of expanded beads, and the resulting quotient was multiplied by 100 to calculate the percentage of expanded beads having cells with a diameter outside the range. Using the obtained percentage, cell unevenness was evaluated according to the following criteria: 1 (good): percentage less than 10%; 0 (bad): The rate is 10% or more.

[0186] (Method for evaluating the surface properties of polystyrene foam molded articles) The surface condition of the polystyrene foam molded article was visually observed. Then, the surface properties were evaluated according to the following criteria: 1 (Good): There is no or only a small amount of melting on the surface and no gaps between the polystyrene foam particles, i.e., the surface is free of mottled patterns; 0 (poor): The surface is melted and there are large gaps between the polystyrene foam particles, that is, the surface has a mottled pattern.

[0187] (Method for evaluating the fusion properties of polystyrene foam molded products) The fusion property of the polystyrene foam molded article was evaluated based on the fusion rate of the foam molded article. The fusion rate of the polystyrene foam molded article was calculated according to the following procedure: (1) The polystyrene foam molding broke; (2) The fracture surface was observed, and the number of all polystyrene-based expanded particles present in the observation field and the number of polystyrene-based expanded particles that were fractured within the polystyrene-based expanded particle itself, rather than at the interface with adjacent polystyrene-based expanded particles, were counted; (3) Using the obtained results, the fusion rate was calculated based on the following formula: Fusion rate (%) = ((number of polystyrene foam particles that are broken not at the interface with adjacent polystyrene foam particles but within the polystyrene foam particles themselves) / total number of polystyrene foam particles in the observation field) × 100.

[0188] Next, the obtained fusion rate was used to evaluate the fusion property of the polystyrene foam molded article based on the following criteria. The higher the fusion rate (the higher the evaluation number), the better the fusion property, and a rating of "1" or higher was considered to be acceptable: 3: fusion rate is 90% or more; 2: Fusion rate is 85% or more but less than 90%; 1: Fusion rate is 70% or more but less than 85%; 0: Fusion rate is less than 70%.

[0189] (Manufacturing example) (Production of acrylic resin (methyl methacrylate / butyl acrylate copolymer)) 0.5 parts of sodium dioctyl succinate and 0.0008 parts of ferrous sulfate (FeSO4·7H2O), previously dissolved in water, along with 0.0032 parts of ethylenediaminetetraacetic acid disodium salt and 0.06 parts of sodium formaldehyde sulfoxylate, were placed in a reactor equipped with a stirrer, and water was added to bring the total volume to 200 parts. The gas in the reactor was then replaced with nitrogen to remove oxygen from the air space and water. The contents were then stirred and heated to 60°C while a mixture of 80 parts of methyl methacrylate and 0.05 parts of t-butyl hydroperoxide was continuously added to the reactor over 170 minutes to polymerize the contents. At 60 and 120 minutes into the mixture addition, 0.2 parts of sodium dodecylbenzenesulfonate were added to the reactor. Furthermore, a mixture of 8 parts of methyl methacrylate, 12 parts of butyl acrylate, and 0.05 parts of t-butyl hydroperoxide (copolymerization components) was continuously added to the reactor over 50 minutes. After the addition of the copolymerization components was completed, the contents were kept at 60°C and the contents were stirred for more than 1 hour to complete the polymerization. The contents were then cooled to obtain an acrylic resin latex.

[0190] To the obtained acrylic resin latex, 4 parts of a 1% (wt / wt%) aqueous calcium chloride solution (25°C) was added to coagulate the acrylic resin. The obtained latex was then heat-treated, dehydrated, washed, and dried to obtain acrylic resin powder. The weight-average molecular weight of the acrylic resin was measured using the method described above and found to be 900,000.

[0191] Example 1 <Production of seed resin> The seed resin was produced by droplet generation polymerization. A 6-liter reactor equipped with a flat-plate agitator and a droplet inlet at the bottom was used as the polymerization reactor. 1.5 liters of an aqueous dispersion medium (aqueous medium) containing 11,000 ppm of tricalcium phosphate as a dispersant and 110 ppm of sodium dodecylbenzenesulfonate as a surfactant was added to the polymerization reactor, and stirring of the aqueous dispersion medium was initiated.

[0192] Next, 5000 g of styrene monomer, 9.7 g of 1,1-di(t-butylperoxy)cyclohexane and 85 g of t-butylperoxy-2-ethylhexanoate as polymerization initiators, and 50 g of liquid paraffin as plasticizer are placed in container 1, which has a supply line to a droplet generating device, and 1000 g of styrene monomer and 1.4 parts by weight of methyl methacrylate / butyl acrylate copolymer (Kane Ace (registered trademark) PA-20 manufactured by Kanegafuchi Chemical Industry Co., Ltd.) as nucleating agent A are placed in container 2. Styrene monomer from container 1 and container 2 was combined in a pipe. The styrene monomer was adjusted to the amount of nucleating agent A shown in Table 1 (in Example 1, container 1: container 2 = 20:1). The mixture was fed at a rate of 90 cc / min to a droplet generator equipped with a plate containing 45 0.17 mm diameter holes. Mechanical vibration at 800 Hz was applied to generate monomer droplets in the aqueous dispersion medium, which was then transferred to the 6-liter reactor. In addition, a separate container from the monomer mixture, equipped with a supply line to the droplet generator, was charged with an aqueous dispersant consisting of 530 g of 3% polyvinyl alcohol and 20 g of sodium nitrite dissolved in 5000 g of pure water. This aqueous dispersant was then transferred to the droplet generator at a rate of 80 cc / min. After 1500 g of the monomer solution had been introduced into the 6-liter reactor, droplet generation was stopped, and the dispersion in the reactor was heated to 90 °C and polymerized for 3 hours. The dispersion was further heated to 120°C and maintained at this temperature for 1 hour to complete the polymerization. The slurry in the reactor was cooled, dehydrated, and dried to obtain polymer particles. The obtained polymer particles were used as a seed resin in the production of expandable polystyrene-based resin particles described below. Styrene was the only monomer used in the production of the seed resin (polymer particles). In other words, the seed resin contained a styrene homopolymer as the base resin. The average particle diameter of the seed resin (polymer particles) was measured by the method described above and was found to be 0.43 mm.

[0193] <Production of Expandable Polystyrene Resin Particles> (Polymerization process) An aqueous suspension was prepared by charging a 6 L autoclave equipped with a stirrer with 92 parts by weight of pure water, 0.38 parts by weight of tribasic calcium phosphate as a dispersant, 0.0104 parts by weight of sodium α-olefin sulfonate as a surfactant, 0.1 parts by weight of sodium chloride as a water-soluble inorganic salt, 15 parts by weight of a seed resin, and 0.45 parts by weight of ethylene bisstearic acid amide as a nucleating agent B. Stirring of the prepared aqueous suspension was then commenced.

[0194] Subsequently, the temperature inside the autoclave was raised to 92° C. Thereafter, (a) 0.256 parts by weight of benzoyl peroxide as a polymerization initiator was added to the autoclave over 4 hours and 50 minutes, and (b) 85 parts by weight of styrene monomer as a styrene-based monomer was added to the autoclave over 5 hours and 30 minutes, thereby polymerizing the styrene monomer.

[0195] Furthermore, 4 hours and 45 minutes after the start of polymerization, 0.13 parts by weight of t-butylperoxy-2-ethylhexyl carbonate was added to the autoclave, and 5 hours and 23 minutes after the start of polymerization, 0.012 parts by weight of divinylbenzene (DVB) as a crosslinking agent was added to the autoclave. The temperature inside the autoclave was then maintained at 92°C for 30 minutes. The temperature inside the autoclave was then raised to 120°C and maintained at 120°C for 1 hour. Through these operations, polystyrene-based resin particles were obtained.

[0196] Thereafter, the temperature inside the autoclave was cooled to 95°C, and 7.8 parts by weight of butane (normal-rich butane (normal butane / isobutane = 70 / 30)) and 2.0 parts by weight of cyclohexane as a plasticizer were added to the autoclave. Subsequently, the temperature inside the autoclave was raised to 120°C, and the temperature inside the autoclave was maintained at 120°C for 3 hours, thereby impregnating the polystyrene-based resin particles with the blowing agent and obtaining expandable polystyrene-based resin particles.

[0197] Thereafter, the temperature inside the autoclave was cooled to room temperature, and the expandable polystyrene resin particles were taken out from the autoclave, washed, dehydrated, and dried.

[0198] (Examples 2 to 5, Comparative Examples 1 to 4) A seed resin was produced and expandable polystyrene-based resin particles were obtained in the same manner as in Example 1, except that the amounts of nucleating agent A and nucleating agent B used were changed as shown in Table 1. Note that the notation "-" in the amount of nucleating agent used indicates that the nucleating agent was not used.

[0199] (Comparative Example 5) A seed resin was produced in the same manner as in Example 1, except that 0.20 parts by weight of ethylene bisstearic acid amide was used as nucleating agent A and the amount of nucleating agent B was changed to 0.25 parts by weight, and expandable polystyrene-based resin particles were obtained.

[0200] The expandable polystyrene resin particles obtained in each of the Examples and Comparative Examples were used to carry out the above-mentioned measurements and evaluations. The results are shown in Table 1.

[0201] [Table 1] [Industrial Applicability]

[0202] According to one embodiment of the present invention, it is possible to provide expandable polystyrene-based resin particles that are excellent in particle size uniformity and that can provide polystyrene-based foamed molded articles in which the occurrence of cell spots is suppressed even after a period of time has passed since production. Therefore, one embodiment of the present invention can be suitably used in fields such as packaging materials (trays) for food containers and the like, packaging materials for transport such as fish boxes, and insulation materials (e.g., hot water storage tanks, roof insulation materials, pipe insulation materials, constant-temperature storage containers, constant-temperature transport containers, etc.).

Claims

1. a polymerization step of polymerizing the styrene-based monomer in an aqueous suspension containing water, a seed resin, a nucleating agent B, a polymerization initiator, and the styrene-based monomer, The seed resin comprises a base resin and a nucleating agent A different from the nucleating agent B.

2. 2. The method for producing expandable polystyrene-based resin particles according to claim 1, wherein the content of the nucleating agent A in the seed resin is 0.05 to 0.33 parts by weight per 100 parts by weight of the base resin.

3. 3. The method for producing expandable polystyrene-based resin particles according to claim 1, wherein the amount of the nucleating agent B used is 0.05 parts by weight to 0.50 parts by weight per 100 parts by weight of the total amount of the seed resin used and the styrene-based monomer used.

4. The method for producing expandable polystyrene-based resin particles according to claim 1 or 2, wherein the nucleating agent A contains an acrylic resin.

5. The method for producing expandable polystyrene-based resin particles according to claim 1 or 2, wherein the nucleating agent B contains ethylene bisstearic acid amide and / or polyethylene wax.

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