Expandable polystyrene-based resin particles, pre-expanded particles thereof, and expanded molded article

Abstract

To provide expandable polystyrene-based resin particles suitable for enabling molding under wider molding conditions while maintaining good energy saving properties and suppressing scratches on the surface of a molded article.SOLUTION: Expandable polystyrene-based resin particles comprising a base resin containing a styrene unit or a styrene unit and an acrylic acid ester unit as structural units, wherein a content of the styrene unit is 97 to 100 parts by weight, and a content of the acrylic acid ester unit is 0 to 3 parts by weight, with respect to a total content of 100 parts by weight of the styrene unit and the acrylic acid ester unit in the base resin, A base material resin contains a specific amount of a plasticizer having a boiling point of 100°C or higher, a foaming agent and a foaming assistant, and the content of an acrylic ester unit in the base material resin and the contents of the plasticizer having a boiling point of 100°C or higher and the foaming assistant satisfy a specific formula.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to expandable polystyrene resin particles, pre-expanded polystyrene particles, and foamed molded articles. [Background technology]

[0002] Expandable polystyrene resin particles are a socially useful material because they are relatively inexpensive, can be expanded using steam or other methods without the need for special methods, and provide high cushioning and heat insulation effects.

[0003] Expandable polystyrene-based resin particles are produced, for example, by a method of impregnating polystyrene resin particles with a blowing agent (i.e., a volatile aliphatic hydrocarbon, such as butane or pentane, that only slightly swells the particles) in an aqueous suspension. The expandable polystyrene-based resin particles produced in this manner are used as a raw material for producing polystyrene-based resin foamed molded articles.

[0004] A common method for industrially and economically producing polystyrene-based resin molded articles of a desired shape is to (1) convert expandable polystyrene-based resin particles into pre-expanded particles using a heating medium such as steam, (2) fill the pre-expanded particles into a closed mold of the desired shape with a large number of small holes in its wall, (3) introduce a heating medium such as steam through the small holes in the mold to heat the pre-expanded particles to a temperature above their softening point, thereby fusing the pre-expanded particles to form a molded article, and (4) cool the mold and then remove it from the mold. Expanded polystyrene-based resin molded articles, especially in-mold foam molded articles, have advantages such as ease of producing molded articles of a desired shape, and are therefore used as packaging materials (trays) for food containers and the like, and transport packaging materials for fish boxes and the like because they are lightweight and have excellent thermal insulation properties.

[0005] When producing polystyrene-based resin foam molded articles using the above method, a large amount of steam is required to convert expandable polystyrene-based resin particles into foam molded articles. Meanwhile, growing interest in environmental issues in recent years has led to a growing demand for greater energy conservation, leading to a demand for resins that can be foamed with less steam by lowering the temperature during pre-expansion and in-mold molding. At the same time, to increase productivity, there is a demand for shortening the cooling time, which accounts for approximately 50% of the molding time.

[0006] Under these circumstances, as in Patent Document 1, the addition of butyl acrylate, a plasticizer, and a foaming aid makes it possible to mold at low temperatures. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-203042 Summary of the Invention [Problem to be solved by the invention]

[0008] However, as a result of intensive research by the present inventors, it has been found that the expandable thermoplastic resin particles described in Patent Document 1 have the following problems. Specifically, the softened resin clogs small holes in the mold, resulting in problems such as increased consumption of heating media such as steam, reduced productivity due to cleaning work to remove the clogs, and scratches on the surface of the molded product. Furthermore, the softened resin may become unable to withstand high-temperature molding, making it difficult to obtain a good molded product.

[0009] That is, the expandable polystyrene-based resin particles described in Patent Document 1 lack a balance among various physical properties, and it is difficult to fully achieve the objective of maintaining good energy saving properties, suppressing scratches on the surface of the molded body, and enabling molding under a wide range of molding conditions.

[0010] An object of one aspect of the present invention is to provide expandable polystyrene resin particles, as well as pre-expanded polystyrene particles and foamed molded articles, which are suitable for enabling molding under a wider range of molding conditions while maintaining good energy-saving properties and suppressing damage to the surface of the molded article. [Means for solving the problem]

[0011] As a result of extensive research to solve the above problems, the present inventors have found that by optimizing the total amount of butyl acrylate, plasticizer, and foaming aid used, it is possible to obtain expandable polystyrene-based resin particles that maintain good energy saving properties, suppress damage to the surface of the molded body, and enable molding under a wider range of molding conditions, and have completed the present invention.

[0012] That is, one embodiment of the present invention includes the following configuration. [1] Expandable polystyrene-based resin particles composed of a base resin containing a styrene unit or a styrene unit and an acrylic acid ester unit as a constituent unit, the content of the styrene units is 97 to 100 parts by weight and the content of the acrylic ester units is 0 to 3 parts by weight relative to 100 parts by weight of the total content of the styrene units and the acrylic ester units in the base resin; For 100 parts by weight of base resin, A total of less than 0.5 parts by weight of plasticizers with a boiling point of 100°C or higher, 3.0 to 8.0 parts by weight of a foaming agent, Contains 1.0 to 2.1 parts by weight of a foaming aid, Expandable polystyrene-based resin particles, which satisfy the following formula (1): (A)+(B)+(C)<4.0...Equation (1); (A) represents the content of acrylic ester units relative to 100 parts by weight of the total content of styrene units and acrylic ester units in the base resin, (B) represents the value obtained by multiplying the content of plasticizers having a boiling point of 100°C or higher per 100 parts by weight of the base resin by 2, (C) indicates the content of the foaming aid relative to 100 parts by weight of the base resin. [2] Expandable polystyrene resin particles according to [1], characterized in that the weight-average molecular weight (Mw) obtained by gel permeation chromatography measurement is 260,000 to 320,000. [3] Expandable polystyrene-based resin particles according to [1] or [2], wherein the base resin contains an acrylic acid ester-based monomer. [4] In pre-expanded polystyrene resin particles obtained by expanding expandable polystyrene resin particles, the wave number of 1600 cm obtained from the infrared absorption spectrum measured by ATR-FTIR -1 Absorbance (A1600) and wavenumber 1730cm -1 The expandable polystyrene resin particles according to [3], characterized in that, when the absorbance ratio (A1730 / A1600) is calculated from the absorbance (A1730) of the polystyrene pre-expanded resin particles, the absorbance ratio α at the surface of the polystyrene pre-expanded resin particles is 0.90 to 5 times the absorbance ratio β at the center of the polystyrene pre-expanded resin particles. [5] Expandable polystyrene-based resin particles according to [3] or [4], characterized in that the acrylic acid ester-based monomer is butyl acrylate. [6] Pre-expanded polystyrene particles, characterized by being obtained by pre-expanding the expandable polystyrene resin particles according to any one of [1] to [5]. [7] A foamed molded article obtained by molding the polystyrene pre-expanded particles described in [6]. [Effects of the Invention]

[0013] According to one aspect of the present invention, expandable polystyrene-based resin particles can be obtained that are suitable for maintaining good energy saving properties, suppressing scratches on the surface of molded articles, and enabling molding under a wider range of molding conditions. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to these, and various modifications are possible within the scope of the description. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments and examples are also included in the technical scope of the present invention. In this specification, unless otherwise specified, "A to B" representing a numerical range means "A or more (including A and greater than A) and B or less (including B and less than B)."

[0015] The expandable polystyrene-based resin particles in one embodiment of the present invention are expandable polystyrene-based resin particles composed of a base resin containing, as a constituent unit, a styrene unit or a styrene unit and an acrylic acid ester unit, the content of the styrene units is 97 to 100 parts by weight and the content of the acrylic ester units is 0 to 3 parts by weight relative to 100 parts by weight of the total content of the styrene units and the acrylic ester units in the base resin; For 100 parts by weight of base resin, A total of less than 0.5 parts by weight of plasticizers with a boiling point of 100°C or higher, 3.0 to 8.0 parts by weight of a foaming agent, Contains 1.0 to 2.1 parts by weight of a foaming aid, Expandable polystyrene-based resin particles, which satisfy the following formula (1): (A)+(B)+(C)<4.0...Equation (1); (A) represents the content of acrylic ester units relative to 100 parts by weight of the total content of styrene units and acrylic ester units in the base resin, (B) represents the value obtained by multiplying the content of plasticizers having a boiling point of 100°C or higher per 100 parts by weight of the base resin by 2, (C) indicates the content of the foaming aid relative to 100 parts by weight of the base resin. The expandable polystyrene resin particles according to one embodiment of the present invention, having the above-described structure, can be molded under a wider range of molding conditions while maintaining energy saving properties and suppressing scratches on the surface of the molded article.

[0016] The base resin constituting the expandable polystyrene-based resin particles of the present invention contains, as a structural unit, a styrene unit, or a styrene unit and an acrylic acid ester unit. In this specification, a "styrene unit" refers to a structural unit derived from a styrene-based monomer, and an "acrylic acid ester unit" refers to a structural unit derived from an acrylic acid ester monomer. That is, the base resin constituting the expandable polystyrene-based resin particles of the present invention contains a homopolymer of a styrene-based monomer, or a copolymer of a styrene-based monomer and an acrylic acid ester monomer. In this specification, the "structural unit" of the base resin may be referred to as a "monomer composition," and the "unit" may be referred to as a "monomer."

[0017] Examples of the styrene-based monomer constituting the expandable polystyrene-based resin particles of the present invention include styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, chlorostyrene, etc. These styrene-based monomers may be used alone or in combination of two or more.

[0018] Examples of the acrylic acid ester monomer constituting the expandable polystyrene resin particles of the present invention include alkyl acrylates such as methyl acrylate and butyl acrylate. These acrylic acid ester monomers may be used alone or in combination of two or more. Among these, butyl acrylate is preferred because it is easily copolymerizable with styrene monomers and has good moldability.

[0019] The monomer composition of the base resin constituting the expandable polystyrene resin particles in the present invention is 97 to 100 parts by weight of styrene monomer and 0 to 3 parts by weight of acrylic ester monomer (100 parts by weight of styrene monomer and acrylic ester monomer), more preferably 97 to 99 parts by weight of styrene monomer and 1 to 3 parts by weight of acrylic ester.

[0020] Adding an acrylic acid ester to the monomer composition of the base resin tends to improve the foaming rate and the appearance of the molded article.

[0021] If the monomer composition of the base resin contains more than 3 parts by weight of acrylic ester monomer, the molded product tends to shrink, especially when highly foamed, and the appearance of the molded product tends to deteriorate. In addition, the average chord length tends to increase, and the fracture displacement of the fracture strength (e.g., bending strength in JIS A9511 or bottom split strength of box-shaped molded product) tends to decrease.

[0022] In addition, the monomer composition of the base resin includes the monomer composition in the seed resin particles when seed suspension polymerization is used as the polymerization method. The base resin may further contain, as structural units, acrylonitrile monomers, carbonate-based monomers, etc., to the extent that the effects of the present invention are not impaired.

[0023] The content of the plasticizer having a boiling point of 100°C or higher in the expandable polystyrene resin particles of the present invention is less than 0.5 parts by weight, preferably less than 0.4 parts by weight, and more preferably 0 parts by weight, per 100 parts by weight of the base resin. When an acrylic acid ester is contained, expandable polystyrene resin particles can be obtained that can be cooled quickly and do not shrink even without a plasticizer, and therefore can maintain a wide molding width.

[0024] If the amount of plasticizer with a boiling point of 100°C or higher is 0.5 parts by weight or more, the molded product tends to shrink, impairing the appearance of the molded product. In addition, the fusion inside the molded product tends to vary. As a result, the strength and fracture displacement tend to decrease.

[0025] In the present invention, a plasticizer having a boiling point of 100° C. or higher may be added during the polymerization step of polystyrene-based resin particles, the step of impregnating the particles with a blowing agent, or the like.

[0026] The plasticizer used in this specification is not particularly limited as long as it has a boiling point of 100°C or higher and has a plasticizing effect on the base resin. Examples of plasticizers having a boiling point of 100°C or higher that can be used in the present invention include toluene, xylene, diisobutyl adipate, dioctyl adipate, dibutyl sebacate, glycerin tristearate, glycerin tricaprylate, coconut oil, palm oil, and rapeseed oil. Among these plasticizers, when used in the medical field or in the field of packaging materials that come into direct contact with food, edible oils are preferred, and coconut oil, palm oil, and rapeseed oil are more preferred.

[0027] The content of the blowing agent in the expandable polystyrene resin particles of the present invention is 3.0 to 8.0 parts by weight, preferably 4.0 to 7.0 parts by weight, based on 100 parts by weight of the base resin.

[0028] If the foaming agent content is less than 3 parts by weight, the foaming power during pre-expansion is low, making it difficult to obtain polystyrene-based pre-expanded particles with a bulk ratio of 65 times. If the foaming agent content is more than 8 parts by weight, the molded product tends to shrink when highly expanded, damaging its appearance. In addition, the foaming power becomes high during pre-expansion, making the cell membrane more likely to tear. As a result, the adhesive surfaces between the pre-expanded particles tend to weaken, reducing the strength and fracture displacement.

[0029] Examples of the blowing agent used in the present invention include aliphatic hydrocarbons such as propane, butane, and pentane; alicyclic hydrocarbons such as cyclobutane and cyclopentane; and halogenated hydrocarbons such as methyl chloride, dichlorodifluoromethane, and dichlorotetrafluoroethane. These blowing agents may be used alone or in combination of two or more. Among these blowing agents, butane is preferred because of its good blowing power.

[0030] The amount of foaming aid used in the present invention is 1.0 to 2.1 parts by weight, and more preferably 1.2 to 1.8 parts by weight, per 100 parts by weight of the base resin.

[0031] If the content of the foaming aid is less than 1.0 part by weight, the pre-expansion time will be longer, and internal fusion during molding and the appearance of the molded product will tend to deteriorate. If the content of the foaming aid is more than 2.1 parts by weight, the molded product will tend to shrink and the appearance of the molded product will tend to be impaired. In addition, the foaming force will be high during molding, making the cell membrane more likely to tear. As a result, the adhesive surfaces between the pre-expanded particles will tend to be weaker, and the strength and breaking displacement will tend to be smaller.

[0032] The foaming aid used in the present invention is a solvent having a boiling point of 50°C or higher but lower than 100°C. Examples include aliphatic hydrocarbons of C6 or higher, such as hexane and heptane, and alicyclic hydrocarbons of C6 or higher, such as cyclohexane and cyclooctane. These foaming aids may be used alone or in combination of two or more. Among these foaming aids, cyclohexane is preferred because of its good foaming power.

[0033] The expandable polystyrene resin particles of the present invention satisfy the following formula (1).

[0034] (A)+(B)+(C)<4.0...Equation (1) (A) indicates the content of acrylic ester units relative to 100 parts by weight of the total content of styrene units and acrylic ester units in the base resin.

[0035] (B) represents a value obtained by multiplying the content of plasticizers having a boiling point of 100° C. or higher by 2 relative to 100 parts by weight of the base resin.

[0036] (C) indicates the content of the foaming aid relative to 100 parts by weight of the base resin.

[0037] In other words, the total amount (A) + (B) + (C) of the twice the amount of plasticizer (A) having a boiling point of 100°C or higher, the acrylic acid ester monomer (B), and the foaming aid (C) in the expandable polystyrene resin particles of the present invention is less than 4.0 parts by weight, preferably 2.0 or more but less than 4.0, and more preferably 3.0 or more but less than 4.0, per 100 parts by weight of the base resin. By keeping this total amount less than 4.0 parts by weight per 100 parts by weight of the base resin, good surface properties can be achieved, and the amount of steam used during molding can be reduced, and molding time can be shortened.

[0038] In this specification, molding time refers to the time from when the mold clamping begins to close until the molded product is removed from the mold. Specifically, the molding process for a molded product includes steps (1) closing the mold from an open state, (2) pouring resin into the closed mold, (3) blowing steam to form the mold, (4) cooling the heated molded product in the mold, and (5) opening the mold and releasing the molded product. The molding time refers to the time required to perform steps (1) to (5) consecutively.

[0039] If the total amount of the plasticizer, acrylic ester monomer, and foaming aid is 4.0 parts by weight or more, the molded product tends to shrink, impairing the appearance of the molded product. Also, the fusion inside the molded product tends to vary. As a result, the strength and fracture displacement tend to decrease.

[0040] The weight average molecular weight Mw of the expandable polystyrene resin particles in the present invention is preferably 260,000 to 320,000, and more preferably 280,000 to 300,000.

[0041] If the weight-average molecular weight Mw of the expandable styrene-based resin particles is less than 260,000, not only will the strength of the expanded molded article be low, but the surface of the molded article will tend to melt easily, which will impair the appearance of the molded article and increase the number of scratches on the surface of the molded article.If the weight-average molecular weight Mw of the expandable polystyrene-based resin particles is 320,000 or more, the expandability will tend to be low and the moldability will tend to deteriorate, that is, the heating temperature required to obtain pre-expanded particles with the desired expansion ratio and the molding temperature required to obtain a molded article with excellent properties will tend to be high.

[0042] The Z-average molecular weight Mz of the expandable polystyrene resin particles in the present invention is preferably from 640,000 to 950,000, and more preferably from 650,000 to 800,000.

[0043] If the Z-average molecular weight Mz of the expandable styrene resin particles is less than 640,000, not only will the strength of the foamed molded article be low, but the surface of the molded article will be more likely to melt, tending to impair the appearance and increase the number of scratches on the surface of the molded article. On the other hand, if the Z-average molecular weight Mz is 950,000 or more, the foamability will be low and moldability will be poor (the heating temperature required to obtain pre-expanded particles with the desired expansion ratio and the molding temperature required to obtain a molded article with excellent fusion properties will be high).

[0044] The weight-average molecular weight Mw and the Z-average molecular weight Mz can be controlled by adjusting the amount of initiator used and the polymerization temperature when polymerizing the polystyrene-based resin particles. For example, Mw and Mz can be reduced by increasing the amount of initiator used and / or increasing the polymerization temperature.

[0045] Here, the weight average molecular weights Mw and Mz of the expandable polystyrene resin particles in the present invention are values ​​measured using a gel permeation chromatograph (hereinafter sometimes abbreviated as "GPC") under the conditions described below.

[0046] The expandable polystyrene resin particles of the present invention are pre-expanded polystyrene resin particles obtained by expanding expandable polystyrene resin particles, and have a wave number of 1600 cm obtained from an infrared absorption spectrum measured by ATR-FTIR. -1 Absorbance (A1600) and wavenumber 1730cm -1 When the absorbance ratio (A1730 / A1600) is calculated from the absorbance (A1730) of the polystyrene pre-expanded resin particle, the absorbance ratio α at the surface of the polystyrene pre-expanded resin particle is 0.90 to 5 times, preferably 0.95 to 3 times, and more preferably 0.95 to 2.0 times the absorbance ratio β at the center of the polystyrene pre-expanded resin particle.

[0047] If the absorbance ratio ratio α / β between the surface and the center is higher than 5, the ratio of acrylic ester on the particle surface to the particle interior is high, making surface melting more likely to occur and damaging the surface appearance, especially when molding at high vapor pressure (high mold temperature).If the absorbance ratio ratio α / β is less than 0.90, the ratio of acrylic ester on the particle surface to the particle interior is low, making molding at low vapor pressure (low mold temperature) difficult, tending to result in internal fusion and poor surface appearance, and also requiring a high heating temperature during pre-expansion.

[0048] In addition, the 1730 cm -1 The absorbance at 1600cm is the absorption spectrum due to the C=O stretching vibration of the carbonyl group, and was taken as the absorbance (A1730). -1 The absorbance is the absorption spectrum of the aromatic benzene ring in-plane vibration, and is taken as the absorbance (A1600).

[0049] The ratio α / β of the absorbance ratio between the surface and the center of the polystyrene-based resin pre-expanded particles can be adjusted by changing the timing of adding the acrylic ester during polymerization of the polystyrene-based resin particles.

[0050] Here, the central portion refers to the center of a cross section of a polystyrene-based resin pre-expanded particle divided into two parts along the center, and a region within a radius of 500 μm from the center.

[0051] In this invention, ATR-FTIR refers to FTIR that utilizes the ATR (Attenuated Total Reflection) method. The ATR method is a technique in which a high-refractive-index crystal is pressed onto the sample surface, and the sample surface is measured with high sensitivity under total reflection conditions, allowing spectra similar to those obtained by the transmission method to be easily obtained. This method is widely used for analyzing general industrial materials that do not transmit light, such as thick polymer films, resins, coatings, paper, and thread.

[0052] Generally, light does not reflect at the interface between the sample and the high refractive index crystal, but penetrates a certain depth into the sample before undergoing total reflection. At this time, in the wavenumber range where the sample does not absorb light, the light is totally reflected, but in the range where the sample absorbs light, the light is not totally reflected at 100%; rather, the intensity of the totally reflected light drops according to the strength of the absorption. The total reflection spectrum can be obtained by measuring this reflected energy.

[0053] However, the penetration depth of light (measurement depth) varies greatly depending on the refractive index of the high-refractive index crystal used, the refractive index of the sample, the angle of incidence of the measurement light, and the wavenumber of the measurement light, so measurement results cannot be compared unless these parameters are specified. The measurement depth in the ATR method is wavenumber dependent, with lower wavenumbers resulting in deeper measurement depths and greater absorption intensity. Therefore, corrections are required when comparing with transmission spectra.

[0054] The expandable polystyrene-based resin particles of the present invention can be produced by either a method of impregnating particles obtained by suspension polymerization in an aqueous medium with a blowing agent, or a method of impregnating pellets produced by bulk polymerization or the like in an aqueous medium with a blowing agent.

[0055] Among these, suspension polymerization is preferred because it can obtain spherical resin particles and, further, expandable polystyrene-based resin particles can be obtained by continuously performing the polymerization step and the blowing agent impregnation step, and therefore has good industrial productivity. That is, as a method for producing expandable polystyrene-based resin particles, a method is preferred in which various monomers constituting a base resin (including a styrene-based monomer, or a styrene-based monomer and an acrylic acid ester-based monomer) are polymerized in the presence of a suspending agent, a polymerization initiator, and, if necessary, other additives, and a blowing agent is added during suspension polymerization, or the particles are impregnated with a blowing agent after polymerization.

[0056] Examples of suspending agents used in the suspension polymerization method of the present invention include water-soluble polymers such as polyvinyl alcohol, methyl cellulose, polyacrylamide, and polyvinylpyrrolidone, and sparingly soluble inorganic substances such as calcium triphosphate and magnesium pyrophosphate. When using a sparingly soluble inorganic substance, the suspension stabilizing effect can be enhanced by using it in combination with an anionic surfactant such as sodium dodecylbenzenesulfonate. It is also effective to use a water-soluble polymer in combination with a sparingly soluble inorganic substance.

[0057] The polymerization initiator used in the suspension polymerization method of the present invention can be a radical-generating polymerization initiator commonly used in the production of polystyrene-based polymers. Representative examples of polymerization initiators include azo compounds such as azobisisobutyronitrile, and peroxides such as benzoyl peroxide, t-butylperoxy-2-ethylhexanoate, lauroyl peroxide-t-butylperoxyisopropyl carbonate, 1,1-bis(t-butylperoxy)cyclohexane, 1,1-bis(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, t-butylperoxybenzoate, and t-butylperoxy-2-ethylhexyl carbonate. These polymerization initiators can be used alone or in combination of two or more.

[0058] Additives that can be added during the suspension polymerization of the present invention include nucleating agents, flame retardants, flame retardant auxiliaries, external additives, etc., within limits that do not impair the effects of the present invention.

[0059] The average chord length of the cells in the cross section of the foam can be controlled by the amount of nucleating agent used: for example, more nucleating agent will decrease the average chord length, and less nucleating agent will increase the average chord length.

[0060] Examples of nucleating agents used in the present invention include methyl methacrylate copolymers, polyethylene wax, talc, fatty acid bisamides, ethylene-vinyl acetate copolymer resins, etc. Specific examples of fatty acid bisamides include methylene bisstearylamide, ethylene bisstearylamide, hexamethylene bispalmitic acid amide, ethylene bisoleic acid amide, etc.

[0061] The expandable polystyrene resin particles of the present invention have an average chord length of cells in a cross section of an expansion molded article obtained by pre-expanding and molding the expandable polystyrene resin particles of 80 to 120 μm, preferably 93 to 110 μm.

[0062] If the average chord length is less than 80 μm, the membrane thickness of the cells constituting the foam will be thin, which will tend to result in internal fusion and a poor surface appearance.If the average chord length is more than 120 μm, the fracture displacement of the fracture strength (for example, bending strength in JIS A9511 or tensile strength of a box-shaped molded body) will be short, which will tend to result in a brittle molded body.

[0063] The flame retardant and flame retardant aid used in the present invention may be any known and commonly used one. Specific examples of the flame retardant include halogenated aliphatic hydrocarbon compounds such as hexabromocyclododecane, tetrabromobutane, and hexabromocyclohexane, brominated phenols such as tetrabromobisphenol A, tetrabromobisphenol F, and 2,4,6-tribromophenol, tetrabromobisphenol A-bis(2,3-dibromopropyl ether), tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether), and tetrabromobisphenol A-diglycerides. Examples of suitable flame retardants include brominated phenol derivatives such as brominated styrene-butadiene ethers and 2,2-bis[4'(2",3"-dibromoalkoxy)-3',5'-dibromophenyl]-propane; brominated styrene-butadiene block copolymers, brominated random styrene-butadiene copolymers, and brominated styrene-butadiene graft copolymers, etc., brominated butadiene-vinyl aromatic hydrocarbon copolymers (e.g., EMERALD3000 manufactured by Chemtura or those disclosed in JP-A-2009-516019). These flame retardants may be used alone or in combination of two or more.

[0064] Specific examples of the flame retardant aid that may be used include initiators such as cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, and 2,3-dimethyl-2,3-diphenylbutane.

[0065] As the external additives used in the present invention, known and commonly used ones can be used.

[0066] Specific examples of external additives include fatty acid triglycerides such as lauric triglyceride, stearic triglyceride, and linoleic triglyceride; fatty acid diglycerides such as lauric diglyceride, stearic diglyceride, and linoleic diglyceride; fatty acid monoglycerides such as lauric monoglyceride, stearic monoglyceride, and linoleic monoglyceride; fatty acid metal salts such as zinc stearate, calcium stearate, magnesium stearate, aluminum stearate, zinc laurate, and calcium laurate; and nonionic surfactants such as polyoxyethylene cetyl ether, polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene laurate, polyoxyethylene palmitate, polyoxyethylene stearate, and polyoxyethylene oleate. These external additives may be used alone or in combination of two or more. Furthermore, these external additives may be added to the aqueous system during the foaming agent impregnation, or may be added and coated after dehydration or drying, regardless of the coating method. A preferred coating method is to apply the coating agent after drying and mix and stir to coat the surface. A preferred external additive is a combination of zinc stearate and hydroxystearic acid triglyceride (also known as castor wax), which is preferred because it can easily suppress blocking during foaming and provide good adhesion during molding. The preferred amount of zinc stearate is 0.12 to 0.25 parts by weight, more preferably 0.15 to 0.21 parts by weight, per 100 parts by weight of the expandable styrene-based resin particles. This range makes it easy to suppress blocking during foaming without deteriorating adhesion during molding. The preferred amount of hydroxystearic acid triglyceride is 0.03 to 0.10 parts by weight, more preferably 0.05 to 0.07 parts by weight, per 100 parts by weight of the expandable styrene-based resin particles. This range makes it easy to obtain good adhesion during molding without causing blocking during foaming or deteriorating the surface appearance of the molded product.

[0067] The content of the monomer component in the expandable polystyrene resin particles of the present invention is less than 0.3% by weight. The contained monomer component tends to volatilize from the foamed molded article obtained by expanding the expandable polystyrene resin particles. In particular, if the content of the monomer component is 0.3% by weight or more, it is not preferable for use in the medical field, in the field of packaging materials that come into direct contact with food, or as automotive or building components. In addition, the surface appearance tends to deteriorate.

[0068] The amount of the monomer component contained can be controlled by a combination of the amount of initiator used and the polymerization temperature when polymerizing the polystyrene-based resin particles. For example, the amount of the monomer component contained can be reduced by increasing the amount of initiator used or the polymerization temperature.

[0069] The pre-expanded polystyrene particles in one embodiment of the present invention are obtained by pre-expanding (primary expanding) the expandable polystyrene resin particles described above.

[0070] As a method for pre-expanding, a conventional method can be adopted, for example, using a cylindrical pre-expanding device and heating the expandable polystyrene-based resin particles with a heating medium such as steam to cause expansion. The device used for pre-expanding and the pre-expanding conditions can be appropriately set depending on the composition of the expandable polystyrene-based resin particles, the desired pre-expansion ratio, etc., and are not particularly limited.

[0071] The foamed molded article according to one embodiment of the present invention is obtained by heating and foaming (secondary foaming) the above-described polystyrene-based pre-expanded particles.

[0072] The method for heat-expanding the polystyrene-based pre-expanded particles can be, for example, a conventional method such as an in-mold foam molding method in which the pre-expanded particles are filled into a mold and heated by blowing in a heating medium such as steam. The apparatus used for heat-expanding and the conditions for heat-expanding can be appropriately set depending on the composition of the expandable polystyrene-based resin particle body, the desired expansion ratio, etc., and are not particularly limited. Foamed molded products, particularly in-mold foamed molded products, have the advantage of being easy to produce molded products of desired shapes, and are therefore suitable for use as, for example, packaging materials (trays) for food containers and the like, and transport packaging materials for fish boxes and the like. [Example]

[0073] EXAMPLES 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. The measurement and evaluation methods were as follows.

[0074] <Blocking rate measurement> Expandable polystyrene resin particles were placed in a pressurized pre-expanding machine (manufactured by Daikai Kogyo Co., Ltd., CH-100) equipped with a stirrer, and pre-expanded (primary expansion) by heating using steam as a heating medium at a blown steam pressure of 0.1 MPa, yielding polystyrene pre-expanded particles with a bulk expansion ratio (apparent expansion ratio) of 65 times. When removing the polystyrene pre-expanded particles from the pre-expanding machine, the pre-expanded particles were passed through a mesh with 1 cm openings, and the pre-expanded particles that did not pass through the mesh were collected and weighed to obtain the amount of blocking. The following calculation formula was used: Blocking rate [wt%] = Blocking amount [g] / Total amount of polystyrene pre-expanded particles [g] × 100 The blocking rate was calculated based on the above, and polystyrene pre-expanded particles having a blocking rate of 1.5% by weight or less were evaluated as passing.

[0075] <Evaluation of foam molded products> A mold measuring 550 mm long, 350 mm wide, and 120 mm high with a bottom thickness of 30 mm and side thicknesses of 25 mm was filled with polystyrene pre-expanded particles pre-expanded 60 times. A molding machine (Daisen Co., Ltd., KR-57) was used to perform in-mold foam molding using steam (water vapor) as the heating medium at a blowing steam pressure of 0.06 MPa to obtain a box-shaped foamed molded article. The resulting foamed molded article was dried at room temperature for 24 hours and then evaluated for surface roughness and scratches. Table 1 shows the amount of steam used and molding time at a blowing steam pressure of 0.06 MPa. Table 1 also shows the evaluation results for the molded width.

[0076] The molding time includes the steps of closing the mold from an open state, pouring the resin into the closed mold, blowing steam into the mold, cooling the heated molded body inside the mold, and opening the mold to release the molded body.

[0077] (1) Evaluation of forming width The conditions for molding were a steam pressure of 0.04 MPa up to 0.12 MPa in 0.02 MPa increments, and the number of times the steam pressure was changed to obtain a good foamed molded product (a molded product that passes both surface properties and fusion properties) was used as the evaluation result, with a score of 4 or higher being considered a pass. A good molded article here is one that exhibits a "surface property" of "4" or higher in the evaluation of (2) surface property below, and a "weldability" of 80% or higher. The "weldability" was determined by breaking the bottom surface of the obtained foamed molded article, observing the fracture surface, determining the area of ​​the entire fracture surface where the particles themselves, not the particle interfaces, were broken, and calculating the percentage of the area where the particles themselves were broken relative to the entire area of ​​the fracture surface.

[0078] (2) Surface evaluation The surface condition of the foamed molded article was visually observed and rated on the following 5-point scale. A higher value indicates fewer gaps between the foamed particles and a cleaner surface, and a value of "4" or higher was judged to be acceptable.

[0079] 5: I can't find any gaps 4: There are gaps in some areas, but overall it is acceptable 3: The gaps are noticeable and the overall appearance is unacceptable. 2: Gaps are noticeable 1: There are many gaps.

[0080] (3) Evaluation of scratches The surface condition of the foam molded article was visually observed and rated on the following 5-point scale. The higher the number, the fewer scratches there were on the surface of the foam molded article and the cleaner the surface was, and a score of "4" or higher was judged to be acceptable.

[0081] 5: No scratches found 4: There are some scratches, but overall it is acceptable 3: There are noticeable scratches in places, and the overall appearance is unacceptable. 2: The scratches are noticeable 1: Many scratches.

[0082] <Measurement of absorbance ratio (A1730 / A1600)> The absorbance ratio of the obtained expandable polystyrene resin particles was determined by randomly selecting 10 pre-expanded particles and performing ATR infrared spectroscopy on the surface and center of each pre-expanded particle under the following conditions to obtain an infrared absorption spectrum. Equipment: FTIR [Shimadzu Corporation, FTIR-8400S] connected to a single-reflection attenuated total reflection (ATR) measurement device [PIKE, MIRacle] ATR prism (high refractive index crystal type): Zinc selenide (ZnSe) Incident angle: 45° Measurement area: 4000cm -1 ~600cm -1 Detector: DLATGS Penetration depth: 1.66 Number of reflections: 1 Resolution: 4cm -1 Accumulation count: 20 times Other: The infrared absorption spectrum measured without contact with the sample was used as the background, and processing that does not affect the measured spectrum was performed.

[0083] In the ATR method, the intensity of the infrared absorption spectrum obtained by measurement changes depending on the degree of contact between the sample and the high refractive index crystal. -1 The degree of contact between the sample and the high refractive index crystal is adjusted so that the absorbance (A696) is 0.08 to 0.12.

[0084] When measuring the surface of a pre-expanded particle, the particle surface was directly brought into close contact with the ATR prism for measurement. When measuring the center of a pre-expanded particle, the particle was divided into two pieces using a razor blade, with the cross section passing through the center of the pre-expanded particle, and the center of the cross section of the two pieces was brought into close contact with the ATR prism for measurement.

[0085] From the infrared absorption spectrum obtained as above, 1600 cm -1 Absorbance (A1600) and 1730cm -1 The absorbance ratio (A1730 / A1600) was calculated from the absorbance (A1730) of the surface and center of ten randomly selected pre-expanded particles. In this measurement, ATR-FTIR measurement was performed on the surface and center of 10 randomly selected pre-expanded particles, and the minimum and maximum absorbance ratios were excluded. The arithmetic mean of the absorbance ratios of the remaining eight particles was then used as the absorbance ratio (A1730 / A1600). The absorbance ratio between the surface and center was calculated using the following formula from the obtained surface absorbance ratio α (A1730 / A1600) and center absorbance ratio β (A1730 / A1600). The ratio of absorbance ratio between the surface and the center = α (surface) / β (center).

[0086] <Weight average molecular weight measurement> 0.02 g of the obtained expandable polystyrene resin particles was dissolved in 20 ml of tetrahydrofuran (hereinafter sometimes abbreviated as "THF"), and then gel permeation chromatography (GPC) was performed under the following conditions to obtain a GPC measurement chart, weight average molecular weight (Mw), and number average molecular weight (Mn). The obtained values ​​are relative values ​​converted into polystyrene. Measurement equipment: Tosoh Corporation, high-speed GPC equipment HLC-8220 Columns used: Tosoh Corporation, SuperHZM-H x 2, SuperH-RC x 2 Column temperature: 40°C, mobile phase: THF (tetrahydrofuran) Flow rate: 0.35ml / min, injection volume: 10μl Detector: RI.

[0087] Example 1 <Production of Expandable Polystyrene Resin Particles> A 6L autoclave equipped with a stirrer was charged with 100 parts by weight of pure water, 0.2 parts by weight of tricalcium phosphate, 0.01 parts by weight of sodium dodecylbenzenesulfonate, 0.16 parts by weight of benzoyl peroxide and 0.20 parts by weight of 1,1-bis(t-butylperoxy)cyclohexane as initiators, 0.4 parts by weight of coconut oil as plasticizer, and 0.04 parts by weight of polyethylene wax as a nucleating agent. Next, 100 parts by weight of styrene monomer was charged while stirring at 250 rpm, and the temperature was raised to 98°C. The mixture was then held at 98°C for 4 hours to obtain polystyrene resin particles.

[0088] Next, 2.1 parts by weight of cyclohexane as a blowing aid and 6.8 parts by weight of butane as a blowing agent were pressurized into the autoclave and heated again to 120°C. After maintaining the temperature at 120°C for 2 hours, the autoclave was cooled to room temperature, and the polymerized slurry was removed from the autoclave. The removed polymerized slurry was washed and dehydrated to obtain expandable polystyrene-based resin particle bodies. A 3 wt% aqueous solution of 0.02 parts by weight of polyoxyethylene oleate (HLB value 10.2, manufactured by Nippon Oil & Fats Co., Ltd.) was added to 100 parts by weight of the obtained expandable polystyrene-based resin particle bodies, and the mixture was mixed and stirred to uniformly coat the surface of the expandable polystyrene-based resin particle bodies. The water was then dried using an airflow dryer, and then heated at 50°C for 20 minutes in a box-type ventilated dryer (manufactured by Tanaka Chemical Machinery Co., Ltd.) to obtain expandable polystyrene-based resin particles.

[0089] The obtained expandable polystyrene resin particles were sieved to separate expandable polystyrene resin particles with particle diameters of 0.6 mm to 1.2 mm. 100 parts by weight of the extracted expandable polystyrene were mixed with 0.18 parts by weight of zinc stearate (Zinc Stearate GF-200, manufactured by NOF Corp.) and 0.06 parts by weight of hydroxystearic acid triglyceride (Castarwax A, manufactured by NOF Corp.) as external additives, and the mixture was stirred for 60 seconds in a super mixer (SMV-20, manufactured by Kawata Corp.).

[0090] <Production of pre-expanded particles> The expandable polystyrene resin particles coated with external additives were pre-expanded to a bulk ratio of 65 times using a pressure pre-expander (manufactured by Daikai Kogyo, BHP) at a steam pressure of 0.08 MPa. Air was introduced into the steam to adjust the steam temperature. The resulting mixture was then left at room temperature for one day to dry and cure.

[0091] <Production of in-mold foamed products> The obtained polystyrene resin pre-expanded particles were filled into a box-shaped mold (550 mm long x 350 mm wide x 120 mm high, with a bottom thickness of 30 mm and side thicknesses of 25 mm) using a molding machine [Daisen, KR-57], and molding was carried out in the mold at a blowing steam pressure of 0.04 to 0.12 MPa to obtain a plate-shaped foamed molded product.

[0092] The resulting expandable polystyrene resin particles, expanded resin particles, and foamed molded articles were evaluated. The results are shown in Table 1.

[0093] Example 2 In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 98 parts by weight, the butyl acrylate monomer was charged at 2 parts by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 1.0 part by weight. The evaluation results are shown in Table 1.

[0094] Example 3 In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 98 parts by weight, the butyl acrylate monomer was charged at 2 parts by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 1.3 parts by weight. The evaluation results are shown in Table 1.

[0095] Example 4 In <Production of Expandable Polystyrene Resin Particles>, expandable polystyrene resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 98 parts by weight, the butyl acrylate monomer was charged at 2 parts by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 1.8 parts by weight. The evaluation results are shown in Table 1.

[0096] Example 5 In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 98 parts by weight, the butyl acrylate monomer was charged at 2 parts by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 1.9 parts by weight. The evaluation results are shown in Table 1.

[0097] Example 6 In the <Production of Expandable Polystyrene Resin Particles>, expandable polystyrene resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 97.1 parts by weight, the butyl acrylate monomer was charged at 2.9 parts by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 1.0 parts by weight. The evaluation results are shown in Table 1.

[0098] (Comparative Example 1) In the <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the amount of coconut oil used as a plasticizer was changed to 0.5 parts by weight. The evaluation results are shown in Table 1.

[0099] (Comparative Example 2) In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 99 parts by weight, the butyl acrylate monomer was charged at 1 part by weight, the coconut oil was added at 1.0 part by weight, and the foaming aid cyclohexane was charged at 1.0 part by weight. The evaluation results are shown in Table 1.

[0100] (Comparative Example 3) In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 99 parts by weight, the butyl acrylate monomer was charged at 1 part by weight, the coconut oil was added at 0 parts by weight, and the foaming aid cyclohexane was charged at 2.5 parts by weight. The evaluation results are shown in Table 1.

[0101] Comparative Example 4 In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged in an amount of 98 parts by weight, the butyl acrylate monomer was charged in an amount of 2 parts by weight, and the foaming aid cyclohexane was added in an amount of 1.5 parts by weight. The evaluation results are shown in Table 1.

[0102] (Comparative Example 5) In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged in an amount of 97 parts by weight, the butyl acrylate monomer was charged in an amount of 3 parts by weight, the coconut oil was added in an amount of 0 parts by weight, and the foaming aid cyclohexane was added in an amount of 0.5 parts by weight. The evaluation results are shown in Table 1.

[0103] (Comparative Example 6) In <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that in the production of expandable polystyrene-based resin particles, the number of parts by weight of styrene monomer charged was 96, the number of parts by weight of butyl acrylate monomer charged was 4, the number of parts by weight of coconut oil added was 0, and the number of parts by weight of cyclohexane added as a foaming aid was 1.0. The evaluation results are shown in Table 1.

[0104] (Comparative Example 7) In the <Production of Expandable Polystyrene-Based Resin Particles>, expandable polystyrene-based resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that in the production of expandable polystyrene-based resin particles, the number of parts by weight of styrene monomer charged was 98, the number of parts by weight of butyl acrylate monomer charged was 2, the number of parts by weight of coconut oil added was 0, the number of parts by weight of cyclohexane added as a foaming aid was 1.8, and the number of parts by weight of the foaming agent charged was 2.5. The evaluation results are shown in Table 1.

[0105] (Comparative Example 8) In the <Production of Expandable Polystyrene Resin Particles>, expandable polystyrene resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that in the above step, the amount of coconut oil added was 0 part by weight, the amount of cyclohexane added as a foaming aid was 1.0 part by weight, and the amount of foaming agent charged was 8.5 parts by weight. The evaluation results are shown in Table 1.

[0106] (Comparative Example 9) In the <Production of Expandable Polystyrene Resin Particles>, expandable polystyrene resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 98 parts by weight, the butyl acrylate monomer was charged at 2 parts by weight, the coconut oil was added at 0.5 parts by weight, the foaming aid cyclohexane was added at 1.0 part by weight, and the foaming agent was charged at 7.0 parts by weight. The evaluation results are shown in Table 1.

[0107] (Comparative Example 10) In the <Production of Expandable Polystyrene Resin Particles>, expandable polystyrene resin particles, pre-expanded particles, and in-mold foamed articles were obtained in the same manner as in Example 1, except that the styrene monomer was charged at 96 parts by weight, the butyl acrylate monomer was charged at 4 parts by weight, the coconut oil was added at 0.3 parts by weight, the foaming aid cyclohexane was added at 1.0 part by weight, and the foaming agent was charged at 7.0 parts by weight. The evaluation results are shown in Table 1.

[0108] [Table 1]

Claims

1. Expandable polystyrene-based resin particles comprised of a base resin containing, as a constituent unit, a styrene unit or a styrene unit and an acrylic acid ester unit, the content of the styrene units is 97.1 to 100 parts by weight and the content of the acrylic ester units is 0 to 2.9 parts by weight, relative to 100 parts by weight of the total content of the styrene units and the acrylic ester units in the base resin; For 100 parts by weight of the base resin, A total of less than 0.5 parts by weight of plasticizers with a boiling point of 100°C or higher, 3.0 to 8.0 parts by weight of a foaming agent, Contains 1.0 to 2.1 parts by weight of a foaming aid, The expandable polystyrene-based resin particles satisfy the following formula (1): 2.9≦(A)+(B)+(C)<4.0...Formula (1); (A) represents the content of the acrylic ester units relative to 100 parts by weight of the total content of the styrene units and the acrylic ester units in the base resin, (B) represents a value obtained by multiplying the content of the plasticizer having a boiling point of 100° C. or higher relative to 100 parts by weight of the base resin by 2, (C) indicates the content of the foaming aid relative to 100 parts by weight of the base resin.

2. 2. The expandable polystyrene-based resin particles according to claim 1, characterized in that the weight average molecular weight (Mw) obtained by gel permeation chromatography measurement is 260,000 to 320,000.

3. The expandable polystyrene-based resin particles according to claim 1 or 2, wherein the base resin contains an acrylic acid ester-based monomer.

4. In the pre-expanded polystyrene resin particles obtained by expanding the expandable polystyrene resin particles, a wave number of 1600 cm obtained from an infrared absorption spectrum measured by ATR-FTIR －１ Absorbance (A1600) and wave number 1730 cm －１ 4. The expandable polystyrene resin particles according to claim 3, wherein, when an absorbance ratio (A1730 / A1600) is calculated from the absorbance (A1730) of the polystyrene pre-expanded particles, the absorbance ratio α at the surface of the polystyrene pre-expanded particles is 0.90 to 5 times the absorbance ratio β at the center of the polystyrene pre-expanded particles.

5. Expandable polystyrene-based resin particles comprised of a base resin containing a styrene unit as a constituent unit, the content of acrylic ester units in the base resin is 0 part by weight, For 100 parts by weight of the base resin, A total of less than 0.5 parts by weight of plasticizers with a boiling point of 100°C or higher, 3.0 to 8.0 parts by weight of a foaming agent, Contains 1.0 to 2.1 parts by weight of a foaming aid, The expandable polystyrene-based resin particles satisfy the following formula (1): 2.0≦(A)+(B)+(C)<4.0...Formula (1); (A) represents the content of the acrylic ester units relative to 100 parts by weight of the total content of the styrene units and the acrylic ester units in the base resin, (B) represents a value obtained by multiplying the content of the plasticizer having a boiling point of 100° C. or higher relative to 100 parts by weight of the base resin by 2, (C) indicates the content of the foaming aid relative to 100 parts by weight of the base resin.

6. 2. The expandable polystyrene-based resin particles according to claim 1, characterized in that the weight average molecular weight (Mw) obtained by gel permeation chromatography measurement is 260,000 to 320,000.

7. 7. Pre-expanded polystyrene resin particles, characterized in that they are obtained by pre-expanding the expandable polystyrene resin particles according to any one of claims 1 to 6.

8. A foamed molded article obtained by molding the pre-expanded polystyrene resin particles according to claim 7.

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