JP7822850B2 - Expandable polystyrene resin particles and manufacturing method - Google Patents

Description

The present invention relates to expandable polystyrene-based resin particles and a method for producing the same.

Polystyrene-based resin foam is a well-balanced foam that combines lightweight properties, thermal insulation, and cushioning performance, and has traditionally been widely used as a thermal insulation material for food containers, cooler boxes, cushioning materials, and homes.

In particular, in recent years, in relation to issues such as global warming, there has been a trend toward energy conservation through improved insulation performance in buildings such as homes, and demand is expected to expand for polystyrene resin foam molded products obtained using expandable polystyrene resin particles. Therefore, various studies are being conducted to improve the foaming and insulation performance of these polystyrene resin foams.

For example, Patent Documents 1 and 2 disclose polystyrene resin foam molded articles that contain a radiation heat transfer inhibitor and a specific amount of isobutane relative to the total amount of pentane and butane, thereby exhibiting excellent foaming and heat insulating properties.

Patent Document 3 discloses that by adjusting the aspect ratio of expandable polystyrene-based resin particles containing a radiant heat transfer inhibitor to 0.95 or less and the sphericity to 0.97 or more, a polystyrene-based resin foam molded article can be obtained that achieves both a high expansion ratio and high thermal insulation performance.

Patent Document 4 discloses a polystyrene resin foam molded article made from expandable polystyrene resin particles impregnated with a flame retardant and a blowing agent in an aqueous medium, with the flame retardant content (wt%) in the surface layer of the expandable resin particles being 1.05 times or more the flame retardant content (wt%) in the entire particle, and with a pentane content of 15 vol% or more to exhibit stable self-extinguishing properties.

JP 2018-145212 A JP 2019-65073 A JP 2020-33481 A JP 2004-346281 A

The expandable polystyrene resin particles described in Patent Documents 1 to 4 have room for improvement, particularly in terms of increasing the expansion ratio.

The present invention provides expandable polystyrene-based resin particles that can be used to produce polystyrene-based resin foamed molded articles that combine a high expansion ratio and high thermal insulation performance.

In general, the use of a radiation heat transfer inhibitor such as graphite in a polystyrene resin foam molded body improves its thermal insulation performance, but the expansion performance of expandable polystyrene resin particles tends to decrease, and there is a particular problem of pre-expanded particles shrinking when expanded at a high expansion ratio.

Patent Documents 1 and 2 specify that the amount of isobutane should be greater than 20% by weight, 50% by weight, or 55% by weight or less relative to 100% by weight of the total amount of pentane and butane. However, the inventors of the present application have found that a larger amount of isobutane is preferable in order to obtain a polystyrene-based resin foam molded article with a high expansion ratio of 90 times or more. From this perspective, the expandable polystyrene-based resin particles of Patent Documents 1 and 2, which contain greater than 20% by weight and 50% by weight or less, or 55% by weight or less, relative to 100% by weight of the total amount of pentane and butane, have room for improvement in order to achieve a higher expansion ratio.

Patent Document 3 specifies that the aspect ratio of expandable polystyrene-based resin particles must be 0.95 or less and the sphericity must be 0.97 or more, but does not address the blowing agent. The inventors of the present application have found that a low amount of isobutane reduces the expansion ratio of the expandable polystyrene-based resin particles, and in particular, when expanded to a high expansion ratio, shrinkage of the pre-expanded particles may occur. In this regard, there is room for improvement to achieve a higher expansion ratio.

Patent Document 4 describes an invention in which excellent flame retardancy is achieved by impregnating the surface layer of polystyrene resin particles with a flame retardant. However, there is a concern that the presence of a large amount of flame retardant, which has a plasticizing effect, in the surface layer may locally reduce the resin viscosity, promoting the escape of the blowing agent and reducing the foaming performance. For this reason, there is room for improvement in terms of achieving a high expansion ratio.

The inventors therefore conducted research to solve the above-mentioned problems, and by incorporating a specific amount of isobutane as a blowing agent and controlling the shape of the expandable polystyrene resin particles, they succeeded in producing a polystyrene resin foam molded article with a high expansion ratio and low thermal conductivity, leading to the completion of the present invention.

That is, the present invention relates to expandable polystyrene-based resin particles comprising a polystyrene-based resin composition containing a radiation heat transfer inhibitor and a blowing agent, wherein the expandable polystyrene-based resin particles have a sphericity of 0.985 or less, the blowing agent contains isobutane, and the content of isobutane is more than 2.7 wt% and not more than 6.0 wt% relative to 100 wt% of the total amount of the polystyrene-based resin composition and the blowing agent (hereinafter, sometimes referred to as "expandable polystyrene-based resin particles of the present invention"). In the expandable polystyrene-based resin particles of the present invention, it is preferable that the content of the flame retardant is more than 1.0 wt% and not more than 6.0 wt% relative to 100 wt% of the polystyrene-based resin composition.
The expandable polystyrene-based resin particles of the present invention preferably contain pentane.
In the expandable polystyrene type resin particles of the present invention, it is preferable that the expandable polystyrene type resin particles are pre-expanded to a bulk magnification of 90 times, and the expanded particles satisfy (Equation 1).
(Formula 1) (A)/(B)×100≦25.0
(A) The bulk expansion ratio of expanded beads pre-expanded to a bulk expansion ratio of 90 times immediately after expansion. (B) The bulk expansion ratio of the pre-expanded beads after curing at 30°C for 24 hours. In the expandable polystyrene-based resin beads of the present invention, it is preferable that the apparent density of the expandable polystyrene-based resin beads is more than 950 kg/ ^m3 and not more than 1,200 kg/ ^m3 .
In the expandable polystyrene-based resin particles of the present invention, it is preferable that the amount of blowing agent in the pre-expanded particles when the expandable polystyrene-based resin particles are pre-expanded to a bulk expansion ratio of 90 times and then cured at 30°C for 24 hours is 5.0% to 6.0%.
In the expandable polystyrene-based resin particles of the present invention, it is preferable that the expandable polystyrene-based resin particles are pre-expanded to a bulk expansion ratio of 90 times, and then cured at 30°C for 24 hours, and when the pre-expanded particles are expanded and molded, the thermal conductivity of the expanded molded product is 0.034 W/m K or less.
The pre-expanded particles of the present invention are pre-expanded particles of the expandable polystyrene resin particles of the present invention, and have a bulk expansion ratio of 90 times or more.

The present invention also relates to a method for producing expandable polystyrene-based resin particles, in which a polystyrene-based resin melt comprising a polystyrene-based resin composition containing a radiant heat transfer inhibitor and a blowing agent is extruded through a die having multiple small holes into pressurized circulating water and then cut into particles with a rotary cutter, wherein the blowing agent contains isobutane, and the isobutane content is more than 2.7 wt % and not more than 6.0 wt % relative to 100 wt % of the total amount of the polystyrene-based resin composition and blowing agent (hereinafter sometimes referred to as the "first production method of the present invention").

In the first production method of the present invention, the radiation heat transfer inhibitor preferably contains a carbon-based radiation heat transfer inhibitor.
In the first production method of the present invention, the blowing agent preferably contains pentane.

The expandable polystyrene-based resin particles of the present invention make it possible to obtain polystyrene-based resin foam molded articles that combine a high expansion ratio with high thermal insulation performance.

[Expandable polystyrene resin particles]
The expandable polystyrene-based resin particles of the present invention are expandable polystyrene-based resin particles containing a radiation heat transfer inhibitor, i.e., polystyrene-based resin particles containing a radiation heat transfer inhibitor and a blowing agent. The expandable polystyrene-based resin particles of the present invention have a sphericity of 0.985 or less, and contain more than 2.7 wt% and 6.0 wt% or less of isobutane relative to 100 wt% of the total amount of the polystyrene-based resin composition and the blowing agent, thereby making it possible to obtain a polystyrene-based resin foam molded article that achieves both a high expansion ratio and high thermal insulation performance.

In general, the use of radiation heat transfer inhibitors such as graphite in polystyrene-based resin foam moldings improves thermal insulation. However, increasing the amount of inorganic substances such as graphite added reduces the expansion performance of expandable polystyrene-based resin particles and causes the expanded pre-expanded particles to shrink. While the exact cause is unclear, it is believed that this problem is primarily caused by the inorganic substances, which create holes in the cell membranes within the pre-expanded particles during pre-expansion. This facilitates the escape of the blowing agent from the resin during expansion, making it impossible to maintain internal pressure. This presumably makes the pre-expanded particles more susceptible to shrinkage after expansion. Although shrinkage of pre-expanded particles can be restored by curing, it is predicted that controlling the expansion ratio after curing will be difficult. Furthermore, if the shrinkage is significant, high-temperature curing is required to restore the expansion ratio, necessitating the use of additional curing silos capable of high-temperature curing. This requires a large amount of thermal energy during curing, which is costly. In particular, if the shrinkage is even greater, the pre-expanded particles will buckle, making it difficult for the expansion ratio to recover even after curing at high temperatures, and the expansion ratio standard will no longer be met, resulting in a decrease in yield.

Furthermore, the higher the expansion ratio of expandable polystyrene resin particles containing a radiant heat transfer inhibitor, the more pronounced the shrinkage of the pre-expanded particles immediately after pre-expansion becomes, making it difficult to achieve an expansion ratio of 90 times or more.

Butane has low solubility in polystyrene resins, so it is prone to supersaturation and is thought to contribute to a high expansion ratio. Furthermore, because butane has a smaller molecular weight than pentane, even a small amount added can contribute to high expansion. Isobutane, in particular, has a bulkier molecular structure than normal butane, which makes it difficult for the blowing agent to escape from the expandable polystyrene resin particles, making a high expansion ratio possible. However, even when the blending ratio of pentane and isobutane is controlled as described in Patent Documents 1 and 2, shrinkage may occur immediately after expansion at a high expansion ratio of 90 times or more.

In general, the higher the sphericity of the expandable polystyrene-based resin particles, the closer they are to a spherical shape, while the lower the sphericity of the expandable polystyrene-based resin particles, the more elliptical they become. Because ellipsoidal shapes have a larger specific surface area than spherical shapes, it is believed that the external additives (anti-blocking agents during pre-expansion and fusion promoters during molding) coated on the surface of the expandable polystyrene-based resin particles work effectively, improving moldability. Furthermore, the amount of external additives used can be reduced, which is thought to lead to cost savings. On the other hand, when expandable polystyrene-based resin particles are expanded at a high expansion ratio of 90 times or more, if the sphericity of the expandable polystyrene-based resin particles is too low, significant shrinkage immediately after expansion is likely to occur.

Therefore, by setting the sphericity of the expandable polystyrene resin particles containing a radiation heat transfer inhibitor to 0.985 or less and by setting the amount of isobutane to more than 2.7% by weight relative to 100% by weight of the total amount of the polystyrene resin composition and blowing agent, it is possible to obtain a polystyrene resin foam molded article that combines a high expansion ratio and high thermal insulation performance.

Furthermore, since the radiation heat transfer inhibitor, which is an inorganic substance, also acts as a nucleating agent for bubbles, expandable polystyrene resin particles containing the radiation heat transfer inhibitor tend to have smaller cell diameters and are easier to suppress radiant heat, making it possible to achieve better thermal insulation performance.

(Polystyrene resin)
The polystyrene resin composition used in the expandable polystyrene resin particles of the present invention contains a polystyrene resin as a base resin. The polystyrene resin may be not only a styrene homopolymer (polystyrene homopolymer) but also a copolymer of styrene with another monomer copolymerizable with styrene or its derivative, as long as the effects of the present invention are not impaired. These may be used alone or in combination of two or more.

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

In the present invention, from the standpoint of impact absorption and heat resistance, it is also possible to blend, for example, diene rubber-reinforced polystyrene, acrylic rubber-reinforced polystyrene, polyphenylene ether resin, etc.

The polystyrene resin used in the present invention preferably contains a styrene homopolymer, as it is relatively inexpensive, can be foam-molded using low-pressure steam or the like without the need for special methods, and has an excellent balance of thermal insulation, flame retardancy, and cushioning properties.

In the present invention, polystyrene-based resin may be used as the main component in combination with other resins, as long as the effects of the present invention are not impaired. Examples of other resins include homopolymers of other monomers copolymerizable with styrene or their derivatives, such as polyolefin-based resins, polyester-based resins, polycarbonate-based resins, and acrylic-based resins, as well as copolymers thereof.

(Radiation heat transfer inhibitor)
In the present invention, a polystyrene-based resin foam molded article having high thermal insulation performance can be obtained by incorporating a radiation heat transfer inhibitor into expandable polystyrene-based resin particles. Here, the radiation heat transfer inhibitor refers to a material that has the property of reflecting, scattering, or absorbing light in the near-infrared or infrared region (e.g., a wavelength region of about 800 to 3000 nm). While there are no particular limitations on the radiation heat transfer inhibitor, examples include carbon materials, metal particles, metal compounds, and metal oxides. Because metal particles, metal compounds, and metal oxides have low affinity for resins and are prone to reducing foaming performance, it is preferable to incorporate a carbon material, i.e., a carbon-based radiation heat transfer inhibitor.

Examples of carbon materials include graphite, graphene, carbon black, expanded graphite, activated carbon, carbon nanotubes, and carbon nanofibers, but graphite is preferred from the standpoints of dispersibility in polystyrene-based resins and cost.

Examples of graphite include flake graphite, amorphous graphite, spherical graphite, and artificial graphite. In this specification, the term "flake" encompasses scaly, thin, or plate-like graphite. These graphites can be used alone or in combination of two or more. Among these, graphite mixtures primarily composed of flake graphite are preferred, with flake graphite being more preferred, due to their high radiation heat transfer suppression effect. From the perspectives of high expansion ratio, thermal insulation, and moldability, the average particle size of the graphite is preferably 1 to 9 μm, and more preferably 2 to 6 μm. The smaller the average particle size of graphite, the higher the production cost. Graphite with an average particle size of less than 1 μm is very expensive due to high production costs, including the cost of milling, and tends to increase the cost of expandable polystyrene-based resin particles. On the other hand, if the average particle size exceeds 9 μm, the cell membranes tend to tear easily when producing pre-expanded particles and polystyrene-based resin foam molded articles from the expandable polystyrene-based resin particles, which can make it difficult to achieve a high expansion ratio, reduce ease of molding, and reduce the compressive strength of the polystyrene-based resin foam molded articles. The average particle size of graphite here refers to the D50 particle size calculated by a laser diffraction/scattering method based on the Mie theory in accordance with JIS Z8825-1.

Examples of metal particles include gold, silver, copper, platinum, palladium, zinc, aluminum, and tin.

Examples of metal compounds include aluminum-based compounds, zinc-based compounds, magnesium-based compounds, titanium-based compounds, antimony-based compounds, calcium-based compounds, and tin-based compounds.

Examples of metal oxides include aluminum oxide, magnesium oxide, titanium oxide, calcium oxide, copper oxide, zinc oxide, and iron oxide.

The content of the radiation heat transfer inhibitor in the expandable polystyrene-based resin particles of the present invention is preferably 2 to 40% by weight based on 100% by weight of the polystyrene-based resin composition. From the standpoint of easy control of the desired expansion ratio and a balance of factors such as the thermal conductivity reduction effect, a content of 3 to 30% by weight is more preferable, and 3 to 20% by weight is even more preferable. A content of the radiation heat transfer inhibitor of 2% by weight or more ensures a sufficient thermal conductivity reduction effect. On the other hand, a content of 40% by weight or less makes it easier to achieve a high expansion ratio and control the expansion ratio because the cell membrane is less likely to rupture when producing pre-expanded particles and polystyrene-based resin foam molded articles from the expandable polystyrene-based resin particles. Here, the term "polystyrene-based resin composition" as used herein refers to a component composition that constitutes the expandable polystyrene-based resin particles, and does not include a blowing agent.

In the present invention, two or more types of radiation heat transfer inhibitors may be added in combination, provided that the effects of the present invention are not impaired.

(Blowing agent)
The expandable polystyrene-based resin particles of the present invention contain butane as a blowing agent. Isobutane is essential as the butane. From the viewpoints of achieving a high expansion ratio by suppressing shrinkage immediately after pre-expansion and production stability, the isobutane content is more than 2.7 wt% and not more than 6.0 wt% relative to 100 wt% of the total amount of the polystyrene-based resin composition and the blowing agent. The isobutane content relative to 100 wt% of the total amount of the polystyrene-based resin composition and the blowing agent is preferably 2.75 wt% or more, more preferably 2.8 wt% or more, and particularly preferably 2.9 wt% or more. Meanwhile, the isobutane content relative to 100 wt% of the total amount of the polystyrene-based resin composition and the blowing agent is preferably 5.0 wt% or less, more preferably 4.5 wt% or less, and particularly preferably 4.2 wt% or less. If the isobutane content exceeds 2.7% by weight, a high expansion ratio can be achieved. On the other hand, if the content is 6.0% by weight or less, when the resin is produced by a melt extrusion method, expansion during the preparation of expandable polystyrene-based resin particles can be suppressed, cutting becomes possible, die clogging is suppressed, and the collection of expandable polystyrene-based resin particles is stabilized.

In addition to isobutane, other hydrocarbon-based blowing agents having 4 to 5 carbon atoms may also be used as the blowing agent in the present invention. Examples include hydrocarbons such as normal pentane, isopentane, normal butane, neopentane, and cyclopentane. When isobutane is used in combination with other hydrocarbon-based blowing agents having 4 to 5 carbon atoms, the amount of the other hydrocarbon-based blowing agent having 4 to 5 carbon atoms added is preferably 1 to 8 parts by weight per 100 parts by weight of the polystyrene-based resin composition. From the perspectives of improved productivity due to a shorter heating time during pre-expansion and improved flame retardancy, 2 to 6 parts by weight is more preferable. Furthermore, since pentane has a stronger plasticizing effect on polystyrene-based resins than isobutane, it is preferable to use isobutane and pentane in combination to shorten the heating time during pre-expansion. Therefore, when isobutane and pentane are used in combination, the amount of pentane added is preferably 1 to 8 parts by weight per 100 parts by weight of the polystyrene-based resin composition. From the perspectives of improved productivity due to a shorter heating time during pre-expansion and improved flame retardancy, 2 to 6 parts by weight is more preferable. As for pentane, it is preferable to use a mixture of normal pentane and isopentane, and more preferably to use normal pentane and isopentane in a weight ratio (normal pentane/isopentane) of 100/0 to 60/40. From the viewpoint of expansion recovery and self-extinguishing properties of the pre-expanded particles after curing at 30°C for 24 hours, a ratio of 98/2 to 60/40 is more preferable.

The amount of blowing agent added is preferably 2.8 to 15 parts by weight per 100 parts by weight of the polystyrene resin composition. When the amount of blowing agent added is 2.8 parts by weight or more, the foaming power is sufficient, making it easy to achieve a high expansion ratio and to produce polystyrene resin foam molded articles with a high expansion ratio. Furthermore, when the amount of blowing agent is 15 parts by weight or less, the flame retardant performance is less likely to deteriorate, and the production time (molding cycle) for producing polystyrene resin foam molded articles is shortened, thereby reducing production costs. The amount of blowing agent added is more preferably 3 to 12 parts by weight, and even more preferably 4 to 10 parts by weight per 100 parts by weight of the polystyrene resin composition.

The expandable polystyrene-based resin particles of the present invention contain a polystyrene-based resin, a radiation heat transfer inhibitor, and a blowing agent, and may optionally contain at least one optional component selected from the group consisting of a flame retardant, a heat stabilizer, a radical generator, an external additive, and other additives. The expandable polystyrene-based resin particles of the present invention preferably contain a polystyrene-based resin, a radiation heat transfer inhibitor, a blowing agent, an external additive, and a flame retardant, and may optionally contain at least one of the optional components listed above excluding the flame retardant. More preferably, the expandable polystyrene-based resin particles contain a polystyrene-based resin, a radiation heat transfer inhibitor, a blowing agent, an external additive, a flame retardant, and a heat stabilizer, and may optionally contain at least one of the optional components listed above excluding the flame retardant and heat stabilizer. Even more preferably, the expandable polystyrene-based resin contains a polystyrene-based resin, a radiation heat transfer inhibitor, a blowing agent, an external additive, a flame retardant, a heat stabilizer, and a nucleating agent, and may optionally contain at least one of the optional components listed above excluding the flame retardant, heat stabilizer, and nucleating agent.

(Flame retardant)
The flame retardant that can be used in the present invention is not particularly limited, and any known flame retardant that has been conventionally used in polystyrene resin foam moldings can be used, but among them, bromine-based flame retardants that have a high flame retardancy-imparting effect are preferred. Examples of brominated flame retardants that can be used in the present invention include brominated bisphenol compounds such as 2,2-bis[4-(2,3-dibromo-2-methylpropoxy)-3,5-dibromophenyl]propane (also known as tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether)) and 2,2-bis[4-(2,3-dibromopropoxy)-3,5-dibromophenyl]propane (also known as tetrabromobisphenol A-bis(2,3-dibromopropyl ether)); brominated butadiene-vinyl aromatic hydrocarbon copolymers such as brominated styrene-butadiene block copolymers, brominated random styrene-butadiene copolymers, and brominated styrene-butadiene graft copolymers (disclosed, for example, in JP-T-2009-516019); and tetrabromocyclooctane. These brominated flame retardants may be used alone or in combination of two or more.

The flame retardant content is preferably more than 1.0 wt% and not more than 6.0 wt%, and more preferably more than 1.0 wt% and not more than 4.0 wt%, per 100 wt% of the polystyrene resin composition, because this makes it easier to control the expansion ratio to the desired level and also balances factors such as flame retardancy when a radiant heat transfer inhibitor is added. If the content is more than 1.0 wt%, the flame retardant effect will not be diminished, and if it is 6.0 wt% or less, the strength of the resulting polystyrene resin foam molded article is less likely to decrease.

(Heat stabilizer)
In the expandable polystyrene-based resin particles of the present invention, by further using a heat stabilizer in combination, it is possible to suppress deterioration of the expandable polystyrene-based resin particles and deterioration of the flame retardant due to decomposition of the flame retardant during the production process.

The heat stabilizers used in the present invention can be used in appropriate combinations depending on the type of polystyrene resin used, the type and content of the foaming agent, the type and content of the radiant heat transfer inhibitor, the type and content of the flame retardant, etc.

Hindered amine compounds, phosphorus-based compounds, and epoxy compounds are preferred as heat stabilizers for use in the present invention, as they allow for the desired control of the 1% weight loss temperature of the polystyrene resin composition in thermogravimetric analysis. Heat stabilizers can be used alone or in combination of two or more.

The amount of heat stabilizer used is preferably 0.5 to 3% by weight per 100% by weight of polystyrene resin composition, as this makes it easier to control the expansion ratio to the desired level and also balances factors such as flame retardancy when a radiation heat transfer inhibitor is added. At 0.5% by weight or more, decomposition of the flame retardant is unlikely to occur and the flame retardancy-imparting effect is not diminished, while at 3% by weight or less, the strength of the resulting polystyrene resin foam molded article is unlikely to decrease.

(External additives)
The external additive that can be used in the present invention is not particularly limited, and any of the known external additives that have been conventionally used for expandable polystyrene-based resin particles can be used. By coating the expandable polystyrene-based resin particles with an external additive, it is possible to suppress blocking (adhesion between expanded particles) during pre-expansion and promote fusion during molding.
Examples of external additives include fatty acid triglycerides such as lauric acid triglyceride, stearic acid triglyceride, and linoleic acid triglyceride; fatty acid diglycerides such as lauric acid diglyceride, stearic acid diglyceride, and linoleic acid diglyceride; fatty acid monoglycerides such as lauric acid monoglyceride, stearic acid monoglyceride, and linoleic acid 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. A preferred coating method for these external additives is to apply them after drying and mix them to coat. A preferred external additive is a combination of zinc stearate and hydroxystearic acid triglyceride, which is preferred because it is easy to achieve both suppression of blocking during pre-expansion and promotion of fusion during molding.

(Other additives)
The polystyrene resin composition of the expandable polystyrene resin particles of the present invention may contain one or more other additives selected from the group consisting of radical generators, processing aids, light resistance stabilizers, nucleating agents, foaming aids, antistatic agents, and colorants such as pigments, as necessary, within the scope of the present invention. Examples of radical generators include cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, 2,3-dimethyl-2,3-diphenylbutane, and poly-1,4-isopropylbenzene. Examples of processing aids include sodium stearate, magnesium stearate, calcium stearate, zinc stearate, barium stearate, and liquid paraffin. Examples of light resistance stabilizers include the above-mentioned hindered amines, phosphorus-based stabilizers, and epoxy compounds, as well as phenolic antioxidants, nitrogen-based stabilizers, sulfur-based stabilizers, and benzotriazoles. Examples of nucleating agents include inorganic compounds such as silica, calcium silicate, wollastonite, kaolin, clay, mica, zinc oxide, calcium carbonate, sodium bicarbonate, and talc; polymeric compounds such as methyl methacrylate copolymers and ethylene-vinyl acetate copolymer resins; olefin waxes such as polyethylene wax; and fatty acid bisamides such as methylene bisstearylamide, ethylene bisstearylamide, hexamethylene bispalmitic acid amide, and ethylene bisoleic acid amide. Examples of foaming aids that can be used include solvents with a boiling point of 200°C or less at atmospheric pressure, such as aromatic hydrocarbons such as styrene, toluene, ethylbenzene, and xylene; alicyclic hydrocarbons such as cyclohexane and methylcyclohexane; and acetate esters such as ethyl acetate and butyl acetate. Antistatic agents and colorants that are used in various resin compositions can be used without particular limitation. These other additives can be used alone or in combination of two or more.

The sphericity of the expandable polystyrene-based resin particles of the present invention is 0.985 or less. To allow the external additives to function effectively, the sphericity is preferably 0.983 or less, and more preferably 0.980 or less. It is believed that a sphericity of 0.985 or less allows the external additives to function effectively, improving moldability and resulting in a foamed molded article with a high expansion ratio. On the other hand, the sphericity of the expandable polystyrene-based resin particles is preferably 0.900 or more, more preferably 0.920 or more, and particularly preferably 0.940 or more. Expandable polystyrene-based resin particles with a sphericity of 0.900 or more do not have an extremely distorted shape, and the significant shrinkage that occurs during high-expansion expansion can be mitigated.

In the expandable polystyrene resin particles of the present invention, the shrinkage percentage of the expanded particles pre-expanded to a bulk ratio of 90 is preferably 25.0 or less, more preferably 22.0 or less, and particularly preferably 20.0 or less. The shrinkage percentage of the pre-expanded particles is calculated by the following formula:
(Formula) Shrinkage rate of pre-expanded particles (%) = (A) / (B) × 100
(A) The bulk expansion ratio of expanded beads pre-expanded to a bulk expansion ratio of 90 times immediately after expansion (B) The bulk expansion ratio after curing the pre-expanded beads at 30° C. for 24 hours When the shrinkage ratio of the pre-expanded beads calculated from the above (formula) is 25.0 or less, the expansion ratio after curing at 30° C. for 24 hours becomes high and the expansion ratio is easily restored. If the shrinkage ratio of the pre-expanded beads is greater than 25.0, the cells of the pre-expanded beads will buckle and the expansion ratio will not be easily restored even if cured at a high temperature. However, since the expandable polystyrene resin beads of the present invention have suppressed shrinkage, there is no need to cure the pre-expanded beads at a high temperature, and control of the expansion ratio after curing becomes easier.

The apparent density of the expandable polystyrene-based resin particles of the present invention is preferably more than 950 kg/ ^m3 and not more than 1200 kg/ ^m3 . From the viewpoint of expandability, it is preferably more than 980 kg/ ^m3 , and more preferably 1000 kg/ ^m3 or more. On the other hand, from the viewpoint of heat insulating performance, it is preferably 1150 kg/ ^m3 or less, more preferably 1100 kg/ ^m3 or less, and particularly preferably 1080 kg/ ^m3 or less.
Here, the density of a typical polystyrene resin is 1050 kg/m ³ to 1060 kg/m ³ , but if the polystyrene resin composition contains a high-density inorganic substance as a radiation heat transfer inhibitor and thereby foaming during extrusion in the production of expandable polystyrene resin particles can be suppressed, the density of the expandable polystyrene resin particles should be higher than the above density of 1050 kg/m ³ to 1060 kg/m ^3. It is believed that foaming during extrusion of a polystyrene resin melt comprising a polystyrene resin composition and a foaming agent is suppressed by having the expandable polystyrene resin particles have an apparent density of more than 950 kg/m ³ and not more than 1200 kg ^/ m 3. Obtaining expandable polystyrene resin particles in which foaming during extrusion is suppressed increases the closed cell ratio in the pre-expanded particles and increases the strength of the cell structure, thereby suppressing shrinkage immediately after pre-expanding and enabling a high expansion ratio.

In the expandable polystyrene-based resin particles of the present invention, after the pre-expanded particles obtained by pre-expanding the expandable polystyrene-based resin particles are cured at 30°C for 24 hours, the amount of blowing agent remaining in the pre-expanded particles is preferably 5.0% to 6.0% by weight, and more preferably 5.2% to 5.8% by weight. Having a blowing agent amount of 5.0% by weight or more in the pre-expanded particles facilitates molding, while having a blowing agent amount of 6.0% by weight or less shortens the molding cycle and provides excellent productivity for foamed molded articles.

In the expandable polystyrene-based resin particles of the present invention, the pre-expanded particles obtained by pre-expanding the expandable polystyrene-based resin particles preferably have a bulk expansion ratio of 90 or more after being cured at 30°C for 24 hours. When the pre-expanded particles have a bulk expansion ratio of 90 or more, the density of the polystyrene-based resin foam molded article obtained by molding the pre-expanded particles decreases, making it possible to produce lighter polystyrene-based resin foam molded articles. Furthermore, increasing the bulk expansion ratio allows for a reduction in the amount of resin used, leading to cost savings.

[Method of producing expandable polystyrene resin particles]
The expandable polystyrene resin particles of the present invention can be obtained by known melt-kneading method, specifically, by melt-kneading polystyrene resin, radiant heat transfer inhibitor and blowing agent in an extruder (melt-kneading step), extruding the melt-kneaded material through a die with small holes attached to the tip of the extruder into a chamber filled with pressurized circulating water (extrusion step), cutting the melt-kneaded material immediately after extrusion with a rotary cutter, and cooling and solidifying with pressurized circulating water (cooling step).By this manufacturing method, it is easy to obtain expandable polystyrene resin particles with sphericity of 0.985 or less.Preferably, it can be obtained by the following manufacturing method of expandable polystyrene resin particles of the present invention.

The method for producing expandable polystyrene-based resin particles of the present invention involves extruding a polystyrene-based resin melt comprising a polystyrene-based resin composition containing a radiant heat transfer inhibitor and a blowing agent through a die having multiple small holes into pressurized circulating water, and cutting the melt into particles with a rotary cutter. The blowing agent contains isobutane, and the amount of isobutane is greater than 2.7% by weight and less than 6.0% by weight, based on 100% by weight of the total amount of the polystyrene-based resin composition and blowing agent (hereinafter, this may be referred to as the "production method of the present invention").

Among the components of the manufacturing method of the present invention, the components described above in the section on "Expandable polystyrene-based resin particles" can also be similarly applied to the manufacturing method of the present invention.

In the manufacturing method of the present invention, from the viewpoint of dispersibility between the polystyrene resin and the various components, it is preferable to first prepare a kneaded mixture by applying a load to the polystyrene resin and the various components using a kneading device equipped with a twin-screw agitator (such as a Banbury mixer), and then feed the resulting kneaded mixture and the polystyrene resin into an extruder to melt-knead and then cut into particles.

In one preferred embodiment of the manufacturing method of the present invention, a polystyrene resin and a radiation heat transfer inhibitor are mixed in a mixer equipped with a twin-screw agitator, such as a Banbury mixer, to prepare a masterbatch mixture. The masterbatch is then melt-mixed in an extruder with new polystyrene resin, a blowing agent, and, if necessary, other components such as a flame retardant. The resulting molten resin is extruded through a die with small holes attached to the tip of the extruder into a cutter chamber filled with pressurized circulating water, where it is cut with a rotating cutter immediately after extrusion and cooled and solidified with the pressurized circulating water. The melt-mixing in the extruder can be performed using a single extruder, multiple connected extruders, or an extruder in combination with a second mixer such as a static mixer or an agitator without a screw, and can be selected as appropriate.

The polystyrene resin and radiation heat transfer inhibitor are preferably kneaded to prepare a masterbatch using a kneading device equipped with a twin-screw agitator, such as an intensive mixer, internal mixer, or Banbury mixer, which can knead the resin under load. In this case, the concentration of the masterbatch is not particularly limited, but a radiation heat transfer inhibitor concentration of 20% to 80% by weight is preferred for balancing kneadability and cost. The prepared masterbatch, polystyrene resin, blowing agent, and, if necessary, flame retardant, thermal stabilizer, and other additives are melt-kneaded in a first extruder and, if necessary, a second kneading device attached to the extruder. The resulting resin melt is cooled to a predetermined temperature and then extruded through a die with small holes into a cutter chamber filled with pressurized circulating water. Immediately after extrusion, the resin is cut into pellets using a rotary cutter, and the resulting pellets (resin particles) are cooled and solidified using pressurized circulating water to obtain expandable polystyrene resin particles. Similarly, for other additives such as flame retardants and heat stabilizers, a masterbatch of the polystyrene resin and other additives may be prepared in advance and then added to the extruder. Furthermore, for radiant heat transfer inhibitors, flame retardants, heat stabilizers, and other additives, the raw materials may be directly added to the extruder without being made into a masterbatch.

When pentane and butane are used together as blowing agents, the method of addition is not particularly important as long as pentane and butane are added; they may be added simultaneously, or one may be added first and then the other.

The amount of isobutane used in the production method of the present invention is more than 2.7% by weight and not more than 6.0% by weight, based on 100% by weight of the total amount of the polystyrene resin composition and blowing agent. When produced by melt extrusion, if the amount of isobutane exceeds 6.0% by weight, it becomes difficult to suppress foaming during the production of expandable polystyrene resin particles, causing the die to clog, and making it difficult to stably obtain expandable polystyrene resin particles.

In the manufacturing method of the present invention, isobutane and pentane may be used in combination. When isobutane and pentane are used in combination, the amount of pentane added is preferably 1 to 8 parts by weight per 100 parts by weight of the polystyrene resin composition. From the viewpoints of improving productivity by shortening the heating time during pre-expansion and improving flame retardancy, an amount of 2 to 6 parts by weight is more preferable.

The set temperature of the melt-kneading section of the extruder is preferably 100°C to 250°C. Furthermore, it is preferable that the residence time in the extruder from the time the polystyrene resin and various components are fed into the extruder until the end of melt-kneading is 10 minutes or less. If the set temperature in the melt-kneading section of the extruder is 250°C or less and/or the residence time in the extruder until the end of melt-kneading is 10 minutes or less, the flame retardant added will not decompose and the desired flame retardancy can be achieved, and there is no need to add an excessive amount of flame retardant to impart the desired flame retardancy. On the other hand, if the set temperature in the melt-kneading section of the extruder is 100°C or higher, the load on the extruder will not be large, extrusion will be stable, and the added components will be well dispersed.

Here, the melt-kneading section of the extruder means the section from the feed section onwards to the tip of the final downstream extruder when the extruder is configured with a single or twin screw. When a second kneading device, such as a static mixer or a mixer without a screw, is used in conjunction with the first extruder, it means the section from the feed section of the first extruder to the tip of the second kneading device.

The water pressure of the pressurized circulating water is preferably 1.0 MPa or more and 2.0 MPa or less, and more preferably 1.1 MPa or more and 1.8 MPa or less. If the water pressure is 1.0 MPa or more, foaming can be suppressed, the bulk density of the expandable polystyrene resin particles increases, and a decrease in expansion ratio and transport efficiency are less likely to occur. On the other hand, if the water pressure is 2.0 MPa or less, the rotary cutter is not pushed back by the water pressure, and the extruded molten resin does not wrap around the rotary cutter, ensuring stable production.

The temperature of the pressurized circulating water is preferably 45°C or higher and 80°C or lower, and more preferably 50°C or higher and 70°C or lower. If the water temperature is 45°C or higher, blockage of the die due to cooling can be suppressed. On the other hand, if the water temperature is 80°C or lower, foaming in the pressurized circulating water can be suppressed, the bulk density of the expandable polystyrene resin particles increases, and a decrease in transport efficiency is less likely to occur.

The die used in the present invention is not particularly limited, but examples include dies with small holes having a diameter of 0.3 mm to 2.0 mm, preferably 0.4 mm to 1.5 mm.

The cutting device that cuts the molten resin extruded into the pressurized circulating water is not particularly limited, but examples include a device in which the resin is cut into small pellets using a rotating cutter that comes into contact with a die lip, and then transported to a centrifugal dehydrator where it is dehydrated and collected.

The method for coating the external additives used in the present invention is not particularly limited, but a preferred coating method is, for example, a method in which the additives are applied after drying and then mixed to coat the surface.

[Polystyrene-based resin foam molded product]
The expandable polystyrene-based resin particles of the present invention are not particularly limited, but can be expanded to a predetermined expansion ratio to form pre-expanded particles, and a polystyrene-based resin foam molded article can be produced by a pre-expanding method in which the expandable polystyrene-based resin particles are expanded to a predetermined expansion ratio to form pre-expanded particles, and these pre-expanded particles are used for molding.

The higher the expansion ratio of a polystyrene-based resin foam molded product, the less raw material expandable polystyrene-based resin particles are required. Therefore, according to the present invention, polystyrene-based resin foam molded products with high expansion ratios can be produced more inexpensively. It should be noted that high expansion ratios were difficult with conventional expandable polystyrene-based resin particles containing graphite. However, with the expandable polystyrene-based resin particles of the present invention and the expandable polystyrene-based resin particles obtained by the manufacturing method of the present invention, high expansion ratios are possible by controlling the isobutane content contained in the expandable polystyrene-based resin particles, making it possible to provide a lightweight, easy-to-handle, and more affordable insulating material.

The expandable polystyrene resin particles of the present invention are subjected to a known pre-expansion process, for example, by expanding them 10 to 120 times using steam to form pre-expanded particles (pre-expansion process), and after curing for a certain period of time as necessary, the pre-expanded particles are molded using steam using a known molding machine to produce a polystyrene resin foam molded article. Depending on the shape of the mold used, complex molded articles or block-shaped molded articles can be obtained.

(Pre-expansion process)
The pre-expansion step can be carried out using a pre-expansion machine in the same manner as in the pre-expansion of conventional expandable polystyrene resin particles.

A known pre-expansion machine can be used, such as a pre-expansion machine equipped with a stirrer, a can containing expandable polystyrene resin particles, a steam chamber installed below the can for supplying steam to the can, and a pre-expanded particle outlet.

The internal pressure (gauge pressure) when steam is introduced is not particularly limited, but is preferably 0.001 to 0.15 MPa, more preferably 0.01 to 0.10 MPa, and even more preferably 0.03 to 0.08 MPa. If the internal pressure is 0.01 MPa or higher, the steam introduction time during pre-expansion can be reduced to 500 seconds or less when a high expansion ratio is desired. If the internal pressure is 0.15 MPa or lower, there is no need to increase the steam pressure, which reduces the occurrence of blocking and increases the pre-expansion yield.

The pre-expansion process can be carried out either continuously or batchwise.

The continuous method involves continuously feeding expandable polystyrene resin particles into a can and discharging pre-expanded particles from a discharge port provided at the top of the can. The expansion ratio of the pre-expanded particles can be adjusted, for example, by appropriately selecting the amount (weight) of expandable polystyrene resin particles fed into the can per unit time. In the case of the continuous method, the residence time in the pre-expanding machine can from the time the expandable polystyrene resin particles are fed into the can until the pre-expanded particles are discharged is defined as the steam feeding time.

The batch method involves placing a predetermined amount of expandable polystyrene resin particles in a can, pre-expanding them to a predetermined expansion ratio, then stopping the supply of steam, and then blowing air into the can as needed to cool and dry the pre-expanded particles, which are then removed from the can. The expansion ratio of the pre-expanded particles can be adjusted by appropriately selecting the amount (weight) of expandable polystyrene resin particles placed in the can per batch. Because the batch method involves pre-expanding the added expandable polystyrene resin particles to a predetermined volume, the smaller the amount placed per batch, the higher the expansion ratio of the resulting pre-expanded particles.

It is also advisable to cure the pre-expanded particles immediately after pre-expansion. Water vapor is present inside the expanded particles during pre-expansion, but this water vapor condenses into water during the cooling process after expansion, resulting in a reduced pressure inside the pre-expanded particles immediately after pre-expansion. If the cell wall strength of the pre-expanded particles is low during this reduced pressure state, shrinkage may occur easily. Furthermore, if the closed cell ratio of the pre-expanded particles is low, the strength of the cell structure decreases, making them more susceptible to shrinkage. For this reason, a curing process is effective, in which the inside of the pre-expanded particles is replaced with air and returned to atmospheric pressure. Because shrinkage immediately after pre-expansion is suppressed in the pre-expanded particles of the polystyrene-based resin expandable particles of the present invention, the curing process can restore the desired expansion ratio.

The temperature during curing is not particularly limited, but is preferably 20 to 80°C, more preferably 25 to 70°C, and even more preferably 30 to 60°C. If the curing temperature is 20°C or higher, air is more easily introduced into the pre-expanded particles, which were previously in a reduced pressure state, and the pressure inside the expanded particles is more easily returned to atmospheric pressure. If the curing temperature is 80°C or lower, the blowing agent present in the pre-expanded particles is less likely to escape, and the expansion power is not reduced, and the surface beauty of the molded product is not impaired.

The polystyrene resin foam molded article of the present invention can be used for a variety of purposes, such as building insulation materials used for floors, walls, roofs, etc.; agricultural and marine product boxes such as boxes for transporting marine products such as fish and agricultural products such as vegetables; bathroom insulation materials; and hot water tank insulation materials.

The present invention will be specifically explained below based on reference examples, examples, and comparative examples, but the present invention is not limited to these.

The measurement and evaluation methods used in the following Reference Examples, Examples, and Comparative Examples are as follows:

(Method for measuring sphericity of expandable polystyrene resin particles)
(1) Measuring device: CAMSIZER P4 manufactured by Retsch Technology
(2) Setting conditions: The feeder and funnel parameters were set to the following conditions.
- Control level when moving forward at high speed: 55
- Maximum time for fast forward movement [seconds]: 60
・Level at the start of measurement: 50
・Maximum control level: 75
・Target coverage area [%]: 0.5
Feeder width [mm]: 60
- A guidance sheet was used. Measurement data was also adopted under the following conditions.
・Basic camera coverage area [%] < 3
・Zoom camera coverage area [%] < 5
However, particles that met the following conditions in the photographed projection were excluded from the measurement data.
・Convexity ≧ 0.99
If particles fall onto each other at the projection measurement point, the shape of each particle cannot be accurately evaluated, so the feeder and funnel parameters were set to the above conditions. Similarly, if a large number of particles fall at the same time, there is a similar possibility that the shape of each particle cannot be accurately evaluated. For this reason, if the number of particles that fell was greater than the set coverage area, the projection and data for that particle were excluded.

Furthermore, in order to eliminate the influence of minute foreign matter such as dust, data with a convexity (surface roughness) of 0.99 or more was excluded from the analysis.
(3) Measurement method: A projection image of about 50 g of expandable polystyrene-based resin particles was photographed using the CAMSIZER P4 set as described above, and the perimeter and area of the projection image of each expandable polystyrene-based resin particle were measured. The average value of sphericity was calculated from the perimeter and area of the projection image of each expandable polystyrene-based resin particle according to the following formula.

In the above formula, S _i is the area (mm ² ) of the i-th particle, and R _i is the perimeter (mm) of the i-th particle.

(Method for measuring apparent density of expandable polystyrene resin particles)
W (kg) of expandable polystyrene resin particles was collected as a measurement sample, and this measurement sample was allowed to fall naturally into a measuring cylinder containing ethanol. The mass (kg) and volume (m ³ ) were measured, and the apparent density was calculated based on the following formula.

Apparent density (kg/m ³ )=weight of measurement sample (W)/volume of measurement sample (V).

(Method for measuring bulk magnification of pre-expanded particles)
W (g) of each pre-expanded particle was collected as a measurement sample, and this measurement sample was allowed to fall naturally into a measuring cylinder, and the measuring cylinder was then struck to keep the apparent volume V (cm ³ ) of the sample constant. The mass (g) and volume (cm ³ ) of the sample were then measured, and the bulk ratio was calculated based on the following formula:

Bulk magnification (cm ³ /g)=volume of measurement sample (V)/weight of measurement sample (W)
The bulk expansion ratio of pre-expanded particles measured within 5 to 10 minutes after the pre-expanded particles were discharged from the pre-expanding machine is defined as the bulk expansion ratio immediately after expansion, when shrinkage occurs after pre-expansion.

For pre-expanded particles, the bulk expansion ratio measured after shrinkage and curing at 30°C for 24 hours is defined as the bulk expansion ratio after curing.

(Calculation method for shrinkage rate of pre-expanded particles)
The value calculated by the following formula is defined as the shrinkage rate of the pre-expanded particles.

Formula: (A)/(B)×100
(A) Bulk expansion ratio of expanded beads pre-expanded to a bulk expansion ratio of 90 times immediately after expansion. (B) Bulk expansion ratio of the pre-expanded beads after curing at 30°C for 24 hours. (Method for measuring the amount of blowing agent in pre-expanded beads)
The expandable polystyrene resin particles were pre-expanded, and after curing at 30°C for 24 hours, the weight _W1 (g) of the expanded particles was measured, heated in an oven at 150°C for 30 minutes, then cooled at room temperature in a desiccator for 30 minutes, and the weight _W2 (g) was measured again. The value calculated using the following formula was used as the amount of blowing agent in the pre-expanded particles.

Amount of foaming agent in pre-expanded particles (wt %)=(W ₁ −W ₂ )/W ₁ ×100
(Expansion ratio of polystyrene resin foam molded product)
The polystyrene-based resin foam molded article was removed from the mold and dried at 30°C for 24 hours, after which the weight (g) of the foam molded article was measured, and the length, width, and thickness were measured using a vernier caliper. The volume ( ^cm3 ) of the polystyrene-based resin foam molded article was calculated from the measured dimensions, and the expansion ratio was calculated according to the following formula.
Expansion ratio (cm ³ /g)=test piece volume (cm ³ )/test piece weight (g)
The expansion ratio "times" of a polystyrene-based resin foam molded article is also conventionally expressed in "cm ³ /g".

(Method for measuring thermal conductivity of polystyrene-based resin foam molded products)
It is generally known that the higher the average temperature at which thermal conductivity is measured, the higher the thermal conductivity value, and so it is necessary to determine the average temperature at which it is measured in order to compare thermal insulation properties. In this specification, the standard used is 23°C, as defined in JIS A9511:2006R, the standard for foamed plastic insulation materials.

Thermal conductivity was measured after the polystyrene resin foam molded body was left to stand at 70°C for 96 hours, a sample for measuring thermal conductivity was cut out, and the sample was then left to stand at 23°C for 24 hours.

More specifically, the polystyrene resin foam molded article was left to stand at 70°C for 96 hours, and then a sample measuring 300 mm long x 300 mm wide x 50 mm was cut out. The sample was then left to stand at 23°C for 24 hours, after which its thermal conductivity was measured using a thermal conductivity measuring device (HC-074, manufactured by Eiko Seiki Co., Ltd.) at an average temperature of 23°C and a temperature difference of 20°C using the heat flow meter method in accordance with JIS A1412-2:1999.

(Evaluation of flame retardancy of polystyrene resin foam molded products)
The flame retardancy was measured by leaving the polystyrene resin foam molded article at 70°C for 96 hours, cutting out a sample for evaluation of flame retardancy, and leaving it at 23°C for 24 hours. The flame retardancy was evaluated in accordance with JIS A9511:2006R (measurement method A was used). The flame extinction time was taken as the average of the measurement results for five test pieces, and the flame retardancy of the polystyrene resin foam molded article was evaluated based on the following criteria.

○: Extinguished within 3 seconds ×: Burning continued for 3 seconds or more (Fusion evaluation of polystyrene resin foam molding)
The obtained polystyrene-based resin foam molded article was broken, and the fracture surface was visually observed to determine the area of the entire fracture surface where the particles themselves, not the particle interfaces, were broken, and the percentage (%) of the area where the particles themselves were broken relative to the entire fracture surface area was calculated. The fusion of the polystyrene-based resin foam molded article was evaluated based on the following criteria.

◯: More than 80% fused △: More than 60% fused but not more than 80% x: Not more than 60% fused The raw materials used in the Reference Examples, Examples and Comparative Examples are listed below.

(styrene-based resin)
(A) Styrene homopolymer [manufactured by PS Japan Co., Ltd., 680]
(Graphite)
(B) Graphite [Flake graphite SGP-40B, manufactured by Marutoyo Foundry Co., Ltd.]
(brominated flame retardants)
(C) 2,2-bis[4-(2,3-dibromo-2-methylpropoxy)-3,5-dibromophenyl]propane [manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., SR-130, bromine content = 66 wt%].

(Heat stabilizer)
(D1) Tetrakis(2,2,6,6-tetramethylpiperidyloxycarbonyl)butane [LA-57, manufactured by ADEKA Corporation]
(D2) Bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite [PEP-36, manufactured by ADEKA Corporation].

(Blowing agent)
(E1) Normal pentane [Wako Pure Chemical Industries, Ltd., reagent]
(E2) Isopentane [Wako Pure Chemical Industries, Ltd., reagent]
(E3) Isobutane [manufactured by Mitsui Chemicals, Inc.]
(Other additives)
(F) Ethylene bisstearic acid amide (NOF Corporation, Alflow H-50S).

(Production Example 1) (Graphite Masterbatch (G))
A Banbury mixer was charged with 49 wt% polystyrene resin (A), 50 wt% graphite (B), and 1 wt% ethylene bisstearamide (F) so that the total weight (A + B + F) was 100 wt%. The raw materials were mixed under a load of 5 kgf/ ^cm² for 20 minutes without heating or cooling. The resin temperature was measured at this time and found to be 180°C. The resin was fed into a ruder and extruded at 250 kg/hr through a die with small holes attached to the tip. The strand-shaped resin was cooled and solidified in a 30°C water bath and then cut to obtain masterbatch (G). The graphite content in masterbatch (G) was 50 wt%.

(Production Example 2) (Masterbatch (H) of a mixture of a brominated flame retardant and a heat stabilizer)
After feeding polystyrene resin (A) into a twin-screw extruder and melt-kneading, a mixture of brominated flame retardant (C), heat stabilizers (D1) and (D2) was fed midway through the extruder and further melt-kneaded. The weight ratio of each material was (A):(C):(D1):(D2) = 70:28.5:0.6:0.9, with (A) + (C) + (D1) + (D2) = 100 wt%. Strand-shaped resin extruded at 300 kg/hr through a die with small holes attached to the tip of the extruder was cooled and solidified in a water bath at 20 °C, and then cut to obtain a masterbatch (H) of a mixture of brominated flame retardant and heat stabilizer. The extruder temperature was set to 170 °C.

(Production Example 3) (Mixture (I) of Brominated Flame Retardant and Heat Stabilizer) Brominated flame retardant (C), heat stabilizers (D1) and (D2) were mixed in a mixer to obtain mixture (I) of brominated flame retardant and heat stabilizer. The weight ratio of each material was (C):(D1):(D2) = 100:2.1:3.2, with (C) + (D1) + (D2) = 100% by weight.

(Reference example 1)
[Preparation of polystyrene-based resin particles]
The polystyrene resin (A), the masterbatch (H), and the graphite masterbatch (G) were each placed in a blender and blended for 10 minutes to obtain a resin mixture.

The weight ratio of each material was (A):(H):(G) = 83.65:8.35:8.00, with (A) + (H) + (G) = 100% by weight. The resulting resin mixture was fed into a 40 mm diameter co-rotating twin-screw extruder and melt-kneaded at a set temperature of 190°C and a screw rotation speed of 230 rpm. The resin was extruded at a rate of 70 kg/hour through a die equipped with 30 small holes, each 1.4 mm in diameter, attached to the tip of the extruder. The resulting strand-like resin was cooled and solidified in a 20°C water bath and then cut with a strand cutter to obtain styrene-based resin particles. The resin temperature at the tip of the extruder was 220°C.

[Preparation of expandable polystyrene resin particles]
Next, 100 parts by weight of the resulting styrene-based resin particles were charged into a 6 L autoclave equipped with a stirrer with 200 parts by weight of deionized water, 1 part by weight of tricalcium phosphate, 0.03 parts by weight of sodium dodecylbenzenesulfonate, and 1 part by weight of sodium chloride, and the pressure vessel was sealed. The mixture was then heated to 105°C over one hour, and 7 parts by weight of mixed pentane (a mixture of 80% normal pentane and 20% isopentane) and 1.5 parts by weight of isobutane were added to the pressure vessel over 30 minutes. The temperature was then raised to 115°C over 10 minutes and maintained at that temperature for four hours. After cooling to room temperature, the resin particles impregnated with the blowing agent were removed from the autoclave, pickled with hydrochloric acid, washed with water, dehydrated using a centrifuge, and then dried to remove moisture adhering to the resin particle surface using an airflow dryer. The resulting expandable polystyrene-based resin particles had a sphericity of 0.988 and an apparent density of 1,030 kg/ ^m3 .

Example 1
[Preparation of Expandable Polystyrene-Based Resin Particles]
The polystyrene resin (A), graphite (B), and the mixture (I) of brominated flame retardant and thermal stabilizer were each fed by a feeder to a tandem two-stage extruder consisting of a 40 mm diameter co-rotating twin-screw extruder (first extruder) and a 90 mm diameter single-screw extruder (second extruder) connected in series, and melt-kneaded at a set temperature of 190°C and a rotation speed of 167 rpm for the 40 mm diameter extruder. The weight ratio of (A):(B):(I) was 93:4.5:2.5, and the total feed rate was 55.7 kg/h. A mixed pentane [a mixture of 80% by weight of normal pentane (E1) and 20% by weight of isopentane (E2)] was injected into a 40 mm diameter extruder (first extruder) at a ratio of 5.2 parts by weight per 100 parts by weight of the melt of the resin mixture (resin composition), and 3.0 parts by weight of isobutane (E3) was injected per 100 parts by weight of the resin composition, resulting in a total of 8.2 parts by weight of a blowing agent. The mixture was then supplied to a 90 mm diameter extruder (second extruder) through a continuation pipe set at 200°C.

The molten resin was cooled to a resin temperature of 160°C using a 90 mm diameter extruder (second extruder), and then extruded through a die equipped with 60 small holes, each 0.65 mm in diameter and 5.0 mm in length, attached to the tip of the second extruder, set at 250°C, into pressurized circulating water at a temperature of 62°C and a pressure of 1.3 MPa. The extruded molten resin was cut and granulated using a rotary cutter with six blades that contacted the die, and then transferred to a centrifugal dehydrator to obtain expandable polystyrene-based resin particles. The residence time in the first extruder was 2 minutes, and the residence time in the second extruder was 5 minutes.

[Preparation of pre-expanded particles]
The resulting expandable polystyrene-based resin particles were stored at 15°C for at least one week, and then dry-blended with 0.04 parts by weight of zinc stearate and 0.1 parts by weight of hydroxystearic acid triglyceride as external additives. 220 g of the expandable polystyrene-based resin particles containing the external additives were placed in a pre-expander [batch-type pre-expander manufactured by Daikai Kogyo Co., Ltd.], the internal pressure was set to 0.05 kg/ ^cm2 to 0.15 kg/ ^cm2 , and 0.10 MPa of steam was introduced into the pre-expander to expand the particles to a bulk expansion ratio of 90, yielding pre-expanded particles.

[Preparation of polystyrene resin foam molded article]
The resulting pre-expanded particles were cured at 30°C for 24 hours and then filled into an in-mold molding die (length 400 mm x width 400 mm x thickness 50 mm) attached to a polystyrene foam molding machine [Daisen Kogyo Co., Ltd., KR-57]. 0.06 MPa of steam was introduced to cause in-mold foaming, and the die was then cooled by spraying water onto it. The polystyrene foam molded body was held in the die until the pressure of the polystyrene foam molded body pressing against the die reached 0.01 MPa (gauge pressure), after which the polystyrene foam molded body was removed to obtain a polystyrene foam molded body. The resulting polystyrene foam molded body had an expansion ratio of 90.5 times, and the thermal conductivity of the polystyrene foam molded body, measured using the above-mentioned method, was 0.03084 W/m K.

The various properties of the produced expandable polystyrene-based resin particles and pre-expanded particles were measured and evaluated using the measurement and evaluation methods described above. The measurement and evaluation results for the expandable polystyrene-based resin particles and pre-expanded particles are shown in Table 1.

Example 2
A polystyrene-based resin foam molded article was produced in the same manner as in Example 1, except that in [Preparation of Expandable Polystyrene-based Resin Particles], the amount of isobutane (E3) was changed to 3.2 parts by weight per 100 parts by weight of the resin composition. The expansion ratio of the obtained polystyrene-based resin foam molded article was 92.6 times, and the thermal conductivity measured by the above-mentioned measuring method was 0.03078 W/m K.

The various properties of the resulting expandable polystyrene resin particles and pre-expanded particles were measured and evaluated in the same manner as in Example 1. The measurement and evaluation results are shown in Table 1.

Example 3
In [Preparation of Expandable Polystyrene Resin Particles], except that the amount of isobutane (E3) was changed to 3.7 parts by weight per 100 parts by weight of the resin composition, and the resin composition was extruded into pressurized circulating water at 1.38 MPa, a polystyrene resin foam molded article was prepared in the same manner as in Example 1. The expansion ratio of the obtained polystyrene resin foam molded article was 92.6 times, and the thermal conductivity, measured by the above-mentioned measuring method, was 0.03088 W/m K.

The various properties of the resulting expandable polystyrene resin particles and pre-expanded particles were measured and evaluated in the same manner as in Example 1. The measurement and evaluation results are shown in Table 1.

(Comparative Example 1)
In [Preparation of Expandable Polystyrene Resin Particles], except that the amount of isobutane (E3) was changed to 2.5 parts by weight per 100 parts by weight of the resin composition, and the resin composition was extruded into pressurized circulating water at 1.25 MPa, a polystyrene resin foam molded article was prepared in the same manner as in Example 1. The expansion ratio of the obtained polystyrene resin foam molded article was 92.1 times, and the thermal conductivity, measured by the above-mentioned measuring method, was 0.03130 W/m K.

The various properties of the resulting expandable polystyrene resin particles and pre-expanded particles were measured and evaluated in the same manner as in Example 1. The measurement and evaluation results are shown in Table 1.

(Comparative Example 2)
In [Preparation of Expandable Polystyrene Resin Particles], except that the amount of isobutane (E3) was changed to 2.8 parts by weight per 100 parts by weight of the resin composition, and the resin composition was extruded into pressurized circulating water at 1.25 MPa, a polystyrene resin foam molded article was prepared in the same manner as in Example 1. The expansion ratio of the obtained polystyrene resin foam molded article was 92.2 times, and the thermal conductivity, measured by the above-mentioned measuring method, was 0.03101 W/m K.

The various properties of the resulting expandable polystyrene resin particles and pre-expanded particles were measured and evaluated in the same manner as in Example 1. The measurement and evaluation results are shown in Table 1.

(Comparative Example 3)
In the [Preparation of Expandable Polystyrene Resin Particles], the weight ratio of polystyrene resin (A): graphite (B): mixture of bromine-based flame retardant and heat stabilizer (I) was changed to 95:4.5:0.5, and the amount of isobutane (E3) was changed to 2.5 parts by weight per 100 parts by weight of the resin composition, and the extrusion was carried out into pressurized circulating water at 1.25 MPa. A polystyrene resin foam molded article was prepared in the same manner as in Example 1. The expansion ratio of the obtained polystyrene resin foam molded article was 90.6 times, and the thermal conductivity measured by the above-mentioned measurement method was 0.03255 W/m K.

The various properties of the resulting expandable polystyrene resin particles and pre-expanded particles were measured and evaluated in the same manner as in Example 1. The measurement and evaluation results are shown in Table 1.

Claims (10)

Expandable polystyrene-based resin particles comprising a polystyrene-based resin composition containing a radiation heat transfer inhibitor and a blowing agent,
the sphericity of the expandable polystyrene-based resin particles is 0.985 or less,
the blowing agent comprises isobutane;
the content of isobutane is more than 2.7% by weight and 6.0% by weight or less relative to 100% by weight of the total amount of the polystyrene-based resin composition and the blowing agent ,
the content of the radiation heat transfer inhibitor is 2 to 40% by weight based on 100% by weight of the polystyrene-based resin composition;
The total amount of the foaming agent is 2.8 to 15 parts by weight based on 100 parts by weight of the polystyrene resin composition . the polystyrene-based resin composition contains a flame retardant,
The expandable polystyrene-based resin particles according to claim 1 , wherein the content of the flame retardant is more than 1.0% by weight and not more than 6.0% by weight, based on 100% by weight of the polystyrene-based resin composition. Expandable polystyrene-based resin particles according to claim 1 or 2, wherein the blowing agent comprises pentane. The expandable polystyrene-based resin particles according to any one of claims 1 to 3 , wherein the expandable polystyrene-based resin particles have an apparent density of more than 950 kg/ ^m3 and not more than 1200 kg/ ^m3 . The expandable polystyrene-based resin particles according to any one of claims 1 to 4, wherein the amount of blowing agent in the pre-expanded particles when the expandable polystyrene-based resin particles are pre-expanded to a bulk expansion of 90 times and then aged at 30°C for 24 hours is 5.0% by weight to 6.0% by weight. The expandable polystyrene-based resin particles according to any one of claims 1 to 5, wherein the expandable polystyrene-based resin particles are pre-expanded to a bulk expansion of 90 times and then cured at 30°C for 24 hours, and the pre-expanded particles are then expanded and molded, resulting in an expansion molded body having a thermal conductivity of 0.034 W/m K or less. 7. Pre-expanded particles of the expandable polystyrene resin particles according to claim 1 , having a bulk expansion ratio of 90 times or more. A method for producing expandable polystyrene-based resin particles, comprising extruding a polystyrene-based resin melt comprising a polystyrene-based resin composition containing a radiation heat transfer inhibitor and a blowing agent through a die having a plurality of small holes into pressurized circulating water, and cutting the melt into particles with a rotary cutter,
the blowing agent comprises isobutane;
isobutane is contained in an amount of more than 2.7% by weight and not more than 6.0% by weight relative to 100% by weight of the total amount of the polystyrene-based resin composition and the blowing agent ,
the content of the radiation heat transfer inhibitor is 2 to 40% by weight based on 100% by weight of the polystyrene-based resin composition;
The method for producing expandable polystyrene-based resin particles , wherein the total amount of the foaming agent is 2.8 to 15 parts by weight based on 100 parts by weight of the polystyrene-based resin composition . The method for producing expandable polystyrene-based resin particles according to claim 8 , wherein the radiative heat transfer inhibitor comprises a carbon-based radiative heat transfer inhibitor. The method for producing expandable polystyrene-based resin particles according to claim 8 or 9 , wherein the blowing agent contains pentane.