JP7538642B2 - Expandable polystyrene resin particles, pre-expanded polystyrene particles, and foamed molded articles. - Google Patents

Description

The present invention relates to expandable polystyrene resin particles, polystyrene expanded particles, and foamed molded articles.

Expandable polystyrene resin particles are a raw material for foamed molded products. They can be expanded and molded using a heating medium such as steam, and are relatively inexpensive, so they are widely used.

Expandable polystyrene resin particles can be produced, for example, by impregnating the particles with a blowing agent (e.g., a highly volatile aliphatic hydrocarbon such as butane or pentane) in an aqueous suspension of polystyrene resin particles.

A method for producing a foamed molded product of a desired shape from expandable polystyrene resin particles generally includes the following steps: (1) a pre-expanding step in which the expandable polystyrene resin particles are pre-expanded using a heating medium to form pre-expanded particles; (2) a filling step in which the pre-expanded particles are filled into a closed mold having a large number of small holes drilled in the wall surface; (3) a molding step in which a heating medium is introduced through the small holes in the mold to heat the pre-expanded particles to a temperature above their softening point and fuse the pre-expanded particles to each other to form them into the desired shape; and (4) a removal step in which the foamed molded product is removed from the mold after cooling.

The foamed molded products produced by this method can be easily molded into the desired shape, are lightweight, and have excellent insulating and cushioning properties, making them suitable for a variety of applications. For example, they are used as packaging materials (trays) for food containers, and as transport packaging materials for fish boxes, etc.

In the above pre-expansion process, the pre-expanded particles may bond (or coalesce) with each other (also called blocking), resulting in the formation of lumps. If blocking occurs, the productivity of the pre-expanded particles decreases. Therefore, in order to eliminate blocking, it is known to coat the surface of the resin particle body, which is the core of the expandable polystyrene resin particle, with an external additive such as a blocking inhibitor made of a specific substance.

For example, Patent Document 1 discloses expandable polystyrene-based resin particles in which the surfaces of the resin particles are coated with a nonionic surfactant and methylphenylpolysiloxane.

On the other hand, Patent Document 2 discloses that the surface of the resin particle body is coated with methylphenyl silicone oil having a refractive index of 1.45 or more at 25°C and a metal salt of a higher fatty acid in order to shorten the molding cycle and increase the strength and surface gloss of the resulting foamed molded body.

Patent Document 3 also discloses that in order to prevent blocking during pre-expansion, the surfaces of the resin particles are coated with a nonionic surfactant, methylphenylpolysiloxane, a fatty acid metal salt, and a fatty acid glyceride compound.

JP 2018-168265 A JP 2007-246705 A JP 2019-65074 A

However, the above-mentioned conventional techniques leave room for improvement in terms of the fluidity of the expandable polystyrene resin particles, prevention of blocking during the pre-expansion process, and reduction of equipment contamination during the molding process.

One embodiment of the present invention has been made in consideration of the above problems, and its purpose is to provide expandable polystyrene resin particles, polystyrene pre-expanded particles, and foamed molded articles that can prevent blocking in the pre-expanding process of expandable polystyrene resin particles over a long period of time, reduce contamination of equipment in the molding process, and provide molded articles with practical strength.

As a result of intensive research into solving the above problems, the present inventors have confirmed that by applying an appropriate amount of a fusion promoter to the surface of the expandable polystyrene-based resin particle body containing an acrylic resin of a specific particle size, blocking during the pre-expanding step of the expandable polystyrene-based resin particles can be prevented for a long period of time, contamination of the device caused by peeling of external additives from the surface of the pre-expanded particles can be suppressed, and an expanded molded product having flexibility and practical strength can be obtained, thereby completing the present invention. That is, one embodiment of the present invention includes the following configuration.
[1] Expandable polystyrene-based resin particles, characterized in that the surface of an expandable polystyrene-based resin particle body containing a base resin including styrene units as constituent units and an acrylic resin is coated with 0.01 to 0.2 parts by weight of a fusion promoter per 100 parts by weight of the expandable polystyrene-based resin particle body, and the acrylic resin is present dispersedly in the expandable polystyrene-based resin particle body as aggregate particles of basic particles having an average particle diameter of 0.2 μm or less.
[2] The expandable polystyrene-based resin particles according to [1], characterized in that the surface of the expandable polystyrene-based resin particle body is further coated with more than 0 parts by weight and not more than 0.2 parts by weight of polyethylene wax per 100 parts by weight of the expandable polystyrene-based resin particle body.
[3] The expandable polystyrene-based resin particles according to any one of [1] to [2], characterized in that the fusion promoter is one or a mixture of a fatty acid triglyceride, a fatty acid diglyceride, and a fatty acid monoglyceride.
[4] The expandable polystyrene-based resin particle body contains 0.01 to 0.5 parts by weight of the acrylic resin per 100 parts by weight of the base resin. [1] to [3].
[5] The expandable polystyrene-based resin particles according to any one of [1] to [4], characterized in that the acrylic resin is at least one selected from the group consisting of a homopolymer of an alkyl acrylate or an alkyl methacrylate, and a copolymer or graft copolymer of an alkyl acrylate and an alkyl methacrylate.
[6] Pre-expanded polystyrene particles obtained by expanding the expandable polystyrene resin particles according to any one of [1] to [5].
[7] The polystyrene pre-expanded particles according to [6], having a cell chord length of 40 to 80 μm.
[8] A foamed molded article obtained by expanding the polystyrene pre-expanded particles according to [6] or [7].

According to one embodiment of the present invention, it is possible to prevent blocking during the pre-expansion process even when the expandable polystyrene resin particles are stored for a long period of time, and to reduce contamination of the equipment, thereby providing expandable polystyrene resin particles, polystyrene pre-expanded particles, and a foamed molded product having practical strength and flexibility.

1 is an electron microscope photograph at 6,000 times magnification of an acrylic resin in the body of an expandable polystyrene resin particle obtained in Example 1 of the present invention. 1 is an electron microscope photograph at 40,000 times magnification of an acrylic resin in the body of an expandable polystyrene resin particle obtained in Example 1 of the present invention. 1 is an electron microscope photograph at 6,000 times magnification of polyethylene wax in the body of an expandable polystyrene-based resin particle obtained in Comparative Example 4 of the present invention.

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

In this specification, the expandable polystyrene resin particles themselves (themselves) are referred to as "expandable polystyrene resin particle bodies", the surfaces of which are coated with external additives (blocking inhibitors) are referred to as "expandable polystyrene resin particles", and particles obtained by pre-expanding expandable polystyrene resin particles are referred to as "polystyrene pre-expanded particles".

[1. Expandable polystyrene resin particles]
[Outline of one embodiment of the present invention]
In the molding process of the foamed molded article, the action of external additives attached to the surface of the pre-expanded particles reduces the fusion strength between the pre-expanded particles, resulting in a reduction in the strength of the foamed molded article. On the other hand, if the fusion strength between the pre-expanded particles is too high, it takes a long time to cool the foamed molded article in the removal process, resulting in a longer molding cycle. In addition, if the foamed molded article is removed from the mold before the temperature has sufficiently decreased, the foamed molded article will be deformed.

The above problems can be solved by using a metal salt of a higher fatty acid, such as zinc stearate, as an external additive. However, this metal salt of a higher fatty acid is prone to clogging filters in equipment used for pneumatic transport of particles and small holes in molds, which can lead to equipment contamination and reduced productivity.

In contrast, the expandable polystyrene resin particles described in the above-mentioned Patent Document 1 contain liquid methylphenylpolysiloxane as a blocking inhibitor, which eliminates the problem of clogging caused by powder, but the fluidity of the resin particles is relatively low. Therefore, there is room for improvement in terms of fluidity and handling.

In addition, the expandable polystyrene resin particles described in Patent Documents 2 and 3 contain higher fatty acid metal salts (especially zinc stearate), which causes the problem of equipment contamination being easily accelerated.

One embodiment of the present invention solves the above-mentioned technical problem. That is, the expandable polystyrene-based resin particles are formed by applying a fusion promoter to the surface of the expandable polystyrene-based resin particle body containing an acrylic resin of a specific particle size, and even if the expandable polystyrene-based resin particles are stored for a long period of time, blocking in the pre-expansion process is prevented, contamination of the equipment caused by peeling of external additives from the surface of the pre-expanded particles is suppressed, and the molded product has practical strength.

[1-1. Expandable polystyrene resin particle body]
The expandable polystyrene-based resin particle according to one embodiment of the present invention has an expandable polystyrene-based resin particle body containing a specific amount of an acrylic resin having a specific particle size. The expandable polystyrene-based resin particle body is a particle made of an expandable resin containing a base resin containing a styrene unit as a constituent unit, an acrylic resin having a specific particle size as a cell chord length adjuster (nucleating agent), and a blowing agent.

(Base resin)
In this specification, the base resin contains a styrene unit as a structural unit. The "styrene unit" constituting the base resin is a structural unit derived from a styrene monomer. When the mass of all structural units contained in the base resin is taken as 100% by weight, the mass of the styrene unit in the base resin is preferably 60% by weight or more, and more preferably 70% by weight or more.

The base resin may be a homopolymer of styrene. The base resin may also be a copolymer of a styrene monomer and a monomer other than a styrene monomer. Examples of monomers other than a styrene monomer include ethylene, butadiene, acrylonitrile, styrene derivatives, and acrylic acid esters. Examples of the styrene derivatives include α-methylstyrene, paramethylstyrene, t-butylstyrene, and chlorostyrene. Examples of the acrylic acid esters include acrylic acid alkyl esters such as methyl acrylate and butyl acrylate. These monomers other than a styrene monomer may be used alone or in combination of two or more.

Specific examples of the copolymers include styrene-ethylene copolymers, styrene-butadiene copolymers, styrene-acrylonitrile copolymers, styrene-α-methylstyrene-acrylonitrile copolymers, and styrene-acrylic acid ester copolymers.

(Acrylic resin)
The expandable polystyrene resin particle body preferably contains 0.01 to 0.5 parts by weight of an acrylic resin relative to 100 parts by weight of the base resin. The acrylic resin functions as a cell chord length adjusting agent (nucleating agent). The acrylic resin is preferably at least one selected from the group consisting of a homopolymer of an acrylic acid alkyl ester or a methacrylic acid alkyl ester, and a copolymer or graft copolymer of an acrylic acid alkyl ester and a methacrylic acid alkyl ester. For example, the acrylic resin is an acrylic resin of a homopolymer, copolymer or graft copolymer of two or more kinds of acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; and an alkyl methacrylate such as ethyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate. A styrene monomer copolymerizable with the alkyl acrylate or alkyl methacrylate may be added.

Such acrylic resins can be obtained by emulsion polymerization, for example, as described in Patent No. 2515014, and Kane Ace PA-20 manufactured by Kaneka Corporation can be used. The acrylic resin product may be made into a powder (secondary particles) having an easy-to-handle particle size by salt-coagulating and drying a latex in which the acrylic resin (primary particles) is dispersed. In other words, an acrylic resin having an average particle size of 0.2 μm or less of the basic particles can also be said to be the average particle size of the primary particles. The particle size in the latex can be adjusted by the amount of dispersant such as an emulsifier during emulsion polymerization. The volume average particle size in the latex can be measured, for example, by using a MICROTRAC UPA150 (manufactured by Nikkiso Co., Ltd.).

The acrylic resin of the present invention is characterized in that it is dispersed in the expandable polystyrene resin particle body as aggregate particles of basic particles having an average particle diameter of 0.2 μm or less. Note that "the acrylic resin is dispersed and present in the expandable polystyrene resin particle body as aggregate particles of basic particles having an average particle diameter of 0.2 μm or less" means that the acrylic resin present in the expandable polystyrene resin particle body is mainly present as aggregate particles (e.g., 60% or more of the basic particles), and does not deny that a part of the acrylic resin is present as basic particles. In other words, "the acrylic resin is dispersed and present in the expandable polystyrene resin particle body as aggregate particles of basic particles having an average particle diameter of 0.2 μm or less" is expressed as meaning that the acrylic resin dispersed and present in the expandable polystyrene resin particle body includes aggregate particles of aggregated basic particles having an average particle diameter of 0.2 μm or less.

The average particle size of the basic particles of the acrylic resin present in the expandable polystyrene resin particle body of the present invention is equal to the volume average particle size of the primary particles of the acrylic resin in the latex, and is 0.2 μm or less, preferably 0.1 μm or less, and more preferably 0.07 μm or less. In addition, the average particle size of the basic particles of the acrylic resin present in the expandable polystyrene resin particle body is preferably 0.04 μm or more. It is presumed that the finer the average particle size, the larger the surface area, and the more stably the foaming agent is localized on the basic particle surface and in the gaps between the particles, and the cell chord length of the pre-expanded particles is uniformly stable for a long period after production. On the other hand, it tends to be difficult to produce particles of less than 0.04 μm by emulsion polymerization due to the increase in viscosity of the latex.

These basic particles aggregate and are uniformly dispersed as aggregate particles in the expandable polystyrene resin body. The average particle diameter of the aggregate particles is preferably 1 to 5 μm. If the average particle diameter of the aggregate particles is less than 1 μm, the cell chord length becomes small, the cycle time during in-mold foaming becomes long, and productivity tends to be poor. If the average particle diameter of the aggregate particles exceeds 5 μm, the cell chord length becomes large, and blocking during pre-expansion tends to increase.

When polyethylene wax, which is commonly used as a nucleating agent, is used as the nucleating agent, particles (basic particles) with an average particle size of 1 to 4 μm are present in a dispersed state in the expandable polystyrene resin particle body, but the basic particles do not form aggregates as in acrylic resin. In this case, the cell chord length of the pre-expanded particles gradually increases over time after the expandable polystyrene resin particle body is manufactured, and blocking during pre-expansion increases.

Thus, even if the average particle size of the nucleating agent dispersed in the expandable polystyrene resin particle body is about the same, it is surprising that when it exists in the form of aggregated particles, as in the acrylic resin of the present invention, the cell chord length does not increase over time, thereby suppressing blocking during pre-expansion.

The dispersion state of the acrylic resin as a nucleating agent and the polyethylene wax in the expandable polystyrene resin particle body can be observed by preparing a thin slice of the expandable polystyrene resin particle body, staining it with _RuO4 vapor, and using a transmission electron microscope (Hitachi: H-7650, accelerating voltage 100 kV). FIG. 1 shows the dispersion state of the acrylic resin in the expandable polystyrene resin particle body at 6000x magnification, and FIG. 2 shows the dispersion state of the polyethylene wax in the expandable polystyrene resin particle body at 6000x magnification. FIG. 3 shows the dispersion state of the polyethylene wax in the expandable polystyrene resin particle body at 6000x magnification. As shown in FIG. 1, the acrylic resin is dispersed in the expandable polystyrene resin of the present invention as aggregate particles having an average particle size of 1 to 5 μm, which are aggregates of basic particles having an average particle size of 0.2 μm or less. On the other hand, as shown in FIG. 3, when polyethylene wax is used as a nucleating agent, particles (basic particles) having an average particle size of 1 to 4 μm are present in a dispersed state in the main body of the expandable polystyrene-based resin particle, and the basic particles do not form aggregates as in the case of acrylic resin used as a nucleating agent.

The average particle size of the basic particles and aggregate particles of the nucleating agent in the expandable polystyrene resin particle body can be read from a photograph observing the dispersion state.

The content of the acrylic resin in the expandable polystyrene resin particle body is preferably 0.01 to 0.5 parts by weight per 100 parts by weight of the base resin. It is more preferable that it is 0.1 parts by weight or more, and more preferably 0.3 parts by weight or less. Within this range, the cell chord length of the pre-expanded particles is adjusted to 40 to 80 μm, the surface is smooth, and the amount of blocking during the pre-expanding process is suppressed.

If the content of the acrylic resin is less than 0.01 parts by weight, the cell chord length of the pre-expanded particles (hereinafter sometimes referred to as pre-expanded particles) becomes non-uniform and coarse, and the amount of blocking increases in the pre-expanding process. On the other hand, even if it exceeds 0.5 parts by weight, the cell chord length tends not to become finer than 40 μm, and the function as a nucleating agent is reduced. The nucleating agent is added during the production of the expandable polystyrene resin particle body, and as long as the nucleating agent is well dispersed in the expandable resin particle body, there are no restrictions on the method of addition, but in order to make the cell chord length of the foamed molded product uniform, it is more preferable to add it before the charging of the styrene monomer and mix and dissolve it uniformly with the styrene monomer.

For example, when polyethylene wax (particle size 40 μm), which is commonly used for expandable styrene resin particles, is used as a nucleating agent, the cell chord length one week after the end of polymerization is 40 to 80 μm, and the amount of blocking is small in the preliminary step, but increases over time, exceeding 100 μm one month after the end of polymerization, and the amount of blocking has increased. On the other hand, in the case of acrylic resin, even after long-term storage, the cell chord length remains stable at 40 to 80 μm, and the amount of blocking remains small.

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

The amount of foaming agent contained in the expandable polystyrene-based resin particle body is selected according to the desired expansion ratio of the final product, which is a foamed molded article, but is preferably 3.0 parts by weight or more, more preferably 3.5 parts by weight or more, and preferably 5.0 parts by weight or less, more preferably 4.5 parts by weight or less, per 100 parts by weight of the base resin. If the amount of foaming agent contained is within the above range, it is possible to prevent the heating time in the pre-expansion process from becoming too long, suppress blocking, and shorten the time required to produce a foamed molded article and the molding cycle of the molding process.

(Method for producing expandable polystyrene-based resin particle body)
The method for producing the expandable polystyrene-based resin particle body may be a known method such as a suspension polymerization method or a seed polymerization method, and is not particularly limited.

The suspension polymerization method includes, for example, the following steps (1) to (4): (1) mixing water, monomers including styrene monomers, a dispersant, a polymerization initiator, and optionally other additives (plasticizers, nucleating agents, flame retardants, flame retardant assistants, etc.) to prepare an aqueous suspension; (2) heating the aqueous suspension to a predetermined temperature; (3) reacting the aqueous suspension at a predetermined temperature for a predetermined time to carry out a polymerization reaction, thereby obtaining a base resin (also called a base resin composition) containing additives; (4) impregnating the base resin composition with a blowing agent during or after the above step (3).

The seed polymerization method includes, for example, the following (1) to (4): (1) mixing water, polystyrene-based resin particles (also called polystyrene-based resin seed particles) as seeds, a dispersant, a polymerization initiator, and optionally other additives (plasticizer, nucleating agent, flame retardant, flame retardant assistant, etc.) to prepare an aqueous suspension; (2) then heating the aqueous suspension to a predetermined temperature; (3) then adding a monomer containing a styrene monomer to the aqueous suspension over a predetermined time at the predetermined temperature, and simultaneously reacting the aqueous suspension to carry out a polymerization reaction, thereby obtaining a base resin (also called a base resin composition) containing additives; (4) the same as the suspension polymerization method. The constituent units of the polystyrene-based resin seed particles are also included in the constituent units of the base resin.

(Volume average particle diameter of expandable polystyrene resin particle body)
The volume average particle diameter of the body of the expandable polystyrene-based resin particles (measured using image processing method Microtrack JPA) can be set appropriately depending on the application of the foamed molded article, but from the viewpoint of moldability, it is preferably 0.5 mm or more, more preferably 0.8 mm or more, and is preferably 1.5 mm or less, and more preferably 1.0 mm or less.

[1-2. Expandable polystyrene resin particles]
The present expandable polystyrene-based resin particles are obtained by applying a specific amount of a fusion promoter to the surface of the expandable polystyrene-based resin particle body. The fusion promoter acts as a fixing agent between the pre-expanded particles in the molding process. In addition, the present expandable polystyrene-based resin particles are preferably further provided with a specific amount of polyethylene wax applied to the surface of the expandable polystyrene-based resin particle body. The polyethylene wax acts as a blocking inhibitor in the pre-expanding process.

(Polyethylene wax)
The polyethylene wax is polyethylene obtained by polymerization of ethylene, decomposition of polyethylene, etc., and acts as a blocking inhibitor in the pre-blocking step. As the polyethylene wax, polyethylene wax particles, for example, a powder obtained by finely pulverizing polyethylene wax into granules, can be suitably used.

The weight-average molecular weight of the polyethylene wax is preferably 600 or more, more preferably 800 or more, and preferably 2500 or less, more preferably 2200 or less. If the weight-average molecular weight of the polyethylene wax exceeds 2500, the blocking suppression effect is enhanced, but practical strength tends to be lost. If the weight-average molecular weight of the polyethylene wax is less than 600, the fusion promotion effect is enhanced, and the foamed molded product shows good fusion properties, but blocking tends to increase. The weight-average molecular weight of the wax is a value measured according to the GPC (gel permeation chromatography) method.

From the viewpoint of improving blocking suppression and fusion promotion, the melting point of the polyethylene wax is preferably 90°C or higher, more preferably 95°C or higher, and is preferably 130°C or lower, more preferably 126°C or lower. The melting point of the wax is a value measured using differential scanning calorimetry (DSC).

The polyethylene wax may be used alone or in combination with two or more polyethylene waxes having different weight average molecular weights and melting points.

The amount of polyethylene wax applied is 0 parts by weight or more, preferably 0.01 parts by weight or more, and preferably 0.2 parts by weight or less, more preferably 0.1 parts by weight or less, per 100 parts by weight of the expandable styrene-based resin particle body. Even if the amount of polyethylene wax applied is 0 parts by weight, the amount of blocking in the pre-expansion process does not increase, but when a pre-expansion machine with a high blowing heating steam pressure is used, the amount of blocking tends to increase. If the amount of polyethylene wax applied exceeds 0.2 parts by weight, the fusion properties of the final product, the foamed molded product, deteriorates, and the strength of the foamed molded product decreases.

(Fusion promoter)
The fusion accelerator constituting the present expandable polystyrene-based resin particles has the effect of bonding the pre-expanded particles together in the molding process and imparting flexibility to the molded product when pressed against it by hand.
The fusion promoter of the present invention is any one or a mixture of fatty acid triglycerides, fatty acid diglycerides, and fatty acid monoglycerides. Examples of the fusion promoter include fatty acid triglycerides such as lauric acid triglyceride, stearic acid triglyceride, linoleic acid triglyceride, and hydroxystearic 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; and vegetable oils such as castor hardened oil (hydroxystearic acid triglyceride). These external additives may be used alone or in combination of two or more. Among them, stearic acid triglyceride and castor hardened oil are preferable for promoting the fusion of the foam and ensuring practical strength.

The amount of the fusion promoter added (amount applied) in the present invention is preferably 0.01 to 0.2 parts by weight, and more preferably 0.03 to 0.1 parts by weight, per 100 parts by weight of the expandable polystyrene resin particle body. If the amount added is less than 0.01 parts by weight, the effect of promoting fusion is small and the molded body does not have practical strength, and if it exceeds 0.2 parts by weight, the surface tends to be eroded by the external additive, impairing the surface beauty.

(Method for producing expandable polystyrene-based resin particles)
The expandable polystyrene-based resin particles of the present invention have a step of applying a specific amount of a fusion promoter to the surface of the expandable polystyrene-based resin particle body of the present invention. That is, the method for producing the expandable polystyrene-based resin particles of the present invention has, in addition to the above-mentioned steps (1) to (4) for producing the expandable polystyrene-based resin particle body, a step (5) of applying a fusion promoter to the surface of the expandable polystyrene-based resin particle body.

The expandable polystyrene-based resin particles of the present invention may be coated with a specific amount of polyethylene wax in addition to the fusion promoter. The expandable polystyrene-based resin particles of the present invention may be coated with an external additive (an attachment applied (adhered) to the surface of the expandable polystyrene-based resin particle body) such as an antistatic agent in addition to the fusion promoter and polyethylene wax.

There are no particular limitations on the method for applying external additives such as polyethylene wax, a fusion promoter, and an antistatic agent to the expandable polystyrene resin particle body, as long as the method can apply the additives evenly to the surface of the expandable polystyrene resin particle body without causing uneven application.

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, and the coating method is not limited. A preferred coating method is to attach the additives after drying and coat them by mixing and stirring. For example, the method of adding the expandable polystyrene resin particles and the external additives to a bag and blending them by shaking them by hand, or the method of using a blender such as a ribbon mixer, super mixer, Nauta mixer, or Pam Apex mixer, etc., may be mentioned. The melting point of the fusion promoter is preferably 40°C or higher. If the melting point is less than 40°C, some of them will become liquid at room temperature, which tends to impair the fusion properties during molding. In addition, the upper limit of the melting point of the external additive is 150°C or lower, preferably 120°C or lower, and more preferably 100°C or lower. If the melting point exceeds the upper limit, they will not melt during molding, which tends to impair the fusion promotion effect.

The expandable polystyrene resin particles of the present invention may contain residual monomer components, solvents, plasticizers, blowing agents, flame retardants, flame retardant assistants, etc. as additives to the extent that the effects of the present invention are not impaired.

(Additives, etc.)
The expandable polystyrene-based resin particles according to one embodiment of the present invention may contain residual monomer components or additives, such as solvents, plasticizers, flame retardants, flame retardant assistants, antistatic agents, water repellents, etc., within the scope that does not impair the effects of the present invention.

The timing and method of adding these additives are appropriately selected depending on their respective functions. Depending on their functions, these additives may be added, for example, during polymerization of the base resin in the production of the expandable polystyrene-based resin particle body, may be added to the expandable polystyrene-based resin particle body before the polyethylene wax and fusion promoter are applied, or may be added to the expandable polystyrene-based resin particles after the application of the polyethylene wax and fusion promoter has been completed.

The residual monomer components are preferably less than 0.3 parts by weight, more preferably less than 0.1 parts by weight, and even more preferably less than 0.01 parts by weight, per 100 parts by weight of the expandable polystyrene resin particle body. The residual monomer components tend to volatilize from the foamed molded product obtained by expanding and molding the expandable polystyrene resin particles. For this reason, expandable polystyrene resin particles with a residual monomer component of less than 0.3 parts by weight can be suitably used in the medical field, the packaging material field such as food containers that come into direct contact with food, the automotive field, and the construction field.

Specific examples of the above solvents and plasticizers include aliphatic hydrocarbons having 6 or more carbon atoms, such as hexane and heptane; alicyclic hydrocarbons having 6 or more carbon atoms, such as cyclohexane and cyclooctane; diisobutyl adipate, dioctyl adipate, dibutyl sebacate, glycerin tristearate, glycerin tricaprylate, coconut oil, palm oil, rapeseed oil; and the like. These solvents and plasticizers may be used alone or in combination of two or more. The content of the solvent and plasticizer is preferably 0.01 parts by weight or more and 2 parts by weight or less, based on 100 parts by weight of the expandable resin particle body. By using the amount of the solvent and plasticizer within this range, the solvent and plasticizer can exert their effects as a solvent and plasticizer without impairing the strength and heat resistance of the foamed molded body.

As the flame retardant, known flame retardants can be used. Specific examples 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-diglycidyl ether. brominated phenol derivatives such as 2,2-bis[4'-(2",3"-dibromoalkoxy)-3',5'-dibromophenyl]-propane; brominated styrene-butadiene block copolymers, brominated random styrene-butadiene copolymers, brominated styrene-butadiene graft copolymers, and other brominated butadiene-vinyl aromatic hydrocarbon copolymers (for example, EMERALD3000 manufactured by Chemtura, or the copolymers described in JP-T-2009-516019); and the like. These flame retardants may be used alone or in combination of two or more.

As the flame retardant auxiliary, a known flame retardant auxiliary can be used. Specific examples include cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, 2,3-dimethyl-2,3-diphenylbutane, etc. These flame retardant auxiliary can be used alone or in combination of two or more.

Specific examples of the above antistatic agents include N-hydroxyethyl-N-2-hydroxyhexadecylamine, N,N-bis(hydroxyethyl)dodecylamine, N,N-bis(hydroxyethyl)tetradecylamine, N,N-bis(hydroxyethyl)hexadecylamine, N,N-bis(hydroxyethyl)octadecylamine, N-hydroxyethyl-N-2-hydroxytetradecylamine, N-hydroxyethyl-N-2-hydroxyhexadecylamine, N-hydroxyethyl-N-2-hydroxyoctadecylamine, and N-hydroxypropyl-N-2-hydroxytetradecylamine. Examples of 1-amino-2-hydroxy compounds include decylamine, N-hydroxybutyl-N-2-hydroxytetradecylamine, N-hydroxypentyl-N-2-hydroxytetradecylamine, N-hydroxypentyl-N-2-hydroxyhexadecylamine, N-hydroxypentyl-N-2-hydroxyoctadecylamine, N,N-bis(2-hydroxyethyl)dodecylamine, N,N-bis(2-hydroxyethyl)tetradecylamine, N,N-bis(2-hydroxyethyl)hexadecylamine, and N,N-bis(2-hydroxyethyl)octadecylamine.

In order to reduce contamination of the device, it is preferable that the amount of higher fatty acid metal salt applied to the expandable polystyrene resin particles is less than 0.1 parts by weight per 100 parts by weight of the expandable polystyrene resin particle body, and it is even more preferable that no coating is applied.

2. Polystyrene-based pre-expanded particles
The polystyrene-based pre-expanded particles according to one embodiment of the present invention can be obtained by pre-expanding (primary expansion) the above-mentioned expandable polystyrene-based resin particles.

As a method for pre-expanding, a conventional method can be used, such as using a cylindrical pre-expanding device and heating the expandable polystyrene resin particles with a heating medium such as steam to cause expansion.

The pre-expansion device and the conditions of the pre-expansion process can be set appropriately depending on the composition of the expandable polystyrene resin particle body, the desired pre-expansion ratio, etc., and are not particularly limited.

[3. Foam molding]
The foamed molded article according to one embodiment of the present invention can be obtained by heating and foaming (secondary foaming) the above-mentioned polystyrene-based pre-foamed particles, followed by molding.

As a method for heating and expanding the polystyrene pre-expanded particles to form them, a conventional method can be used, such as an in-mold foaming method in which the polystyrene pre-expanded particles are filled into a mold and heated by blowing in a heating medium such as steam.

A specific in-mold foam molding method is to fill a mold that can be closed but cannot be sealed with polystyrene pre-expanded particles, and then heat and fuse the polystyrene pre-expanded particles with a heating medium to produce an in-mold foam molded article.

The equipment used for heat expansion and the conditions for heat expansion can be set appropriately depending on the composition of the expandable polystyrene resin particle body, the desired expansion ratio, etc., and are not particularly limited.

The foamed molded products, particularly the in-mold foamed molded products, have the advantage of being easy to produce molded products in the desired shape, 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.

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. Note that "parts" and "%" are by weight unless otherwise specified.

The various measuring and evaluation methods for the expandable polystyrene resin particles, the pre-expanded polystyrene particles, and the foamed molded articles in the examples and comparative examples are as follows.
<Method for measuring the average particle size of elementary particles and aggregate particles in the body of expandable polystyrene resin particles>
The average particle size of the elementary particles of the acrylic resin is measured by the following procedure. A thin section is prepared so as to include the center of the expandable polystyrene resin particle body, and the prepared thin section is stained with _RuO4 vapor, and a photograph is prepared by observing the thin section at 40,000 times with a transmission electron microscope (Hitachi: H-7650, acceleration voltage 100 kV). A straight line is drawn at the center of the cross section of the aggregated particles of the acrylic resin observed in the photograph at 40,000 times, and the number of elementary particles (n) between the fulcrums on the straight line (1 μm) is read and calculated based on the following formula. In the photograph shown in the figure, the area observed as a white sphere is the area where the elementary particles were peeled off when the thin section was prepared.
Formula: Average particle diameter of basic particles (μm) = 1 (μm) / number of particles (n)
The average particle size of the acrylic resin aggregate particles was measured by selecting three aggregate particles from a transmission electron microscope photograph observed at 6000 times in the same manner as in the above-mentioned measurement of the average particle size, reading the major axis diameter and the minor axis diameter of the aggregate particles, and calculating the average value thereof.

The polyethylene wax in the expandable polystyrene resin particle body is dispersed as basic particles, and no aggregate particles are observed. Therefore, the average particle size of the polyethylene wax in the expandable polystyrene resin particle body was measured using the same method as the average particle size of the aggregate particles of the acrylic resin described above, and this was taken as the average particle size of the basic particles.

<Method of producing polystyrene pre-expanded particles>
The expandable polystyrene resin particles were put into a pre-expander equipped with a stirrer (small-sized expander, manufactured by Daikai Kogyo Co., Ltd.). The expandable polystyrene resin particles in the pre-expander were heated using steam (blowing steam pressure 0.1 MPa) as a heating medium to pre-expand (primary expansion), thereby obtaining polystyrene pre-expanded particles with a bulk expansion ratio (apparent expansion ratio) of 25 times.

<Evaluation of Blocking Ratio During Pre-Exposure>
Expandable polystyrene resin particles stored in a FIBC for one week after the end of polymerization, or expandable polystyrene resin particles stored in a FIBC for one month were used. Polystyrene pre-expanded particles with a bulk ratio (apparent ratio) of 25 times were obtained by the above-mentioned method for producing polystyrene pre-expanded particles. Next, when removing the polystyrene pre-expanded particles from the pre-expanding machine, the pre-expanded particles were passed through a mesh with a mesh size of 1 cm, and the polystyrene pre-expanded particles that did not pass through the mesh were collected. The weight of the polystyrene pre-expanded particles that did not pass through the mesh was measured to determine the blocking amount. The blocking ratio was calculated based on the following calculation formula.

Blocking rate [wt %] = blocking amount [g] / total amount of polystyrene pre-expanded particles [g] × 100
Polystyrene pre-expanded particles having a blocking rate of 1.0% by weight or less were evaluated as passing.

<Measurement of cell chord length of pre-expanded particles>
Expandable polystyrene resin particles stored in a FIBC for one week after the end of polymerization, or expandable polystyrene resin particles stored in a FIBC for one month after the end of polymerization, were used to prepare 25 times the amount of pre-expanded polystyrene particles by the above-mentioned method for producing pre-expanded polystyrene particles.
The cell chord length of the prepared 25-fold preliminary particle was calculated according to ASTM-D-2842-97 by cutting the preliminary particle through the center, preparing a photograph of the cut surface of the preliminary particle, drawing a straight line through the center of the cross section of the preliminary particle, reading the number of bubbles (n) between the supporting points on the straight line (1000 μm), and calculating it based on the following formula.
Formula: Cell chord length (μm) = 1000 (μm) / Number of bubbles (n)

<Evaluation of surface properties of pre-formed particles>
Expandable polystyrene resin particles stored in a FIBC for one week after the end of polymerization, or expandable polystyrene resin particles stored in a FIBC for one month after the end of polymerization, were used to prepare 25 times the amount of pre-expanded polystyrene particles by the above-mentioned method for producing pre-expanded polystyrene particles.
The surface irregularities (smoothness) of the prepared particles at 25 times magnification were observed under a microscope and evaluated according to the following three-level scale. The larger the value, the smoother the irregularities. A score of "3" or higher was judged to be acceptable.
3: The surface irregularities are smooth.
2: Surface irregularities are clearly observed. 1: Surface irregularities are severe.

<Method for producing foamed molded product (in-mold foam molding)>
The expandable polystyrene resin particles stored in a flexible container for one month after the end of polymerization were pre-expanded 25 times by the above-mentioned method for producing pre-expanded polystyrene particles, and the obtained pre-expanded polystyrene particles were left at room temperature for 24 hours to cure and dry, and were used for producing an expansion molded article.
Using a molding machine (KR-57, manufactured by Daisen Co., Ltd.), polystyrene pre-expanded particles were filled into a mold measuring 450 mm long x 300 mm wide x 10 mm thick. Next, after reducing the pressure inside the mold (-0.05 MPa), the polystyrene pre-expanded particles in the mold were heated for 8 seconds using steam (blowing steam pressure 0.05 MPa) as a heating medium, and after 10 seconds of cooling in vacuum, the molded product was removed from the mold to obtain a foamed molded product. When the blowing steam pressure was increased, the fusion rate and bending property values of the following evaluation items improved, making it difficult to detect the difference between the levels of each experiment, so the blowing steam pressure was set to 0.05 MPa.

<Fusion rate of foam molded article>
The foamed molded article obtained by the above-mentioned method for producing a foamed molded article was broken, and the broken surface was observed to determine the percentage of the foamed beads that were broken, not the foamed bead interface (fusion rate). When the fusion rate was 30% or more, it was evaluated as acceptable for practical use.

<Flexural properties of foam molded product>
The foamed molded article obtained by the above-mentioned foamed molded article manufacturing method was stored at room temperature for 24 hours and then cut into a size of 300 mm length × 75 mm width × 10 mm thickness. The maximum bending strength and the displacement at break were measured using a strength tester (TG-50kN, manufactured by Minebea Co., Ltd.) according to JIS-7171, with a support distance of 200 mm and a test speed of 20 mm/min.
If the maximum bending strength is within the range of the displacement at break, the molded article is flexible and has an acceptable level for practical use, but if it is below that, the molded article will crack due to small displacement and weak stress, and cannot be used in practice.
Molded articles having a maximum bending strength of 0.4 kPa or more and a deformation at break (bending deformation at break) of 30% or more were evaluated as passing.

<Nucleating agent used>
Acrylic resin: Kane Ace PA-20 (average primary particle size: 0.07 μm, manufactured by Kaneka Corporation)
Measurement of average particle size of primary particles: 1 cc of latex was placed in a 1 L round bottom flask, and the latex was uniformly diluted with pure water. The diluted solution was then placed in a quartz glass container, and the particle size was calculated from the light transmittance of the diluted solution using MICROTRAC UPA150 (manufactured by Nikkiso Co., Ltd.).
Polyethylene: Polywax 1000-80M, molecular weight 1000, melting point 113°C, particle size 40 μm (manufactured by Toyo Adle Co., Ltd.)
<Polyethylene wax used>
PW-655: Molecular weight 655, melting point 99°C (manufactured by Toyo Adle Co., Ltd.)
Polywax 850-80M: molecular weight 850, melting point 107°C (manufactured by Toyo Adle Co., Ltd.)
Polywax 1000-80M: molecular weight 1000, melting point 113°C (manufactured by Toyo Adle Co., Ltd.)
<Fusion promoter used>
Hardened castor oil, melting point 84 degrees (castor wax, Nippon Oil & Fats Co., Ltd., hydroxystearic acid triglyceride)
Example 1
(Production of expandable polystyrene-based resin particle body)
Into a 1500 L pressure vessel equipped with a stirrer, 100 parts by weight of pure water, 0.2 parts by weight of tricalcium phosphate, 0.01 parts by weight of sodium dodecylbenzenesulfonate, and 0.25 parts by weight of benzoyl peroxide (manufactured by NOF Corp.) as an initiator, 0.29 parts by weight of t-butylperoxy-2-ethylhexyl monocarbonate (manufactured by NOF Corp.), and 0.2 parts by weight of acrylic resin (PA-20, manufactured by Kaneka Corp.) as a nucleating agent were charged. Next, 100 parts by weight of styrene monomer was charged while stirring at 60 rpm, and the temperature was raised to 98°C and polymerization reaction was carried out for 4 hours. The polymerization conversion rate of this polymerization reaction was 92%. Next, 4 parts by weight of pentane (normal/iso = 80/20, manufactured by SK Sangyo Co., Ltd.) as a foaming agent was pressurized into the pressure vessel and heated to 120°C. The polymerization slurry was then held at 120° C. for 3 hours, cooled to room temperature, and taken out of the pressure vessel. The polymerized slurry was washed, dehydrated, and dried using an airflow dryer to obtain expandable polystyrene-based resin particle bodies having a volume average particle diameter of 0.9 mm.

(Production of expandable polystyrene-based resin particles)
100 parts by weight of the expandable polystyrene-based resin particle bodies obtained above were put into a super mixer (SMV-20, manufactured by Kawata Co., Ltd.), and 0.05 parts by weight of polyethylene wax (Polywax 850-80M, manufactured by Toyo Adle Co., Ltd.) and 0.05 parts by weight of a fusion accelerator (castor hardened oil, castor wax, manufactured by NOF Corporation) were added over 60 seconds, followed by stirring for a further 120 seconds to obtain expandable polystyrene-based resin particles.

The polystyrene pre-expanded particles and the foamed molded articles were produced in accordance with the above-mentioned methods for producing the polystyrene pre-expanded particles and the foamed molded articles.
The expandable polystyrene resin particles, polystyrene pre-expanded particles, and foamed molded articles obtained above were subjected to evaluation tests. The average particle size of the acrylic resin elementary particles in the expandable polystyrene resin particles was 0.07 μm, and the average particle size of the aggregate particles was 2.5 μm. The evaluation results are shown in Table 1 below.

[Examples 2 to 7, Comparative Examples 1 to 3]
The expandable polystyrene-based resin particle body produced in Example 1 was used, and expandable polystyrene-based resin particles, polystyrene-based pre-expanded particles, and foamed molded articles were obtained in the same manner as in Example 1, except that the types and amounts of the fusion accelerator and polyethylene wax were changed.
The expandable polystyrene resin particles, pre-expanded polystyrene particles, and foamed molded articles obtained were subjected to evaluation tests. The evaluation results are shown in Tables 1 and 2 below.

[Examples 8 and 9]
Except for changing the amount of nucleating agent PA-20 added, expandable polystyrene-based resin bead bodies were produced in the same manner as in Example 1. Using the expandable polystyrene-based resin bead bodies obtained above, expandable polystyrene-based resin particles, polystyrene pre-expanded particles, and foamed molded articles were obtained in the same manner as in Example 1.
The obtained expandable polystyrene resin particles, polystyrene pre-expanded particles, and foamed molded articles were subjected to evaluation tests. The average particle size of the acrylic resin elementary particles in the expandable polystyrene resin particle body was 0.07 μm, and the average particle size of the aggregate particles was 2.5 μm. The evaluation results are shown in Table 2 below. [Comparative Examples 4 and 5]
Expandable polystyrene-based resin particle bodies were prepared in the same manner as in Example 1, except that the nucleating agent PA-20 was not used and the amount of nucleating agent polyethylene wax (Polywax 1000-80M (particle size 40 μm, manufactured by Toyo Adle Co., Ltd.) added was changed. Using the expandable polystyrene-based resin particle bodies obtained above, expandable polystyrene-based resin particles, polystyrene pre-expanded particles and foamed molded articles were obtained in the same manner as in Example 1.
The obtained expandable polystyrene resin particles, polystyrene pre-expanded particles, and foamed molded products were subjected to evaluation tests. The average particle size of the polyethylene wax elementary particles in the resin particle body was 2 μm, and no aggregate particles of elementary particles were observed. The evaluation results are shown in Table 2 below.

Comparative Example 6
Without using a nucleating agent, an expandable polystyrene-based resin bead body was prepared in the same manner as in Example 1. Using the expandable polystyrene-based resin bead body obtained above, expandable polystyrene-based resin particles, polystyrene-based pre-expanded particles, and foamed molded articles were obtained in the same manner as in Example 1.
The expandable polystyrene resin particles, the pre-expanded polystyrene particles, and the foamed molded articles obtained were subjected to evaluation tests. The evaluation results are shown in Table 2 below.

As shown in Tables 1 and 2, the expandable polystyrene resin particles of Examples 1 to 10 had no change in cell chord length, and the surface irregularities of the pre-expanded particles were smooth, even though the pre-expanded particles were made by pre-expanding expandable polystyrene resin particles one month after polymerization, and had an excellent blocking rate in the pre-expanding process. In addition, the foamed molded article obtained from the expandable polystyrene resin particles had good bending properties (flexibility) that could withstand practical use.

In contrast, in Comparative Examples 5 and 6, in which polyethylene wax was used as a nucleating agent, when expandable polystyrene resin particles one month after polymerization were pre-expanded, the cell chord length of the pre-expanded particles increased, the surface irregularities became severe, and the blocking rate during the pre-expanding process increased.

In addition, in Comparative Examples 1 and 2, in which no fusion accelerator was added as an external additive, there was no adhesion between pre-foamed particles during the molding process, the fusion rate was low, and the bending properties were such that the material broke at low displacement and stress, making it unsuitable for practical use. Comparative Example 3, in which the amount of polyethylene wax was excessive, had a low fusion rate and poor bending properties. Comparative Example 4, in which the amount of both the polyethylene wax and the fusion accelerator was excessive, had a high blocking rate during the pre-foaming process and was inferior to the Examples.

These results show that by applying an appropriate amount of fusion promoter to the surface of the expandable polystyrene-based resin particle body containing acrylic resin, and preferably further applying polyethylene wax, it is possible to obtain a foamed molded article with suitable fusion properties and bending characteristics by maintaining a stable cell chord length and suppressing blocking during pre-expansion, even for expandable polystyrene-based resin particles that have been in use for some time since the end of polymerization. In addition, since no higher fatty acid metal salt is used on the surface of the resin particle body, contamination of the equipment is suppressed.

Claims (8)

a surface of an expandable polystyrene-based resin particle body containing a base resin including a styrene unit as a constituent unit and an acrylic resin, and 0.01 to 0.2 parts by weight of a fusion accelerator is applied to 100 parts by weight of the expandable polystyrene-based resin particle body;
the acrylic resin is present in the form of aggregate particles of elementary particles having an average particle diameter of 0.2 μm or less, dispersed in the body of the expandable polystyrene resin particle ;
a surface of the expandable polystyrene-based resin particle body is further coated with more than 0 part by weight and not more than 0.2 part by weight of polyethylene wax per 100 parts by weight of the expandable polystyrene-based resin particle body;
The polyethylene wax has a weight average molecular weight of 600 or more and 2,500 or less . 2. The expandable polystyrene-based resin particles according to claim 1, wherein the coating amount of the higher fatty acid metal salt is less than 0.1 parts by weight per 100 parts by weight of the expandable polystyrene-based resin particle body. Expandable polystyrene-based resin particles according to any one of claims 1 to 2, characterized in that the fusion promoter is one or a mixture of fatty acid triglyceride, fatty acid diglyceride, and fatty acid monoglyceride. The expandable polystyrene-based resin particle according to any one of claims 1 to 3, wherein the expandable polystyrene-based resin particle body contains 0.01 to 0.5 parts by weight of the acrylic resin per 100 parts by weight of the base resin. The expandable polystyrene-based resin particles according to any one of claims 1 to 4, characterized in that the acrylic resin is at least one selected from the group consisting of a homopolymer of an alkyl acrylate or an alkyl methacrylate, and a copolymer or graft copolymer of an alkyl acrylate and an alkyl methacrylate. Pre-expanded polystyrene particles obtained by expanding the expandable polystyrene resin particles according to any one of claims 1 to 5. The polystyrene-based pre-expanded particles according to claim 6, having a cell chord length of 40 to 80 μm. A foamed molded article obtained by expanding the polystyrene pre-expanded particles according to claim 6 or 7.