JP7803481B2 - Expandable methyl methacrylate resin particles - Google Patents

Description

The present invention relates to expandable methyl methacrylate resin particles.

When casting metal, a known method is the lost foam casting (full mold) process, in which a pattern made from a foam molded body is embedded in casting sand, and molten metal is poured into the foam molded body to replace the metal, resulting in a casting. In the full mold process, foam molded bodies made from methyl methacrylate polymers are used to reduce residue during casting. Furthermore, foam molded bodies for casting are often machined from large block molded bodies.

Several technologies are known for producing expandable methyl methacrylate resin particles for producing foamed molded articles from methyl methacrylate polymers.

Patent Document 1 discloses expandable acrylic resin particles that contain specific amounts of a methacrylic acid ester component and a specific acrylic acid ester component and have a glass transition temperature of 112 to 125°C.

Patent Document 2 discloses expandable methyl methacrylate resin particles that contain specific amounts of methyl methacrylate units and acrylic ester units, have an average particle size of 0.6 to 1.0 mm, and have a particle size variation coefficient of 20% or less.

Patent Document 3 discloses a method for producing expandable methyl methacrylate resin particles obtained by suspension polymerization of acrylic monomers containing specific amounts of methyl methacrylate and acrylic esters.

Patent Document 4 discloses expandable methyl methacrylate resin particles obtained by polymerizing specific amounts of methyl methacrylate, an acrylic ester, and a polyfunctional monomer.

Patent Publication No. 2015-183111 WO2020/203537 Patent Publication No. 2018-135407 WO2016/047490

However, the above-mentioned conventional techniques leave room for improvement in terms of the internal fusion properties, castability, and production efficiency of methyl methacrylate resin foam molded articles.

In light of the above circumstances, an object of one embodiment of the present invention is to provide expandable methyl methacrylate resin particles that can efficiently produce methyl methacrylate resin foam molded articles that have excellent internal fusion properties and castability.

The inventors conducted extensive research to solve the above problems, and as a result, they have completed the present invention.

That is, one embodiment of the present invention includes the following configuration.
[1] Expandable methyl methacrylate-based resin particles comprising a base resin containing methyl methacrylate units and acrylic ester units as constituent units, and a foaming agent, and satisfying the following (a) to (d):
(a) the bulk density (A) of the expanded methyl methacrylate resin particles obtained by heating the expandable methyl methacrylate resin particles with steam at 100°C for 300 seconds is 0.0285 g/ ^cm3 or less;
(b) the volume (B) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is 140 ^cm3 or less when 100 ^cm3 of the expanded methyl methacrylate resin beads is heated with steam at 100°C for 30 seconds and then left at 25°C for 1 minute;
(c) the volume (C) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is greater than 160 ^cm3 when 100 ^cm3 of the expanded methyl methacrylate resin beads are heated with steam at 100°C for 180 seconds and then allowed to stand at 25°C for 1 minute; and (d) the glass transition temperature of the base resin is 114.5°C or higher.
[2] The expandable methyl methacrylate resin particles according to [1], wherein the weight average molecular weight of the base resin is 220,000 to 310,000.
[3] A base resin containing a methyl methacrylate unit and an acrylic ester unit as constituent units, and a foaming agent,
The weight average molecular weight of the base resin is 220,000 to 310,000,
The expandable methyl methacrylate resin particles have a glass transition temperature of 114.5°C or higher.
[4] Expandable methyl methacrylate resin particles according to any one of [1] to [3], wherein the acrylic ester unit is a butyl acrylate unit.
[5] In the base resin,
The expandable methyl methacrylate-based resin particles according to any one of [1] to [4], wherein (a) the content of the methyl methacrylate units is more than 97.0 parts by weight and not more than 99.0 parts by weight, and (b) the content of the acrylic ester units is 1.0 part by weight or more and less than 3.0 parts by weight, relative to 100 parts by weight of the total amount of the methyl methacrylate units and the acrylic ester units.

One embodiment of the present invention has the effect of providing expandable methyl methacrylate resin particles that can efficiently produce methyl methacrylate resin foam molded articles with excellent internal fusion properties and castability.

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

In this specification, "expandable methyl methacrylate resin particles" may be referred to as "expandable resin particles," "expanded methyl methacrylate resin particles" may be referred to as "expanded particles," and "expanded methyl methacrylate resin molded article" may be referred to as "expanded molded article."

1. Technical concept of one embodiment of the present invention
A methyl methacrylate resin foam molded article having poor internal fusion properties has poor processability, such that when the methyl methacrylate resin foam molded article is cut, the methyl methacrylate resin foam particles fall off from the cut surface of the methyl methacrylate resin foam molded article.

The main use of methyl methacrylate resin foam molded articles is as castings during metal casting. When used as castings, methyl methacrylate resin foam molded articles are particularly required to have good castability.

The inventors have conducted research and found that the foamed molded articles obtained using the expandable resin particles disclosed in Patent Documents 1 to 5 have room for improvement in terms of internal fusion, castability, and production efficiency.

In light of the above circumstances, the present inventors conducted extensive research with the aim of providing expandable methyl methacrylate resin particles that can efficiently produce methyl methacrylate resin foam molded articles with excellent internal fusion properties and castability.

After extensive research, the inventors discovered the following and completed the present invention: (a) expandable methyl methacrylate resin particles with an excellent expansion rate (expandability) can efficiently provide expanded methyl methacrylate resin particles, which in turn can efficiently provide expanded methyl methacrylate resin articles; (b) expanded methyl methacrylate resin particles with a slow expansion rate and minimal shrinkage after heating can provide expanded methyl methacrylate resin articles with excellent internal fusion; and (c) expandable methyl methacrylate resin particles having a base resin with a high glass transition temperature can provide expanded methyl methacrylate resin articles with excellent castability.

2. Expandable methyl methacrylate resin particles
The expandable methyl methacrylate resin particles according to one embodiment of the present invention include a base resin containing, as constituent units, methyl methacrylate units and acrylic ester units, and a blowing agent, and satisfy the following (a) to (d):
(a) the bulk density (A) of the expanded methyl methacrylate resin particles obtained by heating the expandable methyl methacrylate resin particles with steam at 100°C for 300 seconds is 0.0285 g/ ^cm3 or less;
(b) the volume (B) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is 140 ^cm3 or less when 100 ^cm3 of the expanded methyl methacrylate resin beads is heated with steam at 100°C for 30 seconds and then left at 25°C for 1 minute;
(c) the volume (C) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is greater than 160 ^cm3 when 100 ^cm3 of the expanded methyl methacrylate resin beads are heated with steam at 100°C for 180 seconds and then allowed to stand at 25°C for 1 minute; and (d) the glass transition temperature of the base resin is 114.5°C or higher.

"Expandable methyl methacrylate resin particles according to one embodiment of the present invention" may also be referred to as "the present expandable resin particles" hereinafter.

These expandable resin beads can be expanded by a known method to provide expanded beads. The expanded beads obtained by expanding the expandable resin beads can be molded in a mold by a known method to provide a foamed molded article.

Because the expandable resin particles have the above-described structure, they have the advantage of being able to efficiently produce foamed molded articles with excellent internal fusion properties and castability.

The present expandable resin beads are obtained by expanding the expandable resin beads under specific conditions, and the bulk density (A) of the expanded beads is 0.0285 g/ ^cm3 or less. The smaller the bulk density (A), the higher the bulk ratio of the expanded beads obtained, i.e., the better the expandable resin beads are. Expandable resin beads with excellent expandability can reduce the production time and production costs when using the expandable resin beads to produce expanded beads. Therefore, the present expandable resin beads have the advantage of being able to efficiently provide expanded beads, and as a result, being able to efficiently provide foamed molded articles.

The present expandable resin beads have a volume (B) of 140 ^cm3 or less, obtained by expanding the expandable resin beads. The volume (B) indicates the degree to which the expanded beads expand within a certain period of time and can reflect the expansion rate of the expanded beads. A smaller volume (B) indicates a slower expansion rate and lower expandability of the expanded beads. When the expanded beads are used in in-mold molding, steam can be sufficiently distributed to the expanded beads in the center of the mold, thereby providing a foamed molded article with excellent internal fusion. Therefore, the present expandable resin beads have the advantage of being able to provide a foamed molded article with excellent internal fusion.

The present expandable resin beads have a volume (C) of more than 160 ^cm3 , obtained by expanding the expandable resin beads. The volume (C) indicates the degree of shrinkage of the expanded beads after heating. The larger the volume (C), the less the expanded beads shrink after heating, and the better the shrinkage-inhibiting properties of the expanded beads. Expanded beads with excellent shrinkage-inhibiting properties tend to maintain adhesion (fusion) between the expanded beads inside the expanded molded article during in-mold molding using the expanded beads, or in the expanded molded article obtained by in-mold molding, and as a result, a expanded molded article with excellent internal fusion properties can be provided. Therefore, the present expandable resin beads have the advantage of being able to provide a expanded molded article with excellent internal fusion properties.

The expandable resin particles have a base resin with a glass transition temperature of 114.5°C or higher. Through extensive research, the inventors independently discovered that, surprisingly, when the base resin of the expandable resin particles has a glass transition temperature of 114.5°C or higher, foamed molded articles made using the expandable resin particles have excellent castability. In other words, the expandable resin particles have the advantage of being able to provide foamed molded articles with excellent castability.

The expansion of expandable resin beads can also be called "primary expansion." Therefore, the expansion rate and expandability of expandable resin beads can also be called the expansion rate and expandability of primary expansion, respectively. On the other hand, the expansion of expandable beads can also be called "secondary expansion." Therefore, the expansion rate and expandability of expandable beads can also be called the expansion rate and expandability of secondary expansion, respectively.

(Base resin)
The base resin can be said to be the portion of the expandable resin particles other than the blowing agent and the external additives described below, and can also be said to be the portion that substantially constitutes the expandable resin particles. The base resin contained in the expandable resin particles contains, as structural units, methyl methacrylate units and acrylic ester units. In this specification, a "methyl methacrylate unit" refers to a structural unit derived from a methyl methacrylate monomer, and an "acrylic ester unit" refers to a structural unit derived from an acrylic ester monomer. In this specification, the term "monomer" may be omitted. Therefore, in this specification, for example, when simply referring to "methyl methacrylate" and "acrylic ester," it is intended to mean "methyl methacrylate monomer" and "acrylic ester monomer," respectively.

In the base resin contained in the expandable resin particles, it is preferable that, relative to 100 parts by weight of the total amount of methyl methacrylate units and acrylic ester units, (a) the content of methyl methacrylate units is more than 97.0 parts by weight and not more than 99.0 parts by weight, and the content of acrylic ester units is 1.0 part by weight or more and less than 3.0 parts by weight; (b) the content of methyl methacrylate units is more than 97.0 parts by weight and not more than 98.5 parts by weight, and the content of acrylic ester units is 1.5 parts by weight or more and less than 3.0 parts by weight, more preferably; (c) the content of methyl methacrylate units is more than 97.0 parts by weight and not more than 98.0 parts by weight, and the content of acrylic ester units is 2.0 parts by weight or more and less than 3.0 parts by weight, even more preferably; and (d) the content of methyl methacrylate units is 97.5 parts by weight, and the content of acrylic ester units is 2.5 parts by weight, particularly preferably. When the content of methyl methacrylate units and the content of acrylic ester units in the base resin of the expandable resin particles are each within the above-mentioned ranges, the expandable resin particles have the advantage of efficiently providing foamed molded articles with excellent internal fusion and castability. More specifically, when the content of acrylic ester units in the base resin is 1.0 part by weight or more per 100 parts by weight of the total of methyl methacrylate units and acrylic ester units, the expandable resin particles tend to have excellent expandability. When the content of acrylic ester units in the base resin is 3.0 parts by weight or less per 100 parts by weight of the total of methyl methacrylate units and acrylic ester units, (a) the expanded beads obtained by expanding the expandable resin particles tend to have excellent shrinkage suppression, and (b) the glass transition temperature of the base resin tends to be 114.5°C or higher, which tends to result in excellent castability of the final foamed molded article.

Acrylate ester units according to one embodiment of the present invention include methyl acrylate units, ethyl acrylate units, propyl acrylate units, and butyl acrylate units. Butyl acrylate units are particularly preferred as acrylate ester units. This configuration provides expandable resin particles with excellent expandability and moldability (e.g., shrinkage suppression of expanded particles). Furthermore, butyl acrylate units are highly effective in lowering the glass transition temperature of the base resin and are highly effective in improving castability.

The base resin of the expandable resin particles may contain structural units derived from a crosslinking agent (hereinafter also referred to as crosslinking agent units). When the base resin of the expandable resin particles contains crosslinking agent units, the following advantages are achieved: (a) the expandable resin particles have excellent expandability; (b) expanded particles obtained by expanding the expandable resin particles have low expandability and good shrinkage suppression; and (c) foamed molded articles obtained by molding the expanded beads in a mold have excellent internal fusion and produce little residue upon combustion, resulting in excellent castability. Expandable resin particles containing a base resin that contains structural units derived from a crosslinking agent as structural units also have the advantage of easy molecular weight adjustment during the manufacturing process.

Examples of crosslinking agents include compounds having two or more radically reactive functional groups. Among compounds having two or more radically reactive functional groups, it is preferable to use a bifunctional monomer having two functional groups as the crosslinking agent. In other words, it is preferable that the base resin of the expandable resin particles contains a bifunctional monomer unit, which is a structural unit derived from a bifunctional monomer, as a structural unit derived from a crosslinking agent. This configuration offers the following advantages: (a) the expandable resin particles have excellent expandability; (b) expanded particles obtained by expanding the expandable resin particles have lower expandability and excellent shrinkage suppression; and (c) the foamed molded article obtained by molding the expanded particles in a mold has excellent internal fusion and, due to the reduced residue upon combustion, has excellent castability.

Examples of bifunctional monomers include: (a) ethylene glycol or ethylene glycol oligomers, such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and triethylene glycol di(meth)acrylate, in which both terminal hydroxyl groups are esterified with acrylic acid or methacrylic acid; (b) dihydric alcohols, such as neopentyl glycol di(meth)acrylate, hexanediol di(meth)acrylate (e.g., 1,6-hexanediol diacrylate), and butanediol di(meth)acrylate, in which the hydroxyl groups are esterified with acrylic acid or methacrylic acid; and (c) aryl compounds having two alkenyl groups, such as divinylbenzene. Hexanediol di(meth)acrylate is preferred as a bifunctional monomer due to its ease of molecular weight adjustment. In this specification, "(meth)acrylate" refers to "acrylate and/or methacrylate." For example, hexanediol di(meth)acrylate is intended to mean hexanediol diacrylate and/or hexanediol dimethacrylate.

The content of the bifunctional monomer units in the expanded resin beads is preferably 0.05 to 0.15 parts by weight, and more preferably 0.08 to 0.13 parts by weight, per 100 parts by weight of the total content of the methyl methacrylate units and the acrylic ester units. This configuration offers the advantages of (a) the expandable resin beads having even better expandability, (b) the expanded beads obtained by expanding the expandable resin beads having even lower expandability and even better shrinkage suppression, and (c) the expanded molded article obtained by molding the expanded beads in a mold has even better internal fusion properties and produces even less residue upon combustion, resulting in even better castability.

The base resin of the expandable resin particles may further contain, as a structural unit, a structural unit derived from an aromatic monomer (aromatic unit). Examples of aromatic monomers include aromatic vinyl compounds such as styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, and chlorostyrene. When the base resin of the expandable resin particles contains an aromatic unit, a foamed molded article with excellent strength can be obtained.

On the other hand, from the viewpoint of obtaining a foamed molded article that leaves little residue upon combustion, it is preferable that the amount of structures (e.g., aromatic rings) derived from aromatic monomers contained in the expandable resin particles be as small as possible. Specifically, it is preferable that the amount of aromatic units contained in the base resin of the expandable resin particles be as small as possible. For example, the amount of aromatic units contained in the base resin of the expandable resin particles is preferably 2.5 parts by weight or less, more preferably less than 2.5 parts by weight, more preferably 2.0 parts by weight or less, more preferably 1.5 parts by weight or less, even more preferably 1.0 part by weight or less, and particularly preferably 0 part by weight, per 100 parts by weight of the base resin. In other words, it is particularly preferable that the base resin of the expandable resin particles does not contain aromatic units.

The types and amounts of structural units contained in the base resin are the same as the types and amounts of monomers contained in the monomer mixture used in the polymerization of the base resin (for example, the copolymerization step described below) (provided that the polymerization conversion rate is 100%).

(Blowing agent)
The blowing agent contained in the expandable resin particles is not particularly limited. Examples of the blowing agent include volatile blowing agents such as (a) aliphatic hydrocarbons, which are hydrocarbons having 3 to 5 carbon atoms, such as propane, isobutane, normal butane, isopentane, normal pentane, and neopentane, and (b) hydrofluorocarbons with an ozone depletion potential of zero, such as difluoroethane and tetrafluoroethane. These blowing agents may be used alone or in combination of two or more.

In the expandable resin particles, the content of the blowing agent per 100 parts by weight of the base resin is preferably 5 to 12 parts by weight, and more preferably 7 to 10 parts by weight. This configuration has the advantage of being able to provide expandable resin particles with sufficient expandability and eliminating the need for heavy polymerization equipment.

(Other additives)
In addition to the base resin and the blowing agent, the expandable resin particles may further contain other additives, such as a cell regulator, a solvent, a plasticizer, a flame retardant, a flame retardant assistant, a heat radiation inhibitor, a pigment, a dye, and an antistatic agent.

The solvent and plasticizer are described in detail below in Section 3. "Method for producing expandable methyl methacrylate resin particles." The content of the solvent and plasticizer in the expandable resin particles may be the same as the amount of the solvent and plasticizer used, as described below.

(Foam adjuster)
Examples of the cell regulator include (a) aliphatic bisamides such as methylene bisstearic acid amide and ethylene bisstearic acid amide, and (b) polyethylene wax. The content of the cell regulator per 100 parts by weight of the base resin is preferably 0.01 to 0.50 parts by weight.

(Weight average molecular weight)
The weight-average molecular weight of the base resin contained in the expandable resin beads is preferably 220,000 to 310,000, more preferably 230,000 to 310,000, and even more preferably 240,000 to 300,000. This configuration surprisingly offers the following advantages: (a) the expandable resin beads have good expandability; (b) the expanded beads obtained by expanding the expandable resin beads have low expandability and good shrinkage suppression; and (c) the foamed molded articles obtained by in-mold molding of the expanded beads have excellent internal fusion. After extensive research, the present inventors have surprisingly discovered that by adjusting the amount of methyl methacrylate units and acrylic ester units in the base resin and setting the weight-average molecular weight to 310,000 or less, foamed molded articles with excellent internal fusion can be efficiently provided.

In this specification, the weight-average molecular weight obtained by measurement using the following method is defined as the weight-average molecular weight of the base resin contained in the expandable resin particles: (1) 0.02 g of expandable resin particles are dissolved in 20 ml of tetrahydrofuran (hereinafter sometimes abbreviated as "THF"); (2) The gel component in the resulting solution is then filtered; (3) Only the THF-soluble components (i.e., the filtrate) are used as a sample for GPC measurement using a gel permeation chromatograph (GPC); (4) The weight-average molecular weight (Mw) and number-average molecular weight (Mn) are calculated from the GPC measurement chart obtained by GPC measurement. Note that the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are relative values converted into polystyrene equivalents.

The weight-average molecular weight of the base resin can be adjusted by changing the composition (type and amount) of the monomers used in the polymerization (copolymerization) process of the base resin, the type and amount of chain transfer agent, the polymerization temperature and time, the type and amount of initiator, and the type and amount of crosslinking agent.

(glass transition temperature)
The glass transition temperature of the base resin contained in the present expandable resin particles is 114.5°C or higher. This configuration allows the present expandable resin particles to have the advantage of being able to provide a foamed molded article with excellent castability. Since a foamed molded article with even better castability can be provided, the glass transition temperature is preferably 114.6°C or higher, more preferably 114.8°C or higher, and particularly preferably 115.0°C or higher. From the viewpoint of castability, a higher glass transition temperature is preferable, and the upper limit is not particularly limited, but the glass transition temperature is, for example, 150.0°C or lower.

In this specification, the glass transition temperature of the base resin contained in the expandable resin particles is determined by measuring the glass transition temperature using the following method: (1) The resin obtained by drying the expandable resin particles at 150°C for 30 minutes is used as a sample; (2) 4 mg of the sample is placed in an aluminum container, and an aluminum lid is attached to the container using a compressor to obtain a measurement sample; (3) The measurement sample is heated from 50°C to 150°C (heating rate: 10°C/min) using a DSC measurement device (e.g., Hitachi DSC7000X), cooled from 150°C to 50°C (heating rate: 10°C/min), and then heated again from 50°C to 150°C (heating rate: 10°C/min); (4) The glass transition temperature is calculated using the DSC curve obtained during the second heating. Note that the glass transition temperature here refers to the midpoint glass transition temperature defined in JIS K7121.

The glass transition temperature of the base resin can be adjusted by changing the composition (type and amount) of the monomers used in the polymerization (copolymerization) process of the base resin.

In expanded beads produced using expandable resin beads, the structure of the expandable resin beads changes, but the composition of the expandable resin beads does not. Similarly, in expanded molded articles produced using expandable resin beads, the structure of the expandable resin beads changes, but the composition of the expandable resin beads does not. Therefore, the weight-average molecular weight and glass transition temperature of the expanded beads or expanded molded articles can be considered to be the weight-average molecular weight and glass transition temperature of the base resin contained in the expandable resin beads, which are the raw material for the expanded beads or expanded molded articles. The weight-average molecular weight of the expanded beads or expanded molded articles can be measured using the above-mentioned method for measuring the weight-average molecular weight of the base resin of expandable resin beads, but with "expandable resin beads" or "expanded beads" or "expanded molded articles." The glass transition temperature of the expanded beads or expanded molded articles can be measured using the above-mentioned method for measuring the glass transition temperature of the base resin of expandable resin beads, but with "expandable resin beads" or "expanded beads" or "expanded molded articles."

(Volume average particle size)
The volume average particle diameter of the expandable resin particles is preferably 0.5 mm to 1.4 mm, more preferably 0.6 mm to 1.2 mm, more preferably greater than 0.6 mm and 1.0 mm or less, and even more preferably 0.7 mm to 0.9 mm. When the volume average particle diameter is 0.5 mm or greater, the expandable resin particles are unlikely to exhibit reduced expandability during expansion and/or increased blocking. When the volume average particle diameter is 1.4 mm or less, the expandable resin particles obtained by expanding the expandable resin particles are unlikely to exhibit excessively high expandability, and the surface of the expanded resin particles is slowly formed during molding, allowing steam to penetrate into the interior of the expanded molded article. As a result, the fusion properties of the interior of the expanded molded article are improved. In this specification, the volume average particle diameter of the expandable resin particles is defined as the particle diameter at 50% of the cumulative volume when the particle diameter of the expandable resin particles is measured on a volume basis using a particle size analyzer (e.g., an image processing type Millitrac JPA particle size analyzer), and the obtained results are expressed as a cumulative distribution.

In some cases, the expandable methyl methacrylate resin particles may be sieved to separate out expandable methyl methacrylate resin particles with a particle diameter of 0.5 mm to 1.4 mm. In this case, the volume average particle diameter of the separated expandable methyl methacrylate resin particles will be within the range of 0.5 mm to 1.4 mm.

(Expandability of Expandable Methyl Methacrylate Resin Particles)
The expandable resin particles have a bulk density (A) of 0.0285 g/ ^cm3 or less and exhibit excellent expandability. Since expanded beads can be efficiently (e.g., in a shorter time), and as a result, foamed molded articles can be efficiently (e.g., in a shorter time), the bulk density (A) is preferably 0.0260 g/ ^cm3 or less, more preferably 0.0250 g/ ^cm3 or less, and particularly preferably 0.0222 g/ ^cm3 or less. The smaller the bulk density (A), the more preferable it is, and although there is no particular restriction on the lower limit, the bulk density (A) is at least greater than 0.0000 g/ ^cm3 . The expandable methyl methacrylate resin particles used in measuring the bulk density (A) may have an antiblocking agent applied to their surfaces.

In this specification, the bulk density of expanded beads is a value obtained by carrying out the following steps (1) to (4) in order: (1) measuring the weight of a certain amount of expanded beads (for example, the total amount of expanded beads obtained in the process of measuring bulk density (A)); (2) placing the total amount of expanded beads into a 1000 ^cm3 measuring cylinder; (3) measuring the volume of the expanded beads from the graduations on the measuring cylinder; (4) calculating the bulk density of the expanded beads using the following formula:
Bulk density (g/cm ³ )=weight of expanded beads (g)/volume of expanded beads (cm ³ ).

Alternatively, the weight of the expandable resin particles used to measure the bulk density (A) may be measured in advance and used as the weight of the resulting expanded particles. The weight of the expandable resin particles used to measure the bulk density (A) is, for example, 10 g.

An example of a method for expanding expandable resin particles to measure bulk density (A) is shown below in (1) to (3): (1) A certain amount of expandable resin particles (e.g., 10 g) is weighed out and an anti-blocking agent is applied to the surface of the expandable resin particles; (2) The expandable resin particles are placed in a steamer equipped with an outlet; (3) Steam at 100°C is supplied to the steamer and the expandable resin particles are heated for 300 seconds to obtain expanded particles.

The expanded beads obtained by expanding the expandable resin beads have a volume (B) of 140 ^cm3 or less and have low expandability. Since a foamed molded article having excellent internal fusion properties can be provided, the volume (B) is preferably 138 ^cm3 or less, more preferably 135 ^cm3 or less, and particularly preferably 130 ^cm3 or less. The smaller the volume (B), the better, and the lower limit thereof is not particularly limited, but the volume (B) is at least more than 100 ^cm3 .

Here, the method for measuring the volume (B) of the expanded beads obtained by expanding the present expandable resin beads is not particularly limited, and examples thereof include a method of sequentially performing the following steps (1) to (6): (1) expanding the present expandable resin beads to a bulking ratio of 60 times to prepare expanded beads having a bulking ratio of 60 times; (2) weighing out 100 ^cm3 of the expanded beads and placing them in a steamer (for example, a steamer with an outlet); (3) supplying steam at 100°C to the steamer and heating the expanded beads for 30 seconds; (4) after heating, removing the expanded beads from the steamer and leaving them at 25°C for 1 minute; (5) placing the expanded beads in a 1000 ^cm3 measuring cylinder; (6) measuring the volume (B) of the expanded beads from the graduations on the measuring cylinder.

The expanded beads obtained by expanding the expandable resin beads of the present invention have a volume (C) of more than 160 ^cm3 and are excellent in shrinkage suppression. Since a foamed molded article having excellent internal fusion properties can be provided, the volume (C) is preferably 165 ^cm3 or more, more preferably 168 ^cm3 or more, and particularly preferably 170 ^cm3 or more.

Here, the method for measuring the volume (C) of the expanded beads obtained by expanding the present expandable resin beads is not particularly limited, and examples thereof include a method of sequentially performing the following steps (1) to (6): (1) expanding the present expandable resin beads to a bulking ratio of 60 times to prepare expanded beads having a bulking ratio of 60 times; (2) weighing out 100 ^cm3 of the expanded beads and placing them in a steamer (for example, a steamer with an outlet); (3) supplying steam at 100°C to the steamer and heating the expanded beads for 180 seconds; (4) after heating, removing the expanded beads from the steamer and leaving them at 25°C for 1 minute; (5) placing the expanded beads in a 1000 ^cm3 measuring cylinder; (6) measuring the volume (C) of the expanded beads from the graduations on the measuring cylinder.

In this specification, the bulk ratio of expanded beads is a value obtained by carrying out the following steps (1) to (3) in order: (1) weighing out 10 g of expanded beads and placing them in a 1000 ^cm3 measuring cylinder; (2) measuring the volume of 10 g of expanded beads from the graduations on the measuring cylinder; (3) calculating the bulk ratio of the expanded beads using the following formula:
Bulking ratio (times) = volume of expanded particles (cm ³ )/10 g.

In this specification, the bulk ratio of the expanded beads can also be referred to as the expansion ratio. Although the unit of the bulk ratio is actually ^cm3 /g based on the above formula, in this specification, the unit of the bulk ratio is expressed as "times" for convenience.

The method for producing expanded beads with a bulk ratio of 60 times used to measure volume (B) and volume (C) is not particularly limited, but examples include a method that sequentially performs the following steps (1) to (3): (1) Loading expandable resin beads into a pressurized expansion machine (e.g., a BHP manufactured by Daikai Kogyo Co., Ltd.); (2) Injecting steam (e.g., water vapor) into the expansion machine under conditions of a steam injection pressure of 0.10 MPa to 0.16 MPa and an internal expansion machine pressure of 0.005 MPa to 0.030 MPa to heat the expandable resin beads; (3) Expanding the expandable resin beads to the desired expansion ratio (e.g., a bulk ratio of 60 times) as per step (2) to obtain expanded beads. The expandable resin beads used to produce expanded beads with a bulk ratio of 60 times used to measure volume (B) and volume (C) may be sieved to have a particle diameter of 0.5 mm to 1.4 mm.

(Variation 1)
The present inventors have further independently discovered the following findings in the course of intensive research aimed at providing expandable methyl methacrylate resin particles that can efficiently provide methyl methacrylate resin foamed molded articles excellent in internal fusion property and castability: (i) the greater the content of acrylic ester units in the base material of expandable resin particles, the faster the expansion rate of the expandable resin particles, i.e., the better the expandability, and as a result, the more efficiently the expandable resin particles and foamed molded articles can be provided; (ii) on the other hand, the greater the content of acrylic ester units in the base material of expandable resin particles, the worse the castability of foamed molded articles obtained by expanding the expandable resin particles, and (iii) the greater the content of acrylic ester units in the base material of expandable resin particles, the worse the castability of foamed molded articles obtained by molding the expanded beads (iv) the lower the content of acrylic ester units in the base material of the expandable resin particles, the slower the expansion rate of the expanded beads obtained by expanding the expandable resin particles; and (v) if the content of acrylic ester units in the base material of the expandable resin particles is too low (below a certain value), the expansion rate of the expanded beads obtained by expanding the expandable resin particles becomes too slow, so that the expanded beads do not expand sufficiently during the molding process, resulting in the expanded molded articles obtained by the expandable beads having poor internal fusion properties. In other words, simply adjusting the ratio of methyl methacrylate units to acrylic ester units in the base resin (in other words, the glass transition temperature of the base resin) has not been able to provide expandable methyl methacrylate-based resin particles that can efficiently provide methyl methacrylate-based resin foam molded articles having excellent internal fusion properties and castability.

Therefore, the inventors conducted further intensive research. As a result, they discovered that the above-mentioned objectives can be achieved by adjusting not only the glass transition temperature of the base resin containing methyl methacrylate units and acrylic ester units but also the weight-average molecular weight of the base resin, and thus completed another embodiment of the present invention. Specifically, the inventors discovered the following new findings: By adjusting the weight-average molecular weight of the base resin contained in the expandable resin particles within a specific range and setting the glass transition temperature to a certain temperature or higher, it is possible to provide expandable resin particles that can efficiently produce foamed molded articles with excellent internal fusion properties and castability; and by adjusting the weight-average molecular weight of the base resin contained in the expandable resin particles within a specific range and setting the glass transition temperature to a certain temperature or higher, it is possible to provide expandable resin particles that (a) have excellent expandability, (b) have a slow expansion rate, and (c) exhibit minimal shrinkage after heating.

In other words, expandable methyl methacrylate resin particles according to another embodiment of the present invention have the following configuration: expandable methyl methacrylate resin particles comprising a base resin containing methyl methacrylate units and acrylic ester units as structural units, and a blowing agent, wherein the weight-average molecular weight of the base resin is 220,000 to 310,000, and the glass transition temperature of the base resin is 114.5°C or higher.

According to another embodiment of the present invention, expandable methyl methacrylate resin particles have the above-described configuration, and therefore have the advantage of being able to provide expanded particles that (a) have excellent expandability and (b) have low expandability and excellent shrinkage suppression. Furthermore, expandable resin particles having the above-described configuration have the advantage of being able to efficiently provide foamed molded articles that have excellent internal fusion properties and castability.

For other aspects of the expandable methyl methacrylate resin particles according to another embodiment of the present invention, the above description is incorporated herein by reference as appropriate.

(Variation 2)
The present expandable resin particles can provide a foamed molded article with excellent internal fusion. The internal fusion of the foamed molded article can be evaluated by the proportion of expanded beads that have broken at locations other than their interfaces on the fracture surface obtained by breaking the foamed molded article. For example, if the proportion (D) of expanded beads that have broken at locations other than their interfaces on the fracture surface of a foamed molded article obtained by breaking a foamed molded article obtained by molding expanded beads obtained by expanding the present expandable resin particles is 85% or more relative to the total number of expanded beads (100%) constituting the fracture surface, the foamed molded article can be said to have excellent internal fusion.

That is, expandable methyl methacrylate resin particles according to another embodiment of the present invention have the following configuration: expandable methyl methacrylate resin particles that include a base resin containing methyl methacrylate units and acrylic ester units as structural units, and a blowing agent, and that satisfy the following (a) to (e):
(a) the bulk density (A) of the expanded methyl methacrylate resin particles obtained by heating the expandable methyl methacrylate resin particles with steam at 100°C for 300 seconds is 0.0285 g/ ^cm3 or less;
(b) the volume (B) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is 140 ^cm3 or less when 100 ^cm3 of the expanded methyl methacrylate resin beads is heated with steam at 100°C for 30 seconds and then left at 25°C for 1 minute;
(c) the volume (C) of the methyl methacrylate resin foamed beads obtained by heating 100 ^cm3 of the foamed methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads with steam at 100°C for 180 seconds and then leaving it at 25°C for 1 minute exceeds 160 ^cm3 ; (d) the glass transition temperature of the base resin is 114.5°C or higher; and (e) a methyl methacrylate resin foamed molded article obtained by breaking the foamed methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads has a fracture surface in which the proportion (D) of the foamed methyl methacrylate resin particles broken at a location other than the interface of the foamed methyl methacrylate resin beads is 90% or higher.

Here, the method for measuring the ratio (D) of the foamed particles at the fracture surface of a foamed molded article obtained by in-mold molding of expanded beads obtained by expanding the expandable resin beads is not particularly limited, and examples thereof include a method of sequentially performing the following steps (1) to (4): (1) in-mold molding of the expanded beads obtained by expanding the expandable resin beads using a mold (for example, a mold having a molding space of 2000 mm in length, 1000 mm in width, and 525 mm in thickness) to prepare a foamed molded article; (2) cutting the foamed molded article perpendicular to the thickness direction of the foamed molded article using a hot wire slicer so that the foamed molded article is divided into five equal parts in the thickness direction; (3) bending the plane perpendicular to the thickness direction of one of the middle parts (a portion of the foamed molded article before cutting that is 210 mm to 315 mm in the thickness direction) along the width direction at the center of the length direction to fracture the foamed molded article; (4) visually observing the obtained fracture surface, counting all the particles constituting the fracture surface and the number of expanded particles broken other than at the particle interfaces, and calculating the ratio (D) according to the following formula:
Proportion (D) (%) = number of grains that are broken at a location other than the grain interface on the fracture surface / number of grains constituting the fracture surface × 100.

The manufacturing method of the foamed molded article used to measure the ratio (D) is not particularly limited, but examples include a method in which the following steps (1) to (8) are carried out in order: (1) Expandable resin particles are placed in a pressurized foaming machine (e.g., BHP manufactured by Daikai Kogyo Co., Ltd.); (2) Steam (e.g., water vapor) is blown into the foaming machine under conditions of a steam blowing pressure of 0.10 MPa to 0.16 MPa and an internal pressure of the foaming machine of 0.005 MPa to 0.030 MPa to heat the expandable resin particles; (3) The expandable resin particles are expanded by the method (2) above until the desired expansion ratio (e.g., a bulk ratio of 60 times) is reached; (4) The obtained expanded particles are left at room temperature (e.g., 25°C) for 3 days to obtain expanded particles with a bulk ratio of 60 times; (5) A mold (e.g., a mold having a length of 2000 mm, a width of 1000 mm) is used to expand the expandable resin particles. (6) Steam (e.g., water vapor) is blown into the mold at a steam blowing pressure of 0.15 MPa to 0.25 MPa, and in-mold molding is carried out by vacuum suction and heating under conditions of a pressure inside the mold of 0.030 MPa to 0.060 MPa until the expansion pressure reaches 0.070 MPa to 0.080 MPa, thereby fusing the expanded beads together; (7) After the expansion pressure reaches 0.070 MPa to 0.080 MPa, the expanded beads are left in the mold at 80°C to 110°C for 1,000 seconds, and then the expanded molded article is removed; (8) The removed expanded molded article is left at 60°C for 3 days to obtain a expanded molded article. The expandable resin particles used to produce the foamed molded article used to measure the ratio (D) may be sieved to have a particle diameter of 0.5 mm to 1.4 mm.

The ratio (D) can also be referred to as the fusion rate. To achieve better internal fusion, the ratio (D) is preferably 85% or higher.

3. Method for producing expandable methyl methacrylate resin particles
The method for producing the expandable resin particles of the present invention is not particularly limited, and examples thereof include suspension polymerization in which a monomer mixture is polymerized in an aqueous suspension.

A preferred embodiment of the method for producing expandable resin particles is, for example, the following: a method for producing expandable methyl methacrylate-based resin particles, comprising: a copolymerization step of copolymerizing a monomer mixture containing methyl methacrylate monomer and acrylic ester monomer; and a blowing agent impregnation step of impregnating the resulting copolymer with a blowing agent, wherein the copolymerization step comprises: (a) an initiation step of initiating copolymerization of the monomer mixture in the presence of 0.08 to 0.20 parts by weight of a first poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture; and (b) an addition step of adding 0.08 to 0.50 parts by weight of a second poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture to the reaction mixture when the polymerization conversion rate reaches 35% to 70% after the initiation step; and wherein the copolymer obtained in the copolymerization step has a weight-average molecular weight of 220,000 to 310,000 and a glass transition temperature of 114.5°C or higher.

As used herein, the term "poorly water-soluble inorganic salt" refers to an inorganic salt whose solubility in water at 25°C is 0.1 mg/ml or less.

The preferred embodiment of the method for producing expandable resin particles described above is also one embodiment of the present invention. Below, we will explain the preferred embodiment of the method for producing expandable resin particles described above, i.e., the method for producing expandable methyl methacrylate resin particles according to one embodiment of the present invention. For matters not detailed below, the description in Section [2. Expandable methyl methacrylate resin particles] is incorporated herein by reference as appropriate. Hereinafter, the copolymer (also referred to as the base resin) obtained in the copolymerization step may also be referred to simply as "resin particles." In addition, throughout this specification, "a method for producing expandable methyl methacrylate resin particles according to one embodiment of the present invention" may also be referred to as "the present production method."

In one embodiment of the present invention, the term "aqueous suspension" refers to a liquid in which monomer droplets and/or resin particles are dispersed in water or an aqueous solution using a stirrer or the like. The aqueous suspension may contain (a) dissolved water-soluble surfactants and monomers, and (b) water-insoluble dispersants, polymerization initiators, chain transfer agents, crosslinking agents, cell regulators, flame retardants, solvents, etc. dispersed together with the monomers.

The weight ratio of the monomer and polymer (a methyl methacrylate resin, also known as a copolymer) to the water or aqueous solution in the aqueous suspension is preferably 1.0/0.6 to 1.0/3.0 (the resulting ratio of methyl methacrylate resin to water or aqueous solution). Note that the term "aqueous solution" used here refers to a solution consisting of water and components other than the methyl methacrylate resin.

The copolymerization step according to one embodiment of the present invention includes an initiation step of initiating copolymerization of a monomer mixture in the presence of 0.08 to 0.20 parts by weight of a first poorly water-soluble inorganic salt relative to 100 parts by weight of the monomer mixture. The initiation step is, for example, a step of initiating copolymerization of a monomer mixture using an aqueous suspension containing (a) water, (b) a monomer mixture containing methyl methacrylate monomer and an acrylic ester monomer, (c) 0.08 to 0.20 parts by weight of the first poorly water-soluble inorganic salt relative to 100 parts by weight of the monomer mixture, and optionally (d) a crosslinking agent, a polymerization initiator, a surfactant, a dispersant other than the poorly water-soluble inorganic salt, a chain transfer agent, a cell control agent, a flame retardant, etc.

In this specification, "before the initiation step," i.e., "before the start of the polymerization reaction," is sometimes referred to as the "initial stage of polymerization." The first poorly water-soluble inorganic salt blended (added) to the aqueous suspension in the initiation step, and the optional polymerization initiator, etc., can be considered substances (raw materials) used in the early stage of polymerization.

In the initiation step, the first poorly water-soluble inorganic salt can function as a dispersant. Examples of the first poorly water-soluble inorganic salt used in the initiation step, i.e., the early stage of polymerization, include calcium triphosphate, magnesium pyrophosphate, hydroxyapatite, and kaolin.

In addition, in the initiation step according to one embodiment of the present invention, (a) a water-soluble polymer such as polyvinyl alcohol, methyl cellulose, polyacrylamide, or polyvinylpyrrolidone, and/or (b) an anionic surfactant such as sodium α-olefin sulfonate or sodium dodecylbenzene sulfonate may be used in combination with the first poorly water-soluble inorganic salt.

The first poorly water-soluble inorganic salt used in the initiation step according to one embodiment of the present invention is preferably tribasic calcium phosphate, from the viewpoint of its ability to protect the resin particles and/or monomer droplets. From the viewpoint of dispersion stability of the droplets, the initiation step is preferably a step of initiating copolymerization of a monomer mixture in the presence of the first poorly water-soluble inorganic salt, tribasic calcium phosphate, and the anionic surfactant, sodium α-olefin sulfonate.

In one embodiment of the present invention, the initiation step is preferably a step of initiating copolymerization of the monomer mixture in the presence of preferably 0.08 to 0.20 parts by weight, more preferably 0.10 to 0.19 parts by weight, of the first poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture. When copolymerization of the monomer mixture is initiated in the presence of 0.08 parts by weight or more of the first poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture, there is no risk of the volume average particle diameter of the resulting expandable resin particles becoming too large. When copolymerization of the monomer mixture is initiated in the presence of 0.20 parts by weight or less of the first poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture, there is no risk of the generation of large amounts of fine expandable resin particles. In other words, by initiating copolymerization of the monomer mixture in the presence of the first poorly water-soluble inorganic salt in an amount within the above-mentioned range, expandable resin particles having the desired volume average particle diameter can be obtained with good yield.

In the initiation step according to one embodiment of the present invention, a case where a water-soluble polymer and/or anionic surfactant is used in combination with a first poorly water-soluble inorganic salt will be described. In this case, the concentration of the water-soluble polymer and/or anionic surfactant in the aqueous suspension is preferably 30 ppm to 100 ppm, based on the concentration of the monomer mixture (1,000,000 ppm).

The copolymerization step preferably includes an addition step of adding 0.08 to 0.50 parts by weight of a second poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture to the reaction mixture when the polymerization conversion rate reaches 35 to 70% after the initiation step.

In this specification, "after the initiation step," i.e., "after the start of the polymerization reaction," may also be referred to as "during polymerization." The second poorly water-soluble inorganic salt added to the reaction mixture in the addition step can be considered a substance (raw material) used during polymerization.

In the copolymerization step according to one embodiment of the present invention, when the polymerization (copolymerization) of the monomer mixture is carried out by suspension polymerization, the reaction mixture in the addition step can also be considered an aqueous suspension.

In the addition step according to one embodiment of the present invention, the second poorly water-soluble inorganic salt can function as a dispersant. Examples of the second poorly water-soluble inorganic salt used in the addition step, i.e., during polymerization, include the substances already exemplified as the first poorly water-soluble inorganic salt. The second poorly water-soluble inorganic salt is preferably one or more selected from the group consisting of tribasic calcium phosphate, hydroxyapatite, and kaolin, and more preferably tribasic calcium phosphate. This configuration has the advantage of preventing the resin particles from coalescing after the addition (addition) of the dispersant, and making it easy to obtain expandable resin particles with the desired volume average particle size.

The addition step is preferably a step of adding, to the reaction mixture after the initiation step, at a point when the polymerization conversion rate is 35% to 70%, preferably 0.08 to 0.50 parts by weight, more preferably 0.10 to 0.50 parts by weight, more preferably 0.10 to 0.40 parts by weight, even more preferably 0.10 to 0.30 parts by weight, and particularly preferably 0.10 to 0.20 parts by weight of the second poorly water-soluble inorganic salt per 100 parts by weight of the monomer mixture. When 0.08 parts by weight or more of the second poorly water-soluble inorganic salt is added to the reaction mixture per 100 parts by weight of the monomer mixture in the addition step, there is no risk of the volume average particle diameter of the resulting expandable resin particles becoming too large. When 0.50 parts by weight or less of the second poorly water-soluble inorganic salt is added to the reaction mixture per 100 parts by weight of the monomer mixture in the addition step, the amount of poorly water-soluble inorganic salt used is not excessive, thereby reducing production costs. That is, by adding the second poorly water-soluble inorganic salt to the reaction mixture in an amount within the above-mentioned range in the addition step, expandable resin particles having the desired volume average particle size can be obtained at low production costs.

In the addition step, the second poorly water-soluble inorganic salt is preferably added to the reaction mixture when the polymerization conversion rate is 35% to 70%, more preferably when the polymerization conversion rate is 35% to 60%, and even more preferably when the polymerization conversion rate is 40% to 50%. When the second poorly water-soluble inorganic salt is added to the reaction mixture when the polymerization conversion rate is 35% or higher in the addition step, the volume average particle diameter of the resulting expandable resin particles is unlikely to be too small. When the second poorly water-soluble inorganic salt is added to the reaction mixture when the polymerization conversion rate is 70% or lower in the addition step, the volume average particle diameter of the resulting expandable resin particles is unlikely to be too large. In other words, when the second poorly water-soluble inorganic salt is added to the reaction mixture when the polymerization conversion rate is within the above-mentioned range in the addition step, expandable resin particles having the desired volume average particle diameter can be easily obtained. The method for measuring the polymerization conversion rate in this specification is described in detail in the Examples below.

The copolymerization process according to one embodiment of the present invention is preferably carried out in at least two stages, with the polymerization temperature being changed. For convenience, the two copolymerization processes with different polymerization temperatures are hereinafter referred to as the first copolymerization process and the second copolymerization process. It can also be said that the copolymerization process preferably includes a first copolymerization process and a second copolymerization process that are consecutive and have different polymerization temperatures.

The copolymerization process according to one embodiment of the present invention preferably includes, for example, (a) a first copolymerization process carried out at a polymerization temperature of 70°C to 90°C using a low-temperature decomposition polymerization initiator, and (b) a second copolymerization process carried out consecutively to the first copolymerization process, at a polymerization temperature higher than that of the first copolymerization process (e.g., 90°C to 110°C) using a high-temperature decomposition polymerization initiator. In the copolymerization process, it is preferable that the main polymerization reaction occurs in the first copolymerization process, and that the remaining monomer is reduced in the second copolymerization process. Note that (i) the temperature of the first copolymerization process may be 70°C or higher but lower than 90°C, and the temperature of the second copolymerization process may be 90°C to 110°C, or (ii) the temperature of the first copolymerization process may be 70°C to 90°C, and the temperature of the second copolymerization process may be higher than 90°C but lower than 110°C.

The polymerization initiator can be a radical-generating polymerization initiator commonly used in the production of thermoplastic polymers. Representative radical-generating polymerization initiators include, for example, (a) organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxybenzoate, isopropyl-t-butylperoxycarbonate, butyl perbenzoate, t-butylperoxy-2-ethylhexanoate, t-butyl perpivalate, t-butylperoxyisopropylcarbonate, di-t-butylperoxyhexahydroterephthalate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and t-butylperoxy-2-ethylhexyl monocarbonate; and (b) azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile. These polymerization initiators may be used alone or in combination of two or more.

Of the above-mentioned radical-generating polymerization initiators, (a) benzoyl peroxide, lauroyl peroxide, t-butyl perpivalate, di-t-butylperoxyhexahydroterephthalate, azobisisobutyronitrile, and azobisdimethylvaleronitrile are low-temperature decomposition initiators, while (b) t-butylperoxybenzoate, isopropyl-t-butylperoxycarbonate, butyl perbenzoate, t-butylperoxy-2-ethylhexanoate, t-butylperoxyisopropyl carbonate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and t-butylperoxy-2-ethylhexyl monocarbonate are high-temperature decomposition initiators.

The amount of polymerization initiator used, calculated as the total amount used in the first copolymerization step and the second copolymerization step, is preferably 0.1 to 0.5 parts by weight per 100 parts by weight of the monomer mixture. This configuration results in expandable resin particles with excellent expandability.

The initiation step according to one embodiment of the present invention may be (a) a step of initiating copolymerization of a monomer mixture in the presence of a first poorly water-soluble inorganic salt, a low-temperature decomposition type polymerization initiator, and a high-temperature decomposition type polymerization initiator, or (b) a step of initiating copolymerization of a monomer mixture in the presence of a first poorly water-soluble inorganic salt and a low-temperature decomposition type polymerization initiator. When the initiation step is a step of initiating copolymerization of a monomer mixture in the presence of a first poorly water-soluble inorganic salt and a low-temperature decomposition type polymerization initiator, the high-temperature decomposition type polymerization initiator may be added to the reaction mixture (aqueous suspension) after the initiation step, i.e., during polymerization.

The initiation step may be carried out (started) at a polymerization temperature of 70°C to 90°C, or 70°C or higher and lower than 90°C. If the initiation step is a step of starting copolymerization of the monomer mixture at a polymerization temperature of 70°C to 90°C, or 70°C or higher and lower than 90°C, the polymerization temperature may be changed after the initiation step, i.e., during the polymerization, to a temperature higher than that at the start of polymerization (for example, higher than 90°C and lower than 110°C, or 90°C to 110°C).

In one embodiment of the present invention, a chain transfer agent is preferably used in the copolymerization step. The chain transfer agent is not particularly limited, and well-known substances used in the polymerization of methyl methacrylate resins can be used. Examples of chain transfer agents include (a) monofunctional chain transfer agents such as alkyl mercaptans and thioglycolic acid esters, and (b) polyfunctional chain transfer agents in which the hydroxyl groups of polyhydric alcohols (e.g., ethylene glycol, neopentyl glycol, trimethylolpropane, sorbitol, etc.) are esterified with thioglycolic acid or 3-mercaptopropionic acid. Examples of alkyl mercaptans include n-octyl mercaptan, n-dodecyl mercaptan, and t-dodecyl mercaptan. N-dodecyl mercaptan is preferred as a chain transfer agent due to its ease of molecular weight control.

The molecular weight of the resulting expandable resin particles can be adjusted by changing the amount of chain transfer agent used in the copolymerization step. The amount of chain transfer agent used is, for example, preferably 0.100 parts by weight or more but less than 0.500 parts by weight, more preferably 0.200 parts by weight to 0.400 parts by weight, and even more preferably 0.250 parts by weight to 0.300 parts by weight, per 100 parts by weight of the monomer mixture.

In the blowing agent impregnation step according to one embodiment of the present invention, expandable methyl methacrylate resin particles can be obtained by impregnating the methyl methacrylate resin particles, which are the copolymer obtained in the copolymerization step, with a blowing agent.

The blowing agent impregnation step according to one embodiment of the present invention can be carried out at any time, for example, together with the second copolymerization step or after the second copolymerization step.

In one embodiment of the present invention, the blowing agent impregnation step preferably involves impregnating the resulting copolymer with a blowing agent when the polymerization conversion rate from monomer to copolymer is 80% to 95%. When the copolymer is impregnated with a blowing agent when the polymerization conversion rate is 80% or higher, the blowing agent is adequately impregnated into the interior of the copolymer, eliminating the risk of copolymer aggregation due to softening, and improving production yield. When the copolymer is impregnated with a blowing agent when the polymerization conversion rate is 95% or lower, the blowing agent is sufficiently impregnated into the interior of the copolymer, eliminating the risk of a double cell structure (hard core) forming in the expanded beads obtained by expanding the expandable resin beads. As a result, by molding the expanded beads in a mold, a foamed molded article with excellent surface quality can be obtained.

In the blowing agent impregnation step according to one embodiment of the present invention, the amount (usage) of blowing agent impregnated into the copolymer methyl methacrylate resin particles, including preferred embodiments, is the same as the content of blowing agent in the expandable resin particles described in the section (Blowing Agent) of [2. Expandable Methyl Methacrylate Resin Particles]. This configuration not only produces expandable resin particles with sufficient expandability, but also allows for the safe production of expandable resin particles without causing aggregation of the copolymer during the blowing agent impregnation step.

In the blowing agent impregnation step according to one embodiment of the present invention, the treatment temperature (also referred to as the impregnation temperature) and treatment time (also referred to as the impregnation time) when impregnating the copolymer with the blowing agent are not particularly limited.

In the blowing agent impregnation step according to one embodiment of the present invention, the impregnation temperature when the blowing agent is impregnated into the copolymer is preferably 95°C to 120°C or less, and more preferably 100°C to 117°C or less. When the impregnation temperature is 95°C or higher, the blowing agent is sufficiently impregnated into the interior of the copolymer, and there is no risk of a double cell structure (hard core) forming in the methyl methacrylate resin expanded beads obtained by expanding the resulting expandable resin beads. As a result, by molding the expanded beads in a mold, a foamed molded article with excellent surface quality can be obtained. When the impregnation temperature is 120°C or less, the pressure inside the polymerization machine does not become too high, and therefore, expandable resin beads capable of providing expanded beads with a uniform cell structure can be obtained without the need for heavy-duty impregnation equipment capable of withstanding high pressure.

In order to obtain expandable resin particles that can provide foamed molded articles with excellent strength, aromatic monomers (e.g., aromatic vinyl compounds such as styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, and chlorostyrene) may be used in the preparation of the copolymer in this production method.

On the other hand, in order to obtain expandable resin particles that can provide foamed molded articles with minimal residue upon combustion, the smaller the amount of aromatic monomer used in this production method, the better. In this production method, the content of aromatic monomer per 100 parts by weight of the monomer mixture is preferably 2.5 parts by weight or less, more preferably less than 2.5 parts by weight, more preferably 2.0 parts by weight or less, more preferably 1.5 parts by weight or less, even more preferably 1.0 part by weight or less, and particularly preferably 0 part by weight. In other words, it is particularly preferable that the monomer mixture in this production method does not contain aromatic monomer.

A solvent may also be used in this production method. Solvents are preferably compounds with a boiling point of 50°C or higher, such as (a) aliphatic hydrocarbons with C6 or more (6 or more carbon atoms), such as toluene, hexane, and heptane, and (b) alicyclic hydrocarbons with C6 or more, such as cyclohexane and cyclooctane. To obtain expandable resin particles with excellent expandability, toluene and/or cyclohexane are preferred as solvents.

There are no particular restrictions on the amount of solvent used, but it is preferably between 1.5 and 3.0 parts by weight per 100 parts by weight of monomer. When the amount of solvent used is within this range, expandable resin particles with excellent expandability can be obtained, and expanded beads obtained by expanding these expandable resin particles can be used to obtain foamed molded articles with excellent internal fusion properties.

In this production method, the timing of using the solvent is not particularly limited, and it can be used in the copolymerization step, the blowing agent impregnation step, or both the copolymerization step and the blowing agent impregnation step. The solvent is preferably added to the reaction mixture (aqueous suspension) immediately before the resin particles are impregnated with the blowing agent (i.e., immediately before the blowing agent impregnation step), or simultaneously with the blowing agent.

A plasticizer may also be used in this production method. Preferred plasticizers are those with a high boiling point of 200°C or higher, such as (a) fatty acid glycerides such as stearic acid triglyceride, palmitic acid triglyceride, lauric acid triglyceride, stearic acid diglyceride, and stearic acid monoglyceride; (b) vegetable oils such as coconut oil, palm oil, and palm kernel oil; (c) aliphatic esters such as dioctyl adipate and dibutyl sebacate; and (d) organic hydrocarbons such as liquid paraffin and cyclohexane.

The amount of plasticizer used and the timing of its use are not particularly limited and can be determined as appropriate.

A fatty acid metal salt, a fusion accelerator, an antistatic agent, etc. may be applied to the surface of the expandable resin particles obtained through the above-mentioned copolymerization step and blowing agent impregnation step. That is, the present manufacturing method may further include a coating step in which a fatty acid metal salt, a fusion accelerator, an antistatic agent, etc. is applied to the surface of the expandable resin particles obtained in the blowing agent impregnation step. The fatty acid metal salt, fusion accelerator, antistatic agent, etc. applied to the surface of the expandable resin particles are sometimes referred to as "external additives."

Applying a fatty acid metal salt to the surface of expandable resin particles has the advantage of preventing mutual adhesion (hereinafter referred to as blocking) between expandable resin particles and/or between expandable particles during the manufacturing process of the expandable particles. Examples of fatty acid metal salts include zinc stearate, calcium stearate, magnesium stearate, aluminum stearate, zinc oleate, magnesium oleate, zinc laurate, and calcium laurate. These fatty acid metal salts may be used alone or in combination of two or more. From the viewpoints of affinity with the methyl methacrylate units contained in the base resin, anti-blocking effect, and fusion properties of the foamed molded product, zinc stearate and magnesium stearate are preferred as fatty acid metal salts, with zinc stearate being particularly preferred.

Applying a fusion accelerator to the surface of expandable resin particles has the advantage of ensuring fusion during molding, even when an anti-blocking agent is applied to the surface of the expandable resin particles. Fusion accelerators include (a) fatty acid triglycerides such as lauric acid triglyceride, stearic acid triglyceride, linoleic acid triglyceride, and hydroxystearic acid triglyceride; (b) fatty acid diglycerides such as lauric acid diglyceride, stearic acid diglyceride, and linoleic acid diglyceride; (c) fatty acid monoglycerides such as lauric acid monoglyceride, stearic acid monoglyceride, and linoleic acid monoglyceride; and (d) vegetable oils such as castor oil. These fusion accelerators may be used alone or in combination. Due to their affinity with the methyl methacrylate units contained in the base resin and their excellent fusion-promoting effect, castor oil and stearic acid triglyceride are preferred fusion accelerators, with castor oil being particularly preferred.

Applying an antistatic agent to the surface of expandable resin particles has the advantage of suppressing static electricity-related interference during raw material flow and preventing the expandable particles from adhering to silos. Commonly used antistatic agents include N-hydroxyethyl-N-(2-hydroxyalkyl)amine, N,N-bis(hydroxyethyl)dodecylamine, N,N-bis(hydroxyethyl)tetradecylamine, N,N-bis(hydroxyethyl)hexadecylamine, N,N-bis(hydroxyethyl)octadecylamine, N-hydroxyethyl-N-(2-hydroxytetradecyl)amine, N-hydroxyethyl-N-(2-hydroxyhexadecyl)amine, N-hydroxyethyl-N-(2-hydroxyoctadecyl)amine, N-hydroxypropyl-N-(2-hydroxytetradecyl)amine, and N-hydroxybutyl-N-(2-hydroxypropyl). Examples of suitable antistatic agents include 1-amino-2-hydroxy compounds such as N-hydroxypentyl-N-(2-hydroxytetradecyl)amine, N-hydroxypentyl-N-(2-hydroxyhexadecyl)amine, N-hydroxypentyl-N-(2-hydroxyoctadecyl)amine, 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; glycerin, fatty acid monoglycerides, polyoxyethylene alkyl ethers, and polyoxyethylene fatty acid esters. These antistatic agents may be used alone or in combination of two or more. N-hydroxyethyl-N-(2-hydroxyalkyl)amine is particularly preferred as the antistatic agent due to its excellent antistatic properties.

4. Methyl methacrylate resin foam particles
Expanded particles obtained by expanding the expandable methyl methacrylate resin particles described in Section [2. Expandable methyl methacrylate resin particles] or the expandable methyl methacrylate resin particles produced by the production method described in Section [3. Production method for expandable methyl methacrylate resin particles] also represent one embodiment of the present invention.

"Methyl methacrylate resin expanded particles according to one embodiment of the present invention" may also be referred to as "the present expanded particles" below.

The expandable resin particles can be made into expanded particles using a common expansion method. Specifically, expanded methyl methacrylate resin particles can be obtained, for example, by performing the following steps (1) to (3) in order: (1) Loading the expandable resin particles into a pressurized expansion machine (e.g., a BHP manufactured by Daikai Kogyo Co., Ltd.); (2) Heating the expandable resin particles by blowing steam (e.g., water vapor) into the expansion machine under conditions of a steam blowing pressure of 0.10 MPa to 0.16 MPa and an internal expansion machine pressure of 0.005 MPa to 0.030 MPa; (3) Expanding the expandable resin particles using step (2) until the desired expansion ratio (e.g., a bulk ratio of 60 times) is reached, thereby obtaining expanded particles.

The expansion of expandable methyl methacrylate resin particles can also be considered preliminary expansion for obtaining a methyl methacrylate resin foam molded article, described below, from the expandable methyl methacrylate resin particles. Therefore, the expansion of expandable methyl methacrylate resin particles is sometimes referred to as "pre-expansion," and expanded methyl methacrylate resin particles are sometimes referred to as "pre-expanded methyl methacrylate particles." The expansion machine used to expand expandable methyl methacrylate resin particles (for example, the expansion machine used to measure bulk density (A)) is sometimes referred to as the "pre-expansion machine."

Because the present expanded beads have the above-mentioned configuration, the volume (B) is 140 ^cm3 or less, and therefore the expansion rate is slow. Also, because the present expanded beads have the above-mentioned configuration, the volume (C) is more than 160 ^cm3 , and therefore the present expanded beads have excellent shrinkage suppression properties. In other words, the present expanded beads have the advantage of being able to provide foamed molded articles that are excellent in internal fusion properties and castability.

Regarding the expanded particles, the descriptions in sections [2. Expandable methyl methacrylate resin particles] and [3. Method for producing expandable methyl methacrylate resin particles] are incorporated herein by reference as appropriate, except for the matters mentioned above.

5. Methyl methacrylate resin foam molded article
A foamed molded article obtained by molding the expanded methyl methacrylate resin beads described in the section [4. Expanded methyl methacrylate resin beads] in a mold is also one embodiment of the present invention.

The "methyl methacrylate resin foam molded article according to one embodiment of the present invention" may also be referred to below as "the foam molded article."

These expanded beads can be molded into a foamed molded article using a typical in-mold molding method. Specifically, a foamed molded article can be obtained by, for example, performing the following steps (1) to (3) in order: (1) Filling a molding machine equipped with a mold (e.g., Daisen's PEONY-205DS) with the expanded beads; (2) Blowing steam (e.g., water vapor) into the mold at a steam injection pressure of 0.15 MPa to 0.25 MPa, and performing in-mold molding by vacuum suction and heating under conditions of an in-mold pressure of 0.030 MPa to 0.060 MPa until the foaming pressure reaches 0.070 MPa to 0.080 MPa, thereby fusing the expanded beads together; (3) After the foaming pressure reaches 0.070 MPa to 0.080 MPa, leaving the molded article in the mold at 80°C to 110°C for 1,000 seconds, after which the foamed molded article is removed to obtain the foamed molded article.

Because the foamed molded article has the above-described structure, it has excellent internal fusion properties and castability. In particular, it is preferable that the foamed molded article have (D) of 85% or more. As a result, the foamed molded article can be suitably used as a lost pattern.

The method for evaluating the castability of this foam molded product is described in detail in the examples below. Because residual defects tend to accumulate on the surface of the casting when using the full mold method, casting is generally performed using a foam molded product that is larger than the product dimensions and includes a processing allowance. After casting, the casting is machined to remove the residual defects along with the processing allowance. If the residual defects are large, they may not be completely removed from the casting. This is undesirable because residual defects affect mechanical properties such as strength, and the casting may break after short-term use. If the casting has no residual defects, these problems are eliminated, making it possible to provide an efficient, durable casting.

With regard to the present foamed molded product, the descriptions in sections [2. Expandable methyl methacrylate resin particles], [3. Method for producing expandable methyl methacrylate resin particles], and [4. Expanded methyl methacrylate resin particles] are incorporated herein by reference as appropriate, in addition to the matters described above.

[6. Disappearance model]
A lost model including a methyl methacrylate resin foam molded product described in the section [5. Methyl methacrylate resin foam molded product] is also one embodiment of the present invention.

The lost foam pattern according to one embodiment of the present invention has excellent internal fusion properties and castability, making it suitable for use in a variety of metal castings.

One embodiment of the present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to these.

(Polymerization Conversion Rate of Expandable Methyl Methacrylate Resin Particles)
During polymerization, the aqueous suspension was sampled and filtered. The weight of the resin component remaining on the filter paper was measured, and the obtained weight was defined as the weight before heating. Next, a polymerization inhibitor was added to the resin component, and the resin component was heated at 150°C for 30 minutes to remove volatile components. Thereafter, the weight of the obtained resin component was measured, and the obtained weight was defined as the weight after heating. The polymerization conversion rate was calculated using the following formula.
Polymerization conversion rate (%)=weight after heating/weight before heating×100.

(glass transition temperature)
The glass transition temperature measured by the following method was taken as the glass transition temperature of the base resin contained in the expandable resin particles: (1) The expandable resin particles were dried at 150°C for 30 minutes to obtain a resin sample; (2) 4 mg of the sample was placed in an aluminum container, and an aluminum lid was attached to the aluminum container using a compressor to obtain a measurement sample; (3) The measurement sample was heated from 50°C to 150°C (heating rate: 10°C/min) using a DSC measurement device (Hitachi DSC7000X), cooled from 150°C to 50°C (heating rate: 10°C/min), and then heated again from 50°C to 150°C (heating rate: 10°C/min); (4) The glass transition temperature was calculated using the DSC curve obtained during the second heating. Note that the glass transition temperature here refers to the midpoint glass transition temperature defined in JIS K7121.

(Weight average molecular weight)
The weight-average molecular weight measured by the following method was taken as the weight-average molecular weight of the base resin contained in the expandable resin particles: (1) 0.02 g of expandable resin particles was dissolved in 20 ml of THF; (2) Then, the gel component in the resulting solution was filtered; (3) Next, using only the THF-soluble component (i.e., the filtrate) as a sample, GPC measurement was performed using a gel permeation chromatograph (GPC) under the following conditions; (4) The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were calculated from the GPC measurement chart obtained by the GPC measurement. Note that the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are relative values converted into polystyrene.
<Conditions for GPC measurement>
Measurement equipment: Tosoh Corporation, high-speed GPC equipment HLC-8220
Columns used: Tosoh Corporation, Super HZM-H x 2, Super H-RC x 2 Column temperature: 40°C Mobile phase: THF (tetrahydrofuran)
Flow rate: 0.35ml/min, injection volume: 10μl
Detector: RI.

(Expandability of Expandable Methyl Methacrylate Resin Particles)
The following steps (1) to (6) were carried out in order to calculate the bulk density (A) of the expandable resin particles: (1) 10 g of expandable resin particles were weighed out, and an anti-blocking agent was applied to the surfaces of the expandable resin particles; (2) The expandable resin particles were placed in a steamer equipped with an outlet; (3) Steam at 100°C was supplied to the steamer, and the expandable resin particles were heated for 300 seconds to obtain expanded particles; (4) The obtained expanded particles were placed in a 1000 ^cm3 measuring cylinder; (5) The volume ( ^cm3 ) of the expanded particles was measured from the graduations on the measuring cylinder; (6) The bulk density of the expanded particles was calculated using the following formula:
Bulk density (g/cm ³ )=10 (g)/volume of expanded particles (cm ³ ).

The expandability of the expandable resin particles was evaluated from the obtained bulk density (A) based on the following criteria.
◯ (Good): Bulk density (A) is 0.0285 g/cm ³ or less. × (Poor): Bulk density (A) is greater than 0.0285 g/cm ³ and less than or equal to 0.0333 g/cm ^3. XX (Very Poor): Bulk density (A) exceeds 0.0333 g/cm ³ .

(Expandability of methyl methacrylate resin expanded particles)
The expandable resin particles were sieved to separate expandable resin particles having a particle diameter of 0.5 mm to 1.4 mm. The separated expandable resin particles were used to obtain expanded particles having a bulk ratio of 60 times by carrying out the following steps (1) to (3) in order: (1) the expandable resin particles were placed in a pressure-type expansion machine, BHP, manufactured by Daikai Kogyo Co., Ltd.; (2) steam was blown into the expansion machine under conditions of a steam blowing pressure of 0.10 MPa to 0.16 MPa and an expansion machine internal pressure of 0.005 MPa to 0.030 MPa, to heat the expandable resin particles; (3) the expandable resin particles were expanded until the bulk ratio reached 60 times by the method (2), to obtain expanded particles having a bulk ratio of 60 times.

The resulting expanded beads were subjected to the following steps (1) to (5) in order to measure the volume (B) of the expanded beads: (1) 100 ^cm3 of expanded beads with a bulking ratio of 60 times were weighed out and placed in a steamer equipped with an outlet; (2) 100°C steam was supplied to the steamer, and the expanded beads were heated for 30 seconds; (3) After heating, the expanded beads were removed from the steamer and allowed to stand at 25°C for 1 minute; (4) The expanded beads were placed in a 1000 ^cm3 measuring cylinder; (5) The volume (B) of the expanded beads was measured using the graduations on the measuring cylinder. The expandability of the expanded beads was evaluated from the obtained volume (B) based on the following criteria. Note that the smaller the volume (B), i.e., the lower the expandability of the expanded beads, the higher the evaluation.
◯ (Good): Volume (B) is 140 cm ³ or less. × (Poor): Volume (B) is more than 140 cm ³ and less than 150 cm ^3. XX (Very Poor): Volume (B) is 150 cm ³ or more.

(Shrinkage suppression of methyl methacrylate resin expanded particles)
Expanded beads with a bulking ratio of 60 were obtained using the method described above in the section (Expandability of Methyl Methacrylate-Based Resin Expanded Beads). The resulting expanded beads were subjected to the following steps (1) to (5) in order to measure the volume (C) of the expanded beads: (1) 100 ^cm3 of expanded beads with a bulking ratio of 60 were weighed out and placed in a steamer with an outlet; (2) 100°C steam was supplied to the steamer, and the expanded beads were heated for 180 seconds; (3) After heating, the expanded beads were removed from the steamer and allowed to stand at 25°C for 1 minute; (4) The expanded beads were placed in a 1000 ^cm3 graduated cylinder; and (5) the volume (C) of the expanded beads was measured using the graduated cylinder's scale. The shrinkage-inhibiting properties of the expanded beads were evaluated from the obtained volume (C) based on the following criteria.
◯ (good): Volume (C) is more than 160 cm ³ × (poor): Volume (C) is more than 155 cm ³ and 160 cm ³ or less XX (very poor): Volume (C) is 155 cm ³ or less.

(Internal fusion property of methyl methacrylate resin foam molded article)
The expandable resin particles were sieved to separate expandable resin particles having a particle diameter of 0.5 mm to 1.4 mm.

The separated expandable resin particles were used to obtain a foamed molded article by carrying out the following steps (1) to (8) in order: (1) The expandable resin particles were placed in a pressure-type foaming machine, a BHP manufactured by Daikai Kogyo Co., Ltd.; (2) Steam was blown into the foaming machine under conditions of a steam blowing pressure of 0.10 MPa to 0.16 MPa and an internal pressure of the foaming machine of 0.005 MPa to 0.030 MPa, and the expandable resin particles were heated; (3) The expandable resin particles were expanded by the method (2) above until the bulk ratio reached 60 times; (4) The obtained expanded particles were left at room temperature (25°C) for 3 days to obtain expanded particles with a bulk ratio of 60 times; (5) A mold measuring 2000 mm in length, 1000 mm in width, and 525 mm in thickness was used. A molding machine (PEONY-205DS manufactured by Daisen) was filled with expanded beads with a bulk ratio of 60; (6) steam was blown into the mold at a steam injection pressure of 0.15 MPa to 0.25 MPa, and in-mold molding was performed by vacuum suction and heating under conditions of a pressure inside the mold of 0.030 MPa to 0.060 MPa until the expansion pressure reached 0.070 MPa to 0.080 MPa, thereby fusing the expanded beads together; (7) after the expansion pressure reached 0.070 MPa to 0.080 MPa, the mold was left in the mold at 80°C to 110°C for 1000 seconds, after which the expanded molded article was removed; (8) the removed expanded molded article was left at 60°C for 3 days to obtain a expanded molded article. The resulting expanded molded article was 2000 mm long, 1000 mm wide, and 525 mm thick.

Using the obtained foamed molded article, the following (1) to (3) were carried out in order, and the ratio (D) of the foamed molded article at the fracture surface was measured: (1) Using a hot-wire slicer, the foamed molded article was cut perpendicular to the thickness direction of the foamed molded article so that it was divided into five equal parts in the thickness direction; (2) For one of the middle parts (a portion of the foamed molded article before cutting in the thickness direction of 210 mm to 315 mm), the plane perpendicular to the thickness direction was folded along the width direction at the center of the length to fracture the foamed molded article; (3) The obtained fracture surface was visually observed, and the number of all particles constituting the fracture surface and the number of foamed beads broken other than at the particle interfaces were counted, and the ratio (D) was calculated based on the following formula:
Proportion (D) (%) = number of grains that are broken at a location other than the grain interface on the fracture surface / number of grains constituting the fracture surface × 100.

Based on the obtained ratio (D), the internal fusion property of the foamed molded article was evaluated according to the following criteria.
◯ (Excellent): The ratio (D) is 85% or more. × (Poor): The ratio (D) is 75% or more but less than 85%. XX (Very Poor): The ratio (D) is less than 75%.

(Casting ability of methyl methacrylate resin foam molded article)
The castability of foamed molded articles was measured and evaluated using the following methods: (1) foamed molded articles measuring 2000 mm long x 1000 mm wide x 500 mm thick were produced using foamed beads; (2) the resulting foamed molded articles were processed to produce foamed patterns (lost patterns); (3) the resulting foamed patterns were used to produce castings (castings) using the full mold process. The pouring material was FCD700; (4) the surfaces of the resulting castings were shot-blasted to remove the casting sand used during casting; (5) the casting surfaces, excluding the bottom surface, were then machined 10 mm; (6) the resulting castings were then visually inspected and magnetic particle testing was performed to check for the presence of casting defects such as residues in the center of the castings. Based on the results, castability was evaluated according to the following criteria.

(Casting property evaluation)
The castability of the methyl methacrylate resin foam molded article was evaluated according to the following criteria.
◯ (Excellent): When the casting has no residual defects × (Poor): When the casting has residual defects XX (Very Poor): When the casting has many residual defects (Example 1)
A 6-L autoclave equipped with a stirrer was charged with 150 parts by weight of water, 0.15 parts by weight of tricalcium phosphate as a first poorly water-soluble inorganic salt, 0.0075 parts by weight of sodium α-olefin sulfonate, 0.3 parts by weight of NaCl, 0.08 parts by weight of lauroyl peroxide, 0.1 parts by weight of 1,1-bis(t-butylperoxy)cyclohexane, 0.1 parts by weight of 1,6-hexanediol diacrylate as a crosslinking agent, 0.265 parts by weight of n-dodecyl mercaptan as a chain transfer agent, and 0.026 parts by weight of benzotriazole as an ultraviolet absorber, to prepare a mixed solution containing the first poorly water-soluble inorganic salt. Subsequently, 1.0 parts by weight of toluene and 97.5 parts by weight of methyl methacrylate and 2.5 parts by weight of butyl acrylate as a monomer mixture were charged into the mixed solution to prepare an aqueous suspension. The temperature of the aqueous suspension was then raised to 80°C to initiate polymerization, i.e., the initiation step was carried out. One hour and 45 minutes after the start of polymerization (after the initiation step), the polymerization conversion rate was measured and found to be 40% to 50%. One hour and 45 minutes after the start of polymerization (after the initiation step), 0.10 parts by weight of tribasic calcium phosphate as a second poorly water-soluble inorganic salt was added to the reaction mixture (aqueous suspension) and an addition step was carried out. The initiation step and addition step described above can also be considered as a first copolymerization step.

After another 2 hours and 35 minutes had elapsed, 0.24 parts by weight of tricalcium phosphate, 1.5 parts by weight of cyclohexane, and 9 parts by weight of normal-rich butane as a blowing agent (the weight ratio of normal butane to isobutane in normal-rich butane (normal butane/isobutane) was 70/30) were added to the aqueous suspension. The temperature of the aqueous suspension was then raised to 101°C. The temperature of the aqueous suspension was then maintained at 101°C for 10 hours to carry out copolymerization and impregnation of the copolymer with the blowing agent (copolymerization step (also known as the second copolymerization step) and blowing agent impregnation step). The aqueous suspension was then cooled. After cooling the aqueous suspension, the resulting product was washed, dehydrated, and dried to obtain expandable methyl methacrylate resin particles.

The resulting expandable methyl methacrylate resin particles were sieved using sieves with mesh sizes of 0.500 mm and 1.400 mm. Through this procedure, expandable methyl methacrylate resin particles with particle sizes between 0.500 mm and 1.400 mm were collected.

Next, 0.20 parts by weight of zinc stearate as a fatty acid metal salt and 0.05 parts by weight of hydrogenated castor oil as a fusion promoter were applied to the surface of the resulting expandable methyl methacrylate resin particles.

Next, using the methods described above, the expandability of the expandable methyl methacrylate resin particles, the expandability and shrinkage suppression of the expanded methyl methacrylate resin particles, and the internal fusion and castability of the methyl methacrylate resin foam molded articles were evaluated. The evaluation results are shown in Table 1.

Example 2
The same procedure as in Example 1 was carried out, except that the amount of n-dodecyl mercaptan was changed to 0.300 parts by weight, to obtain expandable methyl methacrylate resin particles having a particle size of 0.500 mm to 1.400 mm and coated with zinc stearate and hydrogenated castor oil on the surface. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 1)
The same procedure as in Example 1 was carried out, except that the monomer mixture used was changed to 95.0 parts by weight of methyl methacrylate and 5.0 parts by weight of butyl acrylate, and the amount of n-dodecyl mercaptan was changed to 0.240 parts by weight, to obtain expandable methyl methacrylate resin particles having a particle size of 0.500 mm to 1.400 mm and coated with zinc stearate and hydrogenated castor oil on the surface. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 2)
The same procedure as in Example 1 was carried out, except that the monomer mixture used was changed to 96.5 parts by weight of methyl methacrylate and 3.5 parts by weight of butyl acrylate, and the amount of n-dodecyl mercaptan was changed to 0.240 parts by weight, to obtain expandable methyl methacrylate resin particles having a particle size of 0.500 mm to 1.400 mm and coated with zinc stearate and hydrogenated castor oil on the surface. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 3)
The same procedure as in Example 1 was carried out, except that the amount of n-dodecyl mercaptan was changed to 0.240 parts by weight, to obtain expandable methyl methacrylate resin particles having a particle size of 0.500 mm to 1.400 mm and coated with zinc stearate and hydrogenated castor oil on the surface. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 4)
The same procedure as in Example 1 was carried out, except that the amount of n-dodecyl mercaptan was changed to 0.330 parts by weight, to obtain expandable methyl methacrylate resin particles having a particle size of 0.500 mm to 1.400 mm and coated with zinc stearate and hydrogenated castor oil on the surface. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 5)
Expandable methyl methacrylate resin particles having particle sizes of 0.500 mm to 1.400 mm were obtained by the same procedure as in Example 1, except that the monomer mixture used was changed to 97.0 parts by weight of methyl methacrylate and 3.0 parts by weight of butyl acrylate, and the amount of n-dodecyl mercaptan was changed to 0.250 parts by weight. Each evaluation item was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

One embodiment of the present invention provides expandable methyl methacrylate resin particles that can efficiently produce foamed molded articles with excellent internal fusion. Therefore, one embodiment of the present invention can be suitably used as a lost pattern when performing metal casting using the full mold method.

Claims (4)

Expandable methyl methacrylate-based resin particles comprising a base resin containing methyl methacrylate units and acrylic ester units as constituent units, and a foaming agent, and satisfying the following (a) to (d):
(a) the bulk density (A) of the expanded methyl methacrylate resin particles obtained by heating the expandable methyl methacrylate resin particles with steam at 100°C for 300 seconds is 0.0285 g/ ^cm3 or less;
(b) the volume (B) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is 140 ^cm3 or less when 100 ^cm3 of the expanded methyl methacrylate resin beads is heated with steam at 100°C for 30 seconds and then left at 25°C for 1 minute;
(c) the volume (C) of the expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads is greater than 160 ^cm3 when 100 ^cm3 of the expanded methyl methacrylate resin beads is heated with steam at 100°C for 180 seconds and then left at 25°C for 1 minute;
and (d) the base resin has a glass transition temperature of 114.5°C or higher. Expandable methyl methacrylate resin particles according to claim 1, wherein the weight-average molecular weight of the base resin is 220,000 to 310,000. The composition includes a base resin containing a methyl methacrylate unit and an acrylic ester unit as constituent units, and a foaming agent,
The weight average molecular weight of the base resin is 220,000 to 310,000,
The glass transition temperature of the base resin is 114.5°C or higher,
the base resin further includes a crosslinker unit as a constituent unit,
In the base resin, (a) the content of the methyl methacrylate units is more than 97.0 parts by weight and not more than 99.0 parts by weight, and (b) the content of the acrylic ester units is 1.0 part by weight or more and less than 3.0 parts by weight, relative to 100 parts by weight of the total amount of the methyl methacrylate units and the acrylic ester units . Expandable methyl methacrylate resin particles according to any one of claims 1 to 3, wherein the acrylic ester units are butyl acrylate units.