Expandable methyl methacrylate resin particles

Abstract

To provide expandable methyl methacrylate resin particles capable of efficiently providing an expanded molding having excellent internal fusion properties and castability.SOLUTION: There are provided expandable methyl methacrylate resin particles which contain a base resin containing a methyl methacrylate unit and an acrylic ester unit and having a specific glass transition temperature and a foaming agent and can provide expanded particles having excellent foaming property, low foaming property and excellent shrinkage suppressing property after heating.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to expandable methyl methacrylate resin particles. [Background technology]

[0002] When metal casting is performed, a lost foam casting method (full mold method) is known in which a pattern made from a foam molded body is buried in casting sand, and molten metal is poured into the foam molded body to replace the foam molded body with the metal, thereby casting a casting. In the full mold method, foam molded bodies of methyl methacrylate polymers are used from the viewpoint of reducing residues during casting. In addition, foam molded bodies for casting are often machined from large block molded bodies.

[0003] Several techniques are known for producing expandable methyl methacrylate resin particles for producing foamed molded articles of methyl methacrylate polymers.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] Patent Document 4 discloses expandable methyl methacrylate resin particles obtained by polymerizing specific amounts of each of methyl methacrylate, an acrylic acid ester, and a polyfunctional monomer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Publication No. 2015-183111 [Patent Document 2] WO2020 / 203537 [Patent Document 3] Patent Publication No. 2018-135407 [Patent Document 4] WO2016 / 047490 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above-mentioned conventional techniques have room for improvement in terms of the internal fusion properties, castability, and production efficiency of methyl methacrylate resin foam molded articles.

[0010] In view of the above circumstances, an object of one embodiment of the present invention is to provide expandable methyl methacrylate resin particles that can efficiently provide methyl methacrylate resin foamed molded articles that have excellent internal fusion properties and castability. [Means for solving the problem]

[0011] The inventors have conducted extensive research to solve the above problems and have completed the present invention.

[0012] 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 / cm 3 Below is; (b) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3 After heating with steam at 100°C for 30 seconds, the volume (B) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 140 cm 3 Below is; (c) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3 After heating with steam at 100°C for 180 seconds, the volume (C) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 160 cm 3 is super; 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 base resin has a weight average molecular weight of 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 base resin having 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 units are butyl acrylate units. [5] In the base resin, The expandable methyl methacrylate 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. [Effects of the Invention]

[0013] According to one embodiment of the present invention, it is possible to provide expandable methyl methacrylate resin particles that can efficiently provide methyl methacrylate resin foam molded articles that have excellent internal fusion properties and castability. DETAILED DESCRIPTION OF THE INVENTION

[0014] One embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the respective configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. All academic literature and patent documents described in this specification are incorporated herein by reference. Furthermore, unless otherwise specified in this specification, the term "A to B" representing a numerical range means "greater than or equal to A (including and greater than A) and less than or equal to B (including and less than B)."

[0015] 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 body" may be referred to as "expanded molded body."

[0016] 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.

[0017] Furthermore, the main use of methyl methacrylate resin foam molded articles is as a casting in metal casting, and when used as a casting, the methyl methacrylate resin foam molded articles are particularly required to have good castability.

[0018] The inventors have conducted studies 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.

[0019] In view of the above circumstances, the present inventors have conducted extensive research with the aim of providing expandable methyl methacrylate resin particles that can efficiently provide methyl methacrylate resin foamed molded articles having excellent internal fusion properties and castability.

[0020] As a result of extensive research, the present inventors have found the following and have completed the present invention: (a) expandable methyl methacrylate resin particles having an excellent expansion rate (expandability) can efficiently provide expanded methyl methacrylate resin particles, and as a result, can efficiently provide expanded methyl methacrylate resin articles; (b) expanded methyl methacrylate resin particles having a slow expansion rate and little shrinkage after heating can provide expanded methyl methacrylate resin articles having 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 having excellent castability.

[0021] [2. Expandable methyl methacrylate resin particles] The expandable methyl methacrylate resin particles according to one embodiment of the present invention comprise 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 / cm 3 Below is; (b) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3After heating with steam at 100°C for 30 seconds, the volume (B) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 140 cm 3 Below is; (c) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3 After heating with steam at 100°C for 180 seconds, the volume (C) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 160 cm 3 is super; and (d) The glass transition temperature of the base resin is 114.5°C or higher.

[0022] The "expandable methyl methacrylate resin particles according to one embodiment of the present invention" may hereinafter be referred to as "the present expandable resin particles."

[0023] The expandable resin particles of the present invention can be expanded by a known method to provide expanded particles. The expandable resin particles of the present invention can be molded in a mold by a known method to provide a foamed molded article.

[0024] The present expandable resin particles have the above-mentioned structure, and therefore have the advantage of being able to efficiently provide foamed molded articles that are excellent in internal fusion properties and castability.

[0025] The expandable resin particles are obtained by expanding the expandable resin particles under specific conditions, and the bulk density (A) of the expanded particles is 0.0285 g / cm 3 The following is true. The smaller the bulk density (A), the higher the bulk ratio of the expanded beads obtained, i.e., the better the expandable resin beads' expandability. Expandable resin beads with excellent expandability can reduce the production time and cost when using the expandable resin beads to produce expanded beads. Therefore, the 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.

[0026] The expandable resin particles are formed by expanding the expandable resin particles, and the volume (B) of the expanded particles is 140 cm 3The 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. The smaller the volume (B), the slower the expansion rate of the expanded beads and the lower the expandability of the expanded beads. When expanded beads with low expandability 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.

[0027] The expandable resin particles are formed by expanding the expandable resin particles, and the volume (C) of the expanded particles is 160 cm 3 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 expanded beads' shrinkage suppression properties. Expanded beads with excellent shrinkage suppression 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.

[0028] The expandable resin particles have a base resin with a glass transition temperature of 114.5°C or higher. Through extensive research, the present inventors have surprisingly independently discovered that when the base resin of the expandable resin particles has a glass transition temperature of 114.5°C or higher, a foamed molded article made using the expandable resin particles has excellent castability. In other words, the expandable resin particles have the advantage of being able to provide a foamed molded article with excellent castability.

[0029] 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.

[0030] (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.

[0031] In the base resin contained in the present expandable resin particles, 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 preferably more than 97.0 parts by weight and not more than 99.0 parts by weight, and the content of acrylic ester units is preferably 1.0 part by weight or more and less than 3.0 parts by weight; (b) the content of methyl methacrylate units is more preferably more than 97.0 parts by weight and not more than 98.5 parts by weight, and the content of acrylic ester units is more preferably 1.5 parts by weight or more and less than 3.0 parts by weight; (c) the content of methyl methacrylate units is even more preferably more than 97.0 parts by weight and not more than 98.0 parts by weight, and the content of acrylic ester units is more than 2.0 parts by weight and less than 3.0 parts by weight; and (d) the content of methyl methacrylate units is particularly preferably 97.5 parts by weight, and the content of acrylic ester units is 2.5 parts by weight. When the contents of methyl methacrylate units and 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, so the castability of the final foamed molded article tends to be excellent.

[0032] Examples of the acrylic acid ester unit according to one embodiment of the present invention include methyl acrylate unit, ethyl acrylate unit, propyl acrylate unit, and butyl acrylate unit. As the acrylic acid ester unit, butyl acrylate unit is particularly preferred. This configuration provides expandable resin particles with excellent expandability and moldability (e.g., shrinkage suppression of expanded particles). The butyl acrylate unit is highly effective in lowering the glass transition temperature of the base resin and is highly effective in improving castability.

[0033] 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, (a) the expandable resin particles have excellent expandability, (b) the expanded particles obtained by expanding the expandable resin particles have low expandability and good shrinkage suppression, and (c) the expanded molded article obtained by molding the expanded particles in a mold has excellent internal fusion and produces little residue upon combustion, resulting in excellent castability. In addition, expandable resin particles containing a base resin containing structural units derived from a crosslinking agent as structural units have the advantage that the molecular weight can be easily adjusted during the manufacturing process.

[0034] Examples of crosslinking agents include compounds having two or more functional groups exhibiting radical reactivity. Among compounds having two or more functional groups exhibiting radical reactivity, 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 present expandable resin particles contains, as a structural unit derived from the crosslinking agent, a bifunctional monomer unit, which is a structural unit derived from a bifunctional monomer. This configuration has 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) a 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.

[0035] 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 because of its ease of molecular weight control. In this specification, the term "(meth)acrylate" refers to "acrylate and / or methacrylate." For example, hexanediol di(meth)acrylate is intended to mean hexanediol diacrylate and / or hexanediol dimethacrylate.

[0036] The content of the bifunctional monomer units in the present expandable resin beads is preferably 0.05 to 0.15 parts by weight, and more preferably 0.08 to 0.13 parts by weight, based on 100 parts by weight of the total content of the methyl methacrylate units and the acrylic ester units. According to the above-mentioned configuration, (a) the expandable resin beads have even better expandability, (b) the expanded beads obtained by expanding the expandable resin beads have 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 and produces even less residue upon combustion, resulting in even better castability.

[0037] The base resin of the present 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 present expandable resin particles contains an aromatic unit, a foamed molded article having excellent strength can be obtained.

[0038] 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 present 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 present 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 present expandable resin particles does not contain aromatic units.

[0039] The types and amounts of the structural units contained in the base resin are the same as the types and amounts of the 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%).

[0040] (foaming 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.

[0041] In the present expandable resin particles, the content of the blowing agent relative to 100 parts by weight of the base resin is preferably 5 parts by weight to 12 parts by weight, more preferably 7 parts by weight to 10 parts by weight. This configuration has the advantage that expandable resin particles having sufficient expandability can be provided and that heavy polymerization equipment is not required.

[0042] (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.

[0043] The solvent and plasticizer will be described in detail in the section "3. Method for producing expandable methyl methacrylate resin particles" below. The contents of the solvent and plasticizer in the expandable resin particles may be the same as the amounts of the solvent and plasticizer used, which will be described later.

[0044] (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, etc. The content of the cell regulator relative to 100 parts by weight of the base resin is preferably 0.01 to 0.50 parts by weight.

[0045] (Weight average molecular weight) The weight-average molecular weight of the base resin contained in the expandable resin particles 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 particles have good expandability; (b) the expanded particles obtained by expanding the expandable resin particles have low expandability and good shrinkage suppression; and (c) the foamed molded articles obtained by molding the expanded particles in a mold 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.

[0046] In this specification, the weight-average molecular weight obtained by measuring 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 the GPC measurement. Note that the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are relative values ​​converted into polystyrene equivalents.

[0047] 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 the chain transfer agent, the polymerization temperature and time, the type and amount of the initiator, and the type and amount of the crosslinking agent.

[0048] (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 of the present expandable resin particles has the advantage of being able to provide a foamed molded article with excellent castability. Since foamed molded articles 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.

[0049] In this specification, the glass transition temperature of the base resin contained in the expandable resin particles is determined by the following method: (1) the expandable resin particles are dried at 150°C for 30 minutes to obtain a resin 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.

[0050] 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.

[0051] 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. Furthermore, in expanded molded articles produced using expandable resin beads, the structure of the expandable beads changes, but the composition of the expandable 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 molded articles" substituted for "expandable resin beads." 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 molded articles" substituted for "expandable resin beads."

[0052] (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 Millitrac JPA particle size analyzer), and the results are expressed as a cumulative distribution.

[0053] In some cases, the expandable methyl methacrylate resin particles are sieved to separate those having 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 is within the range of 0.5 mm to 1.4 mm.

[0054] (Expandability of Expandable Methyl Methacrylate Resin Particles) The expandable resin particles have a bulk density (A) of 0.0285 g / cm 3 The expanded particles can be efficiently (for example, in a shorter time), and as a result, the expanded molded article can be efficiently (for example, in a shorter time). Therefore, the bulk density (A) is 0.0260 g / cm or less. 3 Preferably less than 0.0250 g / cm 3 Less than 0.0222 g / cm is more preferable. 3 The following is particularly preferred: The smaller the bulk density (A), the more preferable, and although there is no particular limitation on the lower limit, the bulk density (A) is at least 0.0000 g / cm 3 The expandable methyl methacrylate resin particles used in measuring the bulk density (A) may have an anti-blocking agent applied to their surfaces.

[0055] 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 the bulk density (A)); (2) measuring the total amount of the expanded beads in a volume of 1000 cm 3 (3) Measure the volume of the expanded particles from the graduated cylinder; (4) Calculate the bulk density of the expanded particles using the following formula: Bulk density (g / cm 3 ) = Weight of foamed particles (g) / Volume of foamed particles (cm 3 ).

[0056] 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.

[0057] An example of a method for expanding expandable resin particles in measuring the bulk density (A) is shown below in (1) to (3): (1) a certain amount (e.g., 10 g) of expandable resin particles 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) 100°C steam is supplied to the steamer, and the expandable resin particles are heated for 300 seconds to obtain expanded particles.

[0058] The expanded particles obtained by expanding the expandable resin particles have a volume (B) of 140 cm 3 Since a foamed molded article having excellent internal fusion properties can be obtained, the volume (B) is 138 cm or less, and the foaming property is low. 3 Less than 135cm is preferable 3 Less than 130cm is preferable. 3 The smaller the volume (B), the more preferable. The lower limit of the volume (B) is not particularly limited, but the volume (B) is at least 100 cm 3 Exceeds.

[0059] Here, the method for measuring the volume (B) of the expanded beads obtained by expanding the present expandable resin beads is not particularly limited, but for example, the following method (1) to (6) are carried out in order: (1) The present expandable resin beads are expanded to a bulk ratio of 60 times to prepare expanded beads having a bulk ratio of 60 times; (2) The expanded beads are expanded to a volume of 100 cm 3 (3) Steam at 100°C is supplied to the steamer and the expanded particles are heated for 30 seconds; (4) After heating, the expanded particles are removed from the steamer and allowed to stand at 25°C for 1 minute; (5) The expanded particles are heated to 1000 cm 3 (6) Measure the volume (B) of the foam particles from the measuring cylinder.

[0060] The expanded particles obtained by expanding the expandable resin particles have a volume (C) of 160 cm 3Since it is possible to provide a foamed molded article with excellent internal fusion properties, the volume (C) is 165 cm 3 Above 168cm is preferred 3 More than 170cm is preferable. 3 The above is particularly preferred.

[0061] Here, the method for measuring the volume (C) of the expanded beads obtained by expanding the present expandable resin beads is not particularly limited, but for example, the following method (1) to (6) are carried out in order: (1) The present expandable resin beads are expanded to a bulk ratio of 60 times to prepare expanded beads having a bulk ratio of 60 times; (2) The expanded beads are expanded to a volume of 100 cm 3 (3) Steam at 100°C is supplied to the steamer and the expanded particles are heated for 180 seconds; (4) After heating, the expanded particles are removed from the steamer and allowed to stand at 25°C for 1 minute; (5) The expanded particles are placed in a 1000cm 3 (6) Measure the volume (C) of the foam particles from the graduated cylinder.

[0062] 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) Weigh out 10 g of expanded beads and measure them in 1000 cm 3 (2) Measure the volume of 10 g of expanded beads from the graduated cylinder; (3) Calculate the bulk ratio of the expanded beads using the following formula: Bulking ratio (times) = Volume of foamed particles (cm 3 ) / 10g.

[0063] In this specification, the bulk ratio of the expanded beads can also be called the expansion ratio. The unit of the bulk ratio is actually cm based on the above formula. 3 / g, but for convenience, the unit of bulk magnification is expressed as "times" in this specification.

[0064] The method for producing the 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 the expandable resin beads into a pressure-type 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 pressure of the expansion machine 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 described in (2) above to obtain expanded beads. The expandable resin beads used to produce the 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.

[0065] (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 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; (iv) the lower the content of acrylic ester units in the base resin 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 resin 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 expanded molded articles having excellent internal fusion properties and castability.

[0066] Therefore, the present inventors conducted further intensive research. As a result, they discovered that the above-mentioned object 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 present inventors discovered the following novel findings: By setting 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 setting 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 little shrinkage after heating.

[0067] That is, 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 base resin has a weight-average molecular weight of 220,000 to 310,000, and a glass transition temperature of 114.5°C or higher.

[0068] The expandable methyl methacrylate resin particles according to another embodiment of the present invention have the above-described structure, and therefore have the advantage of being able to provide expanded particles that have (a) excellent expandability and (b) low expandability and excellent shrinkage suppression. Furthermore, the expandable resin particles having the above-described structure have the advantage of being able to efficiently provide foamed molded articles that have excellent internal fusion properties and castability.

[0069] For other aspects of the expandable methyl methacrylate resin particles according to another embodiment of the present invention, the above description is incorporated as appropriate.

[0070] (Variation 2) The present expandable resin beads can provide foamed molded articles with excellent internal fusion. The internal fusion of a 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 a foamed molded article is obtained by breaking a foamed molded article obtained by molding expanded beads obtained by expanding the present expandable resin beads, and the proportion (D) of expanded beads that have broken at locations other than their interfaces on the fracture surface of the total expanded beads (100%) constituting the fracture surface is 85% or more, the foamed molded article can be said to have excellent internal fusion.

[0071] That is, 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, and satisfying 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 / cm 3 Below is; (b) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3 After heating with steam at 100°C for 30 seconds, the volume (B) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 140 cm 3 Below is; (c) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles 3 After heating with steam at 100°C for 180 seconds, the volume (C) of the methyl methacrylate resin foam particles obtained by leaving it at 25°C for 1 minute is 160 cm 3 (d) the glass transition temperature of the base resin is 114.5°C or higher; and (e) In a fracture surface of a methyl methacrylate resin foamed molded article obtained by breaking a methyl methacrylate resin foamed molded article obtained by molding expanded methyl methacrylate resin beads obtained by expanding the expandable methyl methacrylate resin beads, the proportion (D) of the expanded methyl methacrylate resin beads broken at a location other than the interface of the expanded methyl methacrylate resin beads is 90% or more.

[0072] Here, the method for measuring the ratio (D) of 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) Using a hot-wire slicer, cut the foamed molded article perpendicular to the thickness direction of the foamed molded article so that the foamed molded article is divided into five equal parts in the thickness direction; (3) 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), a plane perpendicular to the thickness direction is folded along the width direction at the center of the length direction to break the foamed molded article; (4) The obtained fracture surface is visually observed, and the number of all particles constituting the fracture surface and the number of expanded beads broken other than at the particle interfaces are counted, and the ratio (D) is calculated according to the following formula: Percentage (D) (%) = Number of particles that are broken at a location other than the particle interface on the fracture surface / Number of particles constituting the fracture surface × 100.

[0073] The method for producing the foamed molded article used to measure the ratio (D) is not particularly limited, and examples thereof include a method in which the following steps (1) to (8) are carried out in order: (1) Expandable resin particles are placed in a pressure-type foaming machine (for example, a BHP manufactured by Daikai Kogyo Co., Ltd.); (2) Steam (for example, 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 a desired expansion ratio (for example, a bulk ratio of 60 times) is reached; (4) The obtained expanded particles are left at room temperature (for example, 25°C) for 3 days to obtain expanded particles with a bulk ratio of 60 times; (5) A mold (for example, 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 performed by vacuum suction and heating under conditions of 0.030 MPa to 0.060 MPa inside the mold until the foaming pressure reaches 0.070 MPa to 0.080 MPa, thereby fusing the foamed particles together; (7) After the foaming pressure reaches 0.070 MPa to 0.080 MPa, the mold is left in the mold at 80°C to 110°C for 1000 seconds, after which the foamed molded product is removed; (8) The removed foamed molded product is left at 60°C for 3 days to obtain a foamed molded product. The expandable resin particles used to produce the foamed molded article used to measure the ratio (D) may be those which have been sieved to have a particle diameter of 0.5 mm to 1.4 mm.

[0074] The ratio (D) can also be called the fusion rate. In order to obtain superior internal fusion properties, the ratio (D) is preferably 85% or more.

[0075] 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.

[0076] A preferred embodiment of the method for producing expandable resin particles includes, for example, the following method: a method for producing expandable methyl methacrylate 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 relative to 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 relative to 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.

[0077] 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.

[0078] The preferred embodiment of the method for producing expandable resin particles described above is also one embodiment of the present invention. A preferred embodiment of the method for producing expandable resin particles described above, i.e., a method for producing expandable methyl methacrylate resin particles according to one embodiment of the present invention, will be described below. For matters other than those detailed below, the description in Section [2. Expandable methyl methacrylate resin particles] is incorporated herein by reference as appropriate. Hereinafter, the copolymer (which may also be referred to as the base resin) obtained in the copolymerization step may also be referred to simply as "resin particles." In addition, in 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."

[0079] 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, etc. The aqueous suspension may contain (a) a water-soluble surfactant and a monomer dissolved therein, and may also contain (b) a water-insoluble dispersant, a polymerization initiator, a chain transfer agent, a crosslinking agent, a cell regulator, a flame retardant, a solvent, etc. dispersed together with the monomer.

[0080] The weight ratio of the monomer and polymer (a methyl methacrylate resin, also called a copolymer) to water or aqueous solution in the aqueous suspension is preferably 1.0 / 0.6 to 1.0 / 3.0 in terms of the resulting ratio of methyl methacrylate resin to water or aqueous solution. Note that the "aqueous solution" referred to here means a solution consisting of water and components other than the methyl methacrylate resin.

[0081] 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 the monomer mixture using an aqueous suspension containing (a) water, (b) a monomer mixture containing methyl methacrylate monomer and an acrylic acid 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.

[0082] In this specification, "before the initiation step," i.e., "before the start of the polymerization reaction," may also be referred to as "the early stage of polymerization." The first poorly water-soluble inorganic salt blended (added) to the aqueous suspension in the initiation step, and the polymerization initiator, which is optionally blended, can be said to be substances (raw materials) used in the early stage of polymerization.

[0083] 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., in the early stage of polymerization, include tricalcium phosphate, magnesium pyrophosphate, hydroxyapatite, and kaolin.

[0084] 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.

[0085] 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 the 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.

[0086] The initiation step according to one embodiment of the present invention is preferably a step of initiating copolymerization of a 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 a first poorly water-soluble inorganic salt relative to 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 relative to 100 parts by weight of the monomer mixture, there is no risk of the volume average particle size 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 relative to 100 parts by weight of the monomer mixture, there is no risk of the generation of large amounts of fine particles of the expandable resin particles. That is, 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 a desired volume average particle size can be obtained with good yield.

[0087] 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).

[0088] 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 to 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.

[0089] 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.

[0090] 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 said to be an aqueous suspension.

[0091] 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 of easily obtaining expandable resin particles having a target (desired) volume average particle size.

[0092] The addition step is preferably a step of adding, to the reaction mixture after the initiation step, at a polymerization conversion rate of 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 relative to 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 relative to 100 parts by weight of the monomer mixture in the addition step, the volume average particle size of the resulting expandable resin particles is unlikely to become too large. When 0.50 parts by weight or less of the second poorly water-soluble inorganic salt is added to the reaction mixture relative to 100 parts by weight of the monomer mixture in the addition step, the amount of the poorly water-soluble inorganic salt used is not excessive, thereby reducing production costs. That is, by adding the second poorly water-soluble inorganic salt in an amount within the above-mentioned range to the reaction mixture in the addition step, expandable resin particles having a desired volume average particle size can be obtained at low production costs.

[0093] 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 size 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 size of the resulting expandable resin particles is unlikely to be too large. That is, 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 size can be easily obtained. The method for measuring the polymerization conversion rate in this specification will be described in detail in the Examples below.

[0094] The copolymerization step according to one embodiment of the present invention is preferably carried out in at least two stages by changing the polymerization temperature. For convenience, the two copolymerization steps having different polymerization temperatures are hereinafter referred to as the first copolymerization step and the second copolymerization step. It can also be said that the copolymerization step preferably includes a first copolymerization step and a second copolymerization step that are successively carried out at different polymerization temperatures.

[0095] The copolymerization step according to one embodiment of the present invention preferably includes, for example, (a) a first copolymerization step carried out at a polymerization temperature of 70°C to 90°C using a low-temperature decomposition polymerization initiator, and (b) a second copolymerization step carried out consecutively to the first copolymerization step, at a polymerization temperature higher than that of the first copolymerization step (e.g., 90°C to 110°C) using a high-temperature decomposition polymerization initiator. In the copolymerization step, it is preferable that the main polymerization reaction occurs in the first copolymerization step, and the remaining monomer is reduced in the second copolymerization step. Note that (i) the temperature of the first copolymerization step may be 70°C or higher but lower than 90°C, and the temperature of the second copolymerization step may be 90°C to 110°C, or (ii) the temperature of the first copolymerization step may be 70°C to 90°C, and the temperature of the second copolymerization step may be higher than 90°C but lower than 110°C.

[0096] As the polymerization initiator, radical-generating polymerization initiators generally used in the production of thermoplastic polymers can be used. Typical radical-generating polymerization initiators include, for example, (a) organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxybenzoate, isopropyl-t-butyl peroxycarbonate, butyl perbenzoate, t-butylperoxy-2-ethylhexanoate, t-butyl perpivalate, t-butylperoxyisopropyl carbonate, di-t-butylperoxyhexahydroterephthalate, 1,1-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.

[0097] 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 type polymerization initiators, and (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 type polymerization initiators.

[0098] The amount of the polymerization initiator used, calculated as the sum of the amount used in the first copolymerization step and the amount used in the second copolymerization step, is preferably, for example, 0.1 to 0.5 parts by weight per 100 parts by weight of the monomer mixture. According to this configuration, expandable resin particles with excellent expandability can be obtained.

[0099] 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 the polymerization.

[0100] 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. When 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.).

[0101] In the copolymerization step according to one embodiment of the present invention, it is preferable to use a chain transfer agent. 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. As a chain transfer agent, n-dodecyl mercaptan is preferred because it allows for easy molecular weight control.

[0102] 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 to 0.400 parts by weight, and even more preferably 0.250 to 0.300 parts by weight, per 100 parts by weight of the monomer mixture.

[0103] 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.

[0104] 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.

[0105] In the blowing agent impregnation step according to one embodiment of the present invention, it is preferable to impregnate 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 the 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.

[0106] In the blowing agent impregnation step according to one embodiment of the present invention, the amount of blowing agent impregnated into the copolymer methyl methacrylate resin particles (usage amount) is the same as the content of the blowing agent in the expandable resin particles described in the section (Blowing Agent) of [2. Expandable Methyl Methacrylate Resin Particles], including preferred embodiments. This configuration allows for the production of expandable resin particles with sufficient expandability, and also allows for the safe production of expandable resin particles without causing aggregation of the copolymer in the blowing agent impregnation step.

[0107] In the blowing agent impregnation step according to one embodiment of the present invention, the treatment temperature (also referred to as impregnation temperature) and treatment time (also referred to as impregnation time) when the copolymer is impregnated with the blowing agent are not particularly limited.

[0108] 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, 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, so there is no risk of a double cell structure (hard core) being formed in the methyl methacrylate resin 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. When the impregnation temperature is 120°C or lower, the pressure inside the polymerization machine does not become too high, so there is no need for heavy-duty impregnation equipment capable of withstanding high pressure, and expandable resin beads capable of providing expanded beads with a uniform cell structure can be obtained.

[0109] 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.

[0110] On the other hand, in order to obtain expandable resin particles that can provide foamed molded articles with little 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.

[0111] In this production method, a solvent may also be used. The solvent is preferably a compound having a boiling point of 50°C or higher, such as (a) aliphatic hydrocarbons having C6 or more (6 or more carbon atoms) such as toluene, hexane, and heptane, and (b) alicyclic hydrocarbons having C6 or more such as cyclohexane and cyclooctane. To obtain expandable resin particles with excellent expandability, toluene and / or cyclohexane are preferred as the solvent.

[0112] The amount of the solvent used is not particularly limited, but is preferably 1.5 parts by weight to 3.0 parts by weight per 100 parts by weight of the monomer. When the amount of the solvent used is within this range, expandable resin particles with excellent expandability can be obtained, and expanded beads obtained by expanding the expandable resin particles can be used to obtain foamed molded articles with excellent internal fusion.

[0113] In this production method, the timing of using the solvent is not particularly limited, and the solvent 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.

[0114] In the present production method, a plasticizer may also be used. Examples of the plasticizer include 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.

[0115] The amount of plasticizer used and the timing of using the plasticizer are not particularly limited and may be set appropriately.

[0116] The surfaces of the expandable resin particles obtained through the copolymerization step and the blowing agent impregnation step described above may be coated with a fatty acid metal salt, a fusion accelerator, an antistatic agent, etc. That is, the present production method may further include a coating step of coating the surfaces of the expandable resin particles obtained in the blowing agent impregnation step with a fatty acid metal salt, a fusion accelerator, an antistatic agent, etc. The fatty acid metal salt, the fusion accelerator, the antistatic agent, etc. coated on the surfaces of the expandable resin particles may also be referred to as "external additives."

[0117] Coating the surfaces of expandable resin particles with a fatty acid metal salt has the advantage of preventing mutual adhesion (hereinafter referred to as blocking) between the expandable resin particles and / or between the expandable particles during the manufacturing process of the expandable beads. 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.

[0118] Applying a fusion accelerator to the surface of expandable resin particles has the advantage of ensuring fusion during molding, even when an antiblocking agent is applied to the surface of the expandable resin particles. Examples of 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 hydrogenated oil. These fusion accelerators may be used alone or in combination of two or more. Due to their affinity with the methyl methacrylate units contained in the base resin and their excellent fusion-promoting effect, castor hydrogenated oil and stearic acid triglyceride are preferred fusion accelerators, with castor hydrogenated oil being particularly preferred.

[0119] Applying an antistatic agent to the surface of expandable resin particles has the advantage of suppressing static electricity-induced interference during raw material flow and preventing the expandable particles from adhering to the silo. Examples of 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)amines are particularly preferred as antistatic agents because they have the best antistatic properties.

[0120] [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.

[0121] The "expanded methyl methacrylate resin particles according to one embodiment of the present invention" may be referred to as "the expanded particles" hereinafter.

[0122] The expandable resin particles can be made into expanded particles by a general expansion method. Specifically, expanded methyl methacrylate resin particles can be obtained by, for example, sequentially performing the following steps (1) to (3): (1) charging the expandable resin particles into a pressure-type expansion machine (e.g., BHP manufactured by Daikai Kogyo Co., Ltd.); (2) 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 expansion machine internal pressure of 0.005 MPa to 0.030 MPa to heat the expandable resin particles; (3) expanding the expandable resin particles according to step (2) until a desired expansion ratio (e.g., a bulk ratio of 60 times) is reached, thereby obtaining expanded particles.

[0123] The expansion of expandable methyl methacrylate resin particles can be considered to be 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." An expansion machine used to expand expandable methyl methacrylate resin particles (for example, an expansion machine used to measure the bulk density (A)) is sometimes referred to as a "pre-expansion machine."

[0124] Since the present expanded beads have the above-mentioned configuration, the volume (B) is 140 cm 3 The volume (C) of the expanded beads is 160 cm or less, and therefore the expansion rate is slow. 3That is, the present expanded beads have the advantage of being able to provide an expanded molded article that is excellent in internal fusion properties and castability.

[0125] Regarding the expanded particles, the descriptions in the 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.

[0126] [5. Methyl methacrylate resin foam molding] 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.

[0127] The "methyl methacrylate resin foam molded article according to one embodiment of the present invention" may hereinafter be referred to as the "present foam molded article."

[0128] The expanded beads can be molded into a foamed molded article by a general 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 (e.g., Daisen's PEONY-205DS) equipped with a mold with the expanded beads; (2) blowing steam (e.g., water vapor) into the mold at a steam blowing pressure of 0.15 MPa to 0.25 MPa, and performing in-mold molding by vacuum suction and heating under conditions of a pressure inside the mold 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 expanded beads in the mold at 80°C to 110°C for 1,000 seconds, and then removing the expanded molded article to obtain the foamed molded article.

[0129] Because the foamed molded article has the above-mentioned structure, it has excellent internal fusion properties and castability. In particular, the foamed molded article preferably has (D) of 85% or more. As a result, the foamed molded article can be suitably used as a lost pattern.

[0130] The method for evaluating the castability of the foamed molded article will be 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 foamed molded article 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. If the casting has residual defects, this is undesirable because it affects mechanical properties such as strength and may cause the casting to break after short-term use. If the casting has no residual defects, these problems are resolved, making it possible to provide a casting that is efficient and durable.

[0131] With respect to the present foamed molded product, the descriptions in the 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, except for the matters described above.

[0132] [6. Vanishing model] The lost model containing 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.

[0133] The evaporative pattern according to one embodiment of the present invention has excellent internal fusion properties and castability, and can therefore be suitably used in a variety of metal castings. [Example]

[0134] An embodiment of the present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these.

[0135] (Polymerization Conversion Rate of Expandable Methyl Methacrylate Resin Particles) During polymerization, a sample of the aqueous suspension was taken and the aqueous suspension was filtered. The weight of the resin component remaining on the filter paper was measured, and the obtained weight was taken as the weight before heating. Next, after adding a polymerization inhibitor to the resin component, the resin component was heated at 150 °C for 30 minutes to remove volatile components. Then, the weight of the obtained resin component was measured, and the obtained weight was taken 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.

[0136] (Glass transition temperature) The glass transition temperature obtained by measurement by the following method was taken as the glass transition temperature of the base resin contained in the foaming resin particles: (1) The resin obtained by drying the foaming resin particles at 150 °C for 30 minutes was used as a sample; (2) After putting 4 mg of the sample into an aluminum container, an aluminum lid was attached to the aluminum container using a compressor to obtain a measurement sample; (3) For the measurement sample, using a DSC measuring machine (DSC7000X manufactured by Hitachi), the temperature was raised from 50 °C to 150 °C (heating rate 10 °C / min), the temperature was lowered from 150 °C to 50 °C (cooling rate 10 °C / min), and the temperature was raised 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. Here, the glass transition temperature is intended to be the midpoint glass transition temperature defined in JIS K7121.

[0137] (Weight average molecular weight) The weight average molecular weight obtained by measurement by the following method was taken as the weight average molecular weight of the base resin contained in the foaming resin particles: (1) 0.02 g of the foaming resin particles was dissolved in 20 ml of THF; (2) Then, the gel component in the obtained solution was filtered; (3) Next, only the component soluble in THF (i.e., the filtrate) was used as a sample, and GPC measurement was performed under the following conditions using gel permeation chromatography (GPC); (4) From the GPC measurement chart obtained by GPC measurement, the weight average molecular weight (Mw) and the number average molecular weight (Mn) were calculated. The weight average molecular weight (Mw) and the number average molecular weight (Mn) are relative values in terms of polystyrene conversion. <Conditions for GPC measurement> Measurement equipment: Tosoh Corporation, high-speed GPC equipment HLC-8220 Columns used: Tosoh Corporation, SuperHZM-H x 2, SuperH-RC x 2 Column temperature: 40°C, mobile phase: THF (tetrahydrofuran) Flow rate: 0.35ml / min, injection volume: 10μl Detector: RI.

[0138] (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 surface of the expandable resin particles; (2) The expandable resin particles were placed in a steamer equipped with an outlet; (3) 100°C steam 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 poured into a 1000 cm 3 (5) The volume of the foam particles (cm 3 ) was measured; (6) The bulk density of the expanded particles was calculated using the following formula: Bulk density (g / cm 3 ) = 10(g) / volume of foam particles (cm 3 ).

[0139] 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 below × (bad): Bulk density (A) is 0.0285 g / cm 3 Greater than 0.0333g / cm 3 below ×× (very poor): Bulk density (A) is 0.0333 g / cm 3 Exceeds.

[0140] (Expandability of methyl methacrylate resin foam particles) The expandable resin particles were sieved to separate those with particle diameters of 0.5 mm to 1.4 mm. The separated expandable resin particles were used to obtain expanded particles with 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, a BHP manufactured by Daikai Kogyo Co., Ltd.; (2) Steam was blown into the expansion machine at a steam injection pressure of 0.10 MPa to 0.16 MPa and an internal pressure of the expansion machine of 0.005 MPa to 0.030 MPa to heat the expandable resin particles; (3) The expandable resin particles were expanded to a bulk ratio of 60 times by the method (2) described above, to obtain expanded particles with a bulk ratio of 60 times.

[0141] The resulting expanded beads were used to measure the volume (B) of the expanded beads by carrying out the following steps (1) to (5) in order: (1) 100 cm of expanded beads with a bulk ratio of 60 times 3 (2) Steam at 100°C was supplied to the steamer and the expanded particles were heated for 30 seconds; (3) After heating, the expanded particles were removed from the steamer and allowed to stand at 25°C for 1 minute; (4) The expanded particles were placed in a 1000 cm 3 (5) The volume (B) of the expanded beads was measured from the graduations on the measuring cylinder. The expandability of the expanded beads was evaluated from the obtained volume (B) based on the following criteria. 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 below × (bad): Volume (B) is 140 cm 3 Super 150cm 3 less than XX (very poor): Volume (B) is 150 cm 3 That's all.

[0142] (Shrinkage suppression of methyl methacrylate resin foam particles) Expanded beads with a bulk ratio of 60 were obtained by the method described in the section (Expandability of methyl methacrylate resin expanded beads). Using the obtained expanded beads, the following (1) to (5) were carried out in order, and the volume (C) of the expanded beads was measured: (1) 100 cm of expanded beads with a bulk ratio of 60 3(2) Steam at 100°C was supplied to the steamer, and the expanded particles were heated for 180 seconds; (3) After heating, the expanded particles were removed from the steamer and allowed to stand at 25°C for 1 minute; (4) The expanded particles were placed in a 1000 cm 3 (5) The volume (C) of the expanded beads was measured from the graduations on the measuring cylinder. The shrinkage suppression ability of the expanded beads was evaluated from the obtained volume (C) based on the following criteria. ○ (Good): Volume (C) is 160 cm 3 super × (bad): Volume (C) is 155 cm 3 Super 160cm 3 below XX (very poor): Volume (C) is 155 cm 3 below.

[0143] (Internal fusion of methyl methacrylate resin foam molded products) The expandable resin particles were sieved to separate expandable resin particles having a particle diameter of 0.5 mm to 1.4 mm.

[0144] The separated expandable resin particles were used to carry out the following steps (1) to (8) in order to obtain a foamed molded article: (1) The expandable resin particles were placed in a BHP pressure foaming machine 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 above step (2) 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 having a length of 2000 mm, a width of 1000 mm, and a thickness of 525 mm was used. A 60x bulk expansion was filled into a molding machine (DAISEN PEONY-205DS); (6) Steam was blown into the mold at a steam injection pressure of 0.15-0.25 MPa, and the mold was heated by vacuum suction until the expansion pressure reached 0.070-0.080 MPa, allowing the expansion particles to fuse together; (7) After the expansion pressure reached 0.070-0.080 MPa, the product was left in the mold at 80-110°C for 1000 seconds, after which the expansion molded product was removed; (8) The removed expansion molded product was left at 60°C for 3 days to obtain a 2000mm long, 1000mm wide, and 525mm thick expansion molded product.

[0145] Using the obtained foamed molded article, the following (1) to (3) were carried out in order, and the proportion (D) of the foamed molded article at the fracture surface was measured: (1) The foamed molded article was cut perpendicular to the thickness direction of the foamed molded article using a hot wire slicer so that the foamed molded article was divided into five equal parts in the thickness direction; (2) One of the middle parts (a portion of the foamed molded article before cutting in the thickness direction of 210 mm to 315 mm) was broken in the width direction at the center of the length direction by bending the plane perpendicular to the thickness direction; (3) The obtained fracture surface was visually observed, and the number of all particles constituting the fracture surface and the number of foamed particles broken other than at the particle interfaces were counted, and the proportion (D) was calculated based on the following formula: Percentage (D) (%) = Number of particles that are broken at a location other than the particle interface on the fracture surface / Number of particles constituting the fracture surface × 100.

[0146] Based on the obtained ratio (D), the internal fusion property of the foamed molded article was evaluated according to the following criteria. ○ (Excellent): Percentage (D) is 85% or more × (defective): Percentage (D) is 75% or more but less than 85% ×× (very poor): Percentage (D) is less than 75%.

[0147] (Casting ability of methyl methacrylate resin foam molded body) 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; (5) The casting surfaces, excluding the bottom surface, were then machined 10 mm; (6) The presence of casting defects such as residues in the center of the castings was confirmed by visual inspection and magnetic particle testing. Based on the results, castability was evaluated according to the following criteria.

[0148] (Casting evaluation) The castability of the methyl methacrylate resin foam molded article was evaluated according to the following criteria. 〇 (Excellent): When there are no residual defects in the casting × (defective): When there are residual defects in the casting ×× (Very poor): When there are many residual defects in the casting Example 1 A 6L 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 crosslinker, 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 mixture containing the first poorly water-soluble inorganic salt. Then, 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 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 the addition step was carried out. The initiation step and addition step described above can also be considered the first copolymerization step.

[0149] 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 (in normal-rich butane, the weight ratio of normal butane to isobutane (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 referred to 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.

[0150] The obtained expandable methyl methacrylate resin particles were sieved using sieves with openings of 0.500 mm and 1.400 mm, and expandable methyl methacrylate resin particles with particle diameters of 0.500 mm to 1.400 mm were collected by this operation.

[0151] Next, 0.20 parts by weight of zinc stearate as a fatty acid metal salt and 0.05 parts by weight of hardened castor oil as a fusion promoter were applied to the surfaces of the obtained expandable methyl methacrylate resin particles.

[0152] Next, 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 according to the methods described above. The evaluation results are shown in Table 1.

[0153] 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 with particle diameters of 0.500 mm to 1.400 mm, the surfaces of which were coated with zinc stearate and hydrogenated castor oil. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0154] (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 added was changed to 0.240 parts by weight, to obtain expandable methyl methacrylate resin particles with particle sizes of 0.500 mm to 1.400 mm, the surfaces of which were coated with zinc stearate and hydrogenated castor oil. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0155] (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 with particle sizes of 0.500 mm to 1.400 mm, the surfaces of which were coated with zinc stearate and hydrogenated castor oil. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0156] (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 with particle diameters of 0.500 mm to 1.400 mm, the surfaces of which were coated with zinc stearate and hydrogenated castor oil. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0157] 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 with particle diameters of 0.500 mm to 1.400 mm, the surfaces of which were coated with zinc stearate and hydrogenated castor oil. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0158] (Comparative Example 5) The same procedure as in Example 1 was carried out, 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, to obtain expandable methyl methacrylate resin particles with particle sizes of 0.500 mm to 1.400 mm. Each evaluation item was evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0159] [Table 1] [Industrial Applicability]

[0160] According to one embodiment of the present invention, it is possible to provide expandable methyl methacrylate resin particles that can efficiently produce foamed molded articles with excellent internal fusion. Therefore, this embodiment of the present invention can be suitably used as a lost pattern when performing metal casting by the full mold method.

Claims

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 water vapor at 100°C for 300 seconds is 0.0285 g / cm ３ The following is true: (b) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles ３ After heating with steam at 100°C for 30 seconds, the volume (B) of the methyl methacrylate resin foamed particles obtained by leaving the mixture at 25°C for 1 minute is 140 cm ３ The following is true: (c) 100 cm of expanded methyl methacrylate resin particles obtained by expanding the expandable methyl methacrylate resin particles ３ After heating with steam at 100°C for 180 seconds, the volume (C) of the methyl methacrylate resin foamed particles obtained by leaving it at 25°C for 1 minute is 160 cm ３ Super; and (d) The glass transition temperature of the base resin is 114.5°C or higher.

2. 2. The expandable methyl methacrylate resin particles according to claim 1, wherein the base resin has a weight average molecular weight of 220,000 to 310,000.

3. 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.

4. 4. The expandable methyl methacrylate resin particles according to claim 1, wherein the acrylic ester unit is a butyl acrylate unit.

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