JP2018135407A - Methyl methacrylate foamed particles - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide methyl methacrylate foamed particles which have good formability with a high expansion ratio, and have no gap (unevenness) of the foamed particles on the surface of a foamed molding, and to provide methyl methacrylate foamed particles suitable for use of a sublimation pattern used in metal casting.SOLUTION: There are provided methyl methacrylate foamed particles, where an average chord length (A) of cells between inside 100 μm and inside 200 μm from the outermost surface layer of the foamed particles is 50-90 μm, an average chord length (B) with a radius of 100 μm from the center point of the foamed particles is 40-80 μm, and an average chord length ratio (A)/(B) of the foamed particles is 0.7-1.3.SELECTED DRAWING: None

Description

  The present invention relates to a methacrylic methyl resin expanded particle. Furthermore, it is related with the disappearance model which consists of a foaming molding of the methyl methacrylate type | system | group foaming particle.

  When performing metal casting, a vanishing model casting method (full mold method) is known in which a model made of a foam molded body is buried in casting sand, and a cast metal is cast by pouring molten metal into the cast sand. In the full mold method, a foamed molded product of methyl methacrylate polymer is used from the viewpoint of reducing residues during casting.

  However, foaming of a methyl methacrylate polymer is generally difficult, the elongational viscosity and the shear viscosity at the time of melting are unsuitable for the foaming behavior, and the ability to retain bubbles is insufficient. For this reason, not only does it not sufficiently foam, the foam cells become uneven, the surface of the obtained foam has a lot of irregularities, and it is difficult to form a smooth surface. It was difficult to obtain a foam having the same.

  As means for improving foaming performance, Patent Document 1 discloses that a thermoplastic polymer particle is suspended in water containing a dispersant for particle size control, and a methacrylic acid ester-based polymer is added to the suspension. A method is disclosed in which the monomer is dropped and polymerized in a specific ratio with respect to the thermoplastic polymer to reduce nonuniformity in the molecular weight distribution of the resulting polymer particles. Moreover, as a method for improving the retention of the foaming agent, Patent Document 2 discloses a method of adjusting the molecular weight in a specific region. In addition, as a method of obtaining a foam having a high foaming ratio, uniform foamed cells, and good appearance at the time of foam molding, the polymer obtained from a specific monomer ratio in Patent Document 3 is adjusted to a specific molecular weight. A method is disclosed.

  Patent Document 4 discloses that the average chord length of the cut foam of the foamed molded product is 70 μm or more and 200 μm or less. When the cell structure inside each foamed particle constituting the foamed molded product is observed, the surface layer of the foamed particle is disclosed. There is a large difference in the average chord length between the inside and the inside, and there are irregularities in the gaps between the foamed particles on the surface of the molded body, which impairs the appearance.

  Patent Document 5 discloses that the average bubbles of acrylic foam particles containing a styrene monomer are 30 to 70 μm. However, the inclusion of styrene increases the residue of the disappearance model, and the disappearance model casting method. Is not preferred.

JP 50-127990 A JP 2001-123001 A JP 2001-233986 A JP 2006-241256 A Publication No. 2006-160354

  The resins described in Patent Documents 1, 2, and 3 have a problem that a large dimensional change is observed in a foamed molded product obtained in a high expansion ratio state. The reason for this is that not only the foamed cells are not sufficiently uniform, but also the cell size is too small, so that the film thickness of the cells constituting the bubbles becomes thin, and the foaming performance is not always sufficient. On the other hand, foamed styrenic resin foam cells are prone to non-uniformity and coarsening, so a method of using a nucleating agent having the ability to make the foam cells uniform and fine has been proposed. Yes. However, even if it has the same problem, it cannot be solved by diverting the technology in the expandable styrene resin to the expandable methyl methacrylate resin particles.

  In view of the above situation, the present invention is to provide methyl methacrylate-based expanded particles that have good moldability at a high expansion ratio and have few gaps (irregularities) in the expanded particles on the surface of the expanded molded article. is there. In particular, an object of the present invention is to provide methyl methacrylate-based expanded particles suitable for use of disappearance models used in metal casting.

  As a result of intensive studies, the present inventors have completed the present invention. That is, the present invention is as follows.

  (1) Methyl methacrylate-based expanded particles having an average chord length (A) of 50 to 90 μm and a radius of 100 μm from the center point of the expanded particles between the innermost 100 μm and inner 200 μm from the outermost surface layer of the expanded particles Methyl methacrylate-based foamed particles having an average chord length (B) of 40 to 80 μm and an average chord length ratio (A) / (B) of the foamed particles of 0.7 to 1.3 .

  (2) Methyl methacrylate-based foamed particles, comprising 100 parts by weight of an acrylic monomer comprising 90% by weight to 98% by weight of methyl methacrylate and 2% by weight to 10% by weight of butyl acrylate; The methyl methacrylate-based expanded particles according to (1), wherein 0.05 to 0.15 parts by weight of a polymer is polymerized.

  (3) The methyl methacrylate-based foamed particles according to (1) to (2), wherein the polystyrene-converted weight average molecular weight measured by gel permeation chromatography (GPC) is from 300,000 to 400,000 .

  (4) A foamed molded article obtained by molding the methyl methacrylate-based foamed particles according to any one of (1) to (3).

  (5) Disappearance model comprising a foamed molded product of methyl methacrylate-based foamed particles according to (4).

  ADVANTAGE OF THE INVENTION According to this invention, the moldability in a high expansion ratio is favorable, and the methyl methacrylate type expanded particle which has few crevice (unevenness | corrugation) in the expanded particle on the surface of a foaming molding can be provided. In particular, it is possible to provide methyl methacrylate-based expanded particles suitable for use in the disappearance model used in metal casting.

  In the methyl methacrylate-based expanded particles of the present invention, the average chord length (A) of the cell between the innermost 100 μm and inner 200 μm from the outermost surface layer of the expanded particles is 50 to 90 μm, and the average chord having a radius of 100 μm from the center point of the expanded particles. A methyl methacrylate-based foamed particle having a length (B) of 40 to 80 μm and an average chord length ratio (A) / (B) of the foamed particle of 0.7 to 1.3 .

  According to ASTM-D-2842-97, the average chord length of the foamed particles of the present invention is the number of bubbles applied on a straight line of the cut surface using a scanning electron micrograph of the cut surface of the foamed particle. Can be measured from

  In general, it is difficult to impregnate methyl methacrylate polymer particles with a foaming agent such as butane, and impregnation unevenness is more difficult than styrene polymer particles. If the butane impregnation into the resin particles is insufficient, the average chord length (A) between the innermost 100 μm and the inner 200 μm from the outermost surface layer of the expanded particles is as large as 100 to 130 μm when the expanded particles are formed. From this point, foamed particles having a double cell structure (referred to as a hard core) having an average chord length (B) having a radius of 100 μm and a fine cell diameter of 50 μm or less are obtained. When foamed particles having a double cell structure such that the average chord length ratio (A) / (B) exceeds 1.3, a foam (unevenness) occurs between the foamed particles on the surface of the molded body, This is not preferable as a casting model.

  On the other hand, as in the present invention, the average chord length (A) of the surface layer portion of the expanded particles is 50 to 90 μm, the average chord length (B) of the center portion of the expanded particles is 40 to 80 μm, and the average chord length of the expanded particles When the length ratio (A) / (B) is 0.7 to 1.3, the foamed particles are uniformly foamed during foam molding in the mold, and a foam molded article having a beautiful surface is obtained.

  In particular, the average chord length ratio (A) / (B) is preferably 0.8 to 1.2, more preferably 0.9 to 1.1, and the bubble diameter inside the expanded particles is uniform. A more beautiful foam-molded article can be obtained.

  As in the present invention, the method for obtaining foamed particles having a uniform cell diameter is important to impregnate a foaming agent when the methyl methacrylate resin particles are soft. For example, it is preferable to impregnate the resin particles with the foaming agent when the polymerization conversion rate of the polymer particles is 80% or more and 95% or less. When the polymerization conversion rate is less than 80%, the foaming agent is easily impregnated, but the resin particles are united by the softening of the resin, and the product yield is deteriorated. On the other hand, when the polymerization conversion rate exceeds 95%, the foaming agent is not impregnated to the inside of the resin particles, a double cell structure (hard core) is formed, and the foam molding in which the gap between the foam particles on the surface of the molded body is uneven. The body is obtained. In order to further impregnate the foaming agent into the resin particles, the impregnation temperature of the foaming agent is preferably 100 ° C. or higher and 120 ° C. or lower. When the impregnation temperature is less than 100 ° C., the foaming agent is not impregnated to the inside of the resin particles, a double cell structure (hard core) is formed, and a foamed molded article having an uneven surface is obtained. When the impregnation temperature exceeds 120 ° C., a uniform cell structure can be obtained, but the can pressure of the polymerization machine becomes high, and a heavy equipment impregnation facility is required.

  The expanded particles of the present invention are composed of 100 parts by weight of an acrylic monomer consisting of 90% by weight to 98% by weight of methyl methacrylate, 2% by weight to 10% by weight of butyl acrylate, and 0.05% by weight of a bifunctional monomer. It is characterized by polymerizing not less than 0.15 parts by weight.

  The ratio of the methyl methacrylate monomer to the butyl acrylate monomer is preferably 92 to 97% by weight of methyl methacrylate and 3 to 8% by weight of butyl acrylate, more preferably methyl methacrylate 93. It is not less than 96% by weight and not more than 96% by weight and not less than 4% by weight and not more than 7% by weight of butyl acrylate. If the ratio of the methyl methacrylate monomer component is large, the foamability and moldability tend to be inferior, making it difficult to obtain a foam molded product having a beautiful surface. If the amount of the butyl acrylate monomer component is excessive, the foam molded product shrinks. It tends to be easy to do.

  When the total of the methyl methacrylate monomer and the butyl acrylate monomer is 100 parts by weight, it is necessary to use a bifunctional monomer in order to reduce residues during combustion and to adjust the molecular weight. Examples of the bifunctional monomer include ethylene glycol such as ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, etc. Esterified with an acid is one obtained by esterifying a hydroxyl group of a divalent alcohol such as neopentyl glycol di (meth) acrylate, hexanediol di (meth) acrylate, butanediol di (meth) acrylate with acrylic acid or methacrylic acid, And aryl compounds having two alkenyl groups such as divinylbenzene.

  The difunctional monomer is preferably hexanediol di (meth) acrylate in terms of ease of molecular weight adjustment, and the amount used is 0.000 with respect to a total of 100 parts by weight of the methyl methacrylate monomer and the butyl acrylate monomer. 05 parts by weight or more and 0.15 parts by weight or less are preferable, and 0.08 parts by weight or more and 0.13 parts by weight or more are more preferable. If the amount of the bifunctional monomer is small, the residue tends to remain and the strength tends to be inferior. If the amount of the bifunctional monomer is large, the foamability and moldability tend to be inferior.

  The methyl methacrylate-based expanded particles of the present invention preferably have a polystyrene-equivalent weight average molecular weight of 300,000 or more and 400,000 or less as measured by gel permeation chromatography (GPC). If the weight average molecular weight is low, residues during casting are likely to remain, and if it is high, it is difficult to obtain a molded article having good surface properties.

  Examples of the method for producing the expandable resin particles of the present invention include suspension polymerization in which polymerization is performed in an aqueous suspension.

  The “aqueous suspension” in the present invention refers to a state in which resin particles and monomer droplets are dispersed in water or an aqueous solution by using stirring or the like. The body may be dissolved, or a water-insoluble dispersant, initiator, chain transfer agent, crosslinking agent, bubble regulator, flame retardant, plasticizer, etc. may be dispersed together.

  The weight ratio of the resin to water is preferably 1.0 / 0.6 to 1.0 / 3.0 as the ratio of the resulting methacrylic resin / water.

  Examples of the dispersant that can be used for suspension polymerization include poorly water-soluble inorganic salts such as tricalcium phosphate, magnesium pyrophosphate, hydroxyapatite, and kaolin, and water-soluble polymers such as polyvinyl alcohol, methyl cellulose, polyacrylamide, and polyvinyl pyrrolidone. In the case of using a poorly water-soluble inorganic salt, it is effective to use an anionic surfactant such as α-olefin sulfonic acid soda and dodecylbenzene sulfonic acid soda together. These dispersants may be added during the polymerization as required.

  The amount of the dispersant used depends on the type, but the water-insoluble inorganic salt is 0.1 to 1.5 parts by weight with respect to 100 parts by weight of water, and 30 ppm for the anionic surfactant or water-soluble polymer. The amount is preferably 100 ppm or less.

  In the suspension polymerization of the present invention, it is preferable to reduce the residual monomer in the second stage polymerization reaction at a higher temperature than the first stage after the first stage polymerization and the main reaction.

  As the polymerization initiator, radical generating polymerization initiators generally used for the production of thermoplastic polymers can be used. Typical examples include benzoyl peroxide, lauroyl peroxide, t-butylperoxy. Benzoate, isopropyl-t-butylperoxycarbonate, butyl perbenzoate, t-butylperoxy-2-ethylhexanoate, t-butylperpivalate, t-butylperoxyisopropylcarbonate, di-t-butylper Oxyhexahydroterephthalate, 1,1-di (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-bis (t-amylperoxy) -3,3,5-trimethylcyclohexane, 1, 1-di (t-butylperoxy) cyclohexane, t-butylperoxy Organic peroxides and the like 2-ethylhexyl monocarbonate, azobisisobutyronitrile, an azo compound such as azo-bis-dimethyl valeronitrile. These polymerization initiators may be used alone or in combination of two or more.

  In the polymerization in the present invention, it is preferable to use a known chain transfer agent used for the polymerization of methyl methacrylate. For example, monofunctional chain transfer agents such as alkyl mercaptans and thioglycolic acid esters, and polyhydric alcohol hydroxyl groups such as ethylene glycol, neopentyl glycol, trimethylolpropane and sorbitol are esterified with thioglycolic acid or 3-mercaptopropionic acid. And polyfunctional chain transfer agents.

  Examples of the blowing agent include aliphatic hydrocarbons that are hydrocarbons having 3 to 5 carbon atoms, such as propane, isobutane, normal butane, isopentane, normal pentane, and neopentane, for example, ozone destruction of difluoroethane, tetrafluoroethane, and the like. Examples thereof include volatile foaming agents such as hydrofluorocarbons having a coefficient of zero. These foaming agents can be used in combination. The amount used is preferably 5 parts by weight or more and 12 parts by weight or less, and more preferably 7 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the expandable methyl methacrylate resin particles. When the amount of the foaming agent is small, it is difficult to obtain the expansion ratio, and when the amount of the foaming agent is large, the resin tends to aggregate in the step of impregnating the foaming agent.

  The impregnation of the foaming agent is preferably performed by impregnating the resin particles with the foaming agent when the polymerization conversion rate of the resin particles is 80% or more and 95% or less. When the polymerization conversion rate is less than 80%, the foaming agent is easily impregnated, but the resin particles are aggregated due to the softening of the resin, and the production yield deteriorates. On the other hand, when the polymerization conversion rate exceeds 95%, the foaming agent is not impregnated to the inside of the resin particles, a double cell structure (hard core) is formed, and a foamed molded article having an uneven surface is obtained.

  The impregnation temperature when impregnating the foaming agent into the resin particles is preferably 95 ° C. or higher and 120 ° C. or lower. More preferably, 100 degreeC or more and 117 degrees C or less are preferable. When the impregnation temperature is less than 95 ° C., the foaming agent is not impregnated to the inside of the resin particles, a double cell structure (hard core) is formed, and a foamed molded article having an uneven surface is obtained. When the impregnation temperature exceeds 120 ° C., a uniform cell structure can be obtained, but the can pressure of the polymerization machine becomes high, and a heavy equipment impregnation facility is required.

  As the additive used in the present invention, a solvent, a plasticizer, a bubble regulator and the like can be used depending on the purpose.

  Examples of the solvent include those having a boiling point of 50 ° C. or higher, and C6 or higher aliphatic hydrocarbons such as toluene, hexane, and heptane, and C6 or higher alicyclic hydrocarbons such as cyclohexane and cyclooctane. Among these, toluene and cyclohexane are preferable for obtaining foaming properties, and it is preferable that 1.5 to 3.0% by weight is contained. When the amount is 1.5% by weight or less, sufficient foaming force cannot be obtained. In order to add a solvent component having a boiling point of 50 ° C. or higher, it is preferable to add it immediately before or simultaneously with impregnating the foaming agent into the resin particles.

  Examples of the plasticizer include high-boiling plasticizers having a boiling point of 200 ° C. or higher. Examples thereof include vegetable oils such as palm kernel oil, aliphatic esters such as dioctyl adipate and dibutyl sebacate, organic hydrocarbons such as liquid paraffin and cyclohexane. Examples of the air conditioner include aliphatic bisamides such as methylene bis stearic acid amide and ethylene bis stearic acid amide, polyethylene wax, and the like.

  The obtained expandable methyl methacrylate resin particles can be made into expanded particles (also referred to as methyl methacrylate expanded particles) by a general prefoaming method. Specifically, it is placed in a container equipped with a stirrer and heated by a heat source such as water vapor to perform preliminary foaming up to a desired foaming ratio.

  Further, the methyl methacrylate-based expanded particles can be formed into a foamed molded article by a general in-mold molding method. Specifically, it is filled in a mold that can be closed but cannot be sealed, and heat-sealed with water vapor to obtain a foamed molded article.

  The foamed molded product of the methyl methacrylate-based expanded particles of the present invention has a low ceiling temperature of the methyl methacrylate-based resin itself, so that the foamed molded product is buried in casting sand, and molten metal is poured into it to replace it. Since the residue of methyl methacrylate resin hardly remains, it can be suitably used as a disappearance model.

  Examples and Comparative Examples are given below, but the present invention is not limited thereby.

(Weight average molecular weight measurement)
0.02 g of expandable methyl methacrylate resin particles are dissolved in 20 cc of tetrahydrofuran, and GPC (HLC-8020 manufactured by Tosoh Corporation), column: TSKgel Super HZM-H, column temperature: 40 ° C., flow rate: 0.35 ml / 1 min. )). The weight average molecular weight was determined as a converted value of standard polystyrene.

  Since the molecular weight of the methyl methacrylate-based expanded particles was not different from the molecular weight of the expandable methyl methacrylate-based resin particles, this was substituted.

(Measurement of polymerization conversion)
Collect polymerized resin particles from a pressure vessel immediately before adding a foaming agent, wipe the water on the surface of the resin particles with filter paper, dissolve in methylene chloride (internal standard cyclopentanol), and gas chromatography manufactured by Shimadzu Corporation. GC-2014 (capillary column: Rtx-1 manufactured by GL Sciences, column temperature condition: 50 → 80 ° C. (3 ° C./min), 80 → 180 ° C. temperature increase (10 ° C./min), carrier gas: helium) The amount of residual monomer contained in the expandable styrene resin particles was quantified. The polymerization conversion rate was measured from the residual monomer amount.

(Manufacture of expanded particles)
The expandable methyl methacrylate resin particles were sieved to obtain expandable methyl methacrylate resin particles having a particle diameter of 0.5 to 1.4 mm.

  The foamed methyl methacrylate resin particles thus separated are pre-foamed to a bulk magnification of 65 times under a condition of a blowing vapor pressure of 0.09 to 0.10 MPa using a pressure type pre-foaming machine “BHP” manufactured by Daikai Kogyo. Then, it was allowed to stand at room temperature for 1 day to obtain methyl methacrylate-based expanded particles.

(Average chord length of bubble diameter of expanded particles)
The average cell diameter of the expanded particles was measured according to ASTM-D-2842-97, from the photograph of the cut surface of the expanded particles projected, and the average chord length was measured from the cell diameter on a straight line of the cut surface. As for the surface layer portion, the average chord length (A) of the cell between the inner 100 μm and the inner 200 μm from the outermost surface layer of the expanded particles, and the average chord length (B) with a radius of 100 μm from the center point of the expanded particles were measured. The average chord length ratio (A) / (B) of the foam particles was estimated as the degree of the hard core of the foam particles.

(Manufacture of foam moldings)
The obtained methyl methacrylate-based expanded particles were blown in-mold at a blown vapor pressure of 0.05 MPa using a molding machine “Daisen, KR-57”, so that a flat plate having a thickness of 150 mm and a length of 400 mm × width of 350 mm A foamed molded product was obtained.

(Surface properties of foamed molded product)
The state of the surface of the foamed molded product was evaluated by visual observation. A larger numerical value indicates a beautiful surface state with less irregularities in the gaps between the particles, and a score of 3 or more expressed as a perfect score of 5 was accepted.

5: Unevenness is not found 4: There is unevenness partially, but is hardly understood 3: Although there are unevenness in some places, it is acceptable as a whole 2: Unevenness is conspicuous 1: There are many unevennesses.

Example 1
In a 6 L autoclave with a stirrer, 150 parts by weight of water, 0.105 part by weight of tribasic calcium phosphate, 0.0075 part by weight of sodium α-olein sulfonate, 0.08 part by weight of lauroyl peroxide, 1,1-bis (t -Butylperoxy) cyclohexane 0.1 part by weight, 1,6-hexanediol diacrylate 0.1 part by weight (0.09 vinyl group mol% with respect to methyl methacrylate monomer), n-dodecyl mercaptan 0 After charging 24 parts by weight, 95 parts by weight of methyl methacrylate, 5 parts by weight of butyl acrylate and 1 part by weight of toluene were charged, and polymerization was carried out at 80 ° C. for 4 hours and 20 minutes. The polymerization conversion at that time was 88%. Thereafter, 1.5 parts by weight of cyclohexane and 9 parts of normal rich butane (normal / iso = 70/30) are charged, the temperature is raised to 102 ° C., polymerization is performed for 10 hours, and after cooling, washing, dehydration and drying are performed. Thus, expandable methyl methacrylate resin particles were obtained.

  The obtained expandable methyl methacrylate resin particles were sieved to collect expandable methyl methacrylate resin particles having a particle diameter of 0.5 to 1.4 mm, and the weight average molecular weight was measured. The resin particles were prefoamed using a pressure type prefoaming machine “BHP, manufactured by Daikai Kogyo Co., Ltd.” to obtain expanded particles having a magnification of 65 times. After the obtained expanded particles were cured at room temperature for 1 day, the average chord length of the cells of the expanded particles was measured. The average cell chord length (A) of the surface layer was 75 μm, and the average chord length (B) of the center portion was measured. Is 65 μm.

  Next, using a Daisen KR-57 molding machine, a foam molded product was obtained with a 300 × 450 × 150 (t) mm size mold, and the surface property of the molded product was visually evaluated. The results are shown in Table 1.

(Example 2)
As shown in Table 1, the same operation as in Example 1 was performed except that methyl methacrylate was changed to 92 parts by weight and butyl acrylate was changed to 8 parts by weight, and expandable methyl methacrylate resin particles, expanded particles, and expanded molding were performed. A body was obtained and evaluated similarly. The evaluation results are shown in Table 1.

Example 3
As shown in Table 1, except for changing to 97 parts by weight of methyl methacrylate and 3 parts by weight of butyl acrylate, the same operation as in Example 1 was carried out to obtain expandable methyl methacrylate resin particles, expanded particles, and expanded molding. A body was obtained and evaluated similarly. The evaluation results are shown in Table 1.

Example 4
As shown in Table 1, except that the impregnation temperature of the foaming agent was changed to 120 ° C., the same operation as in Example 1 was carried out to obtain expandable methyl methacrylate resin particles, expanded particles, and expanded molded articles. Evaluated. The evaluation results are shown in Table 1.

(Example 5)
As shown in Table 1, the same procedure as in Example 1 was carried out except that the polymerization time was 4 hours and 50 minutes and the polymerization conversion rate was changed to 94%, and expandable methyl methacrylate resin particles, expanded particles, A foam molded article was obtained and evaluated in the same manner. The evaluation results are shown in Table 1.

(Comparative Example 1)
As shown in Table 1, the same operation as in Example 1 was performed, the polymerization time was 4:00 hours, and the polymerization conversion rate was 75%. After the foaming agent was added, the temperature reached an impregnation temperature of 102 ° C., and agglomeration between the resin particles was observed, so the polymerization was stopped. Agglomerate is a state in which polymer particles are united.

(Comparative Example 2)
As shown in Table 1, the same operation as in Example 1 was performed, the polymerization time was 4:00 hours, and the polymerization conversion rate was 75%. After the foaming agent was added, the agglomeration between the resin particles was observed after reaching the impregnation temperature of 120 ° C., and thus the polymerization was stopped.

(Comparative Example 3)
As shown in Table 1, the same operation as in Example 1 was performed, the polymerization time was 5 hours and 10 minutes, and the polymerization conversion rate was 98%. Thereafter, a foaming agent was added to obtain an expandable methyl methacrylate resin particle, foamed particle, and foamed molded article at an impregnation temperature of 102 ° C. and evaluated in the same manner. The evaluation results are shown in Table 1.

(Comparative Example 4)
As shown in Table 1, the same operation as in Example 1 was performed, the polymerization time was 5 hours and 10 minutes, and the polymerization conversion rate was 98%. Thereafter, a foaming agent was added, and an impregnation temperature of 120 ° C. was obtained to obtain expandable methyl methacrylate resin particles, expanded particles, and a foamed molded product, and evaluated in the same manner. The evaluation results are shown in Table 1.

Claims (5)

An average chord length (A) of cells between the innermost 100 μm and the inner 200 μm from the outermost surface layer of the expanded foam of methyl methacrylate, with an average chord length of 50 to 90 μm and a radius of 100 μm from the center point of the foamed particles (B) is 40 to 80 μm, and the average chord length ratio (A) / (B) of the expanded particles is 0.7 to 1.3. 100 parts by weight of an acrylic monomer comprising 90 to 98% by weight of methyl methacrylate and 2 to 10% by weight of butyl acrylate, and bifunctional monomer 0 2. The methyl methacrylate-based foamed particles according to claim 1, wherein 0.05 to 0.15 parts by weight are polymerized. 3. The methyl methacrylate-based foamed particles according to claim 1, wherein the polystyrene-reduced weight average molecular weight measured by gel permeation chromatography (GPC) is from 300,000 to 400,000. A foamed molded article obtained by molding the methyl methacrylate-based foamed particles according to any one of claims 1 to 3. A disappearance model comprising a foamed molded product of the methyl methacrylate-based foamed particles according to claim 4.