JP5876790B2 - Foam molded body and resin foam container - Google Patents

Description

  The present invention relates to a foam molded article and a resin foam container. More specifically, the present invention relates to a foamed molded product made of polystyrene resin having excellent surface hardness and a resin foamed container including at least the foamed molded product.

Conventionally, foamed molded articles have been used for transportation packaging materials such as fish boxes and food containers because they are lightweight and have excellent heat insulation properties. Among them, in-mold foam molded articles manufactured using foamable particles as raw materials are often used because of the advantage that a desired shape is easily obtained.
Expandable polystyrene resin particles are widely used as expandable particles that are raw materials for producing a expanded molded article. For example, the expanded molded article is obtained as follows. That is, expandable particles such as expandable polystyrene resin particles are heated with steam and pre-expanded to obtain pre-expanded particles. The obtained pre-expanded particles are filled into the mold cavity. Next, the foamed molded product can be obtained by integrating the prefoamed particles by heat fusion while secondary foaming of the filled prefoamed particles with steam. This method for producing a foam-molded product is generally referred to as a bead method.

Patent No. 3054017

  The foam molded body has been required to further improve the surface hardness from the viewpoint of improving surface scratches and touch feeling.

In the bead method, the pre-expanded particles filled in the cavity of the mold are reheated to re-expand the pre-expanded particles, and the pre-expanded particles are integrated by heat fusion to obtain a foam molded article. . The inventors of the present invention examined the obtained foamed molded product, and found that there was a difference in the multiple in the thickness direction between the portion in contact with the mold and the portion not in contact with the inside.
The inventors have investigated the multiple difference, and surprisingly found that the surface hardness can be improved if the difference between the multiple of the entire foamed molded product and the multiple of the surface layer of the foamed molded product is within a predetermined range. It came to be completed.

Thus, according to the present invention, it is a foamed molded product which is a fusion product of a plurality of expanded polystyrene resin particles, and the foamed molded product has at least a region having a thickness of 0.5 to 10 cm,
The region is, if the multiples of whole W1 and multiples thereof the surface layer portion was W2, 10 times or more, have a relationship of the difference of less than 20 times (W1-W2),
The expanded polystyrene-based resin particles are obtained by impregnating a polystyrene-based resin particle with a foaming agent and then foaming.
The polystyrene resin particles are
(1) The central portion contains more resin components than the surface layer derived from a polymer of a styrene monomer and a monofunctional acrylate ester.
(2) The surface layer contains more resin components derived from a polymer of a styrene monomer and a polyfunctional vinyl monomer than the central portion.
(3) The surface layer has a weight average molecular weight in the range of 250,000 to 600,000.
There is provided a foamed molded product having the following structure .
Furthermore, according to this invention, the resin foam container containing at least the said foaming molding is provided.

ADVANTAGE OF THE INVENTION According to this invention, the foaming molding and resin foam container which were improved in surface hardness can be provided.
Moreover, when the multiple W1 is 15 to 100 times, it is possible to provide a foamed molded article with improved surface hardness.
Further, the expanded polystyrene resin particles are obtained by impregnating a polystyrene resin particle with a foaming agent and then foaming.
The polystyrene resin particles are
(1) The central portion contains more resin components derived from a polymer of a styrene monomer and a monofunctional acrylate ester than the surface layer. (2) The surface layer is composed of a styrene monomer and a polyfunctional vinyl monomer. (3) When the surface layer has a structure having a weight average molecular weight in the range of 250,000 to 600,000, the foam molded body having improved surface hardness is obtained. Can be provided.
When the region is positioned on the side surface of the resin foam container, the surface hardness can be improved and the feel of the surface can be improved by improving the surface hardness.

It is the schematic for demonstrating the measurement procedure of the light absorbency ratio by ATR method. 4 is a graph showing changes in absorbance ratio from the center part of the polystyrene resin particles of Example 1 to the surface layer.

(Foamed molded product)
The foamed molded product of the present invention is a fusion product of a plurality of expanded polystyrene resin particles and has at least a region having a thickness of 0.5 to 10 cm. The region having a thickness of 0.5 to 10 cm has a difference (W1−W2) of 10 times or more and less than 20 times, where W1 is the overall multiple and W2 is the multiple of the surface layer portion. . The foamed molded product can be used, for example, for packing materials such as fish and agricultural products, resin foam containers, heat insulating materials for floor insulation, embankment materials, tatami core materials, and the like.

(1) Multiple The area of 0.5 to 10 cm having a specific multiple difference may be present in at least one part of the foam molded article, and the area other than this area may be thicker than 10 cm. May consist of regions of this thickness. Here, the reason why the thickness is 0.5 to 10 cm is that when the foamed molded body is, for example, in the shape of a container, surface hardness is desired to be improved from the viewpoint of improving surface scratching and feel. is there. In addition, it is more preferable that this area | region is the thickness of the range of 1-8 cm.
The difference between W1 and W2 (W1−W2) is 10 times or more and less than 20 times, but if it is less than 10 times, the surface hardness may not be sufficiently improved, and if it is 20 times or more, the surface hardness The effect of improving is sufficiently obtained, but the internal strength is extremely lowered and the film tends to shrink. The multiple difference (W1−W2) is preferably 10 to 18 times, and more preferably 10 to 15 times. Here, the surface layer portion means a division including the outermost layer when the region is divided into four in the thickness direction. W1 is preferably 15 to 100 times. If it is less than 15 times, the lightweight property of the foamed molded product may be lowered. If it is larger than 100 times, the effect of improving the surface hardness may not be sufficient. In addition, shrinkage may occur in the foamed molded product, which may reduce the appearance. W1 is more preferably 20 to 90 times, and even more preferably 30 to 80 times.

(2) Component (a) Polystyrene resin The polystyrene resin contains a resin component derived from a styrene monomer. The styrene monomer is not particularly limited, and any known monomer can be used. Examples thereof include styrene, α-methylstyrene, vinyl toluene, chlorostyrene, ethyl styrene, isopropyl styrene, dimethyl styrene, bromostyrene, and the like. These styrenic monomers may be one kind or a mixture of plural kinds. A preferred styrenic monomer is styrene.

(B) A resin component derived from a monofunctional acrylic ester and a resin component derived from a polyfunctional vinyl-based monomer The foamed molded article may be composed of only a resin component derived from a styrene-based monomer. To a resin component derived from a monofunctional acrylic ester and / or a resin component derived from a polyfunctional vinyl monomer.
The resin component derived from the monofunctional acrylic ester and / or the resin component derived from the polyfunctional vinyl monomer may or may not be copolymerized with the resin component derived from the styrene monomer. From the viewpoint of further improving the surface hardness, copolymerization is preferred.

(B-1) Specific example of monofunctional acrylic ester Examples of the copolymerizable monofunctional acrylic ester include monomers having an ester moiety having 3 to 20 carbon atoms. By using a monomer of an ester having a carbon number in this range, it is possible to provide a foamed molded article having further improved compressive strength. Specific monofunctional acrylic esters include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, pentyl (meth) acrylate, and (meth) acrylic. Hexyl acid, heptyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, undecyl (meth) acrylate, (meth) acrylic acid Examples include tridecyl, tetradecyl (meth) acrylate, heptadecyl (meth) acrylate, and the like. The alkyl group having 3 or more carbon atoms includes not only a linear alkyl group but also an alkyl group having a structural isomer such as an iso structure, a sec structure or a tert structure.

(B-2) Resin component derived from polyfunctional vinyl monomer The resin component derived from the polyfunctional vinyl monomer is not particularly limited, but a resin component derived from a monomer copolymerizable with a styrene monomer is preferable.
The polyfunctional vinyl monomer is preferably a monomer having 2 to 15 vinyl groups. Resin particles containing a resin component derived from such a polyfunctional vinyl monomer having a specific number of vinyl groups can provide a foamed molded article having a more excellent surface hardness. The number of vinyl groups is more preferably 2 to 5 from the viewpoint of improving foam moldability of the foam molded article.

  Specific polyfunctional vinyl monomers include bifunctional monomers such as divinylbenzene, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, ethoxylation Trifunctional monomers such as trimethylolpropane trimethacrylate and ethoxylated isocyanuric acid triacrylate are exemplified. The polyfunctional vinyl monomer may be used alone or in combination.

(B-3) Content ratio of resin component derived from monofunctional acrylic ester and resin component derived from polyfunctional vinyl monomer Resin component derived from styrene monomer and resin derived from monofunctional acrylic ester constituting polystyrene resin particles The ratio of the component and the resin component derived from the polyfunctional vinyl monomer is preferably in the range of 1: 0.05 to 0.25: 0.6 to 1 (mass ratio).
When the resin component derived from the monofunctional acrylate ester is less than 0.05, it is necessary to maintain a high vapor pressure during foam molding in order to obtain a foam molded article having a desired strength, which may be inferior in energy saving. When it exceeds 0.25, it may be difficult to obtain a high-magnification foamed article.

If the resin component derived from the polyfunctional vinyl monomer is less than 0.6, the desired foamed molded article strength may not be obtained. When it is more than 1, a good foamed molded article may not be obtained with a low molding vapor pressure.
A more preferable ratio is in the range of 1: 0.08 to 0.25: 0.6 to 0.9, and a further preferable ratio is in the range of 1: 0.08 to 0.20: 0.7 to 0.9. is there.
Moreover, it is preferable that the ratio for which the component copolymerized with the styrene-type monomer accounts for 70 mass% or more in the resin component derived from monofunctional acrylate ester. On the other hand, the ratio of the component copolymerized with the styrene monomer in the resin component derived from the polyfunctional vinyl monomer is preferably 70% by mass or more.
The ratio of the resin component derived from the monomer substantially matches the ratio of the monomer as the raw material.

(B-4) Position of presence of resin component derived from monofunctional acrylate ester The foam molded article is composed of a fusion product of a plurality of expanded polystyrene resin particles. Expanded polystyrene resin particles (pre-expanded particles) before fusing are obtained by impregnating polystyrene resin particles with a foaming agent and then foaming. The polystyrene resin particles are preferably particles in which a resin component derived from a monofunctional acrylate ester is present in the following distribution from the viewpoint of further improving the surface hardness.
That is, it is preferable that a large amount of the resin component derived from the monofunctional acrylic acid ester is contained in the central portion of the polystyrene resin particles. By including a large amount in the central portion, it is possible to provide a foamed molded product with improved surface hardness. Here, the central portion means a region within about 15% of the particle radius from the particle center. In addition, in the individual expanded polystyrene resin particles constituting the pre-expanded particles and the fusion molded product of the expanded molded body, the inventors have the same distribution of resin components derived from monofunctional acrylate esters as the polystyrene resin particles. I guess it exists.

The position of the resin component derived from the monofunctional acrylate can be confirmed by various methods. As one of the methods, there is an analysis method in which an infrared absorption spectrum is measured by a single reflection type ATR method using total reflection absorption (Attenuated Total Reflectance). This analysis method is a method in which an ATR prism having a high refractive index is brought into close contact with a sample, infrared light is irradiated to the sample through the ATR prism, and light emitted from the ATR prism is spectrally analyzed.
The ATR method is easy to measure the spectrum simply by bringing the sample and the ATR prism into close contact with each other, and is capable of surface analysis up to a depth of several μm. It is widely used for surface analysis.

The ATR method, was measured, in the infrared absorption spectrum, the ratio of the absorbance D1735 at 1735 cm ^-1 to the absorbance D1600 at 1600cm ^-1 D1735 / D1600 (hereinafter, the absorbance ratio) by, derived from monofunctional acrylic acid ester resin The location of the component can be confirmed. Here, the absorption at 1735 cm ⁻¹ shows a peak derived from stretching vibration between C═O of the ester group contained in the monofunctional acrylate ester. The absorption at 1600 cm ⁻¹ indicates the presence of a peak derived from the in-plane vibration of the benzene ring contained in the polystyrene resin.
Incidentally, the absorbance D1600 and absorbance D1735, when the light measurement wavelength 1600 cm ^-1 is incident from the surface to the target and 1735 cm ^-1 is reflected from the measurement object to a measurement instrument, a region where light can move in the measurement object ( For example, it means a region from the surface to a depth of several μm).

It is preferable that the absorbance ratio has a tendency to decrease from the central part of the polystyrene resin particles toward the surface layer. As a tendency of the decrease, for example, it may be a tendency to decrease linearly from the central portion toward the surface layer, or may be a large decrease in a region close to the central portion or a region close to the surface layer, and a tendency to become a substantially constant value thereafter. Here, the surface layer means a region within about 20% of the particle radius from the particle surface.
The absorbance ratio at the center of the polystyrene resin particles is preferably in the range of 0.40 to 0.80. When the absorbance ratio is less than 0.40, it is necessary to maintain a high vapor pressure during foam molding in order to obtain a foam molded article having a desired surface hardness, which may be inferior in energy saving. When the absorbance ratio is larger than 0.80, a foamed molded article having a desired surface hardness may not be obtained. A preferred absorbance ratio is in the range of 0.40 to 0.70, and a more preferred absorbance ratio is in the range of 0.50 to 0.70.
It can be understood that a large amount of the resin component derived from the monofunctional acrylate ester is present in the central portion by comparing the absorbance ratio of the central portion with the absorbance ratio of the following 50% radius portion.

The absorbance ratio of the polystyrene resin particles having a radius of 50% is preferably in the range of 0.15 to 0.65. When the absorbance ratio is less than 0.15, it is necessary to maintain a high vapor pressure during foam molding in order to obtain a foam molded article having a desired surface hardness, which may be inferior in energy saving. When the absorbance ratio is larger than 0.65, a foamed molded article having excellent surface hardness may not be obtained. A preferred absorbance ratio is in the range of 0.20 to 0.60, and a more preferred absorbance ratio is in the range of 0.20 to 0.50.
Further, the absorbance ratio of the portion having a radius of 50% is preferably a relative value in the range of 0.40 to 0.80, where the absorbance ratio of the central portion is 1. The range of this relative value has shown that many resin components derived from monofunctional acrylate are contained in the center part. When the relative value is less than 0.40, it is necessary to maintain a high vapor pressure during foam molding in order to obtain a foam molded article having a desired surface hardness, which may be inferior in energy saving. When the relative value is greater than 0.80, a foamed molded article having excellent surface hardness may not be obtained. A preferred relative value is in the range of 0.40 to 0.70, and a more preferred relative value is in the range of 0.50 to 0.70.

(B-5) Position of the resin component derived from the polyfunctional vinyl monomer A large amount of the resin component derived from the polyfunctional vinyl monomer is contained in the surface layer of the polystyrene resin particles. By including a large amount in the surface layer, it is possible to provide polystyrene-based resin particles that can give a foamed molded article with improved surface hardness.
The position of the resin component derived from the polyfunctional vinyl monomer can be confirmed by various methods. One method is to measure the weight average molecular weight of all particles and the surface layer by, for example, the GPC method and compare the two. That is, the polyfunctional vinyl monomer also has a role of increasing the molecular weight as a cross-linking agent during the production of polystyrene resin particles. Therefore, comparing the weight average molecular weights of all the particles and the surface layer, if the weight average molecular weight of the surface layer is high with respect to all the particles, it can be inferred that the resin component derived from the polyfunctional vinyl monomer exists in the surface layer.

  The surface layer weight average molecular weight can be in the range of 250,000 to 600,000. When the surface layer weight average molecular weight is less than 250,000, the effect of improving the surface hardness of the foam molded article by the polymer component of the surface layer may not be sufficiently obtained. When it is larger than 600,000, the fusion property between the foamed particles constituting the foamed molded product is lowered, and as a result, the surface hardness may be lowered. A more preferable surface layer weight average molecular weight is in the range of 280,000 to 600,000, and more preferably in the range of 300,000 to 550,000. The weight average molecular weight of the surface layer is preferably 20,000 to 150,000 higher than the weight average molecular weight of all particles.

Since it is difficult to measure the surface average molecular weight from the particles themselves, in this specification, the average molecular weight measured from the surface layer of the foamed molded product obtained from the particles is used. This utilizes the fact that the surface layer of the foamed molded body is a continuous body of particle surface layers. The method for measuring the average molecular weight is described in the Examples section. According to this measuring method, the average molecular weight corresponding to a region of about 15% of the radius from the particle surface is measured.
The total particle weight average molecular weight can range from 230,000 to 400,000. When the total particle weight average molecular weight is less than 230,000, the strength of the foamed molded product may be lowered. When it is larger than 400,000, the fusion property between the expanded particles constituting the expanded molded article is lowered, and as a result, the surface hardness may be lowered. The more preferable total particle weight average molecular weight is in the range of 250,000 to 400,000, and more preferably in the range of 250,000 to 350,000.

(C) Other components Other components include resin components such as polyolefin resins such as polyethylene and polypropylene, polycarbonate resins, and polyesters.
In addition, within the range that does not impair the physical properties, flame retardants, flame retardant aids, plasticizers, lubricants, anti-binding agents, fusion accelerators, anti-static agents, spreading agents, bubble regulators, crosslinking agents, fillers, Additives such as colorants may be included.

As flame retardants, tetrabromocyclooctane, hexabromocyclododecane, trisdibromopropyl phosphate, tetrabromobisphenol A, tetrabromobisphenol A-bis (2,3-dibromo-2-methylpropyl ether), tetrabromobisphenol A- Examples thereof include bis (2,3-dibromopropyl ether).
Examples of the flame retardant aid include organic peroxides such as 2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane, dicumyl peroxide, and cumene hydroperoxide. .
Examples of the plasticizer include glycerin fatty acid esters such as phthalic acid ester, glycerin diacetomonolaurate, glycerin tristearate and diacetylated glycerin monostearate, and adipic acid esters such as diisobutyl adipate.
Examples of the lubricant include paraffin wax and zinc stearate.

Examples of the binding inhibitor include calcium carbonate, silica, zinc stearate, aluminum hydroxide, ethylene bis stearamide, tricalcium phosphate, dimethyl silicon and the like.
Examples of the fusion accelerator include stearic acid, stearic acid triglyceride, hydroxystearic acid triglyceride, stearic acid sorbitan ester, and polyethylene wax.
Examples of the antistatic agent include polyoxyethylene alkylphenol ether, stearic acid monoglyceride, polyethylene glycol and the like.
Examples of the spreading agent include polybutene, polyethylene glycol, and silicone oil.
Examples of the bubble regulator include methacrylic acid ester copolymer, ethylene bis stearamide, polyethylene wax, ethylene-vinyl acetate copolymer, and the like.

(3) Manufacturing method of foaming molding A foaming molding can be obtained, for example with the following method.
Pre-expanded particles are filled in a closed mold with a large number of small holes, heated and foamed with a heat medium (for example, pressurized steam) to fill the gaps between the pre-expanded particles, and the pre-expanded particles are fused to each other By making it integral, a foaming molding can be manufactured. At that time, the density of the foamed molded product can be adjusted, for example, by adjusting the filling amount of the foamed particles in the mold, the amount of steam, the molding time, and the like.
Heating foaming can be performed, for example, by heating with a heat medium of 110 to 150 ° C. for 5 to 50 seconds. Under these conditions, good fusing property between the particles can be ensured. More preferably, the heat foaming can be performed by heating for 10 to 50 seconds with a heat medium of 90 to 120 ° C. Under these conditions, heating foaming is performed under a pressure lower than a typical vapor pressure (for example, 0.06 to 0.08 MPa), which is a molding vapor pressure (gauge pressure) of the heat medium of 0.03 to 0.05 MPa. Can do. Therefore, a foamed molded product can be produced with a small amount of steam.

The pre-expanded particles may be aged, for example, at normal pressure before forming the foamed molded product. The aging temperature of the pre-expanded particles is preferably 20 to 60 ° C. When the aging temperature is low, the aging time of the pre-expanded particles may be long. On the other hand, if it is high, the foaming agent in the pre-expanded particles may be dissipated and the moldability may be lowered.
The pre-expanded particles are obtained by impregnating polystyrene resin particles with a foaming agent to obtain expandable particles, and foaming the expandable particles.

(A) Polystyrene resin particles The method for producing polystyrene resin particles is not particularly limited. For example, resin particles can be obtained by absorbing and polymerizing a monomer mixture containing a styrene monomer into seed particles made of a polystyrene resin.

(A-1) Seed particles Seed particles produced by a known method can be used. For example, (i) a polystyrene resin is melt-kneaded with an extruder, extruded into a strand, and the strand is cut. (Ii) An aqueous medium, a styrene monomer and a polymerization initiator are supplied into an autoclave, and the styrene monomer is suspended and polymerized while heating and stirring in the autoclave to produce seed particles. (Iii) Aqueous medium and polystyrene resin particles are fed into the autoclave, and after the polystyrene resin particles are dispersed in the aqueous medium, the styrene monomer is continuously added while heating and stirring the autoclave. Or intermittently supplying the polymerization initiator while absorbing the styrene monomer in the polystyrene resin particles. Examples thereof include a seed polymerization method in which seed particles are produced by polymerization in the presence.
In addition, the resin particles can be used for part or all of the seed particles. In the case of using a recovered product, production of seed particles by an extrusion method is suitable.

  The average particle size of the seed particles can be adjusted as appropriate according to the average particle size of the resin particles. For example, when obtaining resin particles having an average particle diameter of 1 mm, it is preferable to use seed particles having an average particle diameter of about 0.7 mm to 0.9 mm. Furthermore, the weight average molecular weight of the seed particles is not particularly limited, but is preferably 100,000 to 700,000, and more preferably 150,000 to 500,000.

(A-2) Impregnation step Each monomer is absorbed by the seed particles by supplying a monomer mixture into a dispersion obtained by dispersing the seed particles in an aqueous medium. Examples of the aqueous medium include water and a mixed medium of water and a water-soluble solvent (for example, alcohol).
Each monomer to be used may contain a polymerization initiator. The polymerization initiator is not particularly limited as long as it is conventionally used for monomer polymerization. For example, benzoyl peroxide, t-butyl peroxybenzoate, t-butyl peroxy-2-ethylhexanoate, lauryl peroxide, t-butyl peroxide, t-butyl peroxypivalate, t-butyl peroxyisopropyl Organics such as carbonate, t-butyl peroxyacetate, 2,2-t-butylperoxybutane, t-butylperoxy-3,3,5-trimethylhexanoate, di-t-butylperoxyhexahydroterephthalate Examples thereof include peroxides, azo compounds such as azobisisobutyronitrile, azobisdimethylvaleronitrile, and the like. Among these initiators, in order to reduce the residual monomer, it is preferable to use two or more different polymerization initiators having a decomposition temperature of 80 to 120 ° C. to obtain a half-life of 10 hours. In addition, a polymerization initiator may be used independently or 2 or more types may be used together.

  A suspension stabilizer may be included in the aqueous medium to stabilize the dispersion of monomer droplets and seed particles. The suspension stabilizer is not particularly limited as long as it is conventionally used for monomer suspension polymerization. Examples thereof include water-soluble polymers such as polyvinyl alcohol, methyl cellulose, polyacrylamide, and polyvinyl pyrrolidone, and poorly soluble inorganic compounds such as tricalcium phosphate, magnesium pyrophosphate, and magnesium oxide. And when using a sparingly soluble inorganic compound as said suspension stabilizer, it is preferable to use an anionic surfactant together, and as such anionic surfactant, for example, fatty acid soap, N-acylamino acid or its Salts, carboxylates such as alkyl ether carboxylates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, dialkyl sulfosuccinates, alkyl sulfoacetates, sulfonates such as α-olefin sulfonates; higher alcohol sulfuric acid Ester salts, secondary higher alcohol sulfates, sulfates such as alkyl ether sulfates, polyoxyethylene alkylphenyl ether sulfates, etc .; phosphates such as alkyl ether phosphates, alkyl phosphates, etc. Is mentioned.

(A-3) Polymerization process Although a polymerization process changes with monomer types to be used, a polymerization initiator seed | species, a polymerization atmosphere seed | species, etc., it is normally performed by maintaining 70-130 degreeC heating for 3 to 10 hours. The polymerization step may be performed while impregnating the monomer.
In the polymerization step, the total amount of monomers used may be polymerized in one stage, or may be polymerized in two or more stages (including polymerization during the production of seed particles). It is easier to adjust the positions of the resin component derived from the monofunctional acrylate ester and the resin component derived from the polyfunctional vinyl-based monomer by dividing into two or more stages. Furthermore, adjustment is easier if it is divided into three or more stages. When polymerizing in two or more stages, the impregnation step is usually performed in two or more stages. The polymerization temperature and time of the polymerization process divided into two or more steps may be the same or different. The polymerization process is preferably in three stages. When performed in three stages, it is preferable to adjust the polymerization process as follows.

First, a first monomer mixture containing a styrene monomer and a monofunctional acrylate ester is absorbed into a seed particle of a polystyrene resin and polymerized in the seed particle (first step). Here, the monofunctional acrylic acid ester is preferably added to the polymerization vessel over 1 to 30 minutes.
In the first step, the second step is performed after the polymerization addition rate of seed particles obtained by polymerizing the first monomer mixture containing the styrene monomer and the monofunctional acrylate is increased to 90 to 95%. Is preferred.
Next, the particles obtained through the first step are polymerized while absorbing the styrene monomer (second step).

Further, the particles obtained through the second step are polymerized while absorbing the second monomer mixture containing the styrene monomer and the polyfunctional vinyl monomer (third step). The polyfunctional vinyl monomer is preferably added to the polymerization vessel over 30 to 180 minutes.
In the third step, it is preferable to carry out while maintaining the polymerization conversion rate of the polystyrene-based resin and the second monomer mixture in the range of 75% by mass or more and less than 100% by mass. When the polymerization conversion rate is within this range, the position of the resin component derived from the monofunctional acrylate ester and the resin component derived from the polyfunctional vinyl monomer can be easily adjusted. When it is less than 75% by mass, the surface layer has a high molecular weight component, and thus the effect of improving the surface hardness of the foamed molded product may be lowered. In the case of 100% by mass, the production time of polystyrene-based resin particles becomes long and the production cost may increase. A preferable polymerization conversion rate is in the range of 80 to 98% by mass, and a more preferable range is 80 to 97% by mass.

In the third step, when the time from the start of addition of the polyfunctional vinyl monomer to the end of addition is equally divided into the first to third sections,
The polymerization conversion rate at the start of the first section is in the range of 75% or more and less than 85%,
The polymerization conversion rate at the start of the second section is in the range of 85% or more and less than 90%,
It is preferable to carry out the polymerization conversion at the start of the third section within the range of 88% or more and 93%, and the polymerization conversion at the end of the third section within the range of 93% or more. Thus, by controlling the polymerization conversion rate finely, it is possible to obtain polystyrene resin particles with a higher effect of improving the surface hardness of the foamed molded product.

  Moreover, it is preferable that the usage-amount of the styrene-type monomer used at a 2nd process is 10 mass% or more with respect to the monomer whole quantity used at a 1st process-a 3rd process. When the amount is less than 10% by mass, the resin component derived from monofunctional acrylic ester is contained up to the surface layer of the particle, or the resin component derived from polyfunctional vinyl monomer is contained up to the center of the particle. It may be difficult to obtain polystyrene resin particles having moldability and high strength. The amount of the former styrene monomer used is more preferably 15% by mass or more, and further preferably 20% by mass or more.

(A-4) Shape The shape of the polystyrene resin particles is not particularly limited. For example, spherical shape, cylindrical shape, etc. are mentioned. Of these, a spherical shape is preferable. The average particle diameter of the polystyrene resin particles can be appropriately selected depending on the application, and for example, those having an average particle diameter of 0.2 mm to 5 mm can be used. In consideration of the filling ability into the mold, the average particle diameter is more preferably 0.3 mm to 2 mm, and still more preferably 0.3 mm to 1.4 mm.

(B) Expandable particles Expandable particles are particles obtained by impregnating the polystyrene resin particles with a foaming agent.
The foaming agent is not particularly limited, and any known foaming agent can be used. Particularly, a gaseous or liquid organic compound having a boiling point equal to or lower than the softening point of the polystyrene resin and normal pressure is suitable. For example, hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclopentane, cyclopentadiene, n-hexane, petroleum ether, ketones such as acetone and methyl ethyl ketone, methanol, ethanol, isopropyl alcohol, etc. Low boiling point ether compounds such as alcohols, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, halogen-containing hydrocarbons such as trichloromonofluoromethane, dichlorodifluoromethane, inorganic gases such as carbon dioxide, nitrogen, ammonia, etc. Can be mentioned. These foaming agents may be used alone or in combination of two or more. Among these, it is preferable to use hydrocarbons from the viewpoint of preventing the destruction of the ozone layer and from the viewpoint of quickly replacing with the air and suppressing the time-dependent change of the foamed molded product. Of the hydrocarbons, hydrocarbons having a boiling point of −45 to 40 ° C. are more preferable, and propane, n-butane, isobutane, n-pentane, isopentane and the like are more preferable.

Furthermore, it is preferable that content of a foaming agent is the range of 2-12 mass%. If it is less than 2% by mass, a foamed molded article having a desired density may not be obtained from the expandable particles. In addition, since the effect of increasing the secondary foaming power at the time of in-mold foam molding is reduced, the appearance of the foam molded article may not be good. When the amount is more than 12% by mass, the time required for the cooling step in the production process of the foamed molded product becomes long, and the productivity may decrease. A more preferable content of the blowing agent is 3 to 10% by mass.
The impregnation with the foaming agent may be performed wet simultaneously with the polymerization (for example, the third step), or may be performed wet or dry after the polymerization. When it is carried out in a wet manner, it may be carried out in the presence of a suspension stabilizer and a surfactant exemplified in the polymerization step.

The impregnation temperature of the foaming agent is preferably 60 to 120 ° C. When the temperature is lower than 60 ° C., the time required for impregnating the resin particles with the foaming agent becomes long, and the production efficiency may decrease. On the other hand, when the temperature is higher than 120 ° C., the resin particles may be fused to generate bonded particles. A more preferable impregnation temperature is 70 to 110 ° C.
A foaming aid may be used in combination with a foaming agent. Examples of the foaming aid include isobutyl adipate, toluene, cyclohexane, and ethylbenzene.

(C) Pre-expanded particles Pre-expanded particles are obtained by foaming expandable particles to a desired bulk density using water vapor or the like.
The bulk density of the pre-expanded particles is preferably in the range of 0.01 to 0.07 g / cm ³ . When the bulk density of the pre-expanded particles is smaller than 0.01 g / cm ³ , the effect of improving the surface hardness may not be sufficient. In addition, the heat insulation performance may be reduced. In addition, shrinkage may occur in the foamed molded product, which may reduce the appearance. On the other hand, when the bulk density is larger than 0.07 g / cm ³ , the lightweight property of the foamed molded product may be lowered.
In addition, it is preferable to apply powder metal soaps such as zinc stearate to the surface of the expandable particles before foaming. By applying, the bonding between the foamed particles can be reduced in the foaming process of the foamable particles.

  Hereinafter, specific examples of the present invention will be described by way of examples. However, the following examples are merely illustrative of the present invention, and the present invention is not limited to the following examples. In the following, “part” and “%” are based on mass unless otherwise specified.

<Polymerization conversion>
The method for measuring the amount of monomer contained in seed particles in the middle of nuclear polymerization (hereinafter referred to as growing particles) refers to those measured in the following manner.
That is, the growing particles are taken out from the dispersion, and the water adhering to the surface is wiped off with gauze. 0.08 g of the growing particles are collected, and the collected growing particles are dissolved in 24 ml of toluene to prepare a toluene solution. Next, 10 ml of the Wis reagent, 30 ml of 5% by weight potassium iodide aqueous solution and 30 ml of 1% by weight starch aqueous solution are added to the toluene solution. The result obtained by titrating the obtained solution with an N / 40 sodium thiosulfate solution is defined as a titration constant (milliliter) of the sample. The Wis reagent is obtained by dissolving 8.7 g of iodine and 7.9 g of iodine trichloride in 2 liters of glacial acetic acid. On the other hand, without dissolving the growing particles, 10 ml of Wis reagent, 30 ml of 5% by weight potassium iodide aqueous solution and 30 ml of 1% by weight starch aqueous solution are added to 24 ml of toluene. The resulting solution was titrated with a N / 40 sodium thiosulfate solution to give a blank titration (in milliliters).
From the obtained droplet constant, the amount of unreacted monomer in the growing particles is calculated based on the following formula.
Amount of monomer (% by mass) in growing particles =
0.1322 × (blank drop constant−sample drop constant) / sample drop constant Further, the polymerization conversion is calculated by the following equation.
Polymerization conversion rate (%) =
100 × (sample mass−monomer amount of growing particles) / sample mass

<Average molecular weight>
The surface layer weight average molecular weight of the resin particles is measured from the surface layer of the foam molded article. That is, since the foam molded body is obtained by pre-foaming resin particles and molding in-mold, the resin particle surface layer corresponds to the foam molded body surface layer, and the average molecular weight of the resin particle surface layer is the average molecular weight of the foam molded body surface layer. It corresponds to. The total particle weight average molecular weight of the resin particles is measured from the resin particles themselves.
After the foam molded body having a density of 0.017 g / cm ³ is dried at 50 ° C. for 24 hours, it is cut at a depth of 0.3 mm from the surface of the foam molded body using a ham slicer (Fujishima Koki: FK-18N type). A surface GPC measurement sample is used. As the whole GPC measurement sample, resin particles themselves are used.
0.08 g of the sample is dissolved in 10 ml of tetrahydrofuran, and GPC measurement is performed under the following conditions.
・ Device: High-speed GPC device (HLC-8320GPC) EcoSEC-WorkStation (manufactured by Tosoh Corporation)
Analysis conditions Column: TSKgel SuperMultipore HZ-M × 2
Flow rate: 0.35 ml / min
Detector: RI detector with built-in HLC-8320GPC / UV-8320
Detector conditions: Pol (+), Res (0.5 s) / λ (254 nm), Pol (+), Res (0.5 s)
Concentration: 0.2 wt%
Injection volume: 10 μL
Pressure: 3.5MPa
Column temperature: 40 ° C
System temperature: 40 ° C
Eluent: THF

<Absorbance ratio at 50% of center and radius>
The absorbance ratio (D1735 / D1600) of the central part and 50% of the radius of the polystyrene resin particles is measured as follows.
Ten randomly selected particles are subjected to particle cross-sectional analysis by infrared spectroscopic ATR measurement to obtain an infrared absorption spectrum. In this analysis, an infrared absorption spectrum in the depth range from the sample surface to several μm (about 2 μm) is obtained.
The individual absorbance ratio (D1735 / D1600) is calculated from each infrared absorption spectrum, and the arithmetic average is defined as the absorbance ratio.
The absorbances D1735 and D1600 were measured under the following conditions using a measuring device sold under the trade name “Fourier Transform Infrared Spectrometer MAGNA 560” by Nicolet and a “Thunder Dome” manufactured by Spectra-Tech as an ATR accessory. To do.

(Measurement condition)
High refractive index crystal seed: Ge (germanium)
Incident angle: 45 ° ± 1 °
Measurement region: 4000 cm ^{−1 to} 675 cm ⁻¹
Fraction dependency of measurement depth: uncorrected number of reflections: 1 detector: DTGS KBr
Resolution: 4cm ^-1
Number of integrations: 32 times Others: An infrared absorption spectrum is measured under the following conditions without contacting the sample. The measured spectrum is used as the background. When measuring the sample, the measurement data is processed so that the background does not contribute to the measurement spectrum. In the ATR method, the intensity of the infrared absorption spectrum changes depending on the degree of adhesion between the sample and the high refractive index crystal. Therefore, the maximum load that can be applied by the “Thunder Dome” of the ATR accessory is applied to perform the measurement with a substantially uniform contact degree.

(Background measurement conditions)
Mode: Transparent pixel size: 6.25 μm
Measurement region: 4000 cm ^{−1 to} 650 cm ⁻¹
Detector: MCT
Resolution: 8cm ^-1
Scan / Pixel: 60 times Others: An infrared absorption spectrum obtained by measuring a barium fluoride crystal in the vicinity of the sample where there is no sample is used as a background, and a process that does not relate to the measurement spectrum is performed. 1735 cm obtained from the infrared absorption spectrum Absorbance D1735 at ⁻¹ is an absorbance corresponding to an absorption spectrum derived from stretching vibration between C═O of ester groups contained in the ester. In this absorbance measurement, peak separation is not performed even when other absorption spectra overlap at 1735 cm ⁻¹ . Absorbance D1735 is a straight line connecting the 1680 cm ^-1 and 1785 cm ^-1 as a baseline, means the maximum absorbance between 1680 cm ^-1 and 1785 cm ^-1.
Further, the absorbance D1600 at 1600 cm ⁻¹ obtained from the infrared absorption spectrum is an absorbance corresponding to the absorption spectrum derived from the in-plane vibration of the benzene ring contained in the polystyrene resin. In this absorbance measurement, peak separation is not performed even when other absorption spectra overlap at 1600 cm ⁻¹ . Absorbance D1600 is a straight line connecting the 1565Cm ^-1 and 1640 cm ^-1 as a baseline, means the maximum absorbance between 1565cm ^-1 and 1640 cm ^-1.
The absorbance ratio (D1735 / D1600) at the center is measured as follows.

(A) Preparation of measurement sample Ten randomly selected particles are fixed to an epoxy resin base. Next, a slice sample is obtained by slicing the particles with a diamond knife using an ultramicrotome (LEICA ULTRACUT UCT, manufactured by Leica Microsystems) to a thickness of about 10 μm through the center. The obtained slice sample is sandwiched between two barium fluoride crystals (manufactured by Pure Optex). This is used as a measurement sample.
The image of the slice sample is captured by the CCD attached to the following measuring device. Image capture is performed with the advancing direction of the blade of the ultramicrotome as the Y axis and the vertical direction as the X axis. The particles in the slice sample are slightly crushed in the moving direction of the blade. By matching the Y axis of the captured image with the moving direction of the blade, the measured absorbance ratio is prevented from varying.
For the absorbances D1735 and D1600, an apparatus sold by Perkin Elmer under the trade name “High-Speed IR Imaging System Spectrum Spotlight 300” is used. Using this apparatus, an image of a slice sample is obtained under the following conditions. From the obtained image, an infrared absorption spectrum at each location is obtained under the following measurement conditions.

Measurement condition mode: Transmission pixel size: 6.25 μm
Measurement region: 4000 cm ^{−1 to} 650 cm ⁻¹
Detector: MCT
Resolution: 8cm ^-1
Scan / Pixel: Twice from the captured image, connect the minimum and maximum values of the X coordinate value and the minimum and maximum values of the Y coordinate value of the Y axis with a line as shown in FIG. Let it be the center point A. In image processing, the X and Y coordinate values of the center point are set within a range of ± 20 μm from the center point A.
Next, a straight line passing through the center point A and parallel to the X axis is drawn in the image. A point where this straight line intersects with the position of the end where the particle (resin) exists (maximum value on the X axis) is defined as a point D. An infrared absorption spectrum on a line connecting the points A and D is extracted every 12 ± 2 μm as an X coordinate value. In the present invention, the 50% radius portion refers to a portion of 50% of the distance from the point A to the point D and falls within a range of ± 20 μm.
Absorbances D1735 and D1600 are read from the extracted infrared absorption spectrum, respectively, and the absorbance ratio (D1735 / D1600) at the center and 50% radius is calculated. The arithmetic average of the individual absorbance ratios calculated for 10 particles is taken as the absorbance ratio.

<Bulk density of pre-expanded particles>
The bulk magnification of the pre-expanded particles is measured according to JIS K6911: 1995 “General Test Method for Thermosetting Plastics”. Specifically, first, Wg is collected using pre-expanded particles as a measurement sample, and this measurement sample is naturally dropped into a measuring cylinder. The volume Vcm ³ of the measurement sample dropped into the graduated cylinder is measured using an apparent density measuring instrument based on JIS K6911. By substituting Wg and Vcm ³ into the following formula, the bulk density of the pre-expanded particles is calculated.
Bulk density of pre-expanded particles (g / cm ³ ) = mass of measurement sample (W) / volume of measurement sample (V)

<Multiple of foam molding>
The thickness of the molded product from the foamed molded product (dried at 40 ° C. for 20 hours or more after molding) as the sample thickness, and the mass of a test piece having an arbitrary area (eg, 50 mm × 50 mm × 30 mm thick) Measure (a) and volume (b) so that each of them has three or more significant digits, and obtain a multiple W1 (times) of the foamed molded product by the formula (b) / (a).
Next, the test piece is divided into four equal parts in the thickness direction, and two front and back divided sections including the outermost layer are collected as a surface layer portion. In the same manner as described above, the respective multiples of the outermost layer part are obtained, and the multiple W2 (times) of the outermost layer part is calculated by arithmetic mean.

<Surface hardness>
The surface hardness of the foamed molded product is measured with an Asuka C surface hardness tester. Specifically, the surface hardness is arbitrarily measured at 50 locations on the surface of the foamed molded product having a density of 16.7 kg / m ³ , and the arithmetic average is defined as the surface hardness.
Evaluation: Surface hardness 50 or more: ○
Less than 50: ×

Example 1
(Manufacture of seed particles)
A polymerization vessel equipped with a stirrer with an internal capacity of 100 liters is supplied with 40000 parts by mass of water, 100 parts by mass of magnesium pyrophosphate as a suspension stabilizer, and 5.0 parts by mass of sodium dodecylbenzenesulfonate as an anionic surfactant. After adding 40000 parts by mass and 96.0 parts by mass of benzoyl peroxide and 28.0 parts by mass of t-butylperoxybenzoate as polymerization initiators, the temperature was raised to 90 ° C. to polymerize. And it hold | maintained at this temperature for 6 hours, and also, after heating up to 125 degreeC, it cooled after 2 hours, and obtained the polystyrene-type resin particle (a).
The polystyrene resin particles (a) are sieved, and polystyrene resin particles (b) having a particle diameter of 0.5 to 0.71 mm (average particle diameter of 0.63 mm) having a weight average molecular weight of 250,000 as seed particles. Obtained.

Next, 2350 g of polystyrene-based resin (b) seed particles, 30 g of magnesium pyrophosphate and 10 g of sodium dodecylbenzenesulfonate are supplied to a polymerization vessel equipped with a stirrer with an internal volume of 25 L and dispersed by heating to 72 ° C. while stirring. A liquid was prepared.
Subsequently, a solution prepared by dissolving 45.9 g of benzoyl peroxide and 6.1 g of t-butylperoxybenzoate in a mixture of 850 g of styrene and 150 g of n-butyl acrylate was supplied to the dispersion while stirring.
And it maintained for 60 minutes, after supplying the said solution in a dispersion liquid (1st process).
Thereafter, while the temperature of the dispersion was raised to 88 ° C. over 60 minutes, 2660 g of styrene was charged into the polymerization vessel at a constant rate, and the reaction was carried out while absorbing the seed particles (second step).
Next, while maintaining the dispersion at 88 ° C., a solution of 0.6 g of divinylbenzene (bifunctional monomer, molecular weight 130) dissolved in 4000 g of styrene is charged at a constant rate into the polymerization vessel over 90 minutes and absorbed by the seed particles. The polymerization reaction was carried out (third step).
This third step was divided into three sections, and the polymerization conversion rate at the start of each section during polymerization and the polymerization conversion ratio at the end of the third section were measured.
The respective polymerization conversion rates were 80%, 88%, 90%, and 94%.

After the third step, the dispersion was further heated to 120 ° C. and held for 60 minutes to react unreacted monomers. Next, polystyrene resin particles having an average particle diameter of 1.0 mm were collected from the reaction vessel. Seeking and absorbance D1600 at absorbance D1735 and 1600 cm ^-1 of the resin particles in 1735 cm ^-1 and subjected to ATR method infrared spectroscopy, was calculated D1735 / D1600. As a result, the absorbance ratio at the center was 0.70, and the absorbance ratio at 50% of the radius was 0.51. Furthermore, Mw of the entire polystyrene resin particles was 240,000.
FIG. 2 shows the change in absorbance ratio from the center of the obtained polystyrene resin particles to the surface layer.
Next, the dispersion is kept at 100 ° C., and then 80 g of cyclohexane, 70 g of diisobutyl adipate and 700 g of normal butane are pressed into the polymerization vessel and held for 3 hours, whereby normal butane is contained in the resin particles. Was impregnated. Thereafter, the inside of the polymerization vessel was cooled to 25 ° C. to obtain expandable particles.
Polyethylene glycol was applied as an antistatic agent to the surface of the expandable particles. Thereafter, zinc stearate and hydroxystearic acid triglyceride were further applied to the surface of the expandable particles. After coating, the expandable particles were left in a thermostatic chamber at 13 ° C. for 5 days.
The expandable particles were then heated and pre-expanded to a bulk density of 0.017 g / cm ³ to obtain pre-expanded particles. Pre-expanded particles were aged at 20 ° C. for 24 hours.

Next, the pre-expanded particles were molded under the following conditions.
Molding machine: ACE-3SP made by Sekisui Koki
Molding vapor pressure: 0.04MPa
Mold heating: 3 seconds One-side heating: 10 seconds Double-sided heating: 15 seconds Water cooling: 5 seconds The obtained foamed molded article was dried in a drying chamber at 50 ° C for 6 hours, and then the density was measured. ³ (17 kg / m ³ ). The foamed molded article was excellent in appearance without shrinkage.
Mw of the surface layer of the obtained foamed molded product (corresponding to Mw of the surface layer of polystyrene resin particles) was 280,000. Five test pieces of 50 mm × 50 mm × 30 mm were collected from the obtained foam, and the difference (W1−W2) between the overall multiple W1 and the multiple W2 of the surface layer portion was measured and found to be 11 times. The surface hardness of this foamed molded product was 55 and excellent.

(Example 2)
A foamed molded article was obtained in the same manner as in Example 1 except that the pre-expanded particles were filled in a mold and molded by heating and foaming at a vapor pressure of 0.12 MPa. As a result of measuring the difference (W1−W2) between the overall multiple W1 and the multiple W2 of the surface layer portion, it was 16 times. The surface hardness of this foamed molded product was 55 and excellent.
(Example 3)
In the first polymerization step, a foamed molded article was obtained in the same manner as in Example 1 except that only styrene was used without using butyl acrylate. Mw of the entire polystyrene resin particles was 300,000, and Mw of the surface layer of the foamed molded product was 350,000. A foam-molded article was obtained in the same manner as in Example 1 except that the pre-expanded particles were filled in a mold and molded by heating and foaming at a vapor pressure of 0.12 MPa. As a result of measuring the difference (W1−W2) between the overall multiple W1 and the multiple W2 of the surface layer portion, it was 12 times. The surface hardness of the foamed molded product was excellent at 53.

Example 4
A dispersion liquid is prepared by supplying 2350 g of polystyrene resin (b) seed particles, 30 g of magnesium pyrophosphate and 10 g of sodium dodecylbenzenesulfonate to a polymerization vessel equipped with a stirrer with an internal volume of 25 L and heating to 72 ° C. while stirring. did.
Subsequently, 45.9 g of benzoyl peroxide, 6.1 g of t-butyl peroxybenzoate, 1000 g of styrene, and 50 g of diisobutyl adipate were all fed into the dispersion while stirring.
And it maintained for 60 minutes, after supplying the said solution in a dispersion liquid (1st process). Thereafter, while the temperature of the dispersion was raised to 88 ° C. over 60 minutes, 2660 g of styrene was charged into the polymerization vessel at a constant rate, and the reaction was carried out while absorbing the seed particles (second step).
Next, while maintaining the dispersion at 88 ° C., 4000 g of styrene was charged into the polymerization vessel at a constant rate over 90 minutes, and the polymerization reaction was carried out while absorbing the seed particles (third step).
This third step was divided into three sections, and the polymerization conversion rate at the start of each section during polymerization and the polymerization conversion ratio at the end of the third section were measured.
The respective polymerization conversion rates were 80%, 88%, 90%, and 94%.

After the third step, the dispersion was further heated to 120 ° C. and held for 60 minutes to react unreacted monomers. Next, polystyrene resin particles having an average particle diameter of 1.0 mm were collected from the reaction vessel. Seeking and absorbance D1600 at absorbance D1735 and 1600 cm ^-1 of the resin particles in 1735 cm ^-1 and subjected to ATR method infrared spectroscopy, was calculated D1735 / D1600. As a result, the absorbance ratio at the center was 0.70, and the absorbance ratio at 50% of the radius was 0.51. Furthermore, Mw of the entire polystyrene resin particles was 240,000.
FIG. 2 shows the change in absorbance ratio from the center of the obtained polystyrene resin particles to the surface layer.
Next, the dispersion is kept at 100 ° C., and then 80 g of cyclohexane, 70 g of diisobutyl adipate and 700 g of normal butane are pressed into the polymerization vessel and held for 3 hours, whereby normal butane is contained in the resin particles. Was impregnated. Thereafter, the inside of the polymerization vessel was cooled to 25 ° C. to obtain expandable particles.
Polyethylene glycol was applied as an antistatic agent to the surface of the expandable particles. Thereafter, zinc stearate and hydroxystearic acid triglyceride were further applied to the surface of the expandable particles. After coating, the expandable particles were left in a thermostatic chamber at 13 ° C. for 5 days.
The expandable particles were then heated and pre-expanded to a bulk density of 0.017 g / cm ³ to obtain pre-expanded particles. Pre-expanded particles were aged at 20 ° C. for 24 hours.

Next, the pre-expanded particles were molded under the following conditions.
Molding machine: ACE-3SP made by Sekisui Koki
Molding vapor pressure: 0.12 MPa
Mold heating: 3 seconds One-side heating: 10 seconds Double-sided heating: 15 seconds Water cooling: 5 seconds The obtained foamed molded article was dried in a drying chamber at 50 ° C for 6 hours, and then the density was measured. ³ (17 kg / m ³ ). The foamed molded article was excellent in appearance without shrinkage.
Mw of the surface layer of the obtained foamed molded product (corresponding to Mw of the surface layer of polystyrene resin particles) was 280,000. Five test pieces of 50 mm × 50 mm × 30 mm were collected from the obtained foam, and the difference (W1−W2) between the entire multiple W1 and the multiple W2 of the surface layer portion was measured. As a result, it was 17 times. The surface hardness of the foamed molded product was excellent at 56.

(Comparative Example 1)
A foamed molded article was obtained in the same manner as in Example 1 except that the pre-expanded particles were filled in a mold and molded by heating and foaming at a vapor pressure of 0.06 MPa. As a result of measuring the difference (W1−W2) between the overall multiple W1 and the multiple W2 of the surface layer portion, it was 9 times, and the surface hardness of this foam molded article was inferior to 49.
(Comparative Example 2)
A foamed molded article was obtained in the same manner as in Example 1 except that the pre-expanded particles were filled in a mold and molded by heating and foaming at a vapor pressure of 0.12 MPa. As a result of measuring the difference (W1-W2) between the overall multiple W1 and the bulk multiple W2 of the surface layer portion, it was 22 times, and the surface hardness was also inferior at 49.

  From Table 1, it can be seen that when the multiple difference (W1-W2) is 10 or more and less than 20, a foamed molded article having a high surface hardness is obtained.

Claims (4)

It is a foamed molded product which is a fusion product of a plurality of expanded polystyrene resin particles, and the foamed molded product has at least a region having a thickness of 0.5 to 10 cm,
The region is, if the multiples of whole W1 and multiples thereof the surface layer portion was W2, 10 times or more, have a relationship of the difference of less than 20 times (W1-W2),
The expanded polystyrene-based resin particles are obtained by impregnating a polystyrene-based resin particle with a foaming agent and then foaming.
The polystyrene resin particles are
(1) The central portion contains more resin components than the surface layer derived from a polymer of a styrene monomer and a monofunctional acrylate ester.
(2) The surface layer contains more resin components derived from a polymer of a styrene monomer and a polyfunctional vinyl monomer than the central portion.
(3) The surface layer has a weight average molecular weight in the range of 250,000 to 600,000.
A foamed molded article having the following structure .   The foam molded article according to claim 1, wherein the multiple W1 is 15 to 100 times. Including at least resin foam containers foam molded article according to claim 1 or 2. The resin foam container according to claim 3 , wherein the region is located on a side surface of the resin foam container.