JP2015048356A - Foamable styrenic resin particle, foamed particle, foam molded body and manufacturing method of foamed particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a foamable styrenic resin particle capable of providing a foamed particle capable of foaming with low heat amount and excellent in binding prevention property during foaming.SOLUTION: The problem is solved by a foamable styrenic resin particle containing a resin component derived from monofunctional acrylic ester and a resin component derived from a multifunctional vinyl monomer and a foaming agent, a large amount of the resin component derived from the monofunctional acrylic ester is contained in a center part of the foamable styrenic resin particle, a large amount of the resin component derived from the multifunctional vinyl monomer is contained in a surface layer of the foamable styrenic resin particle, and the foamable styrenic resin particle has low heat amount foamability with 300 sec. or less of time for obtaining a foamed particle when foaming to be the foamed particle having bulk density of 0.0166 g/cmwith heating medium with a flow rate of 0.033 liter/(the foamed styrenic resin particle 1 g)/sec at 180°C (the water vapor content is 50 g/mor less).

Description

  The present invention relates to an expandable styrene resin particle, an expanded particle, an expanded molded article, and a method for producing the expanded particle. More specifically, the present invention is a foamable styrene resin particle that can be foamed with a small amount of heat and can provide foamed particles excellent in anti-bonding properties during foaming, foamed particles obtained from foamable styrene resin particles, The present invention relates to a foamed molded product and a method for producing foamed particles.

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 produced using foamable particles as raw materials are often used because of the advantage that a desired shape is easily obtained.
Expandable styrene resin particles are widely used as expandable particles that are raw materials for producing a expanded molded body. For example, the expanded molded body is obtained as follows. That is, expandable particles such as expandable styrene resin particles are heated with steam and pre-expanded to obtain expanded particles (pre-expanded particles). The obtained pre-expanded particles are filled into the mold cavity. Next, a foamed molded article can be obtained by integrating the pre-expanded particles by heat-sealing while pre-expanding the filled pre-expanded particles with steam.

As described above, in producing a foamed molded product from expandable styrene resin particles, the expandable styrene resin particles are heated by a heating medium to be pre-expanded into pre-expanded particles.
Generally, water vapor is used as a heating medium at the time of foaming, and the obtained pre-foamed particles are cooled from a heated state after the pre-foaming. By cooling, the volume of the foaming gas contained in the pre-expanded particles is reduced, and the water vapor absorbed during the pre-expanding step of the expandable styrene resin particles is dissipated or liquefied. As a result, the inside of the pre-expanded particles is in a reduced pressure state.
When pre-expanded particles in a reduced pressure state are subjected to foam molding, shrinkage occurs in the foamed molded article, and therefore it is desired to eliminate the reduced pressure state. Therefore, the pre-expanded particles are usually subjected to an aging step in which the pre-expanded particles are allowed to stand for 6 to 24 hours at room temperature (about 25 ° C.) and atmospheric pressure. In this aging step, the reduced pressure state in the pre-expanded particles is eliminated by allowing air to flow into the pre-expanded particles and dissipating moisture to the outside.
Since the aging step takes a long time of 6 to 24 hours as described above, the production efficiency of the foamed molded product is lowered. In addition, the aging process has a problem that it requires a space for aging pre-expanded particles and also requires a maintenance cost for an apparatus used for aging pre-expanded particles.
For these problems, Patent Document 1 proposes a foaming method in which heating air and heating steam are used in combination, and molding can be performed without an aging step.

Japanese Patent No. 2013823

  Molding can be performed without the aging step by the technique described in the above publication. However, the heating air has a problem that the amount of heat is lower than that of water vapor, and the heating time during foaming becomes longer. In addition, foaming with heated air has a problem of increased bonding during foaming. Furthermore, in the above publication, since heated steam is used in combination, there is a problem that an aging step is still necessary particularly at a high expansion ratio.

  As a result of earnest research, the inventor of the present invention has an expandable styrenic resin particle containing a resin component derived from a monofunctional acrylate ester in the center and a resin layer derived from a polyfunctional vinyl monomer in the surface layer. Thus, the present inventors have found that foaming occurs quickly even with a low heat quantity, and no aging step is required even with a high expansion ratio, and that binding during foaming can be prevented, leading to the present invention.

Thus, according to the present invention, expandable styrene resin particles containing a resin component derived from a monofunctional acrylate ester, a resin component derived from a polyfunctional vinyl monomer, and a foaming agent,
The resin component derived from the monofunctional acrylic ester is contained in a large amount in the center of the expandable styrene resin particles,
The resin component derived from the polyfunctional vinyl monomer is often contained in the surface layer of the expandable styrene resin particles,
The expandable styrenic resin particles are obtained by a heating medium at 180 ° C. (however, the amount of water vapor is 50 g / m ³ or less) at a flow rate of 0.033 liters / (the expandable styrene resin particles 1 g) / second. In the case of foaming into foam particles having a bulk density of 0166 g / cm ³ , expandable styrene-based resin particles are provided that have a low calorific foaming property in which the time for obtaining the foamed particles is 300 seconds or less. .
Furthermore, according to the present invention, there are provided expanded particles obtained by expanding the expandable styrene resin particles.
Moreover, according to this invention, the foaming molding obtained by carrying out the foam molding of the said foaming particle is provided.
Furthermore, according to the present invention, the foamable styrene resin particles are foamed to obtain foamed particles,
The foaming is performed by using a heating medium having a water vapor amount of 50 g / m ³ or less.

  According to the present invention, it is possible to provide expandable styrenic resin particles that foam quickly even at a low heat quantity, do not require an aging step even at a high expansion ratio, and can prevent bonding during foaming. The inventor believes that this effect is achieved by containing a resin component derived from a monofunctional acrylate ester in the center and a resin component derived from a polyfunctional vinyl monomer in the surface layer. According to the expandable styrenic resin particles of the present invention, even when heated air, the particles can be quickly foamed without being bonded to each other, and thus foaming is possible without reducing productivity.

It is the schematic for demonstrating the measurement procedure of the light absorbency ratio by ATR method. 3 is a graph showing a change in absorbance ratio from the center of the expandable styrene resin particle of Example 1 to the surface layer.

(Expandable styrene resin particles)
Expandable styrenic resin particles (hereinafter also simply referred to as expandable particles)
-Styrenic resin particles comprising a resin component derived from a monofunctional acrylic ester and a resin component derived from a polyfunctional vinyl monomer,
-Many resin components derived from monofunctional acrylic acid esters are contained in the center of the expandable styrene resin particles,
-The resin component derived from a polyfunctional vinyl-type monomer is a particle in which many are contained in the surface layer of an expandable styrene-type resin particle.
The expandable particles are 0.0166 g / cm ³ by a heating medium of 180 ° C. (however, the amount of water vapor is 50 g / m ³ or less) at a flow rate of 0.033 liters / (expandable styrene resin particles 1 g) / second. When foaming into bulk density foamed particles, it has a low heat quantity foaming property in which the time for obtaining the foamed particles is 300 seconds or less. This time is preferably 250 seconds or less, and more preferably 200 seconds or less. The lower limit of this time is usually 100 seconds. The amount of water vapor can be calculated from the humidity measured by a known method.

(1) Constituent Component (a) Styrenic Resin The expandable particle 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) Resin component derived from monofunctional acrylic ester The resin component derived from monofunctional acrylic ester is not particularly limited, but a resin component derived from a monomer copolymerizable with a styrene-based monomer is preferable. The monofunctional acrylic acid ester is preferably an ester having 3 to 20 carbon atoms. By using a monomer having a carbon number within this range, it is possible to provide expandable particles having a lower heat quantity and an improved foaming rate.

  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.

(C) Resin component derived from a polyfunctional vinyl monomer The resin component derived from a 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. By including a resin component derived from such a polyfunctional vinyl monomer having a specific number of vinyl groups, it is possible to provide expandable particles capable of suppressing bonding during foaming. The number of vinyl groups is more preferably 2 to 15 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.

(D) Ratio of resin component The ratio of the resin component derived from the styrenic monomer, the resin component derived from the monofunctional acrylate ester, and the resin component derived from the polyfunctional vinyl monomer constituting the expandable particles is from 1: 0.05 to It is preferably in the range of 0.25: 0.6 to 1 (mass ratio).
When the resin component derived from the monofunctional acrylate is less than 0.05, it is necessary to maintain a long heating time at the time of foaming in order to obtain foamed particles having a desired foaming ratio, which may be inferior in productivity. When it exceeds 0.25, it may be difficult to obtain a high-magnification foamed article.
When the resin component derived from the polyfunctional vinyl monomer is less than 0.6, the bond at the time of foaming may increase. When it is more than 1, expanded particles having a desired expansion ratio may not be obtained.
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.

(E) 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.

(2) Position of the resin component derived from the monofunctional acrylate ester A large amount of the resin component derived from the monofunctional acrylate ester is contained in the center of the foamable particles. By containing a large amount in the center, it is possible to provide expandable particles that can provide a foamed molded article even with a small amount of steam. Here, the central portion means a region within about 15% of the particle radius from the particle center.
The position of the resin component derived from the monofunctional acrylic acid ester 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.
ATR infrared spectroscopic analysis includes organic materials such as polymer materials because of the simplicity of being able to measure the spectrum simply by bringing the sample and the ATR prism into close contact with each other, and the ability to perform surface analysis up to a depth of several μm. It is widely used for surface analysis of various substances.

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 styrene 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).

The absorbance ratio preferably shows a tendency to decrease from the center of the styrene 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 expandable particles is preferably in the range of 0.4 to 0.8. When the absorbance ratio is less than 0.4, 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 the absorbance ratio is larger than 0.8, a foamed molded article having a desired strength may not be obtained. A preferred absorbance ratio is in the range of 0.4 to 0.7, and a more preferred absorbance ratio is in the range of 0.5 to 0.7.

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 50% radius portion of the expandable particles 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 strength, which may be inferior in energy saving. When the absorbance ratio is larger than 0.65, a foamed molded article having excellent strength 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 0.4 to 0.8, 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 0.4, in order to obtain a foam molded article having a desired strength, it is necessary to maintain a high vapor pressure during foam molding, which may be inferior in energy saving. When the relative value is larger than 0.8, a foamed molded article having excellent strength may not be obtained. A preferred relative value is in the range of 0.4 to 0.7, and a more preferred relative value is in the range of 0.5 to 0.7.

(3) 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 expandable particles. By containing a large amount in the surface layer, it is possible to provide expandable particles that can prevent bonding during the manufacture of the expanded particles.
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 when producing expandable 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 viscosity of the resin may be excessively lowered during the production of the expanded particles, and the bond may increase. When it is larger than 600,000, the viscosity at the time of producing expanded particles is high, and heating may take time. 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.
Since it is difficult to measure the weight average molecular weight of the surface layer from the particles themselves, in this specification, the average molecular weight measured from the surface layer of the foam molded article 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 weight average molecular weight of all particles is less than 230,000, the viscosity of the resin may be excessively reduced during the production of expanded particles, and the bond may increase. If it is larger than 400,000, the viscosity at the time of producing the expanded particles is high, and heating may take time. The surface layer weight average molecular weight is more preferably in the range of 250,000 to 400,000, and still more preferably in the range of 250,000 to 350,000.

  The surface layer weight average molecular weight is preferably 2 to 300,000 larger than the total particle weight average molecular weight. If the size is less than 20,000, the viscosity of the resin may decrease too much during the production of the expanded particles, and the bond may increase. When it is larger than 300,000, the viscosity at the time of producing expanded particles is high, and heating may take time. A more preferable size is in the range of 30,000 to 300,000, and more preferably in the range of 50,000 to 250,000.

(4) Shape of expandable particle The shape of the expandable particle 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 expandable particles can be appropriately selected according to 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.

(Method for producing expandable particles)
The method for producing the expandable particles is not particularly limited. For example, expandable particles can be obtained by absorbing a monomer mixture containing a styrene monomer into a seed particle made of a styrene resin, polymerizing the seed particle, and impregnating with a foaming agent.
(1) Seed particles Seed particles produced by a known method can be used. For example, (i) a styrene resin is melt-kneaded with an extruder, extruded into a strand, and the strand is cut. (Ii) An aqueous medium, a styrenic monomer and a polymerization initiator are fed 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 styrenic resin particles are fed into the autoclave, and the styrenic resin particles are dispersed in the aqueous medium, and then the styrene monomer is continuously added while heating and stirring the autoclave. Alternatively, the polymer is supplied intermittently and polymerized in the presence of a polymerization initiator while allowing the styrene resin particles to absorb the styrene monomer. Examples thereof include a seed polymerization method for producing seed particles.
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.

(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, magnesium oxide, and hydroxyapatite. When a sparingly soluble inorganic compound is used as the suspension stabilizer, it is preferable to use an anionic surfactant together. Examples of such anionic surfactant include fatty acid soap, N-acylamino acid or a salt thereof. Carboxylates such as alkyl ether carboxylates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, dialkyl sulfosuccinates, alkyl sulfoacetates, α-olefin sulfonates, etc .; higher alcohol sulfates Salts, secondary higher alcohol sulfates, sulfates such as alkyl ether sulfates, polyoxyethylene alkylphenyl ether sulfates; phosphates such as alkyl ether phosphates, alkyl phosphates, etc. Can be mentioned.

(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 process is usually performed in two 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 seed particles of a styrene resin and polymerized in the seed particles (first step).
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).
Here, it is preferable to add the monofunctional acrylic ester to the polymerization vessel over 1 to 30 minutes and the polyfunctional vinyl monomer over 30 to 180 minutes, respectively.

In the third step, it is preferable to carry out the polymerization conversion while maintaining the polymerization conversion rate of the styrene 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. If less than 75% by mass,
Since the high molecular weight component of the surface layer is reduced, the effect of improving the bending strength of the foamed molded product may be lowered. In the case of 100% by mass, the production time of the styrene 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. By finely controlling the polymerization conversion rate in this way, expandable particles having a high foaming speed can be obtained even with a lower heat quantity.

  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, a resin component derived from a monofunctional acrylate ester is included up to the surface layer of the particle, or a resin component derived from a polyfunctional vinyl-based monomer is included up to the center of the particle. It may be difficult to improve speed. The amount of the former styrene monomer used is more preferably 15% by mass or more, and further preferably 20% by mass or more.

(4) Foaming agent The foaming agent is not particularly limited, and any known foaming agent can be used. In particular, a gaseous or liquid organic compound having a boiling point equal to or lower than the softening point of the styrene resin 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 preferred, and isobutane is still more preferred.

  Furthermore, it is preferable that content of a foaming agent is the range of 4-12 mass%. If the amount is less than 4% by mass, the expansion ratio may not increase when foaming with heated air. 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 6 to 10% by mass.

The expandable particles can be obtained by impregnating the styrene resin particles with a foaming agent. The impregnation 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.

(Foamed particles)
The expanded particles can be obtained by foaming the expandable particles to a desired bulk density using a heating medium having a water vapor amount of 50 g / m ³ or less. Air is usually used as the heating medium. This amount of water vapor means that the heating medium is a medium that does not substantially contain water vapor.
The foamed particles can be used as they are for applications such as cushioning fillers, and can be used as a raw material for foam molded articles for foaming in the mold. In the case of the raw material of the foam molded article, the expanded particles are generally referred to as pre-expanded particles, and the expansion to obtain the expanded particles is generally referred to as pre-expanded.
The bulk density of the expanded particles is preferably in the range of 0.01 to 0.04 g / cm ³ . When the bulk density of the expanded particles is less than 0.01 g / cm ³ , shrinkage may occur in the foamed molded product to be obtained next, and the appearance may be deteriorated. In addition, the heat insulation performance and mechanical strength of the foamed molded product may deteriorate. On the other hand, when the bulk density is larger than 0.04 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.

(Foamed molded product)
The foamed molded product can be used, for example, as a packaging material for fish, agricultural products, etc., a heat insulating material for floor insulation, a banking material, a tatami core material, and the like. According to the present invention, since the aging step of the expanded particles is not required, the productivity of the expanded molded body can be improved.

The foam molded article can be obtained, for example, by the following method.
By filling the expanded particles in a closed mold having a large number of small holes and heating and foaming with a heat medium (for example, pressurized water vapor) to fill the voids between the expanded particles and fuse the expanded particles to each other A foamed molded product can be produced by integrating the components. 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.

  Heat 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. In the present invention, heating foaming is performed under a pressure of the heat medium forming vapor pressure (gauge pressure) of 0.03 to 0.05 MPa, which is lower than a general vapor pressure (for example, 0.06 to 0.08 MPa). Can do. Therefore, a foamed molded product can be produced with a small amount of steam.

  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 expandable particles is measured from the surface layer of the foam molded article. That is, since the foam molded body is obtained by pre-foaming foamable particles and molding in-mold, the foamable particle surface layer corresponds to the foam molded body surface layer, and the average molecular weight of the foamable particle surface layer is the foam molded body surface layer. It corresponds to the average molecular weight. The total particle weight average molecular weight of the expandable particles is measured from the expandable particles themselves.
After the foam molded body having a density of 0.0166 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 Co., Ltd .: FK-18N type). A surface GPC measurement sample is used. The whole GPC measurement sample uses expandable particles themselves.
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 center part of the expandable particles and the 50% part of the radius is measured as follows.
About 100 randomly selected particles, a particle cross-sectional analysis is performed by an infrared spectroscopic analysis ATR measurement method 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.
Absorbance D1735 and D1600 were measured under the following conditions using a measuring device sold under the trade name “Fourier Transform Infrared Spectrometer MAGNA 560” from Nicolet and “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: Infrared absorption spectrum obtained by measuring barium fluoride crystals in the sample-free area near the sample is used as a background, and processing that is not related to the measurement spectrum is performed

The absorbance D1735 at 1735 cm ⁻¹ obtained from the infrared absorption spectrum is the absorbance corresponding to the absorption spectrum derived from the stretching vibration between C═O of the ester group 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 styrene 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: Transparent 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)

<Density of foam molding>
The mass (a) and volume (b) of a test piece (example 75 × 300 × 30 mm) cut out from a foamed molded product (after being molded and dried at 40 ° C. for 20 hours or more) each have three or more significant figures. Then, the density (g / cm ³ ) of the foamed molded product is obtained by the formula (a) / (b).

<Heating time during foaming>
The foamable particles are put into a foaming machine preheated to 100 ° C., and heated at 180 ° C. (0.033 liter / (the above expandable styrene resin particles 1 g) / second, 50 g / m ^3). The time when the following amount of water vapor is fed is set as the starting point, and the time until the desired bulk density (0.0166 g / cm ³ ) is reached is set as the heating time.

(Example 1)
Into a polymerization vessel equipped with a stirrer with an internal volume of 100 liters, 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 are fed and stirred. 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 styrene-type resin particle (a).
The styrene resin particles (a) were sieved to obtain styrene resin particles (b) having a particle diameter of 0.5 to 0.71 mm as seed particles.

Next, 2350 g of seed particles of styrene resin (average particle diameter 0.63 mm; b) having a weight average molecular weight of 250,000, 30 g of magnesium pyrophosphate, and sodium dodecylbenzenesulfonate were placed in a polymerization vessel equipped with a stirrer with an internal volume of 25 L. A dispersion was prepared by supplying 10 g and heating to 72 ° C. while stirring.
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 polymerization 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 completion of the third step, the dispersion was further heated to 120 ° C. and held for 60 minutes to polymerize unreacted monomers. Subsequently, styrene resin particles having an average particle diameter of 1.0 mm were collected from the reaction vessel.
Next, the dispersion liquid is kept at 100 ° C., and then, 120 g of cyclohexane, 80 g of diisobutyl adipate and 1200 g of isobutane are pressed into the polymerization vessel and held for 6 hours, whereby normal butane is contained in the resin particles. Impregnated. Thereafter, the inside of the polymerization vessel was cooled to 25 ° C. to obtain expandable particles. The expandable particles seek and absorbance D1600 at absorbance D1735 and 1600 cm ^-1 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 the 50% radius portion was 0.51. Further, the Mw of the entire expandable particle was 24,000.
FIG. 2 shows the change in the absorbance ratio from the center of the obtained expandable particles (particles having a diameter of about 900 μm) to the surface layer.

On the surface of the expandable particles, 0.05 part by mass of polyethylene glycol as an antistatic agent was applied to 100 parts by mass of the expandable particles. Thereafter, 0.15 parts by mass of zinc stearate and 0.08 parts by mass of 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.
Then, using a foaming machine with an internal volume of 70 liters, expand the jacket temperature of the foaming machine in a state preheated at 100 ° C., and foam with heated air at 180 ° C. (water vapor amount of 50 g / m ³ or less). I let you. Pre-foamed particles were obtained by heating and pre-foaming to a bulk density of 0.0166 g / cm ³ . The heating time was 2 minutes and 45 seconds.
Next, the pre-expanded particles immediately after foaming were filled in a mold and heated and foamed at a vapor pressure of 0.04 MPa to obtain a foamed molded product having a length of 400 mm × width of 300 mm × thickness of 30 mm. After the foamed molded product was dried in a drying chamber at 50 ° C. for 6 hours, the density was measured and found to be 0.0166 g / cm ³ (16.6 kg / m ³ ). The foamed molded article was excellent in appearance without shrinkage. Mw of the surface layer of the foamed molded product was 281,000.

(Example 2)
A foamed molded article was obtained in the same manner as in Example 1 except that the amount of divinylbenzene added was 1.2 g. The absorbance ratio at the center of the expandable particle is 0.71, the absorbance ratio at the 50% radius portion is 0.50, the overall Mw is 26,000,000, and the Mw of the foam molded article surface layer is 50. The heating time was 3 minutes and 5 seconds.
(Comparative Example 1)
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. The Mw of the entire expandable particle was 243,000, and the Mw of the foamed molded product surface layer was 333,000. The heating time during foaming was inferior at 6 minutes.

(Comparative Example 2)
A foamed molded article was obtained in the same manner as in Example 1 except that divinylbenzene was not used in the third polymerization step. The absorbance ratio of the center part of the expandable particle is 0.69, the absorbance ratio of the 50% radius part is 0.50, the overall Mw is 23.9 million, and the Mw of the foam molded article surface layer is 23. The heating time was 2 minutes and 31 seconds, but bonding occurred during foaming.
The results of Examples and Comparative Examples are summarized in Table 1.

  From Table 1, a foaming property in which a resin component derived from a monofunctional acrylate ester is contained in a large amount in the center of the styrene resin particle and a resin component derived from a polyfunctional vinyl monomer is contained in a surface layer of the styrene resin particle in a large amount. Even when the particles are produced using a heating medium having a low heat amount substantially free of water vapor, it is possible to obtain expandable particles in which mutual bonding is suppressed. Further, the foamed particles can be used in the foam molding process without aging.

Claims (5)

A foamable styrene resin particle comprising a resin component derived from a monofunctional acrylic ester, a resin component derived from a polyfunctional vinyl monomer, and a foaming agent,
The resin component derived from the monofunctional acrylic ester is contained in a large amount in the center of the expandable styrene resin particles,
The resin component derived from the polyfunctional vinyl monomer is often contained in the surface layer of the expandable styrene resin particles,
The expandable styrenic resin particles are obtained by a heating medium at 180 ° C. (however, the amount of water vapor is 50 g / m ³ or less) at a flow rate of 0.033 liters / (the expandable styrene resin particles 1 g) / second. Expandable styrenic resin particles characterized by having a low calorific foaming property in which the time for obtaining the foamed particles is 300 seconds or less when foamed into expanded particles having a bulk density of 0166 g / cm ³ .   2. The expandable styrene resin particles according to claim 1, wherein the monofunctional acrylic acid ester is a monomer having 3 to 20 carbon atoms, and the polyfunctional vinyl monomer is a monomer having 2 to 15 vinyl groups. .   Expanded particles obtained by expanding the expandable styrenic resin particles according to claim 1 or 2.   A foam-molded product obtained by foam-molding the foamed particles according to claim 3. A method of obtaining foamed particles by foaming the expandable styrenic resin particles according to claim 1,
The foaming is a method for producing foamed particles, which is performed using a heating medium having a water vapor amount of 50 g / m ³ or less.