JP2731214B2 - Method for producing hollow polymer emulsion - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement in a method for producing a hollow polymer emulsion. More specifically, the present invention
Efficiently produce aqueous emulsions containing hollow polymer particles that are lightweight, scatter well visible light, and have good opacity, for example, useful as pigment components for paper coating and pigment components for various coatings. It is about the method.

2. Description of the Related Art Conventionally, organic pigments are widely used as pigment components for paper coating and pigment components for various paints, for example, since they have characteristics of being lightweight and thermoplastic compared to inorganic pigments. Have been. And in such applications, for the purpose of further improving the lightness and opacity,
In recent years, hollowing out the inside of particles has been performed. In addition, particles whose diameter is on the order of the wavelength of visible light can scatter visible light well, and by hollowing out the interior, it can scatter visible light more efficiently and improve opacity. It has been known.

As a method for producing such hollow particles, for example, (1) a volatile alkali such as ammonia is allowed to act on polymer particles comprising an alkali-soluble core and a sheath covering the core to swell the core. A method of obtaining hollow particles by expanding (JP-A-56-32513), (2) polymerizing a monomer mixture having a solubility parameter different from the polymer by 0.1 or more in the presence of predetermined polymer particles; To obtain hollow particles (JP-A-61-62510),
(3) By polymerizing the monomer mixture in the presence of a non-polymerizable solvent, polymer particles containing a solvent are obtained by utilizing the fact that the solvent undergoes phase separation inside the polymer particles. Method for producing hollow particles by distillation (JP-B-47-25862)
JP-A-61-66710, JP-A-62-127336, and JP-A-63-135409).

However, in the method (1), since a relatively large amount of ammonia or the like must be used, there is a problem that an irritating odor is generated and the working environment is deteriorated. In addition, the permeability of volatile alkali and the extensibility of the sheath polymer are increased. It is necessary to design a polymer in consideration of such factors, and there is a drawback that the operation is inevitably complicated. In order to improve such disadvantages, there are several methods (JP-A-60-69103, JP-A-60-69103 and JP-A-61-69103).
-185505) have been proposed, but none of them has obtained satisfactory results.

Further, in the method (2), although not clear, the hollow particles are hollowed out due to a change in the density when the monomer is converted into a polymer. It has the fatal drawback of having certain limitations.

On the other hand, in the method (3), although the size of the hollow hole can be controlled by the amount of the solvent with respect to the polymer monomer, if the hollow hole is to be enlarged, a large amount of solvent corresponding to the amount is required. However, there is a disadvantage that the production process is complicated and the production cost is inevitably increased by distilling off the solvent.

On the other hand, utilizing the volume expansion when the liquid is vaporized, to create pores inside the particles, as a technology applying a so-called physical foaming method, 0.05 to 10μ containing a volatile organic compound
m foamable fine resin particles are known (JP-A-60-25)
No. 2635). However, the expandable fine resin particles are for producing particles having a homogeneous cell structure, that is, a multi-cell structure, and provide polymer particles having a single hollow pore for the purpose of the present invention. is not. Also known is a method for producing thermoplastic resin polymer particles of 1 to 50 μm having a liquid foaming agent (Japanese Patent Publication No. Sho 42-26524). It is difficult to produce coalesced particles in good yield, and
Simply heating makes it difficult to uniformly form polymer particles of 1 μm or less.

Problems to be Solved by the Invention The present invention overcomes the drawbacks of such a conventional method for producing hollow polymer particles, and provides a hollow polymer particle having a particle size in a range that is lightweight and effective for scattering light. An object of the present invention is to provide a method capable of efficiently producing an aqueous emulsion containing the compound without any troubles due to evaporation of a large amount of solvent or generation of an odor, and without the problem of difficulty in controlling the hollow holes. It is a thing.

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, the physical foaming method has set the size of the hollow hole by appropriately setting the amount of the liquid to be vaporized. Focusing on the fact that it can be controlled arbitrarily, an aqueous emulsion containing expandable polymer particles having a specific particle size obtained by adding a physical blowing agent to specific core / shell type polymer particles is subjected to the physical foaming. After heating to above the boiling point of the agent, it was found that the object could be achieved by shifting to a low pressure range, and the present invention was completed based on this finding.

That is, the present invention relates to a polymerizable monomer containing 1 to 75% by weight of a core portion of a substantially non-crosslinked polymer obtained by polymerizing a polymerizable monomer and 0.3% by weight or more of a crosslinkable monomer. 99 to 25% by weight of a shell portion of a polymer obtained by polymerizing the mixture
Weight average particle diameter of core / shell type polymer particles composed of: and a physical foaming agent impregnated therein.
A method for producing a hollow polymer emulsion, comprising heating an aqueous emulsion containing foamable polymer particles of μm above the boiling point of the physical foaming agent, and then transferring this to a low pressure region. It is.

 Hereinafter, the present invention will be described in detail.

The core / shell weight ratio of the core / shell type polymer particles used in the composition of the present invention is from 1:99 to 7
5:25, preferably 2:98 to 50:50, more preferably 10:
Must be in the range 90 to 43:57. Assuming that both the core and the shell of the polymer particles are truly spherical, the core particle size is usually in the range of 0.01 to 0.9 μm. When the ratio of the core is less than the above range, it is difficult to form hollow holes, and when it is increased, physical destruction occurs at the time of foaming,
Furthermore, the mechanical strength of the hollow particles eventually becomes poor, which is not preferable.

The expandable polymer particles in the method of the present invention are usually obtained by impregnating 1 to 300 parts by weight of a physical blowing agent with respect to 100 parts by weight of the core / shell type polymer particles, and have a weight average particle diameter of Is 0.05 to 1.0 μm, preferably 0.1 to 0.8 μm
m. If the particle diameter is out of the above range, the optical properties and the flow properties deteriorate, which is disadvantageous when used as a pigment or the like.

The physical foaming agent is not particularly limited, and those conventionally used in the production of plastic foams can be used, but those having a boiling point at atmospheric pressure of 150 ° C. or less are preferred. As such, aliphatic hydrocarbons such as ethane, ethylene, propene, propane, butane, butadiene, pentane, hexane, heptane, cyclohexane, isobutene, neopentane,
Chlorinated hydrocarbons such as methyl chloride, methylene chloride, ethyl chloride, chloroform, carbon tetrachloride, trichloroethylene, trichlorofluoromethane, dichlorodifluoromethane, dichlorofluoromethane, chlorodifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, etc. Examples include fluorinated chlorinated hydrocarbons, aromatic hydrocarbons such as toluene and xylene, and volatile monomers such as butadiene, methyl methacrylate, and styrene. Among them, those having a boiling point at atmospheric pressure of 95 ° C. or lower are preferable, and those having a boiling point of 90 ° C. or lower are more preferable. Each of these may be used alone or in combination of two or more. The type and amount used are, for example, the particle size of the polymer particles, the composition and molecular weight of the polymers (A) and (B), the proportion of the toluene-insoluble component described later, and the target polymer particles having hollow holes. It may be appropriately selected depending on the outer diameter, the size of the hollow hole, and the like. Next, the method for producing the hollow polymer emulsion will be described. In the present invention, first, the core / shell type polymer particles are impregnated with the physical blowing agent and have a weight average particle diameter of 0.05 to 1.0 μm. An aqueous emulsion containing polymer particles is prepared. As a method for preparing this emulsion, for example, a method of first producing core / shell type polymer particles and then impregnating the polymer particles with a physical blowing agent, or a method of producing the core / shell type polymer particles In addition, a method of coexisting a physical foaming agent at the time of producing a polymer forming a core or a polymer forming a shell can be used.

The order of producing the core and the shell is not particularly limited, but a method of producing the polymer forming the core and then producing the polymer forming the shell is generally used. However, in some cases, the polymer forming the shell is first produced, and then the polymer forming the core is produced,
It can also be produced by a so-called phase inversion method (phase inversion method). However, in the present invention, since the polymer forming the shell has a crosslinked structure, it is preferable to produce the core and the shell in this order rather than the phase inversion method.

Next, as a specific example, a case where the core and the shell are manufactured in this order will be described. First, the polymer (A) forming the core is manufactured by an emulsion polymerization method. That is, in an aqueous medium, various additives conventionally used in emulsion polymerization, for example, a surfactant, a dispersant, a colloid protective agent, a polymerization initiator,
This is carried out by adding 1 to 75 parts by weight of a polymerizable monomer to the seed particles and heating the mixture while stirring.
In this case, if necessary, the reaction may be carried out in the presence of a physical foaming agent. As a method of coexisting a physical foaming agent, a method of using seed particles which have been swollen in advance with a physical foaming agent, a method of mixing the seed particles with a polymerizable monomer, and subjecting to emulsion polymerization are employed.

Examples of the polymer monomer used for producing the polymer (A) forming the core include aromatic vinyl compounds, acrylates or methacrylates, vinyl esters,
At least one selected from vinyl ethers and other unsaturated monomers is used. Examples of the aromatic vinyl compound include styrene, o-methylstyrene,
m-methylstyrene, p-methylstyrene, ethylstyrene, ar-vinylxylene, ar-chlorostyrene, ar
Styrene derivatives such as -bromostyrene, vinylbenzyl chloride, p-tert-butylstyrene, and acrylates or methacrylates such as methyl-, ethyl-, propyl-, n-butyl, isobutyl-, tert-butyl-, n Each acrylate or methacrylate such as -amyl-, isoamylhexyl-, octyl-, nonyl-, decyl-, dodecyl-, octadecyl-, cyclohexyl-, phenyl-, benzyl-, etc., and vinyl esters such as vinyl acetate and vinyl Butyrate, vinyl stearate, vinyl laurate, vinyl myristate, vinyl propionate, vinyl versatate, and the like.Examples of vinyl ethers include alkyl groups such as methyl, ethyl, propyl, butyl, amyl, and hexyl. That vinyl ethers, and the like.
Further, as other unsaturated monomers, for example, vinyl cyanides such as acrylonitrile and methacrylonitrile, vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene fluoride, vinyl halides such as vinylidene chloride,
Maleic acid, fumaric acid, unsaturated dibasic acid alkyl esters such as monoalkyl esters such as itaconic acid, olefins such as ethylene, glycidyl compounds such as glycidyl (meth) acrylate and methyl glycidyl (meth) acrylate, Amides such as acrylamide and methacrylamide and their N-methylol compounds and alkoxy compounds, and silicon-containing α and β such as vinyltrichlorosilane and vinyltriethoxysilane.
-Ethylenically unsaturated monomers, hydroxyl-containing α, β-unsaturated monomers such as β-hydroxyacrylate and β-hydroxymethacrylate, acrylic acid, methacrylic acid, itaconic acid, citraconic acid, maleic acid, fumaric acid, Α, β-ethylenically unsaturated carboxylic acids such as vinylbenzoic acid and cinnamic acid, unsaturated acids such as vinylsulfonic acid and styrenesulfonic acid, vinylpyridine, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, Basic monomers such as diethylaminoethyl methacrylate can be exemplified.

Each of these monomers may be used alone, or 2
A combination of more than one species may be used. Further, since the polymer (A) forming the core is a substantially non-crosslinked polymer, it is not usually necessary to use a crosslinkable monomer. However, even if a crosslinkable monomer unit is used as desired, it is possible to produce a substantially non-crosslinked polymer (A) by suppressing crosslinking by using an appropriate amount of a chain transfer agent in the polymerization. I can do it. For example, for the purpose of imparting flexibility, when a diene monomer is blended and polymerized as it is, usually a crosslinking reaction also proceeds, but by using a considerable amount of a chain transfer agent, substantially non-crosslinked. A polymer having flexibility can be produced. Examples of such monomers include butadiene, isoprene, chloroprene, and the like.

When polymerizing the core, it is recommended to carry out emulsion polymerization in the presence of a conventionally known chain transfer agent in order to reduce the molecular weight of the polymer. These chain transfer agents are not particularly limited, and may be appropriately selected from those commonly used for controlling the molecular weight of a usual polymerization reaction. The chain transfer constant of this chain transfer agent depends on the type of monomer used, polymerization conditions, and the like. However, even a commonly used organic solvent having a high chain transfer constant has a high chain transfer constant. It can be used as a transfer agent. This chain transfer constant is
Judging based on the polymerization of styrene, 0.5 × 10 ⁻⁴ or more is appropriate. Some of these chain transfer agents are described in "Polymer Handbook", (John
Wheely and Sons, Inc.). Examples of the chain transfer agent include mercaptans having an alkyl group having 1 to 30 carbon atoms, such as propylmethyl mercaptan, butyl mercaptan, and dodecyl mercaptan, and octyl thioglycolate, thioglycolic acid, and diphenyl sulfide.
Examples include 30 organic sulfur compounds, halogenated hydrocarbons having 1 to 20 carbon atoms such as carbon tetrachloride, carbon tetrabromide, and bromotrichloromethane. These chain transfer agents may be used alone or in combination of two or more.

The emulsion polymerization in the present invention can be carried out in the presence of a conventionally known surfactant. As the surfactant, for example, anionic, cationic, nonionic, amphoteric, reactive surfactant, oligosoap, and the like are used. These surfactants may be used alone or in combination of two or more. Usually, anionic and nonionic surfactants are generally used. As the anionic surfactant, for example, an alkali metal salt of alkyl benzene sulfonic acid, an alkali metal salt of alkyl sulfate, an alkali metal salt of polyoxyethylene alkyl phenol sulfate, an alkali metal salt of alkyl diphenyl ether disulfonate, an alkali metal salt of dialkyl sulfosuccinic acid and the like are used. Can be. As the nonionic surfactant, for example, polyoxyethylene alkylphenol ether, polyoxyethylene alkyl ether, polyoxyethylene higher fatty acid ester, ethylene oxide-propylene oxide block copolymer and the like can be used. Further, an acrylic water-soluble oligomer may be used in combination with the anionic surfactant. Further, a water-soluble polymer compound having an action as a colloid protective agent such as polyvinyl alcohol, hydroxyethylcellulose, hydroxypropylcellulose and the like can be used if necessary. As these surfactants and water-soluble polymer compounds increase in use amount, the water resistance of the obtained polymer may decrease, so that the use amount is preferably small, but at least the polymerization stability is low. It is preferable to use the minimum amount necessary to maintain the mechanical stability and the chemical stability of the product. This usage is
Usually, it is selected in the range of 0.1 to 7 parts by weight based on 100 parts by weight of the monomer.

As the polymerization initiator, a compound that generates a radical at a temperature of about 0 to 150 ° C is used. As the polymerization initiator, a water-soluble one is mainly used, but an oil-soluble one may be used. Representative polymerization initiators include ammonium persulfate, sodium persulfate, water-soluble persulfates such as potassium persulfate, inorganic peroxides such as hydrogen peroxide, benzoyl peroxide, cumene hydroperoxide, lauroyl peroxide, Organic peroxides such as tert-butyl peroxide and tert-butylperoxy-2-ethylhexanoate; and azo compounds such as azobisisobutyronitrile are exemplified. In addition, a so-called redox-based initiator that combines the peroxide with a reducing agent to generate a radical in the presence of a trace amount of metal ion can also be used. Examples of this redox type include hydrogen peroxide-ferrous chloride type, cumene hydroperoxide-sodium ascorbate type and the like. The amount of the polymerization initiator to be used is usually selected in the range of 0.1 to 2.5 parts by weight based on 100 parts by weight of the total amount of the monomers. Further, the polymerization initiator may be used alone or in combination of two or more.

In the present invention, the various monomers may be charged all at once into the polymerization reactor, or may be divided stepwise,
Or you may add continuously. Depending on the manner in which such a monomer is added, the polymer may form a layered structure or the composition may change continuously.

The polymerization reaction is usually carried out at a temperature in the range of 0 to 130 ° C., preferably 30 to 100 ° C., under conditions in which a sufficient amount of radicals are supplied from the polymerization initiator system to carry out the polymerization reaction. The polymerization time is not particularly limited, and is usually from 1 to 24.
Hours, preferably 1 to 6 hours.

It is necessary that the polymer (A) forming the core in the present invention is substantially non-crosslinked. Substantially cross-linking results in incomplete formation of hollow holes. The degree of crosslinking of the polymer is
It can be related to the swelling index or the solvent-insoluble matter. In general, since the latter value for measuring the weight of the dry gel is more reproducible than the former value for measuring the weight of the wet gel, it is convenient to express the degree of cross-linking with toluene-insoluble matter for convenience. is there. In the present invention, "substantially non-crosslinked" means that the toluene-insoluble matter is less than 15% by weight. The molecular weight of the polymer (A) forming the core is preferably low, particularly preferably 190,000 or less. This molecular weight can be measured by a viscosity method described later.

On the other hand, the polymer (B) forming the shell is prepared by mixing a monomer mixture containing at least 0.3% by weight of a crosslinkable monomer in the presence of the core and, if necessary, in the presence of a physical blowing agent.
It can be produced by the same emulsion polymerization method as for the polymer (A) forming the core. The polymerizable monomer used at this time can be appropriately selected and used from those exemplified as the polymerizable monomer for producing the polymer (A) forming the core. You. Examples of the crosslinkable monomer used in combination with the polymerizable monomer include compounds having at least two polymerizable unsaturated bonds in the molecule. Examples of such a crosslinkable monomer include ethylene glycol. Dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate, propylene glycol dimethacrylate, divinylbenzene, diallyl phthalate, ethylene glycol acrylate, 1,3-butylene glycol diacrylate, bis (4-acrylic Roxypolyethoxyphenyl) propane, 1,5-pentanediol diacrylate, neopentyl glycol diacrylate, 1,6-
Hexanediol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, tetraethylene glycol diacrylate, other polyethylene glycol diacrylate, polypropylene glycol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, tetramethylol methane tetraacrylate,
Allyl methacrylate and the like can be mentioned. In addition, diene monomers such as butadiene, chloroprene and isoprene, and polyene monomers having three or more double bonds such as trivinylbenzene and triallyl isocyanurate are, of course, crosslinkable monomers. Can be used.
These crosslinkable monomers may be used alone or in combination of two or more. The crosslinkable monomer is at least 0.3% by weight based on the total amount of monomers forming the shell.
Or more, preferably 0.3 to 30% by weight, more preferably 0.5 to 30% by weight.
Used at a rate of 10% by weight. If the amount is less than 0.3% by weight, a sufficiently large hollow hole is not formed at the time of foaming, and if the amount is too large, it becomes difficult to make the physical foaming agent hollow by vaporization.

The amount of the monomer forming the shell is selected in the range of 25 to 99% by weight based on the total amount of the co-existing core polymer (A). Examples of the method of using the physical foaming agent in the emulsion polymerization include, for example, a method using a swelling core impregnated with the physical foaming agent, mixing a monomer and a physical foaming agent, and reacting the mixture with the emulsion polymerization. A method of providing the system can be adopted. Furthermore, if the desired amount of physical blowing agent is included during core production, no additional physical blowing agent needs to be added during shell formation. As specific conditions for the emulsion polymerization, the conditions for producing the core can be employed as they are.

By setting the degree of crosslinking of the shell polymer (B) in an appropriate range, the size of the hollow pores and the mechanical properties of the hollow polymer particles can be controlled. As a guide of a preferable range of the degree of crosslinking, it can be conveniently expressed by the toluene-insoluble content of the core / shell type polymer particles instead of the shell polymer (B), and this value is preferably 15% by weight or more. preferable. If the toluene-insoluble content is less than 15% by weight, the effect of trapping the physical foaming agent when it is vaporized becomes insufficient, so that the gas escapes and the pore diameter does not grow.

In the production of the shell polymer (B), it is necessary to form a substantially cross-linked polymer. Therefore, it is preferable not to use a chain transfer agent that suppresses cross-linking. However, a small amount of a chain transfer agent may be used, if desired, for the purpose of improving the rate of volume increase under mild conditions.

The solid content concentration of the foamable polymer emulsion in the present invention is not particularly limited, but is generally selected from the range of 5 to 70% by weight from the viewpoints of economy, stability and fluidity. Especially in terms of foaming with a physical foaming agent, 10
A range of ~ 55% by weight is preferred.

The core / shell type particles in the present invention may be cationic, anionic or amphoteric in terms of colloidal stability, and are appropriately selected according to the application.
Such a colloidal property can be imparted by using a surfactant or a polymerization initiator having a corresponding property, but can be easily achieved by appropriately selecting a polymerizable monomer. Is done. That is, when it is desired to impart cationicity, a basic monomer is used.When it is desired to provide anionic property, a monomer having a carboxy group or a sulfonic acid group is used. It may be used together.

In the present invention, the polymers (A) and (B) generally have different solubility parameters. In general emulsion polymerization, the larger the solubility parameter, that is, the higher the polarity, the higher the probability of phase separation on the particle surface. Therefore, when determining the production order of (A) and (B), in addition to the weight ratio of the polymer, the presence or absence of the crosslinking agent,
It is desirable to determine in consideration of such points.

The foamable polymer emulsion according to the present invention can be obtained by emulsion polymerization in the presence of a physical foaming agent during the production of the core or shell as described above, but after producing the core / shell type polymer particles. Can also be obtained by absorbing a physical blowing agent. In such a case, a method is generally employed in which a physical foaming agent is added to the polymer emulsion and absorbed while stirring at a predetermined temperature and pressure.

The operation method is not particularly limited, and after mixing a physical foaming agent with the polymer emulsion, heating and pressurizing may be performed, or those in a heated and pressurized state may be mixed. As a method of mixing the polymer emulsion and the physical foaming agent, the physical foaming agent may be added as it is, or may be added after being dispersed in water. In this case, in order to more efficiently absorb the water dispersion, using an ultrasonic homogenizer, a homomixer, or the like, the dispersion diameter of the physical foaming agent is previously reduced to a particle diameter of 4 times or less of the polymer particles,
Preferably, it is advantageous to set it to 1 or less. On this occasion,
Surfactants may be added to stabilize the particles of the physical blowing agent. In addition, in order to prevent early coalescence between oil droplets of a physical foaming agent dispersed in water and to further stabilize the compound, a poorly water-soluble compound having a solubility in water of 0.1% by weight or less is prepared in advance by a physical method. It is advisable to add a small amount to the foaming agent. It is also effective to mix the polymer emulsion and the physical blowing agent and then subject the mixture to ultrasonic treatment. In this case, the obstacle of lowering the solid content is eliminated.

The temperature at which the physical foaming agent is impregnated is not particularly limited, but is usually in the range of 30 to 150 ° C, preferably 30 to 130 ° C. However, as long as the absorption rate is not impaired, a lower temperature is advantageous because fusion and aggregation of the polymer particles do not occur. In addition, when foaming is performed immediately after absorption of a physical foaming agent, the foaming temperature is convenient. There is no particular limitation on the time for absorption.
The time required for a sufficient amount of the physical blowing agent to penetrate into the polymer particles of the polymer emulsion depends on the type of the physical blowing agent, the solubility and the diameter of oil droplets, the composition and the particle size of the polymer particles. It depends on temperature, pressure, etc., and it takes about several minutes for easy absorption and several tens of hours for poor absorption, but it can usually be absorbed in about 20 minutes to 5 hours. It is not preferable to make it longer than necessary. There is no particular limitation on the absorption pressure. The pressure varies depending on the type of the physical foaming agent to be used, depending on the absorption temperature and the like. In consideration of foaming, if desired, the pressure may be increased by an inert gas or the mixture itself may be increased.

As the physical foaming agent, one having a certain degree of compatibility with the polymer particles of the polymer emulsion is used, and the balance between ease of penetration into the polymer particles and swelling of the polymer particles is improved. It is advantageous from the point of view. That is, when the compatibility is extremely small, since the physical blowing agent hardly penetrates into the polymer particles, the power to expand the polymer particles tends to be inferior, while when the compatibility is too high, However, the vaporization power for expanding the particles tends to be low.

It is advantageous to distribute the physical blowing agent at a higher concentration in the core polymer (A) than in the shell polymer (B). From this viewpoint, in the present invention, the polymer (A) is substantially non-crosslinked, and the polymer (B) needs to be crosslinked. Further, (A) preferably has a physical foaming agent and a solubility parameter closer to that of (B). In (A) and (B), as described above, the larger the solubility parameter is, the more likely it is to appear on the surface. Therefore, by using a physical foaming agent having a small solubility parameter, the swelling property with respect to the core can be reduced. It is common to take the technique of raising.

The amount of the physical blowing agent in the present invention is selected in the range of 1 to 300 parts by weight, preferably 5 to 250 parts by weight, per 100 parts by weight of the core / shell type polymer particles. When the amount is less than 1 part by weight, the pressure at the time of gasification is low, the polymer particles hardly expand, and when the amount exceeds 300 parts by weight, the polymer particles temporarily expand well immediately after foaming, Depending on the temperature and pressure at which the polymer particles are placed, or the type of physical blowing agent, they often deform unevenly and deform, or conversely shrink. The hollow diameter of the particles cannot be increased.

Next, a method for producing the hollow polymer particles by foaming the foamable polymer emulsion will be described. The temperature and pressure of the emulsion have a significant effect on the degree of foaming. This temperature must be above the boiling point of the physical blowing agent. At temperatures lower than this, almost no foamed particles can be obtained. At this temperature, the particles, especially the shell polymer, must have sufficient deformation fluidity, but also have low gas permeability to the extent that foaming pressure can be linked to particle enlargement. It is important to have a tensile strength that resists pressure. Therefore, the higher the temperature of the emulsion is, the better it is not. It is preferable that the temperature is not more than 50 ° C. higher than the boiling point of the physical blowing agent. The pressure of the emulsion is not particularly limited. However, since the emulsion is heated to a temperature equal to or higher than the boiling point of the physical foaming agent, it usually exceeds its atmospheric pressure even by its own pressure. Although this self-pressure may be used, particles having a higher foaming rate are more easily obtained when the pressure is maintained at a higher pressure. In addition, since the pressure is closely related to the transition speed when shifting to the low pressure side, it is necessary to select from this viewpoint. Normally used pressure is 1 to 50 kg / cm ² (gauge pressure).

The heated and pressurized polymer emulsion is transferred to a low pressure region to expand the polymer particles. The atmosphere in the low pressure region may be a gas or a liquid. When it is a gas, it is preferably an inert gas such as nitrogen, and when it is a liquid, it is preferably water.
The temperature in the low pressure range is appropriately selected depending on the composition of the polymer particles, the type and amount of the physical blowing agent contained in the particles, and is usually equal to or lower than the temperature of the polymer emulsion.
However, when the polymer particles absorb a large amount of the physical blowing agent, the physical blowing agent remains considerably in the polymer particles even after the migration, so that the polymer emulsion is further vaporized. Often, the temperature is shifted to a low pressure region or higher. When most of the physical blowing agent has been removed and the pressure inside the particles can no longer be maintained high, the temperature of the emulsion is preferably below the glass transition temperature of the polymer particles. The removal of the remaining physical blowing agent is advantageously performed under reduced pressure.

In addition, the transfer of the polymer emulsion from the high-pressure region to the low-pressure region may be performed by reducing the pressure of the container as it is. It can be done from the transfer port.

The foamed particles obtained in this way have an original volume of 20 to
It expands at a volume increase rate of 1000%, forming an almost single hollow hole. In the method of the present invention, the mechanism by which the hollow pores are formed inside the polymer particles is not necessarily clear,
It is considered that when the substantially non-crosslinked polymer foams, the generated minute gas concentrates near the core, thereby forming a hollow inside the particle. At this time, if the polymer particles are composed of a homogenous polymer instead of a core / shell type structure, a foam may be formed, and sometimes a vaporized physical foaming agent may escape to the outside as it is, resulting in a hollow hole. Cannot be formed. In the present invention, since the core has a non-crosslinked core and a crosslinked shell structure, the core has better fluid deformability than the shell, and the shell of the crosslinked structure temporarily suppresses the escape of gas. It is considered that a hollow hole is formed in the hole.

After the foaming, the hollow polymer particles may be used as an emulsion by removing the remaining physical foaming agent, or may be used as a powder through a spray drying step or the like.

Effects of the Invention According to the method of the present invention, the size of the hollow pores of the hollow polymer particles obtained by foaming, the composition of the polymer constituting the expandable polymer particles, the degree of crosslinking of the shell, the type of the physical blowing agent, Composition, amount, or temperature or pressure of the emulsion when transferred to the low pressure region, and further temperature and pressure in the low pressure region,
Since it can be easily controlled by the shape of nozzles and slits that pass at the time of transition, the passing speed, temperature gradient, pressure gradient, post-processing conditions, etc., a hollow weight that can respond to the demands of each application relatively freely The combined particles can be obtained. Further, the method of the present invention has a very large feature that the required amount of the organic liquid with respect to the volume of the formed hollow hole is smaller than that of a conventionally known method. In addition, there are few problems such as odor. The hollow polymer particles produced according to the present invention are lightweight and have a high ability to scatter visible light. , Can be used in place of some of them. In the papermaking field, an inorganic pigment such as talc is used as an internal additive, but the cationic hollow polymer particles obtained by the method of the present invention exhibit excellent performance in such fields, such as light weight and yield. Further, in order to improve the printability of paper and paperboard, smoothness, etc., so-called coated paper coated with a pigment on the surface has been produced.To further improve the performance, the hollow polymer particles are used. And the coated paper is excellent in gloss, whiteness, opacity, rigidity, lightness and the like. In addition, it can be used as an auxiliary for dyeing leather, or as an additive to adhesives, pressure-sensitive adhesives, primers, inks and the like.

EXAMPLES Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

 In addition, the measuring method and evaluation of each characteristic are as follows.

(1) Toluene-insoluble content The polymer emulsion is dried at a temperature higher than the glass transition temperature of the polymer, usually at 120 ° C for 90 minutes, and further at 160 ° C for 3 minutes. 50 ml of toluene was added to 1.00 g of the film thus obtained, and the mixture was shaken for 5 hours, followed by filtration with 325 mesh stainless steel. The solid is dried at 120 ° C. for 50 minutes and the weight (b) is measured. The toluene insoluble content is determined by the following equation.

Toluene-insoluble matter (% by weight) = 100b (2) Weight-average particle diameter From a transmission electron micrograph, a 25,000-fold photograph was projected and the diameter of 300 to 1000 particles was measured.

(3) A toluene-soluble solution of the polymer (A) having a number-average molecular weight of 0.1 to 0.8% by weight was prepared, and the intrinsic viscosity at 30 ° C. was measured using an Ubbelohde viscometer.

η = 11 × 10 ⁻³ [Mn] ^0.725 where η represents the intrinsic viscosity and Mn represents the number average molecular weight.

(4) Volume increase rate (ΔV) ΔV = 100 × (V2−V1) / V1 Here, V1 represents a particle volume before foaming, and V2 represents a particle volume after foaming.

Example 1 Into a separable flask having a capacity of 1, 100 parts by weight of distilled water, 0.2 part by weight of sodium dodecylbenzenesulfonate, 0.5 part by weight of potassium persulfate, and 0.1 part by weight of sodium bicarbonate were added and dissolved. Subsequently, 0.4 part by weight of a 350-g particle size styrene / acrylic acid copolymer latex made from 96 parts by weight of styrene and 4 parts by weight of acrylic acid was added, and the mixture was heated to 70 ° C. while stirring under a stream of nitrogen gas. A monomer mixture consisting of 78.6 parts by weight, 4 parts by weight of t-dodecyl mercaptan, 15 parts by weight of butyl acrylate and 2 parts by weight of methacrylic acid was fed to the separable flask in 5 hours, and polymerization was continued for 1 hour. Next, 0.2 parts by weight of potassium persulfate and 0.05 parts by weight of sodium bicarbonate were dissolved in 20 parts by weight of distilled water, and the resulting mixture was put into a polymerization vessel, and kept at 75 ° C. for 2 hours to obtain polymer particles having a particle size of 0.22 μm. Emulsion (I) was obtained.

The intrinsic viscosity of the emulsion-soluble portion of the polymer particles is 7.
It was 6 ml / g, and the number average molecular weight of the toluene-soluble component calculated by calculation was 8,250.

Next, in a separable flask, 100 parts by weight of distilled water,
0.15 parts by weight of sodium dodecylbenzenesulfonate, 0.5 parts by weight of potassium persulfate, 0.1 parts by weight of sodium bicarbonate,
The polymer emulsion (I) was added in an amount of 12.5 parts by weight in terms of solid content. The flask was replaced with nitrogen gas, and the temperature was raised to 70 ° C. while stirring. 3/5 of a monomer mixture consisting of 50.5 parts by weight of styrene, 7.0 parts by weight of acrylonitrile, 25 parts by weight of methyl methacrylate, and 4 parts by weight of methacrylic acid
The monomer feed was terminated by feeding the flask over time, at which point 1 part by weight of ethylene glycol dimethacrylate was added to the remaining mixed monomer and continuing to feed for an additional 2 hours. Thereafter, the temperature was maintained at 70 ° C. for 1 hour. Dissolve 0.2 parts by weight of potassium persulfate and 0.05 parts by weight of sodium bicarbonate in 20 parts by weight of distilled water,
This was put in a flask, and kept at 75 ° C. for 2 hours to prepare an emulsion of polymer particles having a particle size of 0.387 μm (I
I got. The solids content of this emulsion is 40% by weight
Met.

Next, 20 parts by weight of the polymer emulsion (II) thus obtained and 20 parts by weight of a mixture of pentane and cyclohexane (weight ratio 1/1) are placed in an autoclave, and are placed at 95 ° C. for 2 hours. I shook it. Next, after the temperature was raised to 130 ° C., the valve was opened, and spouted into warm water at 95 to 100 ° C., and stirring was continued for 30 minutes to remove most of the remaining volatile organic compounds. Concentration was carried out under bubbling and reduced pressure of 30 mmHg to obtain hollow polymer particles. Table 1 shows the results.

Examples 2 to 7 The procedure of Example 1 was repeated except that the composition of the monomer mixture used to produce the polymer emulsion (II) was changed as shown in Table 1. Polymer particles were prepared. Table 1 shows the results.

Comparative Examples 1 and 2 The procedure of Example 1 was repeated except that the composition of the monomer mixture used to produce the polymer emulsion (II) was changed as shown in Table 1. Polymer particles were prepared. Table 1 shows the results.

Example 8 Into a separable flask, 100 parts by weight of distilled water, 0.2 part by weight of sodium dodecylbenzenesulfonate, 0.5 part by weight of potassium persulfate, and 0.1 part by weight of sodium bicarbonate were added and dissolved. Next, a styrene / acrylic acid copolymer latex made from 96 parts by weight of styrene and 4 parts by weight of acrylic acid
Add 0.4 parts by weight and stir while stirring under a stream of nitrogen gas.
After the temperature was raised to 0 ° C., a monomer mixture comprising 77.6 parts by weight of styrene, 5.0 parts by weight of t-dodecyl mercaptan, 15 parts by weight of butyl acrylate, and 2 parts by weight of methacrylic acid was added to 5 parts by weight.
The mixture was fed to the separable flask over a period of time, and the polymerization was continued for another hour. Next, 0.2 parts by weight of potassium persulfate and 0.05 parts by weight of sodium bicarbonate are dissolved in 20 parts by weight of distilled water, and the resulting mixture is placed in a flask and kept at 75 ° C. for 2 hours to obtain polymer particles having a particle size of 0.22 μm. Emulsion (III) was obtained.

The solid viscosity of the polymer-soluble portion of the polymer particles is 6.9 ml /
g, and the number average molecular weight of the toluene-soluble component calculated by calculation was 7,220.

Polymer emulsion (III) thus obtained
Is used as a solid content of 12.5 parts by weight, and as monomers, styrene 58.5 parts by weight, acrylonitrile 5 parts by weight,
Using 15 parts by weight of methyl methacrylate, 4 parts by weight of methacrylic acid, and 5 parts by weight of ethylene glycol dimethacrylate, a polymer emulsion (IV) was produced in the same manner as in the step of producing the polymer emulsion (II) in Example 1, Further, hollow polymer particles were prepared in the same manner as in Example 1.
Table 2 shows the results.

Examples 9 to 12 Except that the monomer composition shown in Table 2 was used in the preparation of the polymer emulsion (III) in Example 8, and the amount of t-dodecyl mercaptan was changed as shown in Table 2 A polymer emulsion (III) was produced in the same manner as in Example 8, then a polymer emulsion (IV) was produced, and hollow polymer particles were prepared therefrom. Table 2 shows the results.

Examples 13 to 17, Comparative Examples 3 and 4 Seven kinds of polymer emulsions (V) having the same composition but different particle diameters were produced according to the production steps of the polymer emulsion (I) of Example 1. did. That is, first,
7.5 parts by weight, 2 parts by weight, and 0.5 parts by weight of the styrene / acrylic acid copolymer latex used in Example 1 were used as seed latex, respectively, and polymerized to give particle diameters of 0.060 μm, 0.144 μm, and 0.205, respectively. A μm polymer emulsion was prepared. At this time, the monomer mixture was composed of 79.5% by weight of styrene, 3.5% by weight of t-dodecyl mercaptan, 15% by weight of butyne acrylate,
What consisted of 2% by weight of methacrylic acid was used.

Furthermore, using the emulsion obtained with the particle diameter of 0.144 μm 11 parts by weight, 6.8 parts by weight, 6.2 parts by weight, 4.3 parts by weight as a seed latex, the particle diameter under the same monomer mixture and polymerization conditions as described above. 0.261μm each,
Larger particles of 0.299 μm, 0.310 μm and 0.337 μm were produced.

Using the seven types of polymer emulsions thus obtained, polymerization was carried out in accordance with the production process of the polymer emulsion (II) of Example 1 to produce a polymer emulsion. Incidentally, the composition of the monomer, styrene, acrylonitrile, methyl methacrylate, methacrylic acid, ethylene glycol dimethacrylate, 63/8 by weight, respectively
/ 20/4/5, and the weight ratio (A) / (B) of the polymer (A) and the polymer (B) is 0.00
The monomer mixture was fed in the presence of the polymer emulsion (V) so as to be 5, 0.07, 0.20, 0.40, 0.60, 0.70, 0.93 to produce a polymer emulsion (IV). However, in the polymerization in this step, the amount of the polymerization initiator and the feed time of the monomer mixture were adjusted in proportion to the amount of the monomer.

Using these polymer emulsions, hollow polymer particles were prepared in the same manner as in Example 1. Table 3 shows the results.

Comparative Examples 5 and 6 In the same manner as in Example 1, core / shell particles were produced using the polymer emulsion (I) and the monomer composition shown in Table 4, and the resulting particles were physically expanded. This was heated to 130 ° C. without subjecting the agent to absorption treatment, and then the valve of the autoclave was opened, spouted into warm water maintained at 95 to 100 ° C., and allowed to cool to room temperature. Thus, the particles shown in Table 4 were obtained, but none of them had a clear hollow hole.

Example 18 A single separable flask equipped with a stirrer, a thermometer and a reflux tube was charged with sodium dodecylbenzenesulfonate.
0.2 parts by weight and Newcol 506 (manufactured by Nippon Emulsifier Co., Ltd.)
100 parts by weight of deionized water in which 2 parts by weight were dissolved, and 1.0 part by weight of a charged styrene / acrylic acid copolymer latex as sheet particles were added, and the mixture was purged with nitrogen and heated to 80 ° C. A mixture of 80 parts by weight of styrene, 15 parts by weight of butyl acrylate, 2 parts by weight of methacrylic acid and 3.0 parts by weight of dodecyl mercaptan was added to the flask in 4.5 hours, 0.2 part by weight of sodium dodecylbenzenesulfonate, 0.2 part of Newcol 506 0.15 parts 20 parts by weight of deionized water in which 0.16 parts by weight of caustic soda and 0.8 parts by weight of sodium persulfate were dissolved were fed in 5.0 hours. Thereafter, the temperature was increased to 90 ° C. to complete the polymerization.
The particle size of the obtained polymer was 0.2 μm, and the solid content of the emulsion was 44% by weight.

Next, 65 parts by weight of deionized water obtained by dissolving 0.2 parts by weight of sodium dodecylbenzenesulfonate and 0.2 parts by weight of Newcol 506 in one separable flask equipped with a stirrer, a thermometer, and a reflux tube were obtained. 12.5 parts by weight of the prepared emulsion as a solid content, after nitrogen replacement,
Heated to 85 ° C. Into this flask, a mixed solution consisting of 53.4 parts by weight of styrene, 21.9 parts by weight of methyl methacrylate, 6.1 parts by weight of acrylonitrile, 3.5 parts by weight of methacrylic acid and 2.6 parts by weight of ethylene glycol dimethacrylate was added in an amount of 4.5 parts.
Added over time. However, ethylene glycol dimethacrylate was used for the polymerization reaction by adding it to the monomer mixture 2 hours after the start of the polymerization. In parallel with the addition of these monomers, 20 parts by weight of deionized water in which 0.2 parts by weight of sodium dodecylbenzenesulfonate, 0.15 parts by weight of Newcol 506, 0.16 parts by weight of sodium hydroxide and 0.8 parts by weight of sodium persulfate are dissolved. Was fed in 5.0 hours.
Thereafter, the temperature was increased to 90 ° C. to complete the polymerization. The particle size of the obtained polymer was 0.35 μm, and the solid content was 46% by weight.

Next, 10.0 parts by weight of cyclohexane was added to 46.7 parts by weight (solid content: 42% by weight) of the emulsion, and the mixture was treated with an ultrasonic dispersing machine (Model 450, manufactured by Branson) for 30 seconds. This mixture was placed in a stainless steel autoclave equipped with a stirrer, and stirred at 110 ° C. for 2 hours. During this time, the internal pressure of the autoclave was adjusted to 4 kg / cm ² with nitrogen gas. Next, the contents are purged with nitrogen from the jet nozzle,
It was squirted into a container maintained at atmospheric pressure. The particle size of the obtained emulsion was 0.40 μm. The volume increase rate (ΔV) by this operation is 60%. Observation of the cross section of the particle with a scanning electron microscope photograph showed that the particle size was consistent with the above particle size, and that a hollow hole of approximately 0.30 μm was formed.

Example 19 In a single separable flask equipped with a stirrer, a thermometer, a reflux tube, and an inlet tube, 0.2 part by weight of sodium dodecylbenzenesulfonate and 0.2 part by weight of Newcol 506 (manufactured by Nippon Emulsifier Co., Ltd.) were dissolved. 100 parts by weight of the deionized water thus obtained and 400 parts by weight of a styrene / allylic acid copolymer latex of 400 ° C. as seed particles were added in a charged solid content of 1.0 part by weight, purged with nitrogen, and heated to 80 ° C. In this flask, a mixed solution consisting of 80.0 parts by weight of styrene, 15.0 parts by weight of butyl acrylate, 2.0 parts by weight of methacrylic acid and 3.0 parts by weight of dodecyl mercaptan was added for 4.5 hours, 0.2 parts by weight of sodium dodecylbenzenesulfonate, Newcol 506 0.1
5 parts by weight, 0.16 parts by weight of sodium hydroxide and 20 parts by weight of deionized water in which 0.8 parts by weight of sodium persulfate were dissolved were fed in 5.0 hours. Thereafter, the temperature was increased to 90 ° C. to complete the polymerization. The particle size of the obtained polymer was 0.20 μm, and the solid content of the emulsion was 46% by weight.

Next, 105 parts by weight of deionized water obtained by dissolving 0.2 parts by weight of sodium dodecylbenzenesulfonate and 0.2 parts by weight of Newcol 506 in one autoclave equipped with a stirrer, a thermometer, a reflux tube, and an inlet tube, and The obtained emulsion was charged in an amount of 12.5 parts by weight based on the charged solid content, the atmosphere was replaced with nitrogen, and then heated to 85 ° C. To this autoclave, a mixed solution comprising 53.4 parts by weight of styrene, 21.9 parts by weight of methyl methacrylate, 6.1 parts by weight of acrylonitrile, 3.5 parts by weight of methacrylic acid, 10.0 parts by weight of cyclohexane and 2.6 parts by weight of ethylene glycol dimethacrylate was added over 4.5 hours. However, ethylene glycol dimethacrylate is
Two hours after the start of the polymerization, the mixture was added to the monomer mixture to conduct a polymerization reaction. In parallel with the addition of these monomers, 20 parts by weight of deionized water in which 0.2 parts by weight of sodium dodecylbenzenesulfonate, 0.15 parts by weight of Newcol 506, 0.16 parts by weight of sodium hydroxide and 0.8 parts by weight of sodium persulfate are dissolved. Was fed in 5.0 hours. Thereafter, the temperature was increased to 90 ° C. to complete the polymerization. The particle size of the obtained polymer was 0.35 μm, and the conversion was 100%.

After the polymerization, the autoclave is purged with nitrogen to an internal pressure of 4 kg / cm.
^After adjusting to ² and stirring at 110 ° C. for 2 hours, the mixture was discharged into a low pressure region under a nitrogen atmosphere at 25 ° C. The particle size of the polymer emulsion thus obtained was 0.39 μm. The polymer emulsion was internally added to the resin, and the cut surface was observed. As a result, it was confirmed that the polymer particles had 0.25 μm hollow holes.

Example 20 In a single autoclave equipped with a stirrer, a thermometer, a reflux tube, and an inlet tube, 100 parts by weight of deionized water obtained by dissolving 0.2 parts by weight of sodium dodecylbenzenesulfonate and 0.2 parts by weight of Newcol 506 were used as seed particles. 400 kg of styrene / allylic acid copolymer latex was charged at a solid content of 1.
After putting 0 parts by weight and purging with nitrogen, the mixture was heated to 80 ° C. In this flask, a mixture of 80.0 parts by weight of styrene, 15.0 parts by weight of butyl acrylate, 2.0 parts by weight of methacrylic acid, 10 parts by weight of cyclohexane and 3.0 parts by weight of dodecyl mercaptan was added for 4.5 hours, and 0.2 part by weight of sodium dodecylbenzenesulfonate was added. , Newcol 506 0.2 parts by weight, caustic soda 0.16 parts by weight and sodium persulfate 0.8
20 parts by weight of deionized water in which parts by weight were dissolved were fed in 5.0 hours. Thereafter, the temperature was increased to 90 ° C. to complete the polymerization. The particle size of the obtained polymer is 0.21 μm, and the conversion is 99%.
Met.

Next, 105 parts by weight of deionized water obtained by dissolving 0.2 parts by weight of sodium dodecylbenzenesulfonate and 0.2 parts by weight of Newcol 506 in one autoclave equipped with a stirrer, a thermometer, a reflux tube, and an inlet tube were obtained. 12.5 parts by weight of the obtained emulsion was added as a charged solid content, the atmosphere was replaced with nitrogen, and then heated to 85 ° C. In this autoclave, styrene
A mixture of 53.4 parts by weight, 21.9 parts by weight of methyl methacrylate, 6.1 parts by weight of acrylonitrile, 3.5 parts by weight of methacrylic acid, 8.7 parts by weight of cyclohexane and 2.6 parts by weight of ethylene glycol dimethacrylate was added over 4.5 hours. However, ethylene glycol dimethacrylate is
Two hours after the start of the polymerization, the mixture was added to the monomer mixture to conduct a polymerization reaction. In parallel with the addition of these monomers, 20 parts by weight of deionized water in which 0.2 parts by weight of sodium dodecylbenzenesulfonate, 0.2 parts by weight of Newcol 506, 0.16 parts by weight of sodium hydroxide and 0.8 parts by weight of sodium persulfate are dissolved. Was fed in 5.0 hours. Thereafter, the temperature was increased to 90 ° C. to complete the polymerization. The particle size of the obtained polymer was 0.34 μm, and the conversion was 100%.

After the polymerization, the autoclave is purged with nitrogen to an internal pressure of 4 kg / cm ^2.
After stirring at 110 ° C. for 2 hours, the mixture was discharged into a low pressure region under a nitrogen atmosphere. The particle size of the polymer emulsion thus obtained was 0.38 μm. The polymer emulsion was internally added to the resin, and the cut surface was observed. As a result, it was confirmed that the polymer particles had 0.25 μm hollow holes.

Examples 21 to 30 Hollow polymer particles were produced in the same manner as in Example 19. Table 5 summarizes the results.

Comparative Example 7 Toluene (boiling point: 111 ° C.) instead of cyclohexane
Polymer particles were produced in the same manner as in Example 19, except that was used, and subjected to a foaming test. However, almost no hollow was observed in the obtained polymer particles.

Claims (1)

(57) [Claims] 1. A polymerizable monomer mixture comprising 1 to 75% by weight of a core portion of a substantially non-crosslinked polymer obtained by polymerizing a polymerizable monomer and 0.3% by weight or more of a crosslinkable monomer. Of core / shell type polymer particles composed of 99 to 25% by weight of a shell portion of a polymer obtained by polymerizing a polymer and a physical foaming agent impregnated with the core / shell type polymer particles having a weight average particle diameter of 0.05 to 1.0 μm. A method for producing a hollow polymer emulsion, comprising heating an aqueous emulsion containing a polymer to a temperature equal to or higher than the boiling point of the physical blowing agent, and then transferring it to a low pressure region.