JP2003001098A - Method of manufacturing thermally expandable microcapsule - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient and simple method of manufacturing a thermally expandable microcapsule which has excellent heat resistance and solvent resistance in combination, hardly gives rise to shrinking and foam breaking even under heating conditions of a high temperature and long time and has excellent thermal inflation stability (foaming stability). SOLUTION: The method of manufacturing the thermally expandable microcapsule comprising microcapsulating a monomer component containing >=80 wt.% nitrile-base monomer, <20 wt.%, non-nitrile-base monomer and 0.1 to 1 wt.% tetra- or higher functional crosslinking agent and a volatile inflating agent which turns to a gaseous state at a temperature below the softening point of the polymer obtained from the monomer component described above by polymerization under nitrogen pressurization and the method of manufacturing the thermally expandable microcapsule characterized in that the pressure value in the nitrogen pressurization is 0.4 to 2.0 MPa as well as the method of manufacturing the thermally expandable microcapsule characterized in that the polymer is the polymer obtainable from the monomer component containing 85 to 97 wt.% nitrile-base monomer, 3 to 10 wt.%, non-nitrile-base monomer and 0.2 to 0.5 wt.% tetra- or higher functional crosslinking agent.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing heat-expandable microcapsules, particularly heat-expandable microcapsules excellent in heat resistance and solvent resistance.

[0002]

2. Description of the Related Art A method for producing a heat-expandable microcapsule is known in which a thermoplastic polymer is used to microencapsulate a volatile swelling agent which becomes gaseous at a temperature below the softening point of the polymer.

For example, in Japanese Examined Patent Publication (Kokoku) No. Showa 42-26524, a liquid foaming agent such as an aliphatic hydrocarbon having a low boiling point is added to a monomer, an oil-soluble polymerization initiator is mixed with the monomer component, and then dispersion stabilization is carried out. Disclosed is a method for producing heat-expandable microcapsules, in which spherical monomers containing the liquid foaming agent are produced by adding the monomer component to an aqueous dispersion medium containing the agent while stirring and performing suspension polymerization. Has been done.

However, the heat-expandable microcapsules obtained by the production method disclosed in the above publication are not excellent in heat resistance and foam at a low temperature (about 80 to 130 ° C.) and are kept at a high temperature for a long time. However, there is a problem that the expansion ratio is reduced.

In order to deal with such a problem, for example, Japanese Patent Publication No. 5-15499 discloses a nitrile monomer of 80% by weight or more and a non-nitrile monomer of 20% by weight.
A heat-expandable microcapsule that microencapsulates a volatile expander that becomes gaseous at a temperature below the softening point of the polymer using a polymer obtained from the following and a monomer component containing 0.1 to 1% by weight of a crosslinking agent. A method of making a capsule is disclosed.

The heat-expandable microcapsules obtained by the production method disclosed in the above publication are superior in heat resistance to the heat-expandable microcapsules obtained by conventional production methods, and do not foam at 140 ° C. or lower. Moreover, although it has excellent solvent resistance, it still has a problem that the microcapsules are likely to shrink or break due to heating at a high temperature for a long time, and the stability at the time of foaming is insufficient in the manufacturing process of wallpaper, for example. There is.

[0007]

SUMMARY OF THE INVENTION In view of the above problems, the object of the present invention is to combine excellent heat resistance and excellent solvent resistance and to cause shrinkage and foam breaking even under high temperature and long time heating conditions. An object of the present invention is to provide an efficient and simple method for producing a heat-expandable microcapsule that is difficult and has excellent thermal expansion stability (foaming stability).

[0008]

A method for producing heat-expandable microcapsules according to the present invention is a nitrile monomer of 80% by weight or more and a non-nitrile monomer 20.
A monomer component containing less than 0.1% by weight of a cross-linking agent of less than 4% by weight and tetrafunctionality or more, and a volatile expanding agent which becomes gaseous at a temperature below the softening point of a polymer obtained from the monomer component, It is characterized in that it is polymerized under nitrogen pressure to be microencapsulated.

The method for producing the heat-expandable microcapsules according to the second aspect of the present invention is the method for producing the heat-expandable microcapsules according to the first aspect, wherein the pressure value during nitrogen pressurization is 0.4 to 0.4. It is characterized by being 2.0 MPa.

The method for producing the heat-expandable microcapsules according to the third aspect of the present invention is the method for producing the heat-expandable microcapsules according to the first aspect, wherein the nitrile monomer is acrylonitrile and / or methacrylonitrile. It is characterized by being.

The method for producing the heat-expandable microcapsules according to the invention of claim 4 is the same as the method for producing the heat-expandable microcapsules according to claim 1, wherein the non-nitrile-based monomer is an acrylic acid ester or methacrylic acid. It is characterized by being at least one kind of monomer selected from the group consisting of ester, styrene and vinyl acetate.

Further, the method for producing the heat-expandable microcapsule according to the invention described in claim 5 is the method for producing the heat-expandable microcapsule according to claim 1, wherein the polymer is 85 to 97 wt% of the nitrile monomer. %, 3 to 10% by weight of a non-nitrile-based monomer, and 0.2 to 0.5% by weight of a tetrafunctional or higher cross-linking agent, which is a polymer obtained from a monomer component.

Examples of the nitrile-based monomer used in the present invention include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, fumaronitrile and the like, and they are preferably used. Among them, acrylonitrile and methacrylonitrile are preferable. Nitrile is particularly preferably used. These nitrile-based monomers may be used alone or in combination of two or more kinds.

In the present invention, the amount of the above-mentioned nitrile monomer used is required to be 80% by weight or more in the monomer component, and preferably 85 to 97% by weight.
When the amount of the nitrile-based monomer used is less than 80% by weight in the monomer component, the heat-expandable microcapsules (hereinafter,
The heat resistance and the solvent resistance of "microcapsules" will be insufficient.

The non-nitrile-based monomer used in the present invention may be any non-nitrile-based monomer as long as it can be copolymerized with the above-mentioned nitrile-based monomer, and examples thereof include methyl acrylate, ethyl acrylate and acryl. Acrylic acid esters such as butyl acid and dicyclopentenyl acrylate; Methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobornyl methacrylate; styrene, vinyl acetate and the like, which are preferably used. Among them, methyl acrylate, methyl methacrylate and ethyl methacrylate are particularly preferably used. These non-nitrile-based monomers may be used alone or in combination of two or more kinds.

In the present invention, the amount of the non-nitrile-based monomer used is required to be less than 20% by weight in the monomer component, and preferably 3 to 10% by weight.
The amount of non-nitrile-based monomer used is 20 in the monomer component.
When it is at least wt%, the heat resistance and solvent resistance of the microcapsules become insufficient.

In the present invention, a tetrafunctional or higher functional crosslinking agent is used as the crosslinking agent. By using a tetrafunctional or higher cross-linking agent, the degree of cross-linking of the polymer obtained from the nitrile-based monomer and the non-nitrile-based monomer is sufficiently improved. It is difficult to cause bubbles.

Examples of the tetrafunctional or higher cross-linking agent include, for example,
Pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hexaacrylate, dipentaerythritol hexamethacrylate, and the like are mentioned and preferably used. These tetrafunctional or higher cross-linking agents may be used alone or in combination of two or more kinds.

In the present invention, the amount of the above-mentioned tetrafunctional or higher cross-linking agent used is required to be 0.1 to 1% by weight, preferably 0.2 to 0.5% by weight in the monomer component.
Is. When the amount of the tetrafunctional or more cross-linking agent used is less than 0.1% by weight in the monomer component, the degree of crosslinking of the polymer is not sufficiently improved, and conversely, when it exceeds 1% by weight in the monomer component, The degree of crosslinking of the polymer no longer improves.

The wall material of the microcapsules according to the production method of the present invention comprises a monomer component comprising the above-mentioned nitrile monomer, non-nitrile monomer and a tetrafunctional or more cross-linking agent, and optionally a polymerization initiator if necessary. It is prepared by

Examples of the above-mentioned polymerization initiator include various known polymerization initiators generally used in this field,
Of these, oil-soluble polymerization initiators soluble in nitrile-based monomers and non-nitrile-based monomers are particularly preferably used.

Examples of the oil-soluble polymerization initiator include:
Examples thereof include dialkyl peroxide, diacyl peroxide, peroxyester, peroxydicarbonate and azo compounds. More specifically, methyl ethyl peroxide,
Dialkyl peroxide such as di-t-butyl peroxide and dicumyl peroxide; peroxide such as isobutyl peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide Diacyl; t-butylperoxypivalate, t-hexylperoxypivalate, t-butylperoxyneodecanoate, t-
Hexyl peroxy neodecanoate, 1-cyclohexyl-1-methylethyl peroxy neodecanoate,
1,1,3,3-tetramethylbutyl peroxy neodecanoate, cumyl peroxy neodecanoate,
Peroxyesters such as (α, α-bis-neodecanoylperoxy) diisopropylbenzene; bis (4-
t-butylcyclohexyl) peroxydicarbonate, di-n-propyl-oxydicarbonate, diisopropylperoxydicarbonate, di (2-ethylethylperoxy) dicarbonate, dimethoxybutylperoxydicarbonate, di (3-methyl) Peroxydicarbonate such as -3-methoxybutylperoxy) dicarbonate; 2,2'-azobisisobutyronitrile, 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile, 2, Azo compounds such as 2'-azobis (2,4-dimethylvaleronitrile) and 1,1'-azobis (1-cyclohexanecarbonitrile) are preferably used, and these oil-soluble polymerization initiators are They may be used alone or in combination of two or more.

The wall material of the microcapsule according to the production method of the present invention contains 80% by weight or more of a nitrile monomer, less than 20% by weight of a non-nitrile monomer, and 0.1 to 1% by weight of a tetrafunctional or more cross-linking agent. It is composed of a polymer obtained from a monomer component.

Among the above polymers, a particularly preferable polymer contains 85 to 97% by weight of a nitrile monomer, 3 to 10% by weight of a non-nitrile monomer, and 0.2 to 0.5% by weight of a tetrafunctional or more cross-linking agent. It is a polymer obtained from the monomer component. By forming the wall material from such a polymer, the microcapsules have both excellent heat resistance and excellent solvent resistance.

The volatile swelling agent used in the present invention and contained in the microcapsules is a substance which becomes a gas at a temperature below the softening point of the polymer obtained from the above-mentioned monomer components. When the volatile expansion agent is a substance that becomes a gas at a temperature exceeding the softening point of the polymer, the microcapsules are likely to shrink or break during thermal expansion (foaming).

Examples of the volatile swelling agent include low boiling point organic solvents and compounds which are thermally decomposed into gas when heated, and are preferably used. Among them, low boiling point organic solvents are particularly preferable. Used. These volatile swelling agents may be used alone or in combination of two or more kinds.

Examples of the low boiling point organic solvent include ethane, ethylene, propane, propene, n-butane, isobutane, n-butene, isobutene, n-pentane, isopentane, neopentane, n-hexane, heptane,
Low molecular weight hydrocarbons such as petroleum ether; CCl3F, C
Examples include chlorofluorocarbons such as Cl2F2, CClF3, CClF2-CCl2F2; tetraalkylsilanes such as tetramethylsilane, trimethylethylsilane, trimethylisopropylsilane, trimethyl-n-propylsilane, and the like. Among them, n- is preferably used. Butane, isobutane, n-pentane, isopentane, n-hexane and petroleum ether are particularly preferably used. These low boiling point organic solvents may be used alone or in combination of two or more kinds.

In the production method of the present invention, the monomer component and the volatile swelling agent are polymerized under nitrogen pressure to form microcapsules.

The pressure value when pressurizing with nitrogen is 0.4 to 2.0.
It is preferably MPa. When the pressure value at the time of pressurizing nitrogen is less than 0.4 MPa, the polymerization reactivity of the nitrile monomer is deteriorated, and only microcapsules having a low bulk specific gravity can be obtained. Expansion stability (foaming stability) under heating conditions
When the pressure value during nitrogen pressurization exceeds 2.0 MPa, the thermal expansion stability (foaming stability) of the microcapsules is no longer the same. No further improvement.

The method for polymerizing the monomer component and the volatile swelling agent under nitrogen pressure preferably at a pressure value of 0.4 to 2.0 MPa for microencapsulation is not particularly limited and may be a conventional method. Just go.

A particularly suitable method is, for example, the above-mentioned Japanese Examined Patent Publication No. S42.
As described in JP-A-26524, a monomer component is mixed with a volatile swelling agent and a polymerization initiator, and the mixture is suspended in an aqueous dispersion medium containing an appropriate dispersion stabilizer, auxiliary stabilizer and the like. It is a method of polymerizing.

The suspension polymerization is usually carried out in an aqueous dispersion medium containing a dispersion stabilizer, a co-stabilizer and the like.

Examples of the dispersion stabilizer include silica such as colloidal silica, calcium phosphate, magnesium hydroxide, aluminum hydroxide, ferric hydroxide, barium sulfate, calcium sulfate, sodium sulfate, calcium oxalate, calcium carbonate and carbonic acid. Barium, magnesium carbonate and the like can be mentioned, and they are preferably used. These dispersion stabilizers may be used alone or in combination of two or more kinds. The amount of the dispersion stabilizer used is not particularly limited, but is preferably 0.1 to 20 parts by weight with respect to 100 parts by weight of the monomer component.

Examples of the costabilizer include, for example,
Condensation products such as condensation products of diethanolamine and aliphatic dicarboxylic acids and condensation products of urea and formaldehyde; water-soluble nitrogen-containing compounds such as polyvinylpyrrolidone and polyethyleneimine; polyethylene oxide,
Tetramethylammonium hydroxide, gelatin, methylcellulose, polyvinyl alcohol, dioctyl sulfosuccinate, sorbitan ester, various emulsifiers and the like can be mentioned, and they are preferably used. These costabilizers may be used alone or in combination of two or more kinds.

The aqueous dispersion medium used for suspension polymerization is prepared by blending the above dispersion stabilizer, auxiliary stabilizer, etc. in deionized water. The pH of the aqueous phase during suspension polymerization is appropriately determined depending on the type of dispersion stabilizer, auxiliary stabilizer, etc. used. For example, when silica such as colloidal silica is used as the dispersion stabilizer, suspension polymerization is carried out in an acidic aqueous phase. To make the aqueous phase acidic, an acid such as hydrochloric acid may be added as necessary to adjust the pH of the aqueous phase to 3 to 6. When calcium phosphate, magnesium hydroxide or the like is used as a dispersion stabilizer, suspension polymerization is carried out in an alkaline aqueous phase. To make the aqueous phase alkaline,
If necessary, add a base such as caustic soda to add p to the aqueous phase.
It suffices to adjust H to 8-11.

Colloidal silica (dispersion stabilizer) is an example of a preferable combination of the dispersion stabilizer and the auxiliary stabilizer.
And a condensation product (co-stabilizer). As the condensation product, a condensation product of diethanolamine and an aliphatic dicarboxylic acid is preferable, and among them, a condensation product of diethanolamine and adipic acid and a condensation product of diethanolamine and itaconic acid are particularly preferable. These condensation products may be used alone or in combination of two or more kinds. Furthermore, when an inorganic salt such as sodium chloride or sodium sulfate is added, microcapsules having a more uniform particle shape can be easily obtained.

The amount of colloidal silica used is not particularly limited as long as it can be adjusted according to the particle size, but it is preferably 1 to 20 parts by weight of colloidal silica with respect to 100 parts by weight of the monomer component. It is more preferably 2 to 10 parts by weight. The amount of the condensation product used may be adjusted depending on the type of the condensation product, and is not particularly limited, but it may be 0.05 to 2 parts by weight with respect to 100 parts by weight of the monomer component. preferable.
Further, the amount of the inorganic salt used may be adjusted depending on the kind thereof and is not particularly limited, but it is preferably 100 parts by weight or less of the inorganic salt with respect to 100 parts by weight of the monomer component.

Another example of a preferable combination of the dispersion stabilizer and the auxiliary stabilizer is a combination of colloidal silica (dispersion stabilizer) and a water-soluble nitrogen-containing compound (co-stabilizer). Examples of the water-soluble nitrogen-containing compound include polyvinylpyrrolidone, polyethyleneimine, polyoxyethylene alkylamine, polydialkylaminoalkyl (meth) acrylate represented by polydimethylaminoethyl acrylate and polydimethylaminoethyl methacrylate, and polydimethylaminopropyl. Polydialkylaminoalkyl (meth) acrylamides represented by acrylamide and polydimethylaminopropylmethacrylamide, polyacrylamides, polycationic acrylamides, polyamine sulfones, polyallylamine, and the like are preferably used, and among them, polyvinylpyrrolidone Are particularly preferably used. These water-soluble nitrogen-containing compounds may be used alone or in combination of two or more kinds.

In the case of the above combination, in order to adjust the particle size of the microcapsules, the amount of colloidal silica used is fixed to preferably 1 to 20 parts by weight of colloidal silica with respect to 100 parts by weight of the monomer component, and polyvinyl pyrrolidone is used. It is preferable to adjust the amount by changing the amount used.

Further, as another example of the preferable combination of the dispersion stabilizer and the co-stabilizer, a combination of calcium phosphate or magnesium hydroxide (dispersion stabilizer) and an emulsifier (co-stabilizer) can be mentioned.

The order in which the respective components are mixed in the aqueous dispersion medium is as follows.
Although it is optional and not particularly limited, usually, deionized water, a dispersion stabilizer and, if necessary, a co-stabilizer are charged in a polymerization vessel to prepare an aqueous dispersion medium containing the dispersion stabilizer. In addition, if necessary, an alkali metal nitrite,
Compounds such as stannous chloride, stannic chloride, potassium dichromate are added. The monomer component and the volatile swelling agent may be added individually to the aqueous dispersion medium to form an oily mixture in the aqueous dispersion medium, but usually both are mixed in advance to prepare an oily mixture. And the oily mixture is added to the aqueous dispersion medium. The polymerization initiator may be added in advance to the oily mixture, or may be added after the aqueous dispersion medium and the oily mixture are stirred and mixed in the polymerization vessel. Alternatively, the aqueous dispersion medium and the oily mixture may be mixed in advance in a separate container, stirred and mixed, and then charged in a polymerization vessel.

The particle size of the microcapsules obtained by the production method of the present invention is not particularly limited, but it is preferably about 5 to 50 μm in a state before thermal expansion (unexpanded state). The amount of the volatile swelling agent included in the microcapsules is not particularly limited, but is preferably about 10 to 20% by weight.

According to the production method of the present invention, various combinations exhibiting thermal expansion behavior (foaming behavior) can be obtained by controlling the combination and amount ratio of monomer components and selecting the type of volatile expansion agent. Microcapsules can be manufactured.

[0044]

The microcapsules according to the production method of the present invention are
A wall material is composed of a polymer obtained from a monomer component containing a specific amount of each of a nitrile monomer, a non-nitrile monomer and a tetrafunctional or more cross-linking agent, and vaporizes into a gas at a temperature below the softening point of the polymer. Since the swelling agent is included, it has both excellent heat resistance and excellent solvent resistance, shrinkage or breakage does not easily occur even under high temperature and long time heating conditions, and the expansion ratio does not decrease.

Further, the above-mentioned characteristics are further improved by polymerizing the above-mentioned monomer component and the above volatile swelling agent under nitrogen pressure of a specific pressure value.

[0046]

BEST MODE FOR CARRYING OUT THE INVENTION The following examples are given to illustrate the present invention in more detail, but the present invention is not limited to these examples.

Example 1 First, an oily mixture and an aqueous dispersion medium having the composition shown in Table 1 were prepared. Next, the obtained oily mixture and the aqueous dispersion medium were stirred and mixed with a homogenizer, and then a nitrogen-substituted pressure polymerization vessel (with a content of 20
Liter), charged with nitrogen at a pressure value of 1.0 MPa, and polymerized at 60 ° C. for 16 hours. Next, the obtained polymerized product was repeatedly filtered and washed with water, and then dried to produce microcapsules having an average particle size of 22 μm.

(Example 2) The pressure value during nitrogen pressurization was 2.0.
In the same manner as in Example 1 except that the pressure was set to MPa,
Microcapsules having an average particle size of 17 μm were produced.

(Example 3) The pressure value at the time of pressurizing nitrogen was set to 0.6.
In the same manner as in Example 1 except that the pressure was set to MPa,
Microcapsules having an average particle size of 22 μm were produced.

(Comparative Example 1) The pressure value during nitrogen pressurization was 0.2.
In the same manner as in Example 1 except that the pressure was set to MPa,
Microcapsules having an average particle size of 30 μm were produced.

Comparative Example 2 Microcapsules having an average particle size of 35 μm were produced in the same manner as in Example 1 except that nitrogen pressure was not applied during polymerization.

Examples 1 to 3 and Comparative Example 1
And the expansion ratio of the microcapsules obtained in Comparative Example 2 was measured by the following method. The results are shown in Table 2.

Method for measuring expansion ratio: 1.0 g of microcapsules were placed in a gear type oven and heated under the following heating conditions to expand. Next, each of the obtained foams was put into a graduated cylinder, the volume after foaming was measured, and the volume was divided by the volume when unfoamed to obtain the foaming ratio (times). [Heating conditions] Heating temperature: 140 ° C, 160 ° C, 170 ° C and 180
℃ heating time: 1 minute, 2 minutes, 3 minutes and 4 minutes at each heating temperature

[0054]

[Table 1]

[0055]

[Table 2]

As is clear from Table 2, all of the microcapsules of Examples 1 to 3 produced by the production method of the present invention were smoothly foamed at a heating temperature of 160 ° C. or higher, and at a heating temperature of 180 ° C. Almost no decrease in foaming ratio was observed, and excellent heat resistance and thermal expansion stability (foaming stability) were exhibited.

On the other hand, the pressure value during nitrogen pressurization is 0.4.
The microcapsules of Comparative Example 1 having a pressure of less than 0.2 MPa (0.2 MPa) exhibited a decrease in expansion ratio at a heating temperature of 170 ° C. or higher, and were inferior in heat resistance and thermal expansion stability (foaming stability). Further, the microcapsules of Comparative Example 2 in which nitrogen pressure was not applied at the time of polymerization caused shrinkage and foam breakage at a heating temperature of 160 ° C. or higher, and a good foam could not be obtained.

[0058]

As described above, according to the production method of the present invention, both excellent heat resistance and excellent solvent resistance are provided, and shrinkage and foam breakage hardly occur even under high temperature and long time heating conditions. It is possible to efficiently and simply manufacture microcapsules excellent in thermal expansion stability (foaming stability).

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takahiro Omura             4560 Kaisei-cho, Shinnanyo, Yamaguchi Prefecture Sekisui Chemical Co., Ltd.             Within the corporation (72) Inventor Toshiharu Furukawa             4560 Kaisei-cho, Shinnanyo, Yamaguchi Prefecture Sekisui Chemical Co., Ltd.             Within the corporation F-term (reference) 4G005 AA01 AB14 AB21 BA03 BB02                       BB06 DA05W DA09W DC10W                       DC10Y DC34W DC34Y DD03Z                       DD66Z DD75Z DE08X DE10X                       EA08

Claims (5)

[Claims] 1. A monomer component containing 80% by weight or more of a nitrile monomer, less than 20% by weight of a non-nitrile monomer, and 0.1 to 1% by weight of a tetrafunctional or more cross-linking agent.
A method for producing heat-expandable microcapsules, which comprises polymerizing a volatile expanding agent which becomes a gas at a temperature below the softening point of the polymer obtained from the monomer component under nitrogen pressure to form microcapsules. 2. The pressure value when pressurizing with nitrogen is 0.4 to 2.0 M.
It is Pa, The manufacturing method of the heat-expandable microcapsule of Claim 1 characterized by the above-mentioned. 3. The method for producing thermally expandable microcapsules according to claim 1, wherein the nitrile-based monomer is acrylonitrile and / or methacrylonitrile. 4. The thermal expansion according to claim 1, wherein the non-nitrile-based monomer is at least one kind of monomer selected from the group consisting of acrylic acid ester, methacrylic acid ester, styrene and vinyl acetate. For producing a transparent microcapsule. 5. The polymer is a nitrile-based monomer 85-85.
A polymer obtained from a monomer component containing 97% by weight, 3 to 10% by weight of a non-nitrile-based monomer, and 0.2 to 0.5% by weight of a tetrafunctional or higher cross-linking agent. A method for producing the heat-expandable microcapsule described.