WO1991014741A1

WO1991014741A1 - Plastic pigment and production thereof

Info

Publication number: WO1991014741A1
Application number: PCT/JP1991/000409
Authority: WO
Inventors: Mamoru Sakai
Original assignee: Sansui Co., Ltd.
Priority date: 1990-03-29
Filing date: 1991-03-29
Publication date: 1991-10-03
Also published as: JPH03281577A

Abstract

A white plastic pigment utilized in paper, paint, fiber and so forth, and particularly one comprising silica particles as the core and having a surface coated with a vinylic resin. It can be produced by dispersing silica particles in water in the presence of sulfate radical ions as the emulsifier and adding a vinylic monomer to the dispersion to conduct emulsion polymerization. A plastic pigment of the invention has a hollow or porous layer present in between the core and the coating resin. It can be produced by dispersing silica particles in water in the presence of sulfate radical ions as the emulsifier, adding a vinyl carboxylate monomer to the dispersion to conduct emulsion polymerization to thereby coat the surfaces of the silica particles, further adding thereto a vinylic monomer to conduct emulsion polymerization, and adding a basic compound thereto to neutralize the carboxylic acid. Alternatively it can be produced by dispersing silica particles in water in the presence of sulfate radical ions as the emulsifier, adding a vinyl carboxylate monomer and a cross-linking monomer to the dispersion to conduct emulsion polymerization to thereby coat the surfaces of the silica particles, adding a basic compound thereto to neutralize the carboxylic acid to thereby form a hollow or porous layer, and further adding thereto a vinylic monomer to conduct emulsion polymerization to thereby coat the surfaces of the particles.

Description

Specification

Plastic pigments and their manufacturing technology

The present invention relates to white pigments used for paper, paints, fibers, and the like, and more particularly to a plastic pigment containing silica particles as a core. Background art

According to the Japanese Patent Publication No. 52-138028, a plastic pigment having a particle size of about 30 micron as a polyester resin component is disclosed, but due to its large particle size. It was necessary to increase the viscosity of the solution and to stabilize the dispersion of the particles, which was difficult to handle.

In Japanese Patent Publication No. Showa 56-32 513 or Japanese Patent Publication No. Showa 60-69103, a carboxylic acid resin core or an amino resin core having a small particle size is used as a seed. We have proposed a hollow-type plastic pigment having hollow particles composed of an outer shell and a bull-based resin shell. Japanese Patent Laid-Open Publication No. Showa 63-1354509 has hollow particles in which the core particles of a vinyl resin are swollen with a solvent and then the outer shell is covered with (enclosed with) another resin. A method for producing a hollow plastic pigment has been proposed. However, when these hollow plastic pigments are heated to 100 or more, or even heated to 150 or more, the hollow particles are melted, voids are lost, and consequently the hiding power is reduced and the transparency is further increased. PP l 0, No 1 0. PP 6 9 7-7 0 2 (Oc In 1983), silica particles were dispersed in water using hydroxyv α-bircellose to form a colloid, and the colloidal silica force surface obtained using a styrene monomer was wrapped in styrene resin. A method has been proposed for obtaining a force-Bussel type plastic pigment having force-pressurized particles, but it does not have a function as a pigment having concealment properties. Japanese Patent Laid-Open Publication No. 59-71 13 16 proposes a composition in which the surface of a colloidal sily force is coated with a vinyl resin using a silane coupling agent, but has a hollow structure and concealment properties. It did not reach the point where it could function as a pigment. Disclosure of the invention

The plastic pigment of the present invention has a silica particle as a core, and its surface is coated (enclosed) with a vinyl resin composed of at least one layer, preferably three layers of different vinyl monomer compositions. Things. Preferably, a hollow layer or a porous layer is interposed between the core silicic particles and the vinyl resin coating the particles. For dispersing silica particles in water, silica powder may be used, but an aqueous dispersion solution called colloidal silica force is well known and is particularly preferred in the present invention. The obtained resin having particle size is 0.1 to 10 μm, preferably 0.3 to 3 μm in particle size, and becomes an emulsion-type plastic pigment dispersed in water. When moisture is removed, it becomes a powdery plastic pigment.

This plastic pigment is produced by dispersing silica particles in water using a sulfate radical ion as an emulsifying agent, adding a butyl monomer to the dispersion, and performing emulsion polymerization. Bull unit In the case where a vinyl monomer having a carboxyl group is added as a substance, the emulsion polymerization is followed by further adding a vinyl monomer to carry out emulsion polymerization, and adding a basic compound to neutralize the carboxylic acid. When a vinyl carboxylate monomer and a cross-linkable monomer are added as a vinyl monomer, a basic compound is added after emulsion polymerization to neutralize the carboxylic acid, and then a vinyl monomer is added. And emulsion polymerization *

In the past, various attempts have been made to cover or encapsulate the surface of a Kodidalsil force with a bull resin. However, Japanese Patent Publication No. 59-71 13 16 or a collection of polymer papers Vol. 40, No 1 0. PP 697-1702 (Oct. 1983) As described in the issue, etc., it is intended to coat the surface of the colloidal silica by the usual emulsion polymerization method other than the special method. No micelle formation of the resin monomer occurred, and as a result, the surface of the colloidal silicic force particles could not be covered with the bur resin.

As a result of the study of the present inventors, when emulsion polymerization was carried out on a bullet monomer using sulfuric acid radical ion as an emulsifier, an emulsion polymer of carboxylated particles coated with silica particles as a core was formed. It turned out to be. In order to form the emulsion polymer of hollow particles, silica particles are used as a core and the surface thereof is emulsion-copolymerized with a carboxylic acid vinyl monomer or a vinyl carboxylic acid monomer and another vinyl monomer. To form hollow pores or porous layers by neutralizing the carboxylic acid using ammonia or alkali metal or polyvalent metal or amine, and covering the surface with a vinyl resin. I do.

The thus obtained hollow plastic pigment having the core of the silicic acid particles of the present invention has higher heat resistance than conventionally known hollow particles. It is remarkably excellent in concealing properties. The theoretical basis is not fully understood, but it is presumed that when heated to a high temperature, the effect of preventing the fusion of the vinyl resin is exerted by activating the sily-force component. As for the improvement of the concealing property, it is presumed that light scattering or refraction is caused by the silica core in addition to the hollow layer, and the concealing property is further increased.

In the emulsion polymerization method of the present invention, a conventionally known method can be used except for the emulsifier. In order to deposit a vinyl carboxylate resin or a vinyl resin on the surface of the colloidal silica, a conventionally known noun activator or anion activator may be basically used, but the critical micelle formation concentration (CMC ) Or less, preferably zero. The preferred emulsifier for use is the sulfate radical ion.

(Sulfate Radical Ions:. S0 ₄ has been known as - "described in Th e Application of Synthetic Resin Emulsions J H. Warson Author Ernest Benn Limited London 1972 years Page 15) is, persulfate salts and ferrous sulfate, etc. Sulfate radical ions generated by the decomposition of sodium persulfate, such as ammonium persulfate, sodium persulfate, sulfuric acid diatom, ferrous sulfate, sodium sulfide, sodium sulfate, sodium formaldehyde hydrosulfite, etc. Sulfate radical ions can be obtained by using a reducing agent (for example, ferrous sulfate, hydrosulfites, ascorbic acid, etc.) in combination with heating or low temperature.

When an outer shell layer is further formed on the surface of the obtained vinyl carboxylate-based resin particles that cover (have) the colloidal silicone core, the same conventional activator can be used to form a critical micelle. It is less than rice bran (CMC), and it is preferable to use an activator based on radical ion sulfate. Activator that can be used below CMC Examples of octyl (or noel) phenol are ethylene oxide (E0) adducts, sodium lauryl alcohol sulfate, sodium dodecylbenzenesulfonate, and lauryl alcohol E0 adducts. It is a conventionally known activator such as a sulfate salt or an alkyl sulfosuccinate salt.

Examples of vinyl sulfonic acid monomers that cover the surface of the silica particles (have a covering) (abbreviated symbols in kakko), methacrylic acid (Maa), acrylic acid (Aa) , Itaconic acid, monoalkyl itaconic acid ester, maleic acid, fumaric acid, crotonic acid, 2-carboxyshetyl acrylic acid ester, and the like, and preferably methacrylic acid, acrylic acid, itaconic acid, etc. Acid and the like. Copolymers of these vinyl carbonate monomers with other monomers are also useful, and other monomers that can be used in the copolymerization are na, β-diethylenically unsaturated monomers. , (Meta) acrylic acid esters (meaning methacrylic acid esters and acrylic acid esters): methyl (M (M), ethyl, butyl (Β (Μ) Α), ethyl, butyl ( B (M) A), Octyl (0 (M) A), Alkyl or aryl esters such as sigma hexyl and bezil, butyl ferrate (VA) ', vinyl ethers: methyl, ethyl, 2 -Alkyl or haloalkyl ethers such as -chloroethyl (CEVE), (meth) acrylonitrile ((M) AN), styrene (ST), butyltoluene, α-methylstyrene, butyl chloride, vinylidene chloride , (Meta) acrylyl amide, Ν—methylol (meta) acrylyl , Hydroxyshetyl (meta) Crylate, Glycidyl (meta) Crylate (G (II) Α), Styrenesulfonic acid and its salts, Ethylene glycol (meta) acrylate _Β, such as trimethylolpropane trimethacrylate (Τ «Ρ>) and divinyl benzene (DVB). The monomer used to form the outer shell layer can be the resin monomer deposited on the surface of the core that has already been arrested. The amount of the substance must be within a range that does not dissolve in water due to alkali neutralization, and is preferably a relatively hydrophobic monomer such as methyl methacrylate and styrene. It is a polymer or copolymer that is selected from three or more monomers and polymerized.

In order to form a hollow layer on the surface of the silica core, the first method is to form a carboxylate monomer on the silica core, and then form ammonia or alkali metal after forming the outer shell layer. This can be achieved by adding an aqueous solution of a carboxylic acid or an amine or a polyvalent metal to neutralize the vinyl carboxylate resin layer covering the surface of the silica particles. Neutralization may be performed at room temperature, but preferably heating at 50 to 95 facilitates the neutralization reaction of the carboxylic acid. A second outer shell layer may be further formed on the surface of the neutralized hollow particles, preferably when the formation of the second and third outer shell layers is useful for improving the performance. there is also a _Λ

The second method is to form a copolymer layer of a carboxylic acid vinyl monomer and a cross-linkable monomer on the surface of the silica particles (select a monomer composition that expands by neutralization). _Β) obtained by _neutralizing with a basic compound to form an outer shell layer

The polymerization reaction proceeds with an ordinary radical catalyst. The sulfate radicals already mentioned also serve as activators. For example, persulfates such as ammonium persulfate, sodium persulfate, persulfuric acid vehicles, hydrogen peroxide, peroxysides such as t-butyl hydride π-baroxide, benzoyl peroxide, etc. And as required, ferrous sulfate and sodium as reducing agents Hydrosulfite, sodium formaldehyde, hydrosulfite, and ascorbic acid are also used. The polymerization temperature depends on the decomposition temperature of the catalyst and is preferably between 30 and 100. BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a particle distribution graph chart showing the particle size of cataloid SI-40 obtained in Example 5 measured by the dynamic scattering method, and Fig. 2 is a graph of 0-180 obtained in Example 5. Distribution chart with particle size measured by dynamic scattering method, and Fig. 3 is a particle distribution chart with 0-181 obtained in Example 5 measured by dynamic scattering method. FIG. 4 is a particle distribution graph chart obtained by measuring particles 0-182 obtained in Example 5 by a dynamic scattering method, and FIG. 5 is a graph 0-182 obtained in Example 5 (reference FIG. 3 is a particle distribution chart of Example) measured for particle diameter by dynamic scattering method. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples shown here. The abbreviations used in the examples are the same as those in parentheses described above or below. Example 1. The surface of the colloidal silicic acid particles is coated with << aa-II soldier polymer. Number of the obtained polymer: 0-180

A thermometer, a stirrer, a reflux condenser, a water bath, a drop port, Equipped with nitrogen inlet. A reaction vessel was charged with 475 ml of deionized water and 7.5 g of Kataguchi SI-40 (40% solid content, product of Catalysis Chemical Industry), purged with nitrogen, and ripened to 85. 4 g of an aqueous solution of sodium persulfate (NPS) IO% was added, and a mixture of 6 g of Maa and 9 g of MMA was manually dripped continuously over 20 minutes at 85 ± 1 using a dropping funnel. It was kept at ± 1 in 85 minutes 45 minutes. The mixture was cooled at 30 below, and filtered with a 300-mesh nylon cloth. There was no agglomerate in the filter cloth residue, and an emulsion polymer having a milky white appearance with a residual monomer content of 125 ppm (polymerization rate of 99.5% or more) and a pH of 3.5 was obtained. The unit is gram (g) unless otherwise specified. Example 2 Particles of Maa-M «A copolymer excluding colloidal silicic acid particles are obtained (Reference Example). Number of the obtained polymer: 0-77 The procedure of Example 1 was followed except that the composition was changed to the following composition.

450 ml of deionized water

NPS (10¾) 4

Maa-M «A 6/9

An emulsion polymer having a milky appearance was obtained. The aggregate of the filter cloth residue was 0.2 g. Residual monomers were less than 100 ppm. Example 3: The surface of colloidal silicic particles is coated with an MHA polymer. Number of the obtained polymer: 0-181 The operation was carried out according to Example 1 using the following components.

475 ml of deionized water.

Cataloid SI-40 7.5

NPS (10¾) 4

Mk 15

An emulsion polymer having a milky white appearance with a pH of 6.5 was obtained with an aggregate of 0.05 g of filter cloth residue, residual monomer of 100 ppm or less, and a pH of 6.5. Example 4. The surface of the colloidal silicide particles was co-weighted with Maa-MM. Coalesced and coated with Maa-MMA-BA copolymer. Number of the obtained polymer: 0-182 Emulsion polymer (0-180) 486.5 obtained in Example 1 was charged into a reaction vessel, heated to 80, and 38.5 of 5 (22% aqueous solution) was added thereto. .8 and a mixed monomer solution of MMA106.5 and BA25.8 were continuously dropped into the reaction tank over a period of 30 minutes, and maintained at 80 ± 2 during this time. It was kept at 80 ± 2 for about 35 minutes and aged. Aqueous ammonia (25) 8.5 was added, and the mixture was heated to 90 with 15 minutes. The mixture was cooled to 30 below and controlled using a 100-mesh nap cloth. An emuldion polymer having a milky white appearance having a filter cloth residue agglomerate of 0.1, a blade-attached agglomerate of 0.2, a residual monomer of 350 ppm, and a pH of 7.9 was obtained. Example 5. Colloidal silica particles were coated with a polymer

-Confirmation The emulsion polymer obtained in Examples 1 to 4 and the catalyzed SI-40 (co-idal-siri force) were applied to a dynamic light scattering photometer DL S-700 (manufactured by Union Giken) to obtain a dynamic image. The particle size was measured by the light scattering method (DLS method.) The results are summarized in Table 1 and the attached particle distribution graph chart. ) Polymer numbers: 0-180 and 0-181 indicate that most of the π-idal sily particles were covered by the polymer and grew into large particles. (2) Polymer numbers 0-182 This indicates that the particle diameter grew further than -180, with colloidal silica as the core, the surface was coated with Maa-MMA, and the surface of the obtained particles was further coated with Maa-MMA-BA. This indicates that the particles have become huge.

袠 1 Polymer number ϋίϊ "0-180 0-181 0-182 0-77

SI-40 Reference example 1st beak

Average particle size 11 «19.7 17.3 13.2 74.3 101 Distribution ratio%. Approx. 80 Approx. 1.5 1.0 4.0 90 Second beak

The average particle diameter nm 119.9 201.9 193.6 486.3 868.2 distribution ratio% about 20 to about 98 * 5 99.0 96.0 10.0 drawing number Figure 2 Figure 1 Figure 3 Figure 4 Figure 5 nm · · nanometers - - coater (10 one ⁹ main Example 6. The surface of the colloidal silicide particles is transferred to Haa-MM warfare polymer, << aa-Μ-ΒΑ copolymer and ST polymer (Hollow type plastic pigment). Number of polymer obtained: 0-183 Emulsion polymer (0-182) 670 obtained in Example 4 was added to ammonia water (255 in a reactor) and ripened to 85. NPS

(22% aqueous solution) 10 and add a deionized ice 70 and a monomer emulsion consisting of sodium dodecimebenzensulfonate (purity of 99% or more) 0.15 and ST250 for about 90 minutes using a dropping funnel. The reaction temperature was kept at 85 during this time. After completion of the dropwise addition, the temperature was maintained at 85 at about 15 minutes and cooled at 30 at below. The mixture was filtered using a 100 mesh nylon cloth. Filter cloth Zan查agglomerates about 0.5, residual单量body 420ppm, p H9.5, viscosity 240c _PS (Senchipo I's), an average particle diameter of 0.9 Miku α emission Meters, Emarujo down with milky appearance nonvolatile content 40.2% A polymer was obtained. Example 7. The surface of the colloidal silicic acid particles was changed to ΜΗΑ copolymer and

Coated with a triple resin layer with Maa-MM-.BA copolymer, Maa-MMA-BA copolymer and ST polymer (solid hollow plastic pigment). Obtained polymer number: 0-184 The emulsion polymer (0-181) 545 obtained in Example 3 was charged into a reaction vessel and ripened to 70.> NPS (22% aqueous solution) 38 was added. Then, a mixed monomer of Maa2.8, MMA106.5, and BA25.8 was dropped dropwise over a period of about 50 minutes, and the reaction temperature was kept at 70 and aged for 10 minutes. Aqueous ammonia (25¾) 8 was added for stabilization during the polymerization, and the mixture was heated to 85. Then deionized water 80, sodium dodecylbenzenesulfonate 0.15 and ST250 The resulting monomer emulsion was dropped continuously over 90 minutes, and the simple reaction temperature was maintained at 85, followed by aging for 15 minutes. The mixture was cooled to 30 below and filtered using a 100-mesh nylon cloth. An emulsion polymer having a milky appearance having a filter cloth residue of 0.5, a residual monomer of 150 PPAU, pH 9.4, a viscosity of 9 cps, an average particle diameter of 0.8 micrometer, and a nonvolatile content of 39.8% was obtained. Example 8. Evaluation of concealing property and heat resistance A coating liquid was obtained with the following composition. A hollow plastic pigment emulsion polymer, which is already known and commercially available, was also included in the evaluation for comparison.

Water 20.6

Plastic pigment (35%) 57.1

(Blast pigment to be tested

Adjust the solid content to 35% with water)

A-208 (binder) 21.3

(Aquel-le-Marjon, a major component manufactured by Sansui Co., Ltd.

Bolimmer, 47% solids at minimum deposition temperature 3)

Flora 'FC-149 (1% aqueous solution) 1

(Wetting agent, manufactured by Sumitomo Sriem Co., Ltd.) Plastic pigment emulsion polymer tested

0-183: Example 6 (hollow type with a core)

0-184: Example 7 (non-hollow type with silica core) Primal 0P-42: manufactured by Rohm And Haas Co., Ltd. Hollow type emuljo without colloidal core force Polymer, 40% non-volatile content

Bonco PP-1000: made by Dainippon Ink & Chemicals, Inc.

Hollow-type plastic pigment emulsion polymer without Dalsilica as a core, 45% non-volatile content The obtained coating solution was applied to a 100-micron transparent polyester film on a wire rod bar α8 (manufactured by Etokikisho Co., Ltd.) It was applied using and dried at room temperature. One half of the test piece was cut out, heated in a dryer at 180 for 60 seconds, and subjected to a heat resistance test. The opacity was measured at an absorbance measuring device (Shimadzu UV-160, manufactured by Shimadzu Corporation) at a wavelength of 520ηιη and summarized in Table 2.

Table 2

From the above results, it was shown that the hollow mold of the α) colloidal core has a high concealing property (the colloidal core has little decrease in concealing property (heat resistance) due to heating. Example 9. Crosslinking Hollow plastic pigments

Marsion polymer ST 250 in Example 6 was changed to the composition shown in Table 3 below, and the others were the same. Did one work per week

Table 3

In each case, the same polymerization stability, masking property and heat resistance as those of Polymer No. 0-183 were obtained. Example 10 0. Hollow-type plastic pigment emulsion polymer obtained by copolymerizing another monomer ST 250 in Example 6 was changed to the composition shown in Table 4 below, and the other components were the same in composition and operation. did.

Table 4 Polymer numbers 0-199 0-188 0-189 0-190

ST 245 225 245 245

CEVB 5

BMA 25

OA 5

AN-5 In each case, almost the same results as those of Polymer No. 0-183 were obtained. When Maa 2.8, HMA 106.5 and BA 25.8 of Example 4 were changed to the following composition, almost the same result as that of 0-182 was obtained.Polymer number: 0-191 And then obtained by the same monomer composition and the same operation as in Example 6.

Maa 2.8

MMA 106.5

VA 12.0

BA 12.5 Example 1 1. Powdering and application as an anti-glare paint Polymer number: 0- using a commercially available drybreaker at an inlet temperature of 150 ± 5 and an outlet temperature of 50 ± 3. 183 was powdered. Using the obtained 0-183 powder, 0-183 and titanium oxide, a paint was prepared according to the following formulation. () Is the manufacturing company name

Table 5 Formulation No. 1 Ife 2 i 3 Water 235.0 235.0 235.0 Tribo'tanic acid (10% aqueous solution) 10.0 10.0 10.0 Boise 530 (Kao) 2.0 2.0 2.0 ί-2-0 (Sanyo Chemical Industries) 2.5 5 2.5 2.5 Bukise ft BD (ICI) 1.0 1.0 1.0

The paint thus obtained was evaluated for properties such as paint properties and concealing properties based on Japanese Industrial Standard JISK-5400, and the results are summarized in Table 6 below.

Table 6 Formulation number α 1 2 Να 3 Solid content (%) 50.2 50.2 50.5

PVC (%) Approx. 70 Approx. 70 Approx. 70

Substitution rate of Ti0 _z

(Weight ratio) (%) 14.8 14.8 Tunneling ratio (mirror gloss at 20

Degree) 黑 ground Z white background 0.940 0.942 0.940 60 with specular gloss of 2 or less 2 or less 2 or less Appearance (visual whiteness) good good good but slightly inferior to o.1 and 2 From the above results, heavy Coalescing number 0-183 can be used to improve whiteness without sacrificing any concealing properties, even if a part of titanium oxide is substituted. —Polymer number 0-183 Polymer powder number 0-183 The performance was almost the same as the above, and the performance decrease due to powdering was almost negligible. The invention's effect

The plastic pigment of the present invention is obtained by covering the surface of silica particles with a vinyl resin, and has a practically concealing property. The hollow plastic pigment having silica particles as the core of the present invention exhibits excellent effects in the following points as compared with the conventional hollow plastic pigment having no silica core. (1) In addition to the hollow structure, the light scattering effect of the sili force E is added to further improve the concealment.

<2) Excellent heat resistance at 100 ° C. Even with heating at 100 ° C. or more, there is little fusion of the hollow layer and the concealing property is not lost. (3) Dispersion in water is stable even with a relatively large particle size, and there is no sedimentation of particles over time. (4) Withstands intense drying such as dry spraying when obtained as a powder, and the obtained plastic pigment of powder (1) has a less concealing property than that of a water-dispersed plastic pigment.

The method of the present invention excludes emulsifiers for sily particles. This is convenient because the existing emulsion polymerization method can be used as it is. It has excellent stability during polymerization, and is easy to manufacture with little precipitation or adhesion of aggregates (gels). Industrial applicability

As a white plastic pigment, it can be used in paper, paints, fibers, etc.

Claims

Claimed area. A plastic pigment comprising silica particles as a core, the surface of which is coated with a bur resin.

A plastic pigment comprising a silica particle as a core and a hollow or porous layer interposed between the silica particle and the coating resin. The method for producing a plastic pigment according to claim 1, wherein the silicic acid particles are dispersed in water using sulfuric acid radicalion as an emulsifier, and a vinyl monomer is added to the dispersion to effect emulsion polymerization.

Using sulfuric acid radical as an emulsifier, disperse the silicic acid particles in water, add a carboxylic acid monomer to the emulsion, polymerize the emulsion, coat the surface of the silica particles, and add a vinyl monomer. The method for producing a plastic pigment according to claim 2, wherein after the emulsion polymerization, a basic compound is added to neutralize the carboxylic acid. Disperse silicic acid particles in water using sulfuric acid radical ion as an emulsifier, add vinyl carboxylate monomer and cross-linked monomer into it, emulsion polymerize and coat the surface of silica particles, The method for producing a plastic pigment according to claim 2, wherein the carboxylic acid is neutralized by adding a hydrophilic compound to form a hollow layer or a porous layer, and then a vinyl monomer is added and emulsion polymerization is performed to cover the carboxylic acid. .