JP7510733B2 - Hollow particles and their manufacturing method - Google Patents

Description

The present invention relates to hollow particles and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2022-055547, filed in Japan on March 30, 2022, the contents of which are incorporated herein by reference.

Traditionally, hollow particles, which have an internal space, have been used in a variety of fields. Hollow particles are particles in which gas is trapped inside a shell made of resin, and as such have properties such as being less susceptible to the transmission of heat and vibration, being easy to diffuse light, and being lightweight. Taking advantage of these properties, hollow particles are used in applications such as insulation, soundproofing, light diffusing materials, and weight reducing agents.

Patent Documents 1 and 2 describe methods for producing hollow particles having a shell formed of an organic polymer having a urethane or urea bond. These hollow particles have good affinity with organic resin materials such as urethane resins and urea resins, and are expected to stably maintain the hollow structure without cracking the shell even when added to an organic resin material and subjected to processing such as stirring. They are also said to have little impact on the environment and the human body.

Patent No. 6924533 International Publication No. 2019/181988

However, in the production of hollow particles described in Patent Document 1, lipophilic polymeric MDI is used as the isocyanate compound, which is the raw material for the shell. Since suspension polymerization in water is required to form hollow particles, a large amount of an acrylic polymer surfactant is added to emulsify the isocyanate. There is a concern that the hollow particles will be colored by the colors derived from these raw materials, making it difficult to develop applications such as cosmetics and light diffusing materials.

In addition, in the production of hollow particles described in Patent Document 2, a compound with a boiling point of 150°C or higher (specifically, limonene with a boiling point of 175°C) is blended into the reaction liquid in order to move the isocyanate in the oil phase to the oil-water interface and promote the formation of a hollow structure. This high-boiling compound is encapsulated in the hollow portion of the hollow particles formed in the reaction liquid, so in order to obtain the desired hollow particles, it is necessary to remove it from the hollow portion by heating to about 200°C. However, there is a concern that such high-temperature heating treatment will cause deterioration of the urethane resin that constitutes the shell portion. If the resin performance deteriorates, there is a risk that the dispersibility and durability of the hollow structure will deteriorate when it is blended into materials for various applications.

The present invention provides a method for producing hollow particles that can easily form hollow structures by utilizing the self-emulsifying properties of isocyanate, and hollow particles made from self-emulsifying isocyanate as a raw material.

[1] A hollow particle having an outer shell formed of a polymer having at least a urea bond among urethane bonds and urea bonds, and a hollow portion encapsulated in the outer shell, the polymer being a reaction product of an isocyanate component and at least water among water and a polyol, and the isocyanate component including at least a self-emulsifying isocyanate among a self-emulsifying isocyanate and a non-self-emulsifying isocyanate.
[2] The hollow particle according to [1], wherein the self-emulsifying isocyanate is a polyisocyanate having three or more isocyanate groups formed from at least one of an aliphatic diisocyanate and an alicyclic diisocyanate, and the polyisocyanate has a biuret structure, an isocyanurate structure, or an adduct structure in a molecule and further has a hydrophilic group.
[3] The hollow particle according to [1] or [2], wherein the proportion of the self-emulsifying isocyanate in the total mass of the isocyanate component is 10 to 100 mass%.
[4] The hollow particles according to any one of [1] to [3], having an average particle size of 0.1 to 300 μm.
[5] A method for producing the hollow particles according to any one of [1] to [4], comprising the steps of: heating a mixed liquid containing the isocyanate component, water, and an organic solvent while stirring, and suspension polymerizing the isocyanate component to form the outer shell portion, thereby obtaining a slurry containing resin particles having the water and the organic solvent encapsulated in the outer shell portion; and heating the slurry to remove the organic solvent from the resin particles (excluding the case where the mixed liquid contains 30% by mass or more of xylene and 1% by mass or more of a polyol having a hydroxyl value of less than 20 mg KOH, based on a reference mass obtained by excluding water from a total mass of the mixed liquid).
[6] The method for producing hollow particles according to [5], wherein a ratio (M2/M1) of a mass (M1) of the organic solvent contained in the mixed solution to a total mass (M2) of the isocyanate component contained in the mixed solution is 0.6 to 4.0.
[7] The method for producing hollow particles according to [5] or [6], wherein the temperature to which the slurry is heated is 100° C. or less.
[8] The method for producing hollow particles according to any one of [5] to [7], wherein the organic solvent is toluene, methyl ethyl ketone, or ethyl acetate.
[9] The method for producing hollow particles according to any one of [5] to [8], wherein a ratio (M3/M1) of a mass (M1) of the organic solvent contained in the mixed solution to a mass (M3) of the water contained in the mixed solution is 2.0 to 6.0.
[10] The method for producing hollow particles according to any one of [5] to [9], wherein the isocyanate component contained in the mixed solution contains the non-self-emulsifying isocyanate, and a ratio (M5/M4) of a mass (M4) of the non-self-emulsifying isocyanate contained in the mixed solution to a mass (M5) of the self-emulsifying isocyanate contained in the mixed solution is 1.0 to 10.0.

According to the method for producing hollow particles of the present invention, since a self-emulsifying isocyanate is used, not only is it not necessary to add a large amount of surfactant to emulsify the isocyanate in the reaction solution, but hollow particles can also be easily formed. In addition, since there is no need to use a high-boiling point compound as the organic solvent contained in the hollow portion of the hollow particles formed in the reaction solution, the organic solvent contained in the hollow particles can be easily removed without drying by heating at a high temperature, and there is no concern about deterioration of the target hollow particles due to high temperature heating. Furthermore, since there is no need to use a raw material that causes coloring, a process for decolorizing the hollow particles is also not required.
In the hollow particles of the present invention, the raw materials are simple, so that the inherent properties of the polymer having urethane bonds or urea bonds that constitutes the outer shell are fully exhibited, resulting in good dispersibility when blended with materials for various applications and good durability of the hollow structure.

1 is a SEM image of the hollow particles produced in Example 1. 1 is a SEM image of the hollow particles produced in Example 1.

In this specification, the "hydroxyl value" refers to the amount of potassium hydroxide required to acetylate the hydroxyl groups in a sample and neutralize the acetic acid used for the acetylation, expressed in mg per 1.0 g of the sample. In other words, the hydroxyl value is a measure of the content of hydroxyl groups in a sample. The hydroxyl value is measured using the neutralization titration method specified in JIS K 0070:1992.
The "average particle size" in the present invention refers to a volume-based average particle size (volume average particle size) measured using a laser diffraction particle size distribution analyzer (for example, SALD2100 manufactured by Shimadzu Corporation).
Preferred embodiments of the present invention will be described below, however, the present invention is not limited to the following embodiments.

<Hollow particles>
The first aspect of the present invention is a hollow particle having a shell formed by a polymer having at least a urea bond among urethane bonds and urea bonds, and a hollow portion enclosed in the shell. The hollow portion contains gas such as air, and does not substantially contain liquid such as water under normal dry conditions. When the hollow particle is observed under an electron microscope at a magnification that covers the entire hollow particle, the surface of the shell is basically smooth, and there is no through hole or porous structure that penetrates the shell and connects the hollow portion to the outside. The hollow portion of the hollow particle is often a single space, but may be divided into several spaces. Hollow particles having these structures are completely distinguished from porous particles.

The polymer is a reaction product of an isocyanate component with at least water among water and polyols. Typically, the reaction of water with an isocyanate forms a urea bond, and the reaction of a polyol with an isocyanate forms a urethane bond.
In this specification, polymers having polyurethane bonds or polyurea bonds may be generally referred to as polyurethane polyureas.
The isocyanate component includes at least a self-emulsifying isocyanate out of a self-emulsifying isocyanate and a non-self-emulsifying isocyanate.

<Isocyanate component>
[Self-emulsifying isocyanate]
The self-emulsifying isocyanate is an isocyanate compound that can be dispersed as fine particles in water when added to water and stirred. In one embodiment, when 1 g of the self-emulsifying isocyanate is added to 100 g of ion-exchanged water at 20° C. and gently stirred, the isocyanate compound does not settle to form oil puddles or oil droplets, but is dispersed in water.
Specifically, it is preferable that the compound is a polyisocyanate polymer formed from at least one of an aliphatic diisocyanate and an alicyclic diisocyanate, and has one of the structures of biuret, isocyanurate, and adduct in the molecule, to which a hydrophilic group (e.g., a hydroxy group, an oxyalkylene group, a carboxy group, etc.) is introduced. Here, the isocyanate group is not considered to be a hydrophilic group. In addition, even if the self-emulsifying isocyanate has multiple hydroxyl groups, it is not considered to be a polyol described below.

Of the above structures, the biuret structure and isocyanurate structure are formed by trimerization of diisocyanates (isocyanates having two isocyanate groups in the molecule). Trimers having an isocyanurate structure are sometimes called "nurate bodies." Adduct structures are formed, for example, by addition of diisocyanates to compounds having three or more active hydrogen groups in the molecule (e.g., polyols, polyamines, polythiols).

The self-emulsifying isocyanate is not particularly limited, and known isocyanates can be used. Specific examples of the self-emulsifying isocyanate include Burnock DNW-5500 and DNW-6000 (DIC Corporation), Basonat HW1000, HW180PC, LR9056, and LR9080 (BASF), Takenate WD-720, WD-725, WD-730, WB-700, WB-820, and WB-920 (Mitsui Chemicals, Inc.), Duranate WB40-100, WB40-80D, WT20-100, WT30-100, and WE50-100 (Asahi Kasei Chemicals Corporation), and Bayhydur. 3100, 304, 305, XP2451/1, XP2487/1, XP2547, XP2655, XP2700, DN, DA-L, 401-70 (Sumika Bayer Urethane Co., Ltd.), and the like.
The self-emulsifying isocyanates may be used alone or in combination of two or more kinds.

[Non-self-emulsifying isocyanate]
The isocyanate component may contain an isocyanate other than the self-emulsifying isocyanate, i.e., a non-self-emulsifying isocyanate. When a non-self-emulsifying isocyanate is contained, the average particle size of the hollow particles tends to be large.
The non-self-emulsifying isocyanate is an isocyanate that is not self-emulsifying. As a first embodiment, there is an isocyanate compound that is substantially unable to disperse as fine particles in water when added to water and stirred. As a second embodiment, there is an isocyanate compound that, when 1 g of the non-self-emulsifying isocyanate is added to 100 g of ion-exchanged water at 20° C. and gently stirred, settles and forms oil pools or oil droplets within a few seconds to a few minutes.
The non-self-emulsifying isocyanate is preferably a polyfunctional isocyanate having two or more isocyanate groups.
The non-self-emulsifying isocyanate is not particularly limited, and any known isocyanate can be used. Specific examples include aliphatic isocyanates, aromatic isocyanates, and modified polyisocyanates formed from these isocyanate monomers.

Aliphatic isocyanates include hexamethylene diisocyanate, 2,4-diisocyanate-1-1-methylcyclohexane, diisocyanate cyclobutane, tetramethylene diisocyanate, o-, m- or p-xylylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, dimethyldicyclohexylmethane diisocyanate, lysine diisocyanate, cyclohexane diisocyanate, dodecane diisocyanate, tetramethylxylene diisocyanate or isophorone diisocyanate.

Examples of aromatic isocyanates include isocyanate monomers such as tolylene-2,4-diisocyanate, tolylene-2,6-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3-methyldiphenylmethane-4,4'-diisocyanate, m- or p-phenylene diisocyanate, chlorophenylene-2,4-diisocyanate, naphthalene-1,5-diisocyanate, diphenyl-4,4'-diisocyanate, 3,3'-dimethyldiphenyl-1,3,5-triisopropylbenzene-2,4-diisocyanate carbodiimide-modified diphenylmetha diisocyanate, polyphenylpolymethylene isocyanate, and diphenyl ether diisocyanate.

Examples of modified polyisocyanates include polyurethane polyisocyanates obtained by reacting one or more types of excess isocyanate monomers with polyhydroxy compounds such as various polyhydric alcohols, polyisocyanates containing isocyanurate rings obtained by polymerizing isocyanate monomers, and polyisocyanates containing biuret bonds obtained by reacting isocyanate monomers with water.

Of the total mass of the isocyanate component, the proportion of self-emulsifying isocyanate is preferably 10 to 100% by mass, more preferably 20 to 80% by mass, and even more preferably 30 to 60% by mass. If it is equal to or greater than the lower limit of the above range, it becomes easier to form a single-space hollow structure. If it is less than the upper limit of the above range, 100% by mass, the average particle size of the hollow particles can be increased.

<Polyol>
Examples of polyols include polyethylene glycol (preferably having a mass average molecular weight Mw of 300 to 2000), polyester polyols, polyether polyols, polycarbonate polyols, polybutadiene polyols, acrylic polyols, and polycaprolactone polyols.
The polyol may be used alone or in combination of two or more kinds. The hydroxyl value of the polyol is preferably, for example, 20 to 2000 mgKOH/g.

When the isocyanate component reacts with the polyol to form a urethane bond, the polymer that makes up the outer shell contains a urethane bond, which gives the hollow particles flexibility and elasticity.

<Average particle size>
The average particle size (volume average particle size) of the hollow particles of this embodiment can be, for example, in the range of 0.1 μm to 300 μm, 1.0 μm to 200 μm, 3.0 μm to 100 μm, 5.0 μm to 50 μm, or 8.0 μm to 30 μm. This is appropriately selected depending on the application of the hollow particles.

<<Method for producing hollow particles>>
A second aspect of the present invention is a method for producing hollow particles, comprising a step of heating, with stirring, a mixed liquid (reaction liquid) containing an isocyanate component, water, and an organic solvent, and suspension polymerizing the isocyanate component to form the outer shell portion, thereby obtaining a slurry containing resin particles having the water and the organic solvent encapsulated in the outer shell portion (polymerization step), and a step of heating the slurry and removing the organic solvent from the resin particles (organic solvent removal step).
However, this does not include cases where the reaction liquid contains 30% by mass or more of xylene, or 1% by mass or more of a polyol having a hydroxyl value of less than 20 mg KOH, based on the total mass of the reaction liquid excluding water.
In other words, a reaction liquid containing 30% by mass or more of xylene relative to the reference mass is not used as the reaction liquid of this embodiment.Furthermore, a reaction liquid containing 1% by mass or more of a polyol having a hydroxyl value of less than 20 mg KOH relative to the reference mass is not used as the reaction liquid of this embodiment.
According to the production method of this embodiment, the hollow particles of the first embodiment can be easily produced.

If the reaction liquid contains a polyol with a hydroxyl value of less than 20 mg KOH, hollowing may be difficult. For this reason, the content of polyol with a hydroxyl value of less than 20 mg KOH is preferably as small as possible, more preferably less than 1% by mass, more preferably less than 0.1% by mass, and most preferably substantially absent, relative to the total mass of the reaction liquid excluding water.

<Polymerization step>
The explanation of the isocyanate component used in this step is the same as that of the isocyanate component in the first embodiment, and therefore the overlapping explanation will be omitted here.

The organic solvent contributes to the formation of the hollow structure of the hollow particles, and is encapsulated in the outer shell immediately after the hollow particles are formed in the reaction solution. In other words, particles (called resin particles to be distinguished from the target hollow particles) are formed in which water and the organic solvent are contained in the hollow parts of the target hollow particles. From the viewpoint of facilitating the removal of this organic solvent from the resin particles in a later step, the boiling point of the organic solvent at 1.0 atmospheric pressure is preferably less than 150°C, more preferably 130°C or less, and even more preferably 115°C or less. In addition, organic solvents that have an azeotropic point of 100°C or less when mixed with water in any ratio are preferred.

The smaller the amount of organic solvents (non-azeotropic organic solvents) that are difficult to form an azeotrope at 100°C or less when mixed with water in any ratio, or high-boiling organic solvents (for example, those with a boiling point at 1 atmospheric pressure of more than 115°C, more than 130°C, or more than 150°C), contained in the reaction liquid, the better. The content of non-azeotropic organic solvents or high-boiling organic solvents is preferably less than 30% by mass, more preferably less than 5% by mass, even more preferably less than 1% by mass, and most preferably substantially absent, based on a reference mass obtained by subtracting the mass of water from the total mass of the reaction liquid. For example, the content of xylene based on the reference mass is preferably less than 30% by mass, more preferably less than 5% by mass, even more preferably less than 1% by mass, and most preferably substantially absent.

Suitable organic solvents include, for example, aromatic compounds (e.g., toluene, benzene, etc.), ester compounds (e.g., methyl acetate, ethyl acetate, butyl acetate, etc.), ketone compounds (e.g., acetone, methyl ethyl ketone, etc.), saturated aliphatic hydrocarbons (e.g., n-heptane, n-hexane, n-octane, etc.), etc. Among these, from the above viewpoint, toluene, methyl ethyl ketone, and ethyl acetate are particularly preferred.
The organic solvent in this step is preferably an organic solvent other than polyol.
The organic solvent in this step may be used alone or in combination of two or more kinds.

The total content of the organic solvents relative to the total mass of the reaction liquid excluding the mass of water is preferably 20 to 50 mass %, more preferably 25 to 45 mass %, and even more preferably 30 to 40 mass %.
When the content is within the above preferred range, a hollow structure can be easily formed, and the organic solvent can be easily removed from the resin particles in a subsequent step.

The total content of the isocyanate components relative to the reference mass obtained by subtracting the mass of water from the total mass of the reaction liquid is preferably 50 to 80 mass%, more preferably 55 to 75 mass%, and even more preferably 60 to 70 mass%.
When the content is within the above preferred range, a hollow structure can be easily formed, and hollow particles having an average particle size within the above preferred range can be easily formed.

The content of the self-emulsifying isocyanate relative to the total mass of the reaction liquid excluding the mass of water is preferably 12 to 45 mass%, more preferably 15 to 40 mass%, and even more preferably 18 to 35 mass%.
When the content is within the above preferred range, a hollow structure can be easily formed, and hollow particles having an average particle size within the above preferred range can be easily formed.

When the reaction liquid contains a non-self-emulsifying isocyanate, the content of the non-self-emulsifying isocyanate relative to the reference mass obtained by subtracting the mass of water from the total mass of the reaction liquid is preferably 20 to 50 mass%, more preferably 25 to 45 mass%, and even more preferably 30 to 42 mass%.
When the content is within the above preferred range, a hollow structure can be easily formed, and hollow particles having an average particle size within the above preferred range can be easily formed.

The ratio (M2/M1) of the total mass (M1) of the organic solvents contained in the reaction liquid to the total mass (M2) of the isocyanate components contained in the reaction liquid is preferably 0.6 to 4.0, more preferably 1.0 to 3.0, and even more preferably 1.2 to 2.5.
When the content is within the above preferred range, a hollow structure can be easily formed, and the organic solvent can be easily removed from the resin particles in a subsequent step.

The ratio (M3/M1) of the total mass (M1) of the organic solvents contained in the reaction liquid to the mass (M3) of the water contained in the reaction liquid is preferably 1.5 to 6.0, more preferably 2.5 to 5.0, even more preferably 3.0 to 4.5, and particularly preferably 3.5 to 4.0.
When the content is within the above preferred range, a hollow structure can be easily formed, and the organic solvent can be easily removed from the resin particles in a subsequent step.

When the reaction liquid contains a non-self-emulsifying isocyanate, the ratio (M5/M4) of the mass (M4) of the non-self-emulsifying isocyanate to the mass (M5) of the self-emulsifying isocyanate contained in the reaction liquid is preferably 0.4 to 10.0, more preferably 0.6 to 2.0, even more preferably 0.7 to 1.5, particularly preferably 0.8 to 1.2, and most preferably 0.9 to 1.1.
When the content is within the above preferred range, a hollow structure can be easily formed, and hollow particles having an average particle size within the above preferred range can be easily formed.

The reaction solution thus prepared can be heated while being stirred in the usual manner. The temperature of the reaction solution during heating is preferably, for example, 50 to 80°C. At this preferred temperature, the polymerization reaction can be completed in about 3 to 12 hours. After the reaction is complete, proceed to the next step.

[Suspension stabilizers/surfactants]
A suspension stabilizer and a surfactant that contribute to stabilizing particle synthesis may be added to the reaction liquid.
Examples of suspension stabilizers include water-soluble cellulose resins (e.g., methyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyacrylates, polyvinylpyrrolidone, polyacrylamide, tertiary phosphates, gelatin, etc.
The suspension stabilizer may be used alone or in combination of two or more kinds. The amount of the suspension stabilizer added is preferably 0 to 30 parts by mass per 100 parts by mass of the bead raw material (raw material for forming particles other than organic solvent and water). If the amount of the suspension stabilizer added is within the above range, the suspension state can be sufficiently stabilized.
In order to stabilize the suspension, a surfactant may be added to the water. The surfactant may be any of anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants, and two or more of them may be used.
The suspension stabilizer and surfactant are to be distinguished from the above-mentioned polyol, which is a material for forming particles.

<Organic solvent removal process>
The slurry containing the resin particles in the solution after the reaction obtained in the previous polymerization step is heated to remove the organic solvent contained in the resin particles. From the viewpoint of suppressing deterioration of the outer shell of the resin particles, the heating temperature of the slurry is preferably less than 150° C., more preferably 130° C. or less, and even more preferably 115° C. or less. At this preferred temperature, the organic solvent in the resin particles can be removed preferably for 0.5 to 3 hours, more preferably 1 to 2 hours.
If the organic solvent used in the previous polymerization step has an azeotropic point of 100° C. or less when mixed with water in an arbitrary ratio, it can be removed more easily because it can form an azeotrope with water by heating in this step. It is not necessary to seal and pressurize the slurry when heating it.

<Drying process>
The slurry that has been subjected to the organic solvent removal step in the previous stage contains water and resin particles. Since moisture may remain in the hollows of the resin particles obtained by solid-liquid separation of the slurry, it is preferable to dry the resin particles in a dry atmosphere. The drying process can completely remove water from the hollows. The drying temperature is preferably, for example, 50 to 80°C. At this preferable temperature, the drying process can be completed in about 12 to 24 hours.
Through the above steps, the desired hollow particles are obtained.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

The volume average particle diameter of the hollow particles after drying produced in each example is the volume-based average particle diameter measured using a laser diffraction particle size distribution meter (SALD2100 manufactured by Shimadzu Corporation).

Example 1
A 2 L separable flask equipped with a stirrer was charged with 600 g of water, and 5.0 g of gelatin was dissolved therein to prepare a dispersion medium for dispersing the resin particles formed later in a slurry.
Separately, a prepolymer liquid was prepared by mixing 160.0 g of toluene, 120.0 g of Duranate WB40-100 which is a self-emulsifying isocyanate, 120.0 g of isophorone diisocyanate nurate which is a non-self-emulsifying isocyanate, and 12.0 mg of dibutyltin dilaurate which is a urethanization catalyst.
The prepolymer liquid was added to the dispersion medium while stirring the mixture at a rotation speed of 400 rpm to prepare a polymerization suspension. Next, while continuing stirring, the polymerization suspension was heated to 70°C and reacted for 5 hours to form polyurethane polyurea that becomes the outer shell of the hollow particles, and resin particles containing toluene and water were obtained (polymerization step). After that, the temperature was raised to 100°C and kept for 1 hour to volatilize toluene from the inside of the resin particles (organic solvent removal step). As a result, a slurry in which the resin particles from which toluene had been removed were dispersed in water was obtained.
Next, the slurry was cooled to room temperature, and then subjected to solid-liquid separation by filtration. The recovered resin particles were thoroughly washed with water and then dried at 70°C for 20 hours to thoroughly remove the water remaining inside, obtaining spherical hollow particles with a volume average particle size of 8.6 μm. In order to confirm the internal structure of the hollow particles, SEM images of hollow particles embedded in a fixing resin and sliced into cross sections are shown in Figures 1 and 2. The SEM images confirmed that the hollow particles were hollow particles with an outer shell surrounding a single internal space.

(Examples 2 to 14)
Spherical hollow particles were obtained in the same manner as in Example 1, except that the type or amount of the organic solvent, the type or amount of the polymerization raw material for forming the outer shell, the amount of the urethanization catalyst, the amount of the polyol, and the amount of the dispersion medium were changed as shown in Table 1.
In the table, the "ratio of self-emulsifying isocyanate in the polymerization raw material for forming the outer shell" is the mass % of the self-emulsifying isocyanate for forming the outer shell when the total amount of the self-emulsifying isocyanate for forming the outer shell and the non-self-emulsifying isocyanate for forming the outer shell is taken as 100 mass %.
The "organic solvent ratio" in the table is the mass % of the organic solvent when the total amount of the polymerization raw material for forming the outer shell portion and the organic solvent for forming the hollow portion is taken as 100 mass %.
The non-self-emulsifying isocyanate used in Examples 5 and 6 was hexamethylene diisocyanate nurate.
The polyol used in Examples 12 to 14 was polyethylene glycol (molecular weight: 600, hydroxyl value: 187 mg KOH/g) or polyethylene glycol (molecular weight: 1000, hydroxyl value: 112.2 mg KOH/g).
The hollow particles produced in Examples 2 to 14 were also photographed with an SEM (not shown), and it was confirmed that the outer shell portion surrounded a single internal space.

(Comparative Example 1)
A 2 L separable flask equipped with a stirrer was charged with 600 g of water, and 5.0 g of gelatin was dissolved therein to prepare a dispersion medium.
Separately, a prepolymer liquid was prepared by mixing 160.0 g of toluene, 240.0 g of isophorone diisocyanate nurate, and 12.0 mg of dibutyltin dilaurate as a urethanization catalyst.
The prepolymer liquid was added to the dispersion medium while stirring the mixture at a rotation speed of 400 rpm to prepare a polymerization suspension. Next, while continuing stirring, the polymerization suspension was heated to 70°C and reacted for 5 hours to form polyurethane polyurea that would become the outer shell, thereby obtaining resin particles (polymerization step). After that, the temperature was raised to 100°C and maintained for 1 hour to volatilize toluene that may have been encapsulated inside the resin particles (organic solvent removal step). This resulted in a slurry in which the resin particles were dispersed in water.
Next, the slurry was cooled to room temperature, and then filtered to separate the solid and liquid. The collected resin particles were thoroughly washed with water and then dried at 70° C. for 20 hours to obtain spherical resin particles with a volume average particle size of 185.9 μm. When the internal structure of the resin particles was confirmed using an SEM image (not shown), no hollowing occurred at all. This was thought to be due to the fact that the self-emulsified isocyanate was not mixed into the prepolymer liquid.

(Comparative Example 2)
A 2 L separable flask equipped with a stirrer was charged with 600 g of water, and 5.0 g of gelatin was dissolved therein to prepare a dispersion medium.
Separately, a prepolymer liquid was prepared by mixing 160.0 g of xylene, 120.0 g of Duranate WB40-100, 120.0 g of isophorone diisocyanate nurate, and 12.0 mg of dibutyltin dilaurate as a urethanization catalyst.
The prepolymer liquid was added to the dispersion medium while stirring the stirring machine at a rotation speed of 400 rpm to prepare a polymerization suspension. Next, while continuing stirring, the polymerization suspension was heated to 70 ° C. and reacted for 5 hours to form polyurethane polyurea to become the outer shell, and resin particles were obtained (polymerization step). After that, the temperature was raised to 100 ° C. and maintained for 1 hour to attempt to remove xylene from the inside of the resin particles (organic solvent removal step). However, at 1013 hPa and 100 ° C. or less, xylene did not azeotrope with water and xylene did not volatilize, so a slurry was obtained in which resin particles containing xylene were dispersed in water.
Next, the slurry was cooled to room temperature, and then filtered to separate the solid and liquid. The collected resin particles were thoroughly washed with water and then dried at 70°C for 20 hours to obtain spherical resin particles with a volume average particle size of 10.0 μm. The xylene contained in the resin particles could not be removed, and the desired hollow particles could not be obtained. The reason for this was thought to be that the boiling point of the selected organic solvent was high and it was difficult to form an azeotrope with water.

(Example 8)
A 2 L separable flask equipped with a stirrer was charged with 600 g of water, and 5.0 g of gelatin was dissolved therein to prepare a dispersion medium.
Separately, a prepolymer liquid was prepared by mixing 60.0 g of toluene, 170.0 g of Duranate WB40-100, 170.0 g of isophorone diisocyanate nurate, and 17.0 mg of dibutyltin dilaurate as a urethanization catalyst.
The prepolymer liquid was added to the dispersion medium while stirring the mixture at a rotation speed of 400 rpm to prepare a polymerization suspension. Next, while continuing stirring, the polymerization suspension was heated to 70°C and reacted for 5 hours to form polyurethane polyurea that would become the outer shell, thereby obtaining resin particles (polymerization step). After that, the temperature was raised to 100°C and maintained for 1 hour to volatilize toluene that may have been encapsulated inside the resin particles (organic solvent removal step). This resulted in a slurry in which the resin particles were dispersed in water.
Next, the slurry was cooled to room temperature, and then filtered to separate the solid and liquid. The collected resin particles were thoroughly washed with water and then dried at 70°C for 20 hours to obtain spherical resin particles with a volume average particle size of 6.5 μm. The internal structure of the polyurethane particles was confirmed using multiple SEM images, and although a small number of particles were hollowed out (not shown), the majority of the particles were not hollowed out. This result is thought to indicate that the proportion of hollow particles decreases when the amount of organic solvent added is small.

(Comparative Example 3)
A 2 L separable flask equipped with a stirrer was charged with 600 g of water, and 5.0 g of gelatin was dissolved therein to prepare a dispersion medium.
Separately from this, a prepolymer liquid was prepared by mixing 160.0 g of toluene, 120.0 g of Duranate WB40-100, 120.0 g of isophorone diisocyanate nurate, 8.0 g of polyethylene glycol (molecular weight: 6000, hydroxyl value: 18.7 mg KOH/g), and 12.0 mg of dibutyltin dilaurate as a urethanization catalyst.
The prepolymer liquid was added to the dispersion medium while stirring the mixture at a rotation speed of 400 rpm to prepare a polymerization suspension. Next, while continuing stirring, the polymerization suspension was heated to 70°C and reacted for 5 hours to form polyurethane that would become the outer shell, thereby obtaining resin particles (polymerization step). After that, the temperature was raised to 100°C and maintained for 1 hour to volatilize toluene that may have been encapsulated inside the resin particles (organic solvent removal step). This resulted in a slurry in which the resin particles were dispersed in water.
Next, the slurry was cooled to room temperature, and then filtered to separate the solid and liquid. The collected resin particles were thoroughly washed with water and then dried at 70°C for 20 hours to obtain spherical resin particles with a volume average particle size of 27.0 μm. The internal structure of the polyurethane particles was confirmed using multiple SEM images, and although not shown, almost all of the particles were not hollow. This result is thought to indicate that when the hydroxyl value of the polyol is low, the proportion of hollow particles is extremely reduced.

Claims (10)

The hollow particles have an outer shell formed of a polymer having at least a urea bond among a urethane bond and a urea bond, and a hollow portion enclosed in the outer shell,
The polymer is a reaction product of an isocyanate component with water and a polyol, and
The isocyanate component is a hollow particle containing at least a self-emulsifying isocyanate out of a self-emulsifying isocyanate and a non-self-emulsifying isocyanate. The self-emulsifying isocyanate is a polyisocyanate having three or more isocyanate groups formed from at least one of an aliphatic diisocyanate and an alicyclic diisocyanate,
The hollow particle according to claim 1 , wherein the polyisocyanate has a biuret structure, an isocyanurate structure, or an adduct structure in a molecule, and further has a hydrophilic group. The hollow particles described in claim 2, wherein the proportion of the self-emulsifying isocyanate in the total mass of the isocyanate component is 10 to 100 mass%. Hollow particles according to claim 3, having an average particle diameter of 0.1 to 300 μm. A method for producing the hollow particles according to any one of claims 1 to 4, comprising the steps of:
a step of heating a mixed liquid containing the isocyanate component, water, and an organic solvent while stirring, and suspension-polymerizing the isocyanate component to form the outer shell portion, thereby obtaining a slurry containing resin particles having the water and the organic solvent encapsulated in the outer shell portion;
a step of heating the slurry to remove the organic solvent from the resin particles (excluding the case where the mixed liquid contains 30% by mass or more of xylene and 1% by mass or more of a polyol having a hydroxyl value of less than 20 mg KOH, based on a reference mass obtained by excluding water from a total mass of the mixed liquid). The method for producing hollow particles described in claim 5, wherein the ratio (M2/M1) of the mass (M1) of the organic solvent contained in the mixed liquid to the total mass (M2) of the isocyanate component contained in the mixed liquid is 0.6 to 4.0. The method for producing hollow particles described in claim 6, wherein the temperature to which the slurry is heated is 100°C or less. The method for producing hollow particles according to claim 7, wherein the organic solvent is toluene, methyl ethyl ketone, or ethyl acetate. The method for producing hollow particles described in claim 8, wherein the ratio (M3/M1) of the mass (M1) of the organic solvent contained in the mixed liquid to the mass (M3) of the water contained in the mixed liquid is 2.0 to 6.0. The isocyanate component contained in the mixed liquid contains the non-self-emulsifying isocyanate,
10. The method for producing hollow particles according to claim 9, wherein a ratio (M5/M4) of a mass (M4) of the non-self-emulsifying isocyanate contained in the mixed solution to a mass (M5) of the self-emulsifying isocyanate contained in the mixed solution is 1.0 to 10.0.