JP7518369B2 - Method for producing concave-shaped resin particles - Google Patents

Description

The present invention relates to a method for producing concave-shaped resin microparticles.

Irregular particles have excellent light diffusion properties due to their unique shape, and are expected to be used in light diffusion sheets, light guide plates, etc. (Patent Document 1). Particles of various shapes are known as irregular particles, but for example, particles with irregularities have high water and oil absorption due to their shape, and are considered to be suitable for use in cosmetics, etc. (Patent Document 2, Patent Document 3).

Patent No. 3827617 Patent No. 3229011 Patent No. 3574673

Due to their unique shape, irregularly shaped particles are often superior in terms of functionality compared to typical spherical particles, but they often contain a large amount of impurities during the manufacturing process, in which case a process is required to remove the impurities after the particles are obtained by polymerization. The manufacturing methods for irregularly shaped microparticles in Patent Documents 1 to 3 also require the addition of large amounts of hydrophobic substances that are not involved in the polymerization reaction during the manufacturing process, and removing these hydrophobic substances makes the manufacturing process complicated. Furthermore, if a process for removing these hydrophobic substances is not provided, the hydrophobic substances remaining in the particles will become impurities and may cause defects.

The inventors have conducted extensive research to find a simple method for obtaining irregularly shaped particles, and have discovered that by using a surfactant with a specific structure and a water-soluble initiator in the polymerization of resin microparticles, concave-shaped resin microparticles can be obtained without adding hydrophobic substances that are not involved in the polymerization, such as liquid saturated hydrocarbons such as n-hexane, n-heptane, and liquid paraffin, or silicone oil.

The present invention aims to provide a simple method for producing concave-shaped resin microparticles.

[1] A method for producing concave-shaped resin microparticles, comprising the steps of dispersing, suspending, and polymerizing a polymerizable monomer having a polymerizable unsaturated hydrocarbon group in a dispersion medium mainly composed of water to obtain concave-shaped resin microparticles, using at least a surfactant represented by formula (1) as a dispersant and using a water-soluble initiator, comprising the steps of:
2. A method for producing concave-shaped resin microparticles, wherein the amount of the water-soluble initiator used is less than 3.0 parts by mass per 100 parts by mass of the polymerizable monomer used.
T ¹ O-(RO) _n (EO) _m -T ² ...(1)
In formula (1), ^T1 is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or an alkenyl group having 2 to 18 carbon atoms; ^T2 is a hydrogen atom, a sulfonic acid group, a sulfonate group, a carboxylate group, a carboxylate group, a phosphoric acid group, a phosphate group, an amino group, or an ammonium group; RO is an oxyalkylene group having 3 to 18 carbon atoms; n is an integer from 1 to 50; EO is an oxyethylene group; and m is an integer from 0 to 200.
[2] The method for producing concave-shaped resin microparticles according to [1], wherein the water-soluble initiator is an inorganic peroxide.
[3] The method for producing concave-shaped resin microparticles according to [2], wherein the inorganic peroxide is at least one selected from the group consisting of potassium peroxodisulfate, ammonium peroxodisulfate, sodium peroxodisulfate, and hydrogen peroxide.
[4] The method for producing concave-shaped resin microparticles according to any one of [1] to [3], wherein in formula (1), T ¹ is an alkenyl group having 1 to 18 carbon atoms.
[5] The method for producing hollow shaped resin microparticles according to any one of [1] to [4], wherein the hollow shaped resin microparticles have an average particle size of 0.5 to 500 μm.
[6] The method for producing concave-shaped resin microparticles according to any one of [1] to [5], further comprising adding an aqueous solution containing the water-soluble initiator to a first dispersion in which the polymerizable monomer and the surfactant are dispersed, to prepare a second dispersion in which the polymerizable monomer constitutes a continuous phase and water constitutes a dispersed phase.
[7] The method for producing concave-shaped resin microparticles described in [6], comprising mixing the second dispersion with an aqueous solution containing a second surfactant to obtain a third dispersion having water as a continuous phase and the polymerizable monomer as a dispersed phase.

The present invention provides a simple method for producing concave-shaped resin microparticles.

FIG. 2 is a diagram for illustrating a schematic diagram of step 1 of the method for producing concave-shaped resin microparticles according to the embodiment of the present invention. FIG. 2 is a diagram for illustrating a schematic diagram of step 1 of the method for producing concave-shaped resin microparticles according to the embodiment of the present invention. FIG. 2 is a diagram for illustrating a schematic diagram of step 1 of the method for producing concave-shaped resin microparticles according to the embodiment of the present invention. FIG. 2 is a diagram for illustrating a schematic diagram of step 2 of the method for producing concave-shaped resin microparticles according to an embodiment of the present invention. FIG. 2 is a diagram for illustrating a schematic diagram of step 2 of the method for producing concave-shaped resin microparticles according to an embodiment of the present invention. FIG. 2 is a diagram for illustrating a schematic diagram of step 3 of the method for producing concave-shaped resin microparticles according to an embodiment of the present invention. 2 is a scanning electron microscope photograph of concave-shaped resin fine particles of Example 1. 1 is a scanning electron microscope photograph of concave-shaped resin fine particles of Example 4. 1 is a scanning electron microscope photograph of hollow-shaped resin fine particles of Example 5. 1 is a scanning electron microscope photograph of the resin fine particles of Comparative Example 1. 1 is a scanning electron microscope photograph of the resin fine particles of Comparative Example 2.

Below, a form for implementing the present invention will be described, but the present invention is not limited to the embodiments described below, and various modifications are possible as long as they do not change the gist of the present invention.
In the present invention, the numerical range expressed using "to" includes the numerical values on both sides of "to".
In the present invention, "(meth)acrylic" is a general term for "acrylic" and "methacrylic". Similarly, "(meth)acrylic group" is a general term for "acrylic group" and "methacrylic group", "(meth)acrylic acid" is a general term for acrylic acid and methacrylic acid, and "(meth)acrylate" is a general term for acrylate and methacrylate.
In the present invention, the "polymerization initiator" may be referred to as an "initiator".

[Method of manufacturing concave-shaped resin fine particles]
A method for producing concave-shaped resin microparticles according to one embodiment of the present invention comprises dispersing a polymerizable monomer having a polymerizable unsaturated hydrocarbon group in a dispersion medium mainly composed of water, suspending it, and polymerizing it to obtain concave-shaped resin microparticles, using at least a surfactant represented by formula (1) as a dispersant and using a water-soluble initiator in the polymerization of the resin microparticles.
T ¹ O-(RO) _n (EO) _m -T ² ...(1)
In formula (1):
^T1 is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or an alkenyl group having 2 to 18 carbon atoms.
^T2 is a hydrogen atom, a sulfonic acid group, a sulfonate group, a carboxylic acid group, a carboxylate group, a phosphoric acid group, a phosphate group, an amino group or an ammonium group.
RO is an oxyalkylene group having 3 to 18 carbon atoms.
EO is an oxyethylene group.
m is an integer from 0 to 200.
n is an integer from 1 to 50.

According to the method for producing concave-shaped resin microparticles of this embodiment, it is possible to obtain resin microparticles having a smooth particle surface, partially open particles, and concave shapes having a hollow structure inside the particles.
In this embodiment, the term "concave-shaped resin fine particles" is defined to include resin fine particles having a concave shape with a part of the particle being open and having a hollow structure inside the particle.

The term "water-based dispersion medium" is not particularly limited as long as it does not impair the objectives and effects of the present invention, but the water content in the dispersion medium is usually 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more.

The components used in the method for producing concave-shaped resin microparticles according to this embodiment, the production method, and the resulting concave-shaped resin microparticles are described in detail below.

[Structural component]
Surfactant A
T ¹ O-(RO) _n (EO) _m -T ² ...(1)
In formula (1):
^T1 is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or an alkenyl group having 2 to 18 carbon atoms.
^T2 is a hydrogen atom, a sulfonic acid group, a sulfonate group, a carboxylic acid group, a carboxylate group, a phosphoric acid group, a phosphate group, an amino group or an ammonium group.
RO is an oxyalkylene group having 3 to 18 carbon atoms.
EO is an oxyethylene group.
m is an integer from 0 to 200.
n is an integer from 1 to 50.

Examples of commercially available anionic surfactants for the surfactant A represented by formula (1) include AQUALON KH series and HITENO XJ-630S (both manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and RAMTEL PD-104 and RAMTEL PD-105 (both manufactured by Kao Corporation).
Examples of commercially available nonionic surfactants include the Noigen XL series (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), Ramtel PD-420, Ramtel PD-430, Ramtel PD-450, Emulgen LS series, Emulgen MS series, and Emulgen PP series (all manufactured by Kao Corporation), and the Finesurf NDB series, IDEP series, Wondersurf NDR series, ID series, and S series (all manufactured by Aoki Oil Industries Co., Ltd.).

The amount of the surfactant used in the present embodiment is not particularly limited, but is preferably 0.05 to 5.0 parts by mass with respect to 100 parts by mass of the polymerizable monomer described below.
When the amount of surfactant A used is within the above range, concave shaped resin fine particles can be easily obtained stably.
When the amount of surfactant A used is within the above range, the hollow structure of the concave-shaped resin fine particles is likely to be complete, and the properties of the resin constituting the particles are unlikely to be impaired.
Furthermore, in the case where a surfactant remains in the obtained resin fine particles having concave shapes, T ¹ is preferably an alkenyl group having 2 to 18 carbon atoms, since bleeding can be suppressed.

In this specification, surfactant A may be referred to as "surfactant represented by formula (1) (first surfactant)" or as "second surfactant."
Surfactant A is sometimes referred to as the "first surfactant" or the "second surfactant" because, in the manufacturing process described in detail below, surfactant A may be used separately multiple times to prepare resin microparticles.
That is, in the preparation of resin microparticles, for example, when surfactant A is added in two separate steps, when it is necessary to distinguish between the two steps, it may be referred to as the "first surfactant" and the "second surfactant" for ease of understanding.
As the "second surfactant", a surfactant different from the above-mentioned surfactant A may be used as long as it does not impair the effects of the present invention.
In addition, instead of the "second surfactant", an "optional dispersant" described later may be used within a range that does not impair the effects of the present invention.
The "second surfactant" and the "optional dispersant" may be used alone or in combination of two or more kinds.
In addition, in the preparation of resin microparticles, for example, even when surfactant A is added in two separate steps, the total amount of surfactant A used is preferably 0.05 to 5.0 parts by mass relative to 100 parts by mass of polymerizable monomer A described later.

<Polymerizable monomer>
The polymerizable monomer is a polymerizable monomer having a polymerizable unsaturated hydrocarbon group, and examples thereof include a (meth)acrylic monomer and an aromatic vinyl monomer.
In this specification, the "polymerizable monomer having a polymerizable unsaturated hydrocarbon group" may be simply referred to as the "polymerizable monomer".

Examples of (meth)acrylic monomers include monofunctional (meth)acrylates and polyfunctional (meth)acrylates having difunctional or higher polymerizable functional groups.

Specific examples of the monofunctional (meth)acrylate include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, and octyl (meth)acrylate; (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-(n-propoxy)ethyl (meth)acrylate, 2-(n-butoxy)ethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 2-(n-propoxy)propyl (meth)acrylate, and 2-(n-butoxy)propyl (meth)acrylate; and (meth)acrylic acid esters having an alicyclic structure such as cyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and isobornyl (meth)acrylate. These may be used alone or in combination of two or more.

Examples of the polyfunctional (meth)acrylate include (poly)ethylene glycol di(meth)acrylate with 1 to 9 ethylene oxide (EO), alkylene glycol di(meth)acrylate with 4 to 9 carbon atoms, and (meth)acrylic acid esters having at least two or more (meth)acrylic groups, such as trimethylolpropane tri(meth)acrylate. These may be used alone or in combination of two or more.

Examples of the aromatic vinyl monomer include styrene-based monomers and divinylbenzene, with styrene-based monomers being preferred.
Examples of the styrene monomer include styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, etc. These may be used alone or in combination of two or more.

In the polymerizable monomer according to the present embodiment, the (meth)acrylic monomer may be used in combination with the monofunctional (meth)acrylate and the polyfunctional (meth)acrylate, or the (meth)acrylic monomer may be used in combination with an aromatic vinyl monomer.
The ratio of the monofunctional (meth)acrylate, the polyfunctional (meth)acrylate, and the aromatic vinyl monomer in the polymerizable monomer when producing the hollow resin microparticles according to this embodiment is not particularly limited, but may be, for example, 20 to 95 parts by mass of the monofunctional (meth)acrylate, 1 to 30 parts by mass of the polyfunctional (meth)acrylate, and 10 to 50 parts by mass of the aromatic vinyl monomer.
The other monomer is not particularly limited as long as it is copolymerizable with the acrylic monomer without impairing the effects of the present invention. For example, vinyl acetate, vinyl propionate, vinyl chloride, acrylonitrile, etc. may be used.
If necessary, additives such as a chain transfer agent may be used.

<Oil-soluble initiator>
Examples of oil-soluble initiators include peroxide-based initiators such as benzoyl peroxide and lauroyl peroxide, and oil-soluble azo-based initiators such as 2,2'-azobis(2-methylbutyronitrile) and 2,2'-azobis(isobutyronitrile).

The amount of the oil-soluble initiator used is preferably 0.05 to 3.0 parts by mass, more preferably 0.1 to 2.0 parts by mass, and even more preferably 0.2 to 1.5 parts by mass, based on 100 parts by mass of the polymerizable monomer.
When the amount of the oil-soluble initiator used is 0.05 parts by mass or more, the proportion of unreacted monomers in the polymerizable monomer can be further reduced.
On the other hand, when the amount of the oil-soluble initiator used is 3.0 parts by mass or less, it is possible to further suppress the decomposition products of the oil-soluble initiator from remaining as impurities.

<Water-soluble initiator>
Examples of the water-soluble initiator include inorganic peroxides such as potassium peroxodisulfate (potassium persulfate), ammonium peroxodisulfate, sodium peroxodisulfate, and aqueous hydrogen peroxide.
The water-soluble initiators may be used alone or in combination of two or more kinds.
The water-soluble initiator is preferably at least one selected from the group consisting of potassium peroxodisulfate (potassium persulfate), ammonium peroxodisulfate, sodium peroxodisulfate, and aqueous hydrogen peroxide, and more preferably potassium peroxodisulfate.

The amount of the water-soluble initiator used is preferably 0.05 to 3.0 parts by mass, more preferably 0.1 to 2.0 parts by mass, and even more preferably 0.2 to 1.0 parts by mass, based on 100 parts by mass of the polymerizable monomer.
When the amount of the water-soluble initiator used is 0.05 parts by mass or more with respect to 100 parts by mass of the polymerizable monomer, particles having a concave shape can be obtained.
On the other hand, when the amount of the water-soluble initiator used is 3.0 parts by mass or less with respect to 100 parts by mass of the polymerizable monomer, the particle shape can be maintained.

<Optional Surfactant>
As the surfactant other than surfactant A, any of the surfactants shown below (surfactants different from surfactant A) may be used as long as the effects of the present invention are not impaired.
In other words, in addition to the surfactant A, an appropriate surfactant may be added as long as the effects of the present invention are not impaired.

The optional surfactant may be, for example, an anionic surfactant or a cationic surfactant.

Specific examples of anionic surfactants include higher fatty acid salts such as sodium oleate and sodium stearate; alkyl (or aryl) sulfonates such as sodium dodecylbenzenesulfonate and disodium dodecyldiphenylethersulfonate; alkyl (or alkenyl) sulfates such as sodium lauryl sulfate and sodium oleyl sulfate; polyoxyethylene alkyl (or alkenyl) ether sulfates such as sodium polyoxyethylene lauryl ether sulfate, ammonium polyoxyethylene oleyl ether sulfate, and ammonium polyoxyethylene styrenated phenyl ether sulfate; polyoxyethylene alkylaryl ether sulfates such as sodium polyoxyethylene nonylphenyl ether sulfate; alkyl sulfosuccinates such as sodium monooctyl sulfosuccinate, sodium di-2-ethylhexyl sulfosuccinate, sodium dioctyl sulfosuccinate, and sodium polyoxyethylene lauryl sulfosuccinate, or derivatives thereof. These anionic surfactants may be used alone or in combination of two or more.

Specific examples of cationic surfactants include quaternary ammonium salts such as alkylbenzylmethylammonium salts, such as dodecylbenzylmethylammonium chloride; alkyltrimethylammonium salts, such as dodecyltrimethylammonium chloride, stearyltrimethylammonium chloride, and cetyltrimethylammonium chloride; dialkyldimethylammonium salts, such as didecyldimethylammonium chloride and distearyldimethylammonium chloride; and alkylbenzyldimethylammonium salts, such as dodecylbenzyldimethylammonium chloride. These cationic surfactants may be used alone or in combination of two or more.

When the optional surfactant is used, the amount thereof is preferably 0.05 to 5.0 parts by mass, more preferably 0.05 to 3.0 parts by mass, and even more preferably 0.05 to 1.5 parts by mass, based on 100 parts by mass of the polymerizable monomer.
When the amount of the optional dispersant used is within the above range, concave particles can be obtained stably and the amount of remaining impurities can be reduced.

<Optional Dispersant>
In order to improve the stability of the polymerization, any dispersant may be used within a range that does not impair the effects of the present invention. Examples of the optional dispersant include the following.
Examples of the organic dispersant include polyvinyl alcohol, cellulose, and polyvinylpyrrolidone.
Examples of inorganic dispersants include calcium tertiary phosphate and calcium carbonate.
These dispersants may be used alone or in combination of two or more kinds.

When the optional dispersant is used, the amount thereof is preferably 0.05 to 5.0 parts by mass, more preferably 0.05 to 3.0 parts by mass, and even more preferably 0.05 to 1.5 parts by mass, based on 100 parts by mass of the polymerizable monomer.
When the amount of the optional dispersant used is within the above range, the amount of remaining impurities can be reduced while maintaining stability during polymerization.

<Dispersion medium>
As the dispersion medium, known water such as ion-exchanged water may be used alone, or a known solvent miscible with water such as alcohol may be used in combination as appropriate.
In addition, the "dispersion medium mainly composed of water" in this embodiment may be a combination of water and a known solvent that is miscible with water, such as an alcohol, within a range that does not impair the object and effect of the present invention. Although not particularly limited, the water content in the dispersion medium may be 50% or more, 70% or more, or 90% or more, or the dispersion medium may be only water.
In this specification, like the surfactant, the water used as the dispersion medium or solvent may be referred to as "first water" or "second water".
For example, when water is added in two separate steps, the water may be referred to as "first water" and "second water" to distinguish between the two steps for ease of understanding.

<Method for producing concave-shaped resin fine particles>
Schematic diagrams and schematic diagrams of the suspension polymerization method according to this embodiment are shown in FIGS. 1A to 1F.
The detailed process is as follows.

(Step 1)
A mixed liquid (oil-soluble initiator-polymerizable monomer 4-surfactant A mixed liquid, 1A mixed liquid, first dispersion) 1 of an oil-soluble initiator, a polymerizable monomer 4, and a surfactant A (first surfactant) represented by formula (1) is prepared, and the oil-soluble initiator-polymerizable monomer 4-surfactant A mixed liquid 1 is stirred using a stirrer 100 such as a homomixer ( FIG. 1A ).
Water having a water-soluble initiator dissolved therein (second A mixed liquid) 2 is added to the stirred oil-soluble initiator-polymerizable monomer 4-surfactant A mixed liquid 1, and dispersed to prepare a dispersion 11 (a W/O (Water in Oil) type dispersion, a second dispersion) having a continuous phase of the polymerizable monomer 4 and a dispersed phase of water 6 (FIGS. 1B and 1C).
The amount of the water-soluble initiator used is usually less than 3.0 parts by mass, preferably 0.01 to 2.0 parts by mass, and more preferably 0.01 to 1.0 parts by mass, based on 100 parts by mass of the polymerizable monomer 4 used.
That is, a dispersion 11 is obtained in which the second A mixture 2 is dispersed in the oil-soluble initiator-polymerizable monomer-surfactant A mixture 1.

(Step 2)
After step 1, water mixed with a surfactant A (second surfactant) represented by formula (1) (surfactant A-water mixed liquid, third A mixed liquid) 3 is added to the dispersion 11 being stirred by a stirrer 100, thereby producing a suspension (W/O/W (Water in Oil in Water) suspension, third dispersion) 12 that has undergone phase inversion (dispersed phase inversion) so that the continuous phase is water 6 and the dispersed phase is polymerizable monomer 4 (FIGS. 1D and 1E).

That is, a suspension 12 is obtained in which the continuous phase is water 6 and the dispersed phase is polymerizable monomer 4 (polymerizable monomer 4 is dispersed in water 6).
In other words, in this embodiment, when preparing the suspension, as (step 1), a 1A mixed liquid 1 containing a surfactant A (first surfactant), a polymerizable monomer 4, and the oil-soluble initiator, and a 2A mixed liquid 2 containing water and the water-soluble initiator are prepared, and the 2A mixed liquid 2 is added to the 1A mixed liquid 1. As (step 2), a 3A mixed liquid 3 further containing a surfactant A (second surfactant) is added to the dispersion liquid 11 formed by the 1A mixed liquid 1 and the 2A mixed liquid 2, thereby preparing the suspension 12.

(Step 3)
Furthermore, after (step 2), the suspension 12 is subjected to suspension polymerization, thereby producing resin microparticles 15 having a smooth particle surface, a hollow structure inside the particle due to some openings in the particle, and a concave shape (FIG. 1F).
When suspension polymerization is carried out, water 6 may be added to the suspension 12 to adjust the concentration of the suspension 12 to be polymerized, as shown in FIG.

(Conjecture on the principle of concave shape)
By including a water-soluble initiator in the aqueous phase of the second dispersion 11 (W/O type dispersion), emulsion polymerization also proceeds simultaneously inside the polymerizable monomer 4 droplets in the oil phase during suspension polymerization of the suspension 12 (W/O/W type suspension). Therefore, while the polymerizable monomer 4 droplets are originally suspended and polymerized while maintaining their spherical shape to become spherical resin microparticles, it is presumed that the polymerizable monomer 4 is consumed from the inside of the polymerizable monomer 4 droplets due to emulsion polymerization occurring inside the polymerizable monomer 4 droplets, resulting in a shortage of resin components that can maintain the spherical shape, and the polymerizable monomer 4 droplets during polymerization cannot maintain their spherical shape, resulting in concave particles. Therefore, if the amount of water-soluble initiator added is small, the consumption of polymerizable monomer 4 inside the polymerizable monomer 4 droplets is small, so that a shape close to a sphere can be maintained. On the other hand, if the amount of water-soluble initiator added is large, the consumption of polymerizable monomer 4 inside the polymerizable monomer 4 droplets is large, and the shape of the particles cannot be maintained, resulting in a shape like a broken film.

[Concave shaped resin particles]
The concave-shaped resin fine particles obtained by the production method according to this embodiment are fine particles having a smooth particle surface, partially open particles, and a hollow structure inside the particle, with the particles having a concave shape.
As will be shown in the examples below, the concave-shaped resin fine particles according to the present embodiment have superior light diffusibility compared to spherical resin fine particles, and are suitable for use in light diffusion sheets, light guide plates, etc. In addition, since they have better oil absorption than spherical particles, they can also be used in cosmetics.

Average particle size
The average particle size of the concave-shaped resin fine particles according to this embodiment is not particularly limited, but is preferably 0.5 to 500 μm, more preferably 0.5 to 100 μm, and even more preferably 1 to 50 μm.
When the average particle diameter of the concave-shaped resin microparticles according to this embodiment is within this range, the resin microparticles can be polymerized stably.
The average particle diameter of the concave shaped resin fine particles according to this embodiment is a 50% volume average particle diameter measured by a laser diffraction particle size distribution measuring device (manufactured by Shimadzu Corporation).

<Light diffusion>
The light diffusion properties of the concave-shaped resin microparticles in this embodiment are evaluated by preparing a test piece in which the concave-shaped resin microparticles are dispersed in a resin having the same refractive index as the concave-shaped resin microparticles, and measuring the total light transmittance and haze value of the obtained test piece using a haze meter (manufactured by Murakami Color Research Laboratory) according to the following criteria.
(Rating A) The total light transmittance is 80% or less and the haze value is 30% or more.
(Rating B) The total light transmittance or haze value is outside the above range (not "total light transmittance 80% or less and haze value 30% or more").
In addition, an A rating may indicate that the result of the light diffusion test is good, and a B rating may indicate that the result of the light diffusion test is not good.

<Oil absorption>
The oil absorption of the concave-shaped resin microparticles according to this embodiment is the oil absorption of refined linseed oil, measured in accordance with JIS K 5101-13-1:2004.
The oil absorption of the concave shaped resin microparticles according to the present embodiment is not particularly limited, but is preferably 60 g/100 g. If the oil absorption is 60 g/100 g or more, when the concave shaped resin microparticles according to the present embodiment are blended in, for example, cosmetics, the effect of improving adhesion while maintaining slipperiness to the skin can be expected.

The concave-shaped resin fine particles according to this embodiment are concave-shaped resin fine particles having a space inside the particle.
The concave-shaped resin microparticles according to the present embodiment have, for example, superior light diffusion properties, a large surface area, and a high oil absorption capacity, compared to general spherical particles (for example, particles that are densely formed without having an internal space or hollow structure).

<Application>
The applications of the concave-shaped resin microparticles obtained by the production method according to this embodiment are not particularly limited, but can be used, for example, as light reflectors, light diffusing materials such as light diffusing plates for liquid crystal backlights, cosmetics, etc.

<Action and effect>
The concave-shaped resin microparticles obtained by the method for producing concave-shaped resin microparticles according to this embodiment exhibit excellent light diffusing effect when used as a light diffusing material, for example, in a light diffusing plate for a liquid crystal backlight.
Furthermore, when incorporated into cosmetics, it is expected to have the effect of improving adhesion while maintaining the smoothness to the skin.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.
The particles produced in the examples and comparative examples are shown below.

[Example 1]
Preparation of fine resin particles
Resin fine particles according to Example 1 were prepared according to the procedure described below and the composition shown in Table 1.
To polymerizable monomers (methyl methacrylate, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., 97 parts by mass and bifunctional (meth)acrylate (ethylene glycol dimethacrylate (EGDMA), manufactured by Tokyo Chemical Industry Co., Ltd.), 3 parts by mass), a surfactant (Noigen XL-400D, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) was added in an amount of 0.2 part by mass and an oil-soluble initiator (azobisisobutyronitrile (AIBN), manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was added in an amount of 1.0 part by mass relative to 100 parts by mass of the polymerizable monomers, to prepare a mixed liquid (1A mixed liquid).
The prepared 1A mixture was placed in a homogenizer and stirred.
Next, a mixed solution (2A mixed solution) was prepared containing 0.5 parts by mass of a water-soluble initiator (potassium peroxodisulfate, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) relative to 100 parts by mass of the polymerizable monomer, and 10 parts by mass of water (deionized distilled water, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) relative to 100 parts by mass of the polymerizable monomer.
Next, the prepared 2nd A mixed solution was added to the 1st A mixed solution while stirring, thereby obtaining a dispersion 11 (a W/O (Water in Oil) type dispersion) having the polymerizable monomer as a continuous phase and water as a dispersed phase.
Next, a mixed solution (3A mixed solution) was prepared by mixing 150 parts by mass of water (deionized distilled water, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) with 1.0 part by mass of a surfactant (Noigen XL-400D, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) with respect to 100 parts by mass of the polymerizable monomer.
Next, the prepared 3A mixed liquid was added to the dispersion liquid 11 while stirring it, thereby obtaining a suspension 12 (a W/O/W (Water in Oil in Water) type suspension) in which phase inversion had occurred so that the continuous phase was water and the dispersed phase was the polymerizable monomer.
Next, the obtained suspension 12 was charged into a four-neck flask equipped with a stirrer, a condenser, a thermometer and a nitrogen inlet tube, and the temperature was raised to 75° C. under nitrogen gas sealing, and the reaction was carried out at 75° C. for 4 hours.
Thereafter, the obtained suspension was filtered, washed with tap water in an amount of 250 parts by mass relative to 100 parts by mass of the polymerizable monomer, and then dried at 60° C. for 12 hours to obtain resin fine particles 15.

<Particle shape>
The produced resin fine particles were collected, and the particle surfaces and particle shapes were observed under a scanning electron microscope (manufactured by JEOL Ltd.).
As a result, concave-shaped resin fine particles were observed, which had a smooth particle surface and a concave shape with a space inside the particle due to partial openings in the particle.
Table 1 shows the evaluation results of particle shape, which was evaluated according to the following criteria.
A...Concave shape B...Partially concave shape C...Concave shape (partly torn)
D: Broken E: Spherical FIG. 2 shows a transmission electron microscope photograph.

Average particle size
The average particle size of the resulting resin fine particles was measured using a laser diffraction type particle size distribution measuring device (manufactured by Shimadzu Corporation) and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
2.0 g of the obtained hollow resin microparticles and 45 g of a 40% by mass toluene solution of polymethyl methacrylate resin were placed in a glass bottle and mixed for 30 minutes with a shaker to obtain a resin toluene solution-hollow resin microparticle dispersion. The dispersion was applied to a 1 mm thick glass plate with an applicator having a gap of 150 μm, and dried at 100° C. for 10 minutes to obtain a test piece. The total light transmittance and haze value of the obtained test piece were measured with a haze meter (manufactured by Murakami Color Research Laboratory) to evaluate the light diffusion property. In Table 1, the case where the result of the light diffusion property test was good is indicated as "A", and the case where the result of the light diffusion property test was not good is indicated as "B".
A good result (rating A) in the light diffusion test was defined as a test result of a total light transmittance of 80% or less and a haze value of 30% or more, and a bad result (rating B) in the light diffusion test was defined as a test result outside the above range (a result that does not satisfy "a total light transmittance of 80% or less and a haze value of 30% or more").
The total light transmittance of the obtained test piece was 72.3%, the haze value was 40.2%, and the light diffusion property of the resin fine particles was evaluated as A. Table 1 shows the light diffusion property evaluation results.
The following examples and comparative examples were also evaluated in the same manner.

<Oil absorption>
The oil absorption of refined linseed oil of the obtained resin fine particles was measured in accordance with JIS K 5101-13-1:2004, and was found to be 89 g/100 g.
Table 1 shows the measurement results of the oil absorption.

[Example 2]
Preparation of resin fine particles
Resin microparticles were prepared in the same manner as in Example 1, except that 90 parts by mass of methyl methacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 10 parts by mass of a bifunctional (meth)acrylate (ethylene glycol dimethacrylate (EGDMA), manufactured by Tokyo Chemical Industry Co., Ltd.) were used as the polymerizable monomer.

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, concave-shaped resin fine particles were observed, which had a smooth particle surface and a concave shape with a space inside the particle due to partial opening of the particle (evaluation: A).
Table 1 shows the evaluation results of particle shape.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
A test piece was prepared in the same manner as in Example 1, and the light diffusion property of the test piece obtained was evaluated using a haze meter. The total light transmittance was 71.8%, the haze value was 43.2%, and the light diffusion property of the resin fine particles was evaluated as A. Table 1 shows the light diffusion evaluation results.

<Oil absorption>
In the same manner as in Example 1, the oil absorption of refined linseed oil of the resulting resin fine particles was measured and found to be 92 g/100 g.
Table 1 shows the measurement results of the oil absorption.

[Example 3]
Preparation of resin fine particles
Resin microparticles were prepared in the same manner as in Example 1, except that 60 parts by mass of methyl methacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 30 parts by mass of styrene (styrene monomer, manufactured by Denka Co., Ltd.), and 10 parts by mass of a bifunctional (meth)acrylate (ethylene glycol dimethacrylate (EGDMA), manufactured by Tokyo Chemical Industry Co., Ltd.) were used as polymerizable monomers.

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, concave-shaped resin fine particles were observed, which had a smooth particle surface and a concave shape with a space inside the particle due to partial opening of the particle (evaluation: A).
Table 1 shows the evaluation results of particle shape.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
A test piece was prepared in the same manner as in Example 1, and the light diffusion property of the test piece obtained was evaluated using a haze meter. The total light transmittance was 74.2%, the haze value was 38.5%, and the light diffusion property of the resin fine particles was evaluated as A. Table 1 shows the light diffusion evaluation results.

<Oil absorption>
In the same manner as in Example 1, the oil absorption of refined linseed oil of the resulting resin fine particles was measured, and the oil absorption was found to be 84 g/100 g.
Table 1 shows the measurement results of the oil absorption.

[Example 4]
Resin fine particles were prepared in the same manner as in Example 2, except that 0.01 parts by mass of a water-soluble initiator (potassium peroxodisulfate) was used per 100 parts by mass of the polymerizable monomer.

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, fine resin particles having a smooth particle surface and a partially concave shape with an opening in part and a space inside the particle were observed (evaluation: B).
Table 1 shows the evaluation results of particle shape.
FIG. 3 shows a transmission electron microscope photograph.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
A test piece was prepared in the same manner as in Example 1, and the light diffusion property of the test piece obtained was evaluated using a haze meter. The total light transmittance was 79.2%, the haze value was 31.8%, and the light diffusion property of the resin fine particles was evaluated as A. Table 1 shows the light diffusion evaluation results.

<Oil absorption>
In the same manner as in Example 1, the oil absorption of refined linseed oil of the resulting resin fine particles was measured and found to be 62 g/100 g.
Table 1 shows the measurement results of the oil absorption.

[Example 5]
Resin fine particles were prepared in the same manner as in Example 2, except that 2.5 parts by mass of the water-soluble initiator (potassium peroxodisulfate) was used per 100 parts by mass of the polymerizable monomer.

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, resin fine particles having a concave shape (partially broken) with a smooth particle surface and a concave shape with an opening in part and a space inside the particle were observed (evaluation: C).
Table 1 shows the evaluation results of particle shape.
FIG. 4 shows a transmission electron microscope photograph.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
A test piece was prepared in the same manner as in Example 1, and the light diffusion property of the test piece obtained was evaluated using a haze meter. The total light transmittance was 73.4%, the haze value was 39.1%, and the light diffusion property of the resin fine particles was evaluated as A. Table 1 shows the light diffusion evaluation results.

<Oil absorption>
In the same manner as in Example 1, the oil absorption of refined linseed oil of the resulting resin fine particles was measured and found to be 90 g/100 g.
Table 1 shows the measurement results of the oil absorption.

[Comparative Example 1]
Resin fine particles were prepared in the same manner as in Example 2, except that 3.0 parts by mass of the water-soluble initiator (potassium peroxodisulfate) was used per 100 parts by mass of the polymerizable monomer.

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, broken resin particles were observed (evaluation: D).
Table 1 shows the evaluation results of particle shape.
FIG. 5 shows a transmission electron microscope photograph.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusivity, oil absorption>
The light diffusion property and oil absorption were not measured.

[Comparative Example 2]
Resin microparticles were prepared in the same manner as in Example 2, except that the surfactant was changed from Noigen XL-400D (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., corresponding to the compound represented by formula (1)) to ADEKA REASOAP SR-10 (manufactured by ADEKA Corporation, ether sulfate type ammonium salt, not corresponding to the compound represented by formula (1)).

<Particle shape>
In the same manner as in Example 1, the produced resin fine particles were recovered, and the particle surfaces and particle shapes were observed by a scanning electron microscope.
As a result, spherical resin particles were observed (evaluation: E).
Table 1 shows the evaluation results of particle shape.
FIG. 6 shows a transmission electron microscope photograph.

Average particle size
The average particle size of the resulting resin fine particles was measured in the same manner as in Example 1, and was found to be 10 μm.
Table 1 shows the measurement results of the average particle size.

<Light diffusion>
A test piece was prepared in the same manner as in Example 1, and the light diffusion property of the test piece obtained was evaluated using a haze meter. The total light transmittance was 91.1%, the haze value was 6.9%, and the light diffusion property of the resin fine particles was evaluated as B. Table 1 shows the light diffusion evaluation results.

<Oil absorption>
In the same manner as in Example 1, the oil absorption of refined linseed oil of the resulting resin fine particles was measured and found to be 55 g/100 g.
Table 1 shows the measurement results of the oil absorption.

The abbreviations in Table 1 represent the following compounds or product names.
MMA: methyl methacrylate EGDMA: ethylene glycol dimethacrylate AIBN: azobisisobutyronitrile XL-400D: Noigen XL-400D (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
SR-10: Adeka Rear Soap SR-10 (manufactured by Adeka Corporation)
In Table 1, the amount of each component is expressed in parts by mass. Furthermore, the blank spaces for each component mean that the component was not used.

As shown in Table 1, the resin fine particles obtained by the production methods of Examples 1 to 5 were concave-shaped resin fine particles.
In contrast, in Comparative Example 1 in which the amount of the surfactant represented by formula (1) used was 3.0 parts by mass per 100 parts by mass of the polymerizable monomer, the resin microparticles were broken, and concave-shaped resin microparticles could not be produced.
Moreover, the resin microparticles obtained by the production method according to Comparative Example 2, in which the surfactant represented by formula (1) was not used, were spherical resin microparticles, and concave-shaped resin microparticles could not be produced.

The concave-shaped resin microparticles of Examples 1 to 5 had the same average particle diameter as the spherical resin microparticles of Comparative Example 2, but were superior in light diffusion. From these results, the concave-shaped resin microparticles produced by the production method of the present invention are expected to be used in light diffusion materials, etc. In addition, since the oil absorption is higher than that of spherical resin microparticles, they are also expected to be used in cosmetics, etc.

Although the embodiments of the present invention have been described above, each configuration and their combinations in the embodiments are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the scope of the present invention. Furthermore, the present invention is not limited to the embodiments.

The method for producing concave shaped resin microparticles of the present invention is a simple process and can produce concave shaped resin microparticles that have excellent light diffusivity and large oil absorption compared to spherical resin microparticles of the same particle size.
The concave-shaped resin microparticles obtained by the method for producing concave-shaped resin microparticles of the present invention are, for example,
It is expected that this material will be used in light reflectors, light diffusion materials such as light diffusion plates for LCD backlights, cosmetics, etc.

1...Oil-soluble initiator-polymerizable monomer-surfactant A mixed solution (first A mixed solution, first dispersion)
2... Water in which a water-soluble initiator is dissolved (second A mixed solution)
3...Surfactant A-water mixture (third A mixture)
4... Polymerizable monomer 6... Water 11... W/O type (water in oil type) dispersion (containing a water-soluble initiator) (second dispersion)
12...W/O/W type (water in oil in water type) suspension (containing a water-soluble initiator) (third dispersion)
15... Concave-shaped resin fine particles (resin fine particles having a concave shape)
100...Mixer

Claims (5)

A method for producing concave-shaped resin microparticles, comprising the steps of dispersing, suspending, and polymerizing a polymerizable monomer having a polymerizable unsaturated hydrocarbon group in a dispersion medium mainly composed of water to obtain concave-shaped resin microparticles, using at least a surfactant represented by formula (1) as a dispersant and using a water-soluble initiator, the method comprising the steps of:
the amount of the water-soluble initiator used is less than 3.0 parts by mass per 100 parts by mass of the polymerizable monomer used;
adding an aqueous solution containing the water-soluble initiator to a first dispersion in which the polymerizable monomer, the surfactant, and the oil-soluble initiator are dispersed, to prepare a second dispersion in which the polymerizable monomer constitutes a continuous phase and water constitutes a dispersed phase;
A method for producing concave-shaped resin microparticles, comprising mixing the second dispersion with an aqueous solution containing a second surfactant to obtain a third dispersion having water as a continuous phase and the polymerizable monomer as a dispersed phase .
T ¹ O-(RO) _n (EO) _m -T ² ...(1)
[ In formula (1), ^T1 is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or an alkenyl group having 2 to 18 carbon atoms, ^T2 is a hydrogen atom, a sulfonic acid group, a sulfonate group, a carboxylate group, a carboxylate group, a phosphoric acid group, a phosphoric acid group, an amino group, or an ammonium group, RO is an oxyalkylene group having 3 to 18 carbon atoms, n is an integer of 1 to 50, EO is an oxyethylene group, and m is an integer of 0 to 200. ] The method for producing concave-shaped resin microparticles according to claim 1, wherein the water-soluble initiator is an inorganic peroxide. The method for producing concave-shaped resin microparticles according to claim 2, wherein the inorganic peroxide is at least one selected from the group consisting of potassium peroxodisulfate, ammonium peroxodisulfate, sodium peroxodisulfate, and hydrogen peroxide. 4. The method for producing concave shaped resin microparticles according to claim 1, wherein in formula (1), T ¹ is an alkenyl group having 2 to 18 carbon atoms. The method for producing concave-shaped resin microparticles according to any one of claims 1 to 4, wherein the average particle diameter of the concave-shaped resin microparticles is 0.5 to 500 μm.

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