Water-in-oil type emulsion composition

A water-in-oil emulsion composition that is significantly excellent in emulsification stability, and also excellent in non-stickiness is provided. The water-in-oil emulsion composition includes (a) a polyoxyalkylene/alkyl co-modified silicone; (b) a core-corona particle; (c) an oil-phase component; and (d) an aqueous-phase component.

RELATED APPLICATION

This application is the U.S. National Stage of PCT/JP2019/039459, filed Oct. 7, 2019, which claims the priority to Japanese Patent Application No. 2018-190665, filed on Oct. 9, 2018, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a water-in-oil emulsion composition, and more specifically to a water-in-oil emulsion composition obtained by emulsifying a core-corona particle and a specific lipophilic surfactant.

BACKGROUND OF THE INVENTION

In recent years, polymeric microparticles having various characteristics have been produced, and are widely used in fields of pharmaceuticals and cosmetics. Polymeric microparticles are produced by heterogeneous polymerization methods such as a microemulsion polymerization method, and are classified into several types by their compositions and forms.

A core-corona particle that is one of them is developed by the present inventors (Patent Literatures 1, 7). It has a spherical body consisting of a polymer of which a hydrophilic group is graft-bonded to a hydrophobic polymer skeleton, and has a structure of which a corona part consisting of a hydrophilic group is disposed around a hydrophobic center part (core part).

The core-corona particle is used as an emulsifier for producing an oil-in-water emulsion composition since it can stably disperse in water by hydrophilicity of the corona part (e.g., Patent Literatures 2, 7), although the core part of the core-corona particle has an excellent affinity to organic solvents. Pickering emulsion is known as an oil-in-water emulsified particle using a powder as an emulsifier, and the emulsified particles easily coalesce by stirring or impact; whereas, the oil-in-water emulsified particles emulsified by the core-corona particles are significantly stable to physical impacts. Moreover, in general, although the emulsion system obtained by emulsifying with a surfactant is greatly affected by temperature, an emulsion system obtained by the core-corona particle is hardly affected by temperature change.

Furthermore, the core-corona particle is known to be capable of being used as a clouding agent for clouding cosmetics (Patent Literature 3), and a capsule agent using swelling ability exhibited by organic solvents (Patent Literatures 4, 5).

As describe above, the core-corona particles were widely known in applications as emulsifiers that generate oil-in-water emulsion systems (e.g., Patent Literatures 2, 7), clouding agents of aqueous systems (Patent Literature 3), and inclusion complexes for blending slightly water-soluble components to aqueous systems (Patent Literature 4).

CITATION LIST

Patent Literatures

DISCLOSURE OF THE INVENTION

Technical Problem

The emulsion compositions that are emulsified by surfactants generally have a problem of stickiness that occurs when the blending amount of the surfactant is increased in order to improve emulsification stability, and thus feeling of use deteriorates. This problem was more serious for the water-in-oil emulsion composition having an oil phase as the continuous phase than the oil-in-water emulsion composition having an aqueous phase as the continuous phase. This is because the oil-in-water emulsion composition having the aqueous phase as the continuous phase can gain contribution of stabilization such as hydration repulsion between emulsified particles or electric double-layer ability; whereas, the water-in-oil emulsion composition cannot gain such contribution of stabilization, and the oil-water interface film needs to be strengthened by blending a large amount of a high-molecular weight surfactant having stronger stickiness, and stabilization of emulsified particles needs to be secured.

The present invention has been made for the problem of the above-described water-in-oil emulsion composition, and the object thereof is to provide a water-in-oil emulsion composition that is significantly excellent in emulsification stability and excellent in non-stickiness.

Solution to Problem

The present inventors have diligently studied the above-described problem, and as a result, they have surprisingly found that a core-corona microparticle that is known to produce a stable oil-in-water emulsion system produces a significantly stable water-in-oil emulsion system under coexistence with a specific oil-soluble surfactant. Furthermore, they have also found that a water-in-oil emulsion composition obtained with the core-corona particle is excellent in non-stickiness, and completed the present invention.

That is, the present invention comprises the following.

(a) 0.5 to 3% by mass of a polyoxyalkylene/alkyl co-modified silicone;(b) 0.01% by mass or greater of a core-corona particle;(c) an oil-phase component; and(d) an aqueous-phase component,
wherein the core-corona particle (b) is obtained by radically polymerizing a polyethylene oxide macromonomer represented by the following formula (1), and a hydrophobic monomer selected from a group consisting of an acrylate/methacrylate derivative monomer represented by the following formula (2) and an acrylamide/methacrylamide derivative monomer represented by the following formula (3), under presence or absence of a crosslinking polymer represented by the following formula (4) and under the following conditions (A) to (E);(A) a mole ratio expressed by a feed mole amount of the polyethylene oxide macromonomer/a feed mole amount of the hydrophobic monomer is 1:10 to 1:250;(B) a feed amount of the crosslinking monomer relative to a feed amount of the hydrophobic monomer is 0.1 to 1.5% by mass;(C) the acrylate/methacrylate derivative monomer represented by the following formula (2) has a monomer composition of which one type or two or more types of a methacrylic acid derivative having a 1-8C alkyl group is mixed; and
the acrylamide/methacylamide derivative monomer represented by the following formula (3) is an acrylamide derivative or a methacrylamide derivative having a substituent having a 1-18C alkyl group;(D) a polymerization solvent is a water-alcohol mixed solvent, and the alcohol is one type or two or more types selected from a group consisting of ethanol, dipropylene glycol, 1,3-butylene glycol, and isoprene glycol; and(E) solvent composition of the water-alcohol mixed solvent is water:alcohol=90 to 10:10 to 90 in a mass ratio at 20° C.

(1) R1is H or a 1-3C alkyl group, and n is a number of 8 to 200. X is H or CH3.

(3) R4is H or a 1-3C alkyl group, and R5and R6are H or substituents having a 1-18C alkyl group.

(4) R7and R8are independently a 1-3C alkyl group, and m is a number of 0 to 2.
[2] The water-in-oil emulsion composition of [1], wherein the component (a) is one type or two or more types of a polyoxy alkylene/alkyl co-modified silicone selected from a group consisting of cetyl PEG/PPG-10/1 dimethicone and lauryl PEG-9 polydimethylsiloxyethyl dimethicone.
[3] The water-in-oil emulsion composition of [1] or [2], wherein 20 to 80% by mass of the oil-phase component (c) is comprised.

Effects of Invention

A water-in-oil emulsion composition that is significantly excellent in emulsification stability and excellent in non-stickiness is provided by the present invention.

In the water-in-oil emulsion composition according to the present invention, emulsification state is not deteriorated by stirring or shaking like in conventional Pickering emulsions, and temperature stability is also good since changes in physical properties of surfactants due to temperature is small like in emulsions obtained by conventional surfactants.

DESCRIPTION OF EMBODIMENTS

The water-in-oil emulsion composition is produced by using the oil-phase component (c) and the aqueous component (d), and the polyoxyalkylene/alkyl co-modified silicone and the core-corona particle (b) as emulsifiers.

In the present invention, a polyoxyalkylene/alkyl co-modified silicone type can be used as an emulsifier used together with the core-corona particle (b).

Examples of the polyoxyalkylene/alkyl co-modified silicones include: linear polyoxyalkylene/alkyl co-modified silicones such as cetyl PEG/PPG-10/1 dimethicone, or the like; and branched polyoxyalkylene/alkyl co-modified silicones such as lauryl PEG-9 polydimethylsiloxyethyl dimethicone, or the like. These surfactants can be used alone or in combination of a plurality of types. PEG is an abbreviation of polyethylene glycol, and PPG is an abbreviation of polypropylene glycol.

Among them, cetyl PEG/PPG-10/1 dimethicone and lauryl PEG-9 polydimethylsiloxyethyl dimethicone can be used preferably as the component (a) in the present invention. By using cetyl PEG/PPG-10/1 dimethicone and/or lauryl PEG-9 poly dimethylsiloxyethyl dimethicone together with the following component (b), a particularly stable water-in-oil emulsion composition can be obtained.

The core-corona particle that can be used preferably in the present invention is a particle that has a core part consisting of a polymer having a relatively high hydrophobicity, and the corona part is stabilized by a polyethylene oxide chain that is a nonionic polymer. It is excellent in dispersion stability in water, acid resistance and salt resistance because of the polyethylene oxide chain of the corona part. The particle size is preferably almost uniform; and the average particle size may be within a range of 50 to 400 nm, and preferably 100 to 300 nm. Moreover, the degree of dispersion may be less than 0.2, and preferably less than 0.05.

The production methods of the core-corona particle are reported in Patent Literatures 2, 5, 6 and 7. It is disclosed that the core-corona particle is obtained by radically polymerizing a specific polyethylene oxide macromonomer and a specific hydrophobic monomer, under presence or absence of a specific crosslinking monomer, in a water-ethanol mixed solvent. Moreover, by dialyzing the obtained polymerization solution to water, the dispersion can be substituted to water.

The core-corona particle that can be used in the present invention can be obtained by radically polymerizing the monomers represented by the following formulae (1) to (4) under specific conditions.

For example, commercially available products manufactured by Sigma-Aldrich Co. LLC, Blemmer® manufactured by NOF Corporation, or the like can be used as the polyethylene oxide macromonomer represented by the formula (1)

The molecular weight (i.e., value of n) of the polyethylene oxide part needs to be n=8 to 200.

Examples of such macromonomers include PME-400, PME-1000, and PME-4000 that are methoxy polyethylene glycol monomethacrylates (n values in the formula (1) are n=9, n=23, n=90, respectively; all manufactured by NOF corporation).

(1) R1is a 1-3C alkyl group, and n is a number of 8 to 200. X is H or CH3.

For example, commercially available products manufactured by Sigma-Aldrich Co. LLC or Tokyo Chemical Industry Co., Ltd can be used as the acrylate/methacrylate derivative monomer represented by the formula (2).

R3is a 1-12C alkyl group, and more preferably a 1-8C alkyl group.

These acrylate/methacrylate derivative monomers are generally-used raw materials, and can be easily obtained as general industrial raw materials.

For example, t-butylacrylamide, N,N-dimethylacrylamide, N-[3-(dimethylamino)propyl]acrylamide, t-butylmethacrylamide, octylacrylamide, octylmethacrylamide, and octadecylacrylamide, or the like can be used preferably as the acrylamide/methacrylamide derivative monomer represented by the formula (3). Among these, t-butylacrylamide, N,N-dimethylacrylamide and N-[3-(dimethylamino)propyl]acrylamide are particularly preferred.

These acrylamide/methacrylamide monomers are available as commercial products or industrial raw materials.

(3) R4is H or a 1-3C alkyl group, and R5and R6are H or substituents having a 1-18C alkyl group.

The crosslinking monomer represented by the formula (4) is available as commercial products or industrial raw materials. This crosslinking monomer is preferably hydrophobic.

The value of m is preferably 0 to 2. To be specific, ethylene glycol dimethacrylate (may be abbreviated as EGMA hereinbelow) manufactured by Sigma-Aldrich Co. LLC., Blemmer® PDE-50 manufactured by NOF Corporation, or the like are used preferably.

(4) R7and R8are independently a 1-3C alkyl group, and m is a number of 0 to 2.

The core-corona particle dispersion that is a raw material for cosmetics according to the present invention may be one that is obtained by radically-polymerizing the above-identified monomers under the following conditions (A) to (E).(A) A mole ratio expressed by a feed mole amount of the polyethylene oxide macromonomer/a feed mole amount of the hydrophobic monomer is 1:10 to 1:250.(B) A feed amount of the crosslinking monomer relative to a feed amount of the hydrophobic monomer is 0.1 to 1.5% by mass.(C) The hydrophobic monomer represented by the formula (2) has a monomer composition of which one type or two or more types of a methacrylic acid derivative having a 1-8C alkyl group is mixed.(D) A polymerization solvent is a water-alcohol mixed solvent, and the alcohol is one type or two or more types selected from a group consisting of ethanol, dipropylene glycol, 1,3-butylene glycol and isoprene glycol.(E) Solvent composition of the water-alcohol mixed solvent is water:alcohol=90 to 10:10 to 90 in a mass ratio at 20° C.

When the crosslinking monomer represented by the formula (4) is used in the present invention, “a feed amount of the crosslinking monomer relative to a feed amount of the hydrophobic monomer” is defined as a crosslink density (% by mass). In such case, with respect to the crosslink density of the core-corona particles used in the present invention, the feed amount of the crosslinking monomer relative to the feed amount of the hydrophobic monomer should be 0.1 to 1.5% by mass by Condition (B).

Next, each condition in cases when the crosslinking monomer represented by the formula (4) is used is described in detail.

The feed mole amounts of the polyethylene oxide macromonomer and the hydrophobic monomer can be polymerized when the feed mole amount of the polyethylene oxide macromonomer:the feed mole amount of the hydrophobic monomers is in the range of 1:10 to 1:250 (mole ratio). The feed mole amount is preferably in a range of 1:10 to 1:200, and more preferably in a range of 1:25 to 1:100.

When the mole amount of the hydrophobic monomer is less than 10 times of that of the polyethylene oxide macromonomer, the polymerized polymer becomes water soluble and does not form a core-corona polymer particle. In addition, when the mole amount of the hydrophobic monomer is 250 times or greater of that of the polyethylene oxide macromonomer, dispersion stabilization by the polyethylene oxide macromonomer becomes insufficient, so that the hydrophobic polymer by the insoluble hydrophobic monomer aggregates and precipitates.

A particle having the hydrophobic polymer of the core part crosslinked can be polymerized by co-polymerizing the crosslinking polymer.

When the feed amount of the crosslinking polymer is 0.10% by mass or less of that of the hydrophobic monomer, the crosslink density becomes low, and the particle may collapse upon swelling. When the feed amount exceeds 1.5% by mass, particles aggregate and preferred particles of narrow particle size distribution cannot be polymerized. The feed amount of the crosslinking monomer is preferably 0.2 to 1.0% by mass, more preferably 0.2 to 0.8% by mass, and most preferably 0.2 to 0.5% by mass.

The hydrophobic monomer represented by the formula (2) needs to have a monomer composition of which one type or two or more types of a methacrylate derivative having a 1-12C alkyl group is mixed. When the number of carbon atoms is zero (when it is a monomer without a terminal ester bond), the monomer may be too hydrophilic to be emulsion-polymerized adequately. Whereas, when the number of carbon atoms is 13 or greater, it may become steric hindrance upon polymerization, and a crosslink structure may not be constructed adequately.

The polymerization solvent needs to be a water-alcohol mixed solvent. An alcohol that can dissolve the hydrophobic monomer represented by formula (2) and the crosslinking monomer represented by the formula (3) is preferred. The alcohol used in the present invention needs to be ethanol, dipropylene glycol, 1,3-butylene glycol, or isoprene glycol.

When considering of being industrially manufacturable, that is using the polymerization solution without purification processes such as dialysis as a raw material as it is, the solvent to be mixed with water needs to be an alcohol that can be generally blended to cosmetics, not organic solvents that irritation upon application to skin is concerned such as ethanol, propanol, and butanol.

Solvent composition of the water-alcohol mixed solvent that is the polymerization solvent needs to be water:alcohol=90 to 10:10 to 90 in the mass ratio at 20° C. Solvent composition of the water-alcohol mixed solvent is preferably water:alcohol=90 to 10:10 to 90 (volume ratio at 20° C.), and more preferably water:alcohol=80 to 20:20 to 80 (volume ratio at 20° C.).

The polymerization solvent needs to be added with an alcohol in order to dissolve the hydrophobic monomer uniformly. The mixing ratio of the alcohol is 10 to 90 volume ratio. When the mixing ratio of the alcohol is less than 10 volume ratio, the dissolving ability of the hydrophobic monomer becomes extremely poor, polymerization proceeds in a monomer droplet state to form a gigantic lump, and thus the core-corona particle may not be formed. When the mixing ratio of the alcohol exceeds 90 volume ratio, an emulsion of the hydrophobic monomer by hydrophobic interaction may not be formed, emulsion-polymerization does not proceed, and thus the core-corona particle may not be obtained.

The polymerization solvent having a high monomer solubility is preferred. When production and purification processes (distillation, or the like) are considered, the viscosity and the boiling point are preferably not too high.

In the present invention, ethanol, dipropylene glycol, 1,3-butylene glycol, isoprene glycol or the like can be used preferably as the alcohol to be used as the polymerization solvent. Among the above, ethanol is particularly preferred.

Commercially available polymerization initiators used in common water-soluble thermal radical polymerization can be used as a polymerization initiator used in a polymerization system. When polymerization is performed without strictly controlling the stirring condition particularly in this polymerization system, an extremely narrow particle size distribution of the polymerized particle can be achieved.

When the crosslinking monomer represented by the formula (4) is not used, it may be polymerized under the condition described in Japanese Unexamined Patent Publication No. 2017-175011A, for example.

The core-corona particle (b) according to the present invention can be used together with the component (a) as an emulsifier for producing water-in-oil emulsion composition.

It was conventionally known that the core-corona particle emulsifies the oil-phase component and the aqueous-phase component to form the oil-in-water emulsion composition having a structure where the core-corona particle emulsifier is adsorbed onto the oil droplets of the oil-phase component dispersed in the aqueous-phase component (Patent Literature). In the present invention, it is considered that a change of some kind happens to the core-corona particle by coexistence of the polyoxyalkylene/alkyl co-modified silicone type surfactant (a); and it makes not the oil-phase component but the aqueous component into droplets to be extremely stably adsorbed onto the water droplets.

The blending amount of the core-corona particle (b) for producing the water-in-oil emulsion composition based on the pure content of the particle relative to the total amount of the composition is preferably 0.01 to 10% by mass, more preferably 0.03 to 5% by mass, and most preferably 0.05 to 1% by mass. When the blending amount is less than 0.01%, a stable cosmetic may be difficult to obtain. When the blending amount exceeds 10%, phase inversion to an oil-in-water emulsion may occur over time when the preparation is stored for a long term.

Moreover, the blending amount of the polyoxyalkylene/alkyl co-modified silicone (a) added for this purpose is preferably 0.5 to 3% by mass, more preferably 0.7 to 2% by mass, and most preferably 1% by mass. When the blending amount is less than 0.5% by mass, a stable water-in-oil emulsion may not be obtained. When the blending amount exceeds 3% by mass, feeling of use may deteriorate due to strong stickiness.

The water-in-oil emulsion composition of the present invention can be produced in accordance with common procedures. It may be produced by: mixing and dissolving an oil-phase component to which the polyoxyalkylene/alkyl co-modified silicone (a) is dissolved and other components; and adding thereto a dispersion obtained by mixing and dispersing the core-corona particle (b) to water or the aqueous-phase component, followed by emulsification by stirring and application of shear force.

Examples of the oil-phase components include, but not limited to, hydrocarbon oils, higher fatty acids, higher alcohols, synthetic esters, silicone oils, liquid fats and oils, solid fats and oils, waxes, and perfumes that are commonly used in cosmetics, quasi-drugs, or the like.

Selection of the perfume is not limited in particular; examples include natural perfumes from animals or plants, synthetic perfumes prepared by means of chemical synthesis, and perfume blends thereof. By blending perfume, a cosmetic having a superior durability of fragrance can be obtained.

In emulsion compositions obtained by conventional surfactants, the physical properties of surfactants and the physical properties of oil components greatly affect emulsifiability, and measures such as changing the types of surfactants needed to be taken when changing the oil-phase component. However, since the water-in-oil emulsion composition of the present invention is emulsified by the core-corona particle (b), emulsifiability and stability are less affected by the types of oil components, and the oil component in a wider range than before can be blended.

Water, water soluble alcohols, thickeners, or the like commonly used in cosmetics, quasi-drugs, or the like can be blended as the aqueous-phase component; in addition, appropriate amounts of moisturizers, chelating agents, preservatives, pigments, or the like can also be blended as desired.

Water contained in the oil-in-water emulsion composition of the present invention is not limited in particular; and examples thereof include, but not limited to, purified water, ion-exchanged water, and tap water.

Examples of amino acids include, but not limited to, neutral amino acids (e.g., threonine and cysteine), and basic amino acids (e.g., hydroxylysine). Examples of amino acid derivatives include, but not limited to, sodium acyl sarcosinate (sodium N-lauroyl sarcosinate), acyl glutamate, sodium acyl β-alanine, and glutathione.

Examples of pH adjusters include, but not limited to, buffers such as lactic acid-sodium lactate, citric acid-sodium citrate, and succinic acid-sodium succinate.

The blending amounts of the oil-phase components and the aqueous-phase components blended to the water-in-oil emulsion composition of the present invention are not particularly limited. By using the polyoxyalkylene/alkyl co-modified silicone (a) and the core-corona particle (b) together as the emulsifier, a water-in-oil emulsion composition having a wide range of the oil-phase component/aqueous-phase component ratio can be obtained; the water-in-oil emulsion composition ranging from embodiments having a small oil-phase component/aqueous-phase component ratio, i.e., the oil-phase component is blended at a small amount (emulsions, or the like), to embodiments of which the oil-phase component is blended at a large amount (cleansing creams, sunscreens, hair creams, etc.), can be obtained.

Other components commonly used in cosmetics and quasi-drugs can be blended as necessary in the water-in-oil emulsion composition according to the present invention as long as the effect of the present invention is not adversely affected; examples of such components include, but not limited to, ultraviolet absorbents, powders, organic amines, polymer emulsions, vitamins, and antioxidants.

Examples of vitamins include, but not limited to, vitamins A, B1, B2, B6, C and E as well as their derivatives, pantothenic acid and its derivatives, and biotin.

Examples of antioxidants include, but not limited to, tocopherols, dibutyl hydroxytoluene, butyl hydroxyanisole, and gallic acid esters.

Moreover, not as the emulsifier, but for the purpose of controlling tactile sensations during use, controlling drug permeation and such, or improving washing ability when blended into washing agents for skin or hair, other surfactants can be blended as the aqueous-phase or oil-phase component to the water-in-oil emulsion composition of the present invention.

An ampholytic surfactant has at least one cationic functional group and one anionic functional group, is cationic when the solution is acidic and anionic when the solution is alkaline, and has characteristics similar to a nonionic surfactant around the isoelectric point.

Ampholytic surfactants are classified, based on the type of the anionic group, into the carboxylic acid type, the sulfuric ester type, the sulfonic acid type, and the phosphoric ester type. The carboxylic acid type, the sulfuric ester type, and the sulfonic acid type are preferred in the present invention. The carboxylic acid type is further classified into the amino acid type and the betaine type. The betaine type is particularly preferred.

Anionic surfactants are classified into the carboxylate type such as fatty acid soaps, N-acyl glutamates, and alkyl ether acetates, the sulfonic acid type such as α-olefin sulfonates, alkane sulfonates, and alkylbenzene sulfonates, the sulfuric ester type such as higher alcohol sulfuric ester salts, and the phosphoric ester salt type. The carboxylate type, the sulfonic acid type, and the sulfuric ester salt type are preferred; and the sulfuric ester salt type is particularly preferred.

A nonionic surfactant is a surfactant that is not ionized to bear an electric charge in an aqueous solution. Specific examples include, but not limited to, glycerol fatty acid esters, ethylene oxide derivatives of glycerol fatty acid esters, polyglycerol fatty acid esters, propylene glycol fatty acid esters, ethylene oxide derivatives of propylene glycol fatty acid esters, polyethylene glycol fatty acid esters, polyethylene glycol alkyl ethers, polyethylene glycol alkyl phenyl ethers, polyethylene glycol castor oil derivatives, and polyethylene glycol hydrogenated castor oil derivatives.

Applications of the water-in-oil emulsion composition of the present invention are not limited in particular; however, it is preferred as skin cosmetics, hair cosmetics, skin external preparations, or the like.

EXAMPLES

The present invention will be described with reference to the following examples, but the present invention is not limited thereto. The blending amounts are expressed with “% by mass” unless otherwise specified. The blending amounts of the core-corona particles in Tables 3 to 5 are the converted values of the pure content of the particles.

Test Example 1: Production of a Core-Corona Particle

Production Example 1

A polyethyleneoxide macro monomer, a hydrophobic monomer, and a crosslinking monomer were added to a water-alcohol mixed solvent in a three-neck flask equipped with a reflux tube and a nitrogen feeding tube. After sufficient dissolution or dispersion, 1 mol % of the polymerization initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride, relative to the total amount of the monomers, was dissolved in a small amount of water and added, and further dissolution or dispersion was carried out. The uniformly dissolved or dispersed polymerization solution was put through nitrogen substitution for 20 minutes to remove dissolved oxygen, followed by 8 hours of polymerization with stirring by means of a magnetic stirrer while the temperature was maintained at 65 to 70° C. in an oil bath. After the completion of polymerization, the polymer solution was returned to room temperature to obtain a core-corona particle dispersion (production example 1).

In the production of the core-corona particle dispersion, Blemmer PME-4000 (manufactured by NOF CORPORATION; n≈9 in the macromonomer represented by Formula (1)) was used as the polyethylene oxide macro-monomer. Methyl methacrylate (MMA) and butyl methacrylate (n-BMA) were used as the hydrophobic monomer. Ethylene glycol dimethacrylate (EGDMA) was used as the crosslinking monomer.

Production Example 2

A polyethyleneoxide macro monomer and a hydrophobic monomer were added to a water-alcohol mixed solvent in a three-neck flask equipped with a reflux tube and a nitrogen feeding tube. After sufficient dissolution or dispersion, dissolved oxygen was removed by nitrogen substitution for 20 minutes. Then, 1 mol % of the polymerization initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride, relative to the total amount of the monomers, was dissolved in a small amount of water and added, and further dissolution or dispersion was carried out. The uniformly dissolved or dispersed polymerization solution was put through nitrogen substitution for 20 minutes to remove dissolved oxygen, followed by 8 hours of polymerization with stirring by means of a magnetic stirrer while the temperature was maintained at 65 to 70° C. in an oil bath. After the completion of polymerization, the polymer solution was returned to room temperature to obtain a core-corona particle dispersion.

In the above, Blemmer PME-4000 (manufactured by NOF CORPORATION) was used as the polyethylene oxide macro-monomer. Methyl methacrylate (MMA), butyl methacrylate (n-BMA), t-butylacrylamide (t-BAA), and N-[3-(dimethylamino)propyl]acrylamide (DMAPA) were used as the hydrophobic monomer.

<Method for Measuring the Particle Size and the Degree of Dispersion>

The particle size of the core-corona particle (hereinafter may be referred simply as “particle”) was measured with a Zetasizer manufactured by Malvern Instruments Ltd. A measurement sample of the core-corona particle dispersion having the particle concentration of about 0.1% was prepared by dilution with water. After removing dust with a 0.45 μm filter, the scattering intensity at 25° C. was measured at the scattering angle of 173° (back-scattered light), and the average particle size and the degree of dispersion were calculated with analysis software installed on the measurement apparatus. The particle size was analyzed by the cumulant analysis method, and the degree of dispersion is a normalized value of the second-order cumulant value obtained by the cumulant analysis. This degree of dispersion is a commonly used parameter, and can be automatically analyzed with a commercial dynamic light scattering measurement apparatus. For the viscosity of the solvent, which is necessary for the particle size analysis, the viscosity of pure water at 25° C., i.e., 0.89 mPa s, was used.

The polymerization conditions used in Production examples 1 and 2 are shown in Tables 1 to 3 below. The numerical values in Table 1 are all in g (grams). Moreover, EtOH in the tables is an abbreviation of ethanol.

The particle sizes and the degrees of dispersion were 206.1 nm and 0.052 in Production example 1, and 210.3 nm and 0.018 in Production example 2, respectively.

Test Example 2: Production of an Emulsion Composition

Emulsion compositions were prepared with the core-corona particle produced in Test Example 1, and (1) a phase state, and (2) emulsification stability were analyzed in accordance with the following methods. The formulations and results are shown in Table 4.

An oil-phase component (c) and a polyoxyalkylene/alkyl co-modified silicone (a) were mixed. In addition, the core-corona particle dispersion (b) produced in Test example 1 was added to an aqueous-phase component (d), and mixed with stirring to uniformly disperse the core-corona particles in the aqueous-phase component. The mixed solution consisting of the components (c) and (a) was added to an aqueous dispersion consisting of the components (b) and (d), and was subjected to shear mixing with a homomixer until homogeneous.

An oil-phase component (c) and a polyoxyalkylene/alkyl co-modified silicone (a) were mixed. In addition, the core-corona particle dispersion (b) produced in Test example 1 was added to an aqueous-phase component (d) and mixed with stirring to uniformly disperse the core-corona particles in the aqueous-phase component. This aqueous dispersion was added to the mixed solution consisting of the components (c) and (a), and was subjected to shear mixing with a homomixer until homogeneous.

Evaluation

(1) Phase State

A phase state of the sample was observed with an optical microscope.

After subjecting the sample to a centrifugal treatment with a centrifugal separator (3500 rpm, 120 minutes), the emulsification state was observed with an optical microscope. It was evaluated in accordance with the following criteria. Stability of the emulsion particles to stirring or shaking can be evaluated by this analysis. A or greater was regarded as acceptable in the present invention.A: The emulsion particles were homogeneous, and no coalescence or aggregation was observed.B: The emulsion particles were mostly homogeneous, but slight coalescence or aggregation was observed.C: The emulsion particles were not homogeneous, and significant coalescence or aggregation was observed.

As described above, the core-corona particle is widely known as an emulsifier for producing an oil-in-water emulsion composition (Patent Literature 1, or the like). As shown in Table 4, the composition (Comparative example 1) of which the oil component was emulsified with the core-corona particle in accordance with the general production method for the oil-in-water emulsion composition (Production method 1) became an oil-in-water emulsion composition having a high emulsification stability.

Whereas, the composition (Example 1) of which lauryl PEG-9 poly dimethylsiloxyethyl dimethicone, which is the polyoxyalkylene/alkyl co-modified silicone, was added to the formulation of Comparative example 1 surprisingly became a water-in-oil emulsion composition, even if it was produced in accordance with the production method for the oil-in-water emulsion composition (Production method 1). Moreover, this water-in-oil emulsion composition was excellent in emulsification stability.

When production of an emulsion was tried in accordance with the general production method for the water-in-oil emulsion composition (Production method 2) with the formulation of Comparative example 1, emulsification was insufficient, and thus an emulsion could not be obtained.

Accordingly, it was shown that the core-corona particle that is known to produce an oil-in-water emulsion system produced a stable water-in-oil emulsion system under coexistence with a specific surfactant.

Test Example 3: Investigation on Components

The surfactant that produces a water-in-oil emulsion when used together with the core-corona particle was investigated. The emulsion compositions were produced by the same method as Test example 2, and, in addition to the above-identified items, the following items were evaluated in accordance with the following criteria. The results are shown in Tables 5 and 6.

Evaluation

(3) Emulsion Particle Size

The emulsion particle size of the sample was measured with an optical microscope.

The sample was put into a test tube to be subjected to centrifuge treatment at 3500 rpm for two hours. Then, the state of the sample was visually observed in accordance with the following criteria. Dynamic stability (coalescence stability) of the emulsion system can be evaluated by this analysis. A or greater was regarded as acceptable in the present invention.A: The sample maintained the emulsification state at the time of production.B: Due to coalescence of some emulsion particles, the aqueous phase was slightly separated.C: All emulsion particles coalesced, and the aqueous phase was completely separated.
(2-2) Emulsification Stability

After storing the sample at 50° C. for four weeks, the state of the sample was visually observed in accordance with the following criteria. Temperature stability and stability over-time of the emulsion system can be evaluated by this analysis. When providing this composition in commercial base, this index is more important than (2-1). A or greater was regarded as acceptable in the present invention.A: The sample maintained the emulsification state at the time of production.B: Due to coalescence of some emulsion particles, the aqueous phase was slightly separated.C: All emulsion particles coalesced, and the aqueous phase was completely separated.
(4) Non-Stickiness

Non-stickiness when the sample was applied on the skin was evaluated by 10 professional panelists based on the following criteria. B or greater was regarded as acceptable in the present invention.A: 7 or more panelists answered “stickiness was not felt”.B: 5 or more and 6 or less panelists answered “stickiness was not felt”.C: 3 or more and 4 or less panelists answered “stickiness was not felt”.D: 2 or less panelists answered “stickiness was not felt”.

As shown in Table 5, when the polyoxy alkylene/alkyl co-modified silicone type and the core-corona particle were used together as the emulsifier, the water-in-oil emulsion compositions that are significantly excellent in emulsification stability and excellent in non-stickiness were obtained (Examples 2 to 7). These emulsion compositions were resistant against stirring and impact, and were significantly high in temperature stability ((2-1) and (2-2) were all “A”).

Moreover, it became significantly stable water-in-oil emulsion systems in a wide range of the oil phase ratio ranging from 20 to 80% by mass (Examples 2 to 7). The core-corona particle is known to produce a good oil-in-water emulsion composition (Patent Literature) even if the ratio of oil-phase/aqueous-phase is high (i.e., the ratio of the oil-phase component in the composition is high); however, it is shown that addition of the polyoxyalkylene/alkyl co-modified silicone surfactant enables to produce a significantly stable water-in-oil emulsion composition in a formulation having a wide range of oil-phase/aqueous-phase ratio (Example 5).

Comparative examples 3 to 12 are compositions prepared with formulations having a non-silicone surfactant (Comparative example 3 to 8), amodimethicone (Comparative examples 9, 10), and PEG-modified silicone (no alkyl modification) (Comparative examples 11, 12) as the surfactant. It became clear from Table 6 that these compositions were low in emulsification stability regardless of presence/absence of the core-corona particle.

Comparative examples 13 and 14 are compositions prepared with formulations in which only the core-corona particle was excluded from the formulations of Examples 2 and 3, respectively. Table 6 shows that, when the compositions of these comparative examples are compared to the compositions of the examples comprising the core-corona particle, there was almost no difference in emulsion particle size and non-stickiness; however, emulsification stability deteriorated remarkably. Furthermore, in the composition in which the blending amount of the polyether-modified silicone (a) was increased up to 5% by mass, emulsification stability improved, but strong stickiness occurred (Comparative example 15).

As stated above, by using the core-corona particle (b) and the polyoxyalkylene/alkyl co-modified silicone (a) in combination as the emulsifier, it is shown that a water-in-oil emulsion composition that is significantly excellent in emulsification stability and also excellent in non-stickiness can be obtained in a formulation having a wide oil-phase/aqueous phase ratio.