JP2024064415A - Method for producing aqueous dispersion of polyolefin resin particles, aqueous dispersion of polyolefin resin particles, and polyolefin resin particles - Google Patents

Abstract

[Problem] To provide an aqueous dispersion of polyolefin resin particles having a small diameter and a narrow particle size distribution. [Solution] A method for producing an aqueous dispersion of polyolefin resin particles, comprising the steps of heating (A) a polyolefin resin, (B) a nonionic surfactant, (C) an ionic surfactant having a molecular weight of 1000 or less, and (D) water under stirring to above the melting point of the polyolefin resin, and then cooling to below the crystallization temperature of the polyolefin resin. [Selected Figures] None

Description

The present invention relates to a method for producing an aqueous dispersion of polyolefin resin particles, an aqueous dispersion of polyolefin resin particles, and polyolefin resin particles.

Particles mainly composed of polyolefin resins are used as coating agents, adhesives, inks, toners, resin additives, etc., taking advantage of their thermal properties and excellent water resistance and chemical resistance. In particular, they are used to coat a wide variety of substrates, such as metals and resin films (see, for example, Patent Documents 1 and 2).

Methods for producing aqueous dispersions of small-sized polyolefin resin particles include, for example, a method for obtaining an aqueous dispersion of polyolefin resin particles by emulsion polymerization (see Patent Document 3), and a method for heating and stirring polyolefin resin, water, and a basic compound in a sealable container (see Patent Document 2). In addition, methods for producing aqueous dispersions of polyolefin resin particles with somewhat large particle sizes include a method for dissolving polyolefin resin in a solvent and then slowly cooling the solution to precipitate the polyolefin resin (see Patent Document 1), and a method for dissolving polyolefin resin in an organic solvent, adding water to cause phase inversion emulsification, and then removing the organic solvent (see Patent Document 4).

JP 2004-059869 A JP 2004-051661 A JP 2003-221267 A Japanese Patent Application Laid-Open No. 3-250005

However, the known methods for producing aqueous dispersions of small-sized polyolefin resin particles described in the above patent documents require the use of organic solvents and basic compounds, and require specialized equipment that can handle the injection of high-pressure gas. Furthermore, production methods that use organic solvents, etc., impose a large burden on the environment.

The present invention aims to provide a method for producing an aqueous dispersion of polyolefin resin particles that does not use organic solvents or basic compounds and does not require specialized equipment for sealing high-pressure gas, and to provide an aqueous dispersion of polyolefin resin particles and polyolefin resin particles that are small in diameter and have a narrow particle size distribution.

In order to solve the above problems, the present invention has the following configuration.
The present invention relates to a method for producing an aqueous dispersion of polyolefin resin particles, the method comprising the steps of heating (A) a polyolefin resin, (B) a nonionic surfactant, (C) an ionic surfactant having a molecular weight of 1,000 or less, and (D) water under stirring to a temperature equal to or higher than the melting point of the polyolefin resin, and then cooling the mixture to a temperature lower than the crystallization temperature of the polyolefin resin.

The manufacturing method of the present invention does not require special equipment and can produce an aqueous dispersion of polyolefin resin particles with small diameters and narrow particle size distribution by an aqueous process with low environmental impact. Since the polyolefin resin particles have small diameters and narrow particle size distribution, it is possible to produce a uniform and smooth thin film by coating on various substrates. Due to the thermal properties, water resistance, and chemical resistance unique to polyolefin resins with a high degree of crystallinity, it is possible to impart heat sealability, rust prevention, liquid repellency, chemical resistance, etc. by coating on various substrates, and it can also be suitably used as an ink or resin additive. In addition, since it does not contain additives that have a significant impact on the environment and human body, it is also easy to use in applications such as medical materials and cosmetics.

The following describes in detail preferred embodiments of the present invention. Note that the present invention is not limited to the embodiments described below, and should be understood to include various modifications that are implemented within the scope of the present invention.

The present invention is a method for producing an aqueous dispersion of polyolefin resin particles, which comprises a step of heating (A) a polyolefin resin, (B) a nonionic surfactant, (C) an ionic surfactant having a molecular weight of 1000 or less, and (D) water under stirring to above the melting point of the polyolefin resin, and then cooling to below the crystallization temperature of the polyolefin resin. By using a nonionic surfactant in combination with an ionic surfactant having a molecular weight of 1000 or less, it has become possible to obtain small particles with a volume average particle size of less than 1.5 μm and a narrow particle size distribution, which was difficult to achieve using conventional methods, without the need for special equipment and through an aqueous process with low environmental impact.

(1) (A) Polyolefin Resin The (A) polyolefin resin of the present invention is not particularly limited, and examples thereof include polyethylene-based resins, polypropylene-based resins, polyolefin-based thermoplastic elastomers, etc. Among these, polyethylene-based resins are preferred because they have a low melting point and are easy to use.
The polyethylene-based resin includes polyethylene and/or a polyethylene copolymer.
Polyethylene refers to a resin obtained by polymerizing only ethylene, and modified versions of such resins. Examples of polyethylene include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, oxidized polyethylene, and polyethylene wax.
The polyethylene may be one produced by a conventional method or a commercially available product. Specific examples of commercially available polyethylene include EVOLUE (manufactured by Mitsui Chemicals), EVOLUE H (manufactured by Mitsui Chemicals), HI-ZEX MILLION (manufactured by Mitsui Chemicals), LUBMER (manufactured by Mitsui Chemicals), SANWAX (manufactured by Sanyo Chemical Industries, Ltd.), SUMIKACENE (manufactured by Sumitomo Chemical Co., Ltd.), SUMIKACENE-L (manufactured by Sumitomo Chemical Co., Ltd.), SUMIKACENE-E (manufactured by Sumitomo Chemical Co., Ltd.), SUMIKACENE-EP (manufactured by Sumitomo Chemical Co., Ltd.), EXCELLEN (manufactured by Sumitomo Chemical Co., Ltd.), NOVATEC HD (manufactured by Japan Polyethylene Co., Ltd.), NOVATEC LD (manufactured by Japan Polyethylene Co., Ltd.), and NOVATEC LL (manufactured by Japan Polyethylene Co., Ltd.).
The polyethylene copolymer is a copolymer of ethylene and other monomer components, and examples of the other monomer components include α-olefins, unsaturated carboxylic acids, unsaturated dicarboxylic anhydrides, unsaturated carboxylic esters, vinyl acetate, etc., and it is also possible to polymerize two or more selected from these in combination. The α-olefin is not particularly limited as long as it is copolymerizable with ethylene, and examples thereof include propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-pentene, and 1-heptene. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, methylmaleic acid, methylfumaric acid, mesaconic acid, citraconic acid, and glutaconic acid, examples of unsaturated dicarboxylic anhydrides include maleic anhydride, itaconic anhydride, and citraconic anhydride, and examples of unsaturated carboxylic acid esters include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, glycidyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, hydroxyethyl methacrylate, and glycidyl methacrylate, etc. Among polyethylene resins, ethylene-acrylic acid copolymers or ethylene-methacrylic acid copolymers have carboxyl groups in the molecule, have good affinity with water, and have reduced interfacial tension, so that the size of the oil droplets during emulsification becomes small, and it is easy to obtain polyolefin resin particles with small diameters and narrow particle size distribution.

The copolymerization amount of the monomer component other than ethylene in the polyethylene copolymer may be within a range that does not impair the properties of polyethylene, but when the total of the copolymer constituent units in the polyethylene copolymer is taken as 100% by mass, it is preferably 30% by mass or less, more preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less. The above-mentioned products are commercially available and easy to use.
As the polyethylene copolymer, commercially available products can be used. Specific examples of commercially available polyethylene copolymers include Admer (manufactured by Mitsui Chemicals), Nucrel (manufactured by Mitsui-Dow Polychemicals), Bondfast (manufactured by Sumitomo Chemical), Acryft (manufactured by Sumitomo Chemical), Fusabond (manufactured by Dow Chemical), Rotader (manufactured by Arkema), and Evaflex (manufactured by Mitsui-Dow Polychemicals).

In the present invention, the melt flow rate (MFR) of the polyolefin resin at 190°C and a load of 2160 g can be appropriately selected depending on the particle size of the polyolefin resin particles to be obtained, but is preferably 15 g/10 min or more, more preferably 45 g/10 min or more, and particularly preferably 100 g/10 min or more. Since polyolefin resins with an MFR within the above range have low viscosity, the oil droplet size during emulsification becomes small, and it is easy to obtain particles with a small diameter and a narrow particle size distribution.
The melt flow rate of the polyolefin resin is measured using a melt indexer (F-B01 manufactured by Toyo Seiki Seisakusho) at a temperature of 190° C. and a load of 2160 g in accordance with a method in accordance with JIS K7210-2014.

The amount of polyolefin resin used in the present invention is preferably 1 part by mass or more, more preferably 10 parts by mass or more, and particularly preferably 20 parts by mass or more, assuming the total amount of (A) to (D) to be 100 parts by mass. By using polyolefin resin at or above the lower limit, the resulting aqueous dispersion of polyolefin resin particles can be used directly for applications such as coating without being concentrated, thereby reducing the number of manufacturing steps. There is no particular upper limit to the amount of polyolefin resin, and it is sufficient that it is within a range in which (A) to (D) can be stably emulsified.

(2) (B) Nonionic Surfactant The (B) nonionic surfactant in the present invention is a surfactant having a hydrophilic group such as a hydroxyl group or an ether bond that does not ionize in water. The nonionic surfactant may be any surfactant that has the effect of emulsifying the molten polyolefin resin and water, and may include polyethylene glycol type surfactants, polyhydric alcohol type surfactants, etc. In addition, polymer surfactants such as polyvinyl alcohol and its copolymers, polyvinylpyrrolidone and its copolymers, etc. may also be used. Among these, polyethylene glycol type surfactants containing a structural unit derived from ethylene oxide are preferred.

The average number of moles of ethylene oxide added in a polyethylene glycol surfactant is preferably 10 or more in total per molecule, more preferably 50 or more, and most preferably 100 or more. By having the degree of polymerization of ethylene oxide within the above range, sufficient emulsifying power can be obtained.

Examples of polyethylene glycol surfactants include ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylphenols, ethylene oxide adducts of fatty acids, ethylene oxide adducts of polyhydric alcohol fatty acid esters, ethylene oxide adducts of fatty acid amides, ethylene oxide adducts of oils and fats, and ethylene oxide adducts of polypropylene glycols. From the viewpoint of the emulsifying power of polyolefin resins, ethylene oxide adducts of polypropylene glycols are preferred.

The copolymerization ratio of the polypropylene glycol ethylene oxide adduct is not particularly limited, but the ethylene oxide ratio is preferably 50% by mass or more, more preferably 60% by mass or more, particularly preferably 70% by mass or more, and most preferably 80% by mass or more. By having the ethylene oxide ratio within the above range, the emulsifying power is improved.

Examples of polyhydric alcohol surfactants include fatty acid esters of glycerol, fatty acid esters of pentaerythritol, fatty acid esters of sorbitol and sorbitan, fatty acid esters of sucrose, alkyl ethers of polyhydric alcohols, fatty acid amides of alkanolamines, etc.

The amount of the nonionic surfactant used in the present invention is preferably 0.1 parts by mass or more, more preferably 1.0 parts by mass or more, assuming that the total amount of (A) to (D) is 100 parts by mass. When the amount of the nonionic surfactant is equal to or more than the lower limit, the surface area of the polyolefin resin that can be covered by the nonionic surfactant during emulsification increases, i.e., small oil droplets can be formed, making it possible to obtain small-diameter polyolefin resin particles. Furthermore, the upper limit of the nonionic surfactant is preferably 20 parts by mass or less, more preferably 10 parts by mass or less. When the amount of the nonionic surfactant is equal to or less than the upper limit, the viscosity increase of the resulting polyolefin resin particle aqueous dispersion is suppressed.

(3) (C) Ionic Surfactant The (C) ionic surfactant in the present invention is a surfactant having a hydrophilic group that undergoes ionic dissociation in water, and has a molecular weight of 1000 or less. If an ionic surfactant with a molecular weight of more than 1000 is used, the affinity with the polyolefin resin decreases, and sufficient emulsifying power may not be exhibited when used in combination with a nonionic surfactant. Ionic surfactants can be broadly classified into anionic surfactants, amphoteric surfactants, and cationic surfactants.

The molecular weight of ionic surfactants is measured using mass spectrometry. If multiple surfactants are present and separation is required, a liquid chromatography mass spectrometer or a capillary electrophoresis mass spectrometer can be used. If there is a distribution of molecular weights, such as with polyoxyethylene-based ionic surfactants, the value at the peak top of the obtained mass spectrum is taken as the molecular weight.

Known anionic surfactants include carboxylates, sulfates, sulfonates, and phosphates. Examples of sulfate and sulfonate surfactants include alkyl sulfates, alkylbenzene sulfonates, dialkyl succinate sulfonates, alkyl diphenyl ether disulfonates, polyoxyethylene alkyl ether sulfates, and polyoxyethylene alkyl phenyl ether sulfates. Among these, alkyl sulfates such as sodium lauryl sulfate and sodium hexadecyl sulfate are preferred.

Amphoteric surfactants include amino acid and betaine types, such as lauryl betaine, sodium hydroxyethyl imidazoline sulfate, and sodium imidazoline sulfonate.

Amine salt types and quaternary ammonium salt types are known as cationic surfactants, and examples thereof include alkylpyridinium chloride, alkyltrimethylammonium chloride, dialkyldimethylammonium chloride, and alkyldimethylbenzylammonium chloride.

As the ionic surfactant, a fluorosurfactant such as perfluoroalkyl carboxylate, perfluoroalkyl sulfonate, perfluoroalkyl phosphate, perfluoroalkyl betaine, or perfluoroalkoxy fluoroammonium carboxylate can also be used.

The amount of the ionic surfactant having a molecular weight of 1000 or less used in the present invention is preferably 0.01 parts by mass or more, more preferably 0.1 parts by mass or more, assuming that the total amount of (A) to (D) is 100 parts by mass. By having the amount of the ionic surfactant having a molecular weight of 1000 or less be equal to or more than the lower limit, it becomes possible to obtain polyolefin resin particles having a small diameter and a narrow particle size distribution. The upper limit of the amount of the ionic surfactant having a molecular weight of 1000 or less is preferably 10 parts by mass or less, more preferably 5 parts by mass or less. By having the amount of the ionic surfactant having a molecular weight of 1000 or less be equal to or less than the upper limit, it becomes easy to adjust the pH when using the obtained aqueous dispersion of polyolefin resin particles.

(4) (D) Water and Other Components The water used in the present invention is not particularly limited, but pure water or ultrapure water such as distilled water, RO water, and ion-exchanged water can be suitably used.
The amount of water used in the present invention is preferably 10 parts by mass or more, with the total amount of (A) to (D) being 100 parts by mass. By using an amount of water that is equal to or more than the lower limit, the dispersion stability of the polyolefin resin particles is improved.
In the present invention, small amounts of components other than (A) to (D) may be contained, but the total amount of these components is preferably 1 part by mass or less, and more preferably 0.1 parts by mass or less, relative to 100 parts by mass of the total amount of (A) to (D).

(5) Manufacturing process The manufacturing method of the present invention comprises the steps of heating (A) polyolefin resin, (B) nonionic surfactant, (C) ionic surfactant having a molecular weight of 1000 or less, and (D) water under stirring to a temperature equal to or higher than the melting point of the polyolefin resin, and then cooling to a temperature lower than the crystallization temperature of the polyolefin resin. A known method can be used for the stirring. In particular, when stirring with a stirring blade, the stirring speed can be appropriately adjusted according to the shape of the stirring blade, but it is sufficient as long as the polyolefin resin is emulsified.
Specific examples of the stirring blade include a propeller type, a paddle type, a flat paddle type, a turbine type, a double cone type, a single cone type, a single ribbon type, a double ribbon type, a screw type, and a helical ribbon type, but are not limited to these as long as they can apply a sufficient shear force to the system. Furthermore, in order to perform efficient stirring, a baffle plate or the like can be installed in the tank.

The heating temperature in the present invention is equal to or higher than the melting point of the polyolefin. If it is lower than the melting point of the polyolefin, the polyolefin will not melt sufficiently, and small-diameter particles will not be obtained. The lower limit of the heating temperature is not particularly limited as long as it is equal to or higher than the melting point of the polyolefin, and can be adjusted appropriately according to the particle size of the polyolefin resin particles to be obtained, but is preferably 20°C or higher than the melting point of the polyolefin resin, more preferably 30°C or higher, particularly preferably 40°C or higher, and most preferably 50°C or higher. The higher the heating temperature, the smaller the melt viscosity of the polyolefin, making it easier to form small-diameter oil droplets during emulsification, and ultimately making it easier to obtain small-diameter particles. The upper limit of the heating temperature is not particularly limited, and may be within a range in which the polyolefin resin does not undergo thermal decomposition (below the thermal decomposition temperature).

The melting point of polyolefin resin is measured by a differential scanning calorimeter according to the method described in JIS K7121-1987. For example, in the case of polyethylene, the temperature is raised from room temperature to 150°C at a heating rate of 10°C/min (1st Run), cooled from 150°C to room temperature at a heating rate of 10°C/min, and then heated from room temperature to 150°C at a heating rate of 10°C/min (2nd Run). The peak top temperature (°C) of the endothermic peak of melting is read as the melting point from the DSC chart obtained from the heating measurement in the 2nd Run.

In addition, (A) to (D) are heated to a temperature equal to or higher than the melting point of polyolefin resin (A), and the heating time at the maximum temperature is not particularly limited as long as the polyolefin resin is completely melted and a fine emulsion can be formed, but is preferably 10 minutes or more.

The cooling rate in the present invention is not particularly limited, but is preferably 0.5°C/min or more, since this allows for the production of an aqueous dispersion of particles with a uniform particle size.

In the present invention, (A) to (D) are heated under stirring to a temperature equal to or higher than the melting point of the polyolefin resin, and then cooled to a temperature below the crystallization temperature of the polyolefin resin.
The crystallization temperature of the polyolefin resin is measured by a differential scanning calorimeter according to the method described in JIS K7121-1987. The crystallization temperature is determined by the peak top temperature (°C) of the exothermic peak that appears when the temperature is increased from room temperature to 150°C at a rate of 10°C/min (1st Run) and then decreased from 150°C to room temperature at a rate of 10°C/min.

(6) Polyolefin resin particle aqueous dispersion As one embodiment of the present invention, there can be mentioned an aqueous dispersion of polyolefin resin particles composed of (A) polyolefin resin particles, (B) polyolefin resin particles containing a nonionic surfactant and (C) an ionic surfactant having a molecular weight of 1000 or less, and (D) water. Such an aqueous dispersion of polyolefin resin particles can be obtained, for example, by the above-mentioned production method. The aqueous dispersion of polyolefin resin particles preferably has a nonvolatile content of 1% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more.

The non-volatile content of the polyolefin resin particle aqueous dispersion is calculated from the weight of the residue after heating at 140°C.

When obtained by the manufacturing method of the present invention, the aqueous dispersion of polyolefin resin particles can be concentrated by a conventional method such as filtration, centrifugation, or evaporation, if necessary, or dried and used as a powder.

When the particles are extracted from the aqueous dispersion of polyolefin resin particles obtained by the manufacturing method of the present invention and used, they can be filtered using a filter that collects the particles, and then washed with water in an amount of at least 10 times the weight of the obtained wet cake, so that the total amount of (B) nonionic surfactant and (C) cationic surfactant having a molecular weight of 1000 or less contained in the particles is 5% by mass or less, preferably 1% by mass or less, based on the total weight of the particles.

In addition, as long as it does not interfere with solving the problems of the present invention, the pH of the polyolefin resin particle aqueous dispersion may be adjusted as necessary to improve dispersibility, or other components such as inorganic particles may be added to the polyolefin resin particle aqueous dispersion.

(7) Polyolefin resin particles As an embodiment of the present invention, small-sized polyolefin resin particles having a volume average particle diameter of preferably less than 100.0 μm, more preferably less than 10.0 μm, and particularly preferably less than 1.5 μm, and polyolefin resin particles having a particle diameter distribution index (D90/D10) of less than 2.9 and a narrow particle size distribution can be mentioned. By having a volume average particle diameter of the above upper limit or less, a smooth coating film can be formed. Such polyolefin resin particles can be obtained, for example, by the above-mentioned manufacturing method. The lower limit of the volume average particle diameter of the polyolefin resin particles is preferably 0.1 μm or more, more preferably 0.3 μm or more, and particularly preferably 0.5 μm or more. By having a lower limit of the volume average particle diameter of the above lower limit or more, handling properties are improved. The polyolefin resin particles contain (A) a polyolefin resin, (B) a nonionic surfactant, and (C) an ionic surfactant having a molecular weight of 1000 or less, and the (B) nonionic surfactant and the (C) ionic surfactant having a molecular weight of 1000 or less are contained in a total amount of 5% by weight or less, preferably 1% by mass or less, based on the entire particles. When the polyolefin resin particles contain (B) and (C) at or below the upper limit values, particle aggregation is suppressed, and a smooth coating film can be formed.

The volume average particle size of polyolefin resin particles can be measured using a particle size distribution measuring device that uses the laser diffraction/scattering method as its measurement principle. Examples of such particle size distribution measuring devices include the Microtrac MT3300EXII (manufactured by Nikkiso Co., Ltd.). The particle size distribution index (D90/D10) of polyolefin resin particles is calculated by measuring the particle size distribution in the same manner as the volume average particle size of polyolefin resin particles, determining a cumulative curve with the total volume of the obtained particles set at 100%, and calculating the value at which the cumulative curve from the small particle size side is 10% (D10) and the value at which the cumulative curve is 90% (D90).

(8) Uses of Polyolefin Resin Particle Aqueous Dispersion The polyolefin resin particle aqueous dispersion obtained by the production method of the present invention can be suitably used in various applications such as paints, adhesives, inks, toners, and cosmetic additives. In particular, the surface properties can be modified by coating substrates such as wood, metals, ceramics, carbon materials, glass, and plastics to form a coating film. The polyolefin resin particles obtained by the production method of the present invention have a small diameter and a narrow particle size distribution, so that a uniform and smooth coating film can be formed by coating, thereby imparting high rust prevention, liquid repellency, solvent resistance, adhesion, and the like.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. First, the measurement method will be described.

(1) Volume average particle size and particle size distribution index (D90/D10) of polyolefin resin particles
The polyolefin resin particle water dispersion was added to a laser diffraction type particle size distribution measuring device (Microtrac MT3300EXII) manufactured by Nikkiso Co., Ltd. until it reached a measurable concentration, and ultrasonic dispersion was performed in the measuring device at 30 W for 60 seconds, after which the volume average particle size was measured for a measurement time of 10 seconds. Measurement was also performed using the same method, and the particle size at which the cumulative frequency from the small particle size side of the particle size distribution was 90% was defined as D90, and the particle size at which the cumulative frequency was 10% was defined as D10. The refractive index during measurement was 1.60, and the refractive index of the medium (ion-exchanged water) was 1.333.

(2) Heat Residue (Solid Content Concentration) of Polyolefin Resin Particle Aqueous Dispersion
The heating residue of the aqueous dispersion of polyolefin resin particles at 140° C. was measured in accordance with JIS K5601-1-2:2008.

Example 1
60 g of ethylene-methacrylic acid copolymer a (manufactured by Mitsui Dow Polychemicals, MFR: 100 g/10 mL, melting point: 95° C., acid content: 11% by weight), 18 g of polypropylene glycol ethylene oxide adduct (manufactured by ADEKA, ethylene oxide ratio 80% by weight), 3 g of sodium lauryl sulfate (manufactured by Sigma-Aldrich), and 219 g of ion-exchanged water were added to a 1 L autoclave, sealed, and then substituted with nitrogen up to 0.7 MPa. The pressure of the system was adjusted to 0.03 MPa while releasing nitrogen, and the temperature was raised to 160° C. while stirring at 600 rpm. At this time, the pressure of the system reached 0.6 MPa. After holding at 160° C. for 30 minutes, the temperature was lowered to 120° C. at a rate of 2° C./min, and then lowered to 50° C. at a rate of 1° C./min. The obtained slurry was filtered through a nylon mesh with an opening of 77 μm to complete the preparation of a polyolefin resin particle aqueous dispersion. The volume average particle size of the obtained polyolefin resin particles was 1.4 μm, and the particle size distribution index (D90/D10) was 2.3. The heating residue (solid content concentration) of the particle aqueous dispersion was 26 mass%.

Example 2
A polyolefin resin particle aqueous dispersion was obtained in the same manner as in Example 1, except that the ethylene-methacrylic acid copolymer was changed to an ethylene-methacrylic acid copolymer b (manufactured by Dow Mitsui Polychemicals, MFR: 500 g/10 mL, melting point: 95° C., acid content: 10 mass%) having different physical properties. The volume average particle size of the obtained polyolefin resin particles was 0.9 μm, and the particle size distribution index (D90/D10) was 2.2. The heating residue (solid content concentration) of the particle aqueous dispersion was 27 mass%.

Example 3
A polyolefin resin particle aqueous dispersion was obtained in the same manner as in Example 2, except that the amount of sodium lauryl sulfate was changed to 1.5 g and the amount of ion-exchanged water was changed to 229.5 g. The volume average particle size of the obtained polyolefin resin particles was 1.0 μm, and the particle size distribution index (D90/D10) was 2.4. The heating residue (solid content concentration) of the particle aqueous dispersion was 23 mass%.

Example 4
A polyolefin resin particle aqueous dispersion was obtained in the same manner as in Example 3, except that sodium lauryl sulfate was replaced with sodium hexadecyl sulfate. The volume average particle size of the obtained polyolefin resin particles was 1.0 μm, and the particle size distribution index (D90/D10) was 2.4. The heating residue (solid content concentration) of the particle aqueous dispersion was 24 mass%.

Comparative Example 1
A polyolefin resin particle aqueous dispersion was obtained in the same manner as in Example 1, except that sodium lauryl sulfate was not used and the amount of ion-exchanged water was changed to 222 g. The volume average particle diameter of the obtained polyolefin resin particles was 2.3 μm, and the particle diameter distribution index (D90/D10) was 2.9, indicating that the particle diameter could not be reduced without an ionic surfactant. The heating residue (solid content concentration) of the particle aqueous dispersion was 26 mass%.

Comparative Example 2
The same procedure as in Example 2 was repeated except that no ethylene oxide-propylene oxide copolymer was used and the amount of ion-exchanged water was changed to 237 g. However, polyolefin resin lumps were obtained and it was not possible to obtain an aqueous dispersion of polyolefin resin particles.

The polyolefin resin particle aqueous dispersion obtained by the manufacturing method of the present invention has high chemical resistance and water resistance inherent to highly crystalline polyolefins, and because the particles have a small diameter and narrow particle size distribution, it can be suitably used as an additive for paints, adhesives, inks, toners, cosmetics, etc. In particular, because of the small diameter and narrow particle size distribution, it is possible to produce a uniform and smooth thin film by coating on various substrates such as films and metal surfaces, and it can impart high rust resistance, liquid repellency, solvent resistance, heat sealability, etc.

Claims (10)

A method for producing an aqueous dispersion of polyolefin resin particles, comprising the steps of heating (A) a polyolefin resin, (B) a nonionic surfactant, (C) an ionic surfactant having a molecular weight of 1000 or less, and (D) water under stirring to a temperature equal to or higher than the melting point of the polyolefin resin, and then cooling the mixture to a temperature lower than the crystallization temperature of the polyolefin resin. The method for producing an aqueous dispersion of polyolefin resin particles according to claim 1, wherein the ionic surfactant is an anionic surfactant. The method for producing an aqueous dispersion of polyolefin resin particles according to claim 1, wherein the ionic surfactant is an alkyl sulfate ester salt. The method for producing an aqueous dispersion of polyolefin resin particles according to any one of claims 1 to 3, wherein the polyolefin resin is any one of polyethylene, ethylene copolymer, polypropylene, and propylene copolymer, or a mixture thereof. The method for producing an aqueous dispersion of polyolefin resin particles according to claim 4, wherein the polyolefin resin is an ethylene-(meth)acrylic acid copolymer. The method for producing an aqueous dispersion of polyolefin resin particles according to any one of claims 1 to 3, wherein the nonionic surfactant contains structural units derived from ethylene oxide, and the degree of polymerization thereof is 100 or more in total. An aqueous dispersion of polyolefin resin particles comprising (A) polyolefin resin particles, (B) a nonionic surfactant, (C) an ionic surfactant having a molecular weight of 1000 or less, and (D) water. The polyolefin resin particle aqueous dispersion according to claim 7, wherein the volume average particle diameter of the polyolefin resin particles is less than 1.5 μm. The polyolefin resin particle aqueous dispersion according to claim 7 or 8, wherein the particle size distribution index (D90/D10) of the polyolefin resin particles is less than 2.9. Polyolefin resin particles containing (A) a polyolefin resin, (B) a nonionic surfactant, and (C) an ionic surfactant having a molecular weight of 1000 or less, the polyolefin resin particles containing the (B) nonionic surfactant and the (C) ionic surfactant having a molecular weight of 1000 or less in a total amount of 5 mass% or less relative to the entire particle.