Aqueous polyolefin resin dispersion, method for producing same, and coating film

Abstract

The purpose of the present invention is to provide an aqueous polyolefin resin dispersion which has excellent stability in a high temperature environment and is capable of forming a coating film that is excellent in terms of surface smoothness, and which is capable of forming a coating film that has high transparency even if the coating film is formed at a low temperature.　An aqueous polyolefin resin dispersion according to the present invention contains acid-modified polyolefin resin particles and an aqueous medium. An acid-modified polyolefin resin that constitutes the acid-modified polyolefin resin particles contains 50 mass% or more of a structural unit that is derived from propylene in 100 mass% of all structural units. The acid-modified polyolefin resin particles in the aqueous dispersion have a median diameter (D50) of 0.03 μm to 1.00 μm, a 90% particle diameter (D90) of 2.00 times the median diameter or less, and a 95% particle diameter (D95) of 2.20 times the median diameter or less in the volume-based cumulative particle size distribution as determined by a dynamic light scattering method.

Description

Aqueous polyolefin resin dispersion, its production method, and coating film

[0001] The present invention relates to an aqueous polyolefin resin dispersion, a method for producing the same, and a coating film.

[0002] Polyolefin resins, particularly propylene resins, are used in large quantities mainly in the automotive and electrical fields, packaging, and daily necessities due to their excellent electrical, mechanical, and chemical properties, etc. In particular, aqueous dispersions of polyolefin resins are widely used in adhesives and coating agents (see, for example, Patent Document 1).

[0003] International Publication No. 2004 / 104090

[0004] In order to use aqueous polyolefin resin dispersions in a wider range of applications, the aqueous polyolefin resin dispersions are required to be stable in high-temperature environments and to have surface smoothness for coating films formed from the aqueous polyolefin resin dispersions. In particular, in order to use aqueous polyolefin resin dispersions for bonding substrates that are susceptible to heat, it is required that coating films from the aqueous polyolefin resin dispersions can be formed at low temperatures. However, coating films formed at low temperatures using conventional aqueous polyolefin resin dispersions have the problem of being difficult to obtain sufficient transparency.

[0005] An object of the present invention is to provide an aqueous polyolefin resin dispersion that is excellent in stability under high-temperature environments, can form a coating film with excellent surface smoothness, and can form a coating film with high transparency even when formed at low temperatures.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that by using an acid-modified polyolefin resin containing 50% by mass or more of structural units derived from propylene based on 100% by mass of all structural units, and by having the acid-modified polyolefin resin particles in the aqueous dispersion have a specific particle size distribution, it is possible to obtain an aqueous polyolefin resin dispersion that can form a coating film that is stable under high temperature conditions and has excellent surface smoothness, and that can form a coating film that is highly transparent even when formed at low temperatures.The present inventors have also found that by stirring a composition containing the acid-modified polyolefin resin and an aqueous medium using a Max Blend impeller as an impeller at a stirring speed of 350 rpm or more, acid-modified polyolefin resin particles having a specific particle size distribution can be obtained.The present invention was completed through further research based on this finding.

[0007] That is, the present invention provides the following aspects: Item 1. An aqueous polyolefin resin dispersion containing acid-modified polyolefin resin particles and an aqueous medium, wherein the acid-modified polyolefin resin constituting the acid-modified polyolefin resin particles contains 50% by mass or more of structural units derived from propylene, based on 100% by mass of all structural units, and the acid-modified polyolefin resin particles in the aqueous dispersion have a median diameter (D50) of 0.03 to 1.00 μm, a 90% particle diameter (D90) of 2.00 times or less the median diameter, and a 95% particle diameter (D95) of 2.20 times or less the median diameter, in a volume cumulative particle size distribution obtained by measurement using a dynamic light scattering method. Item 1. The aqueous polyolefin resin dispersion according to Item 1, wherein the acid-modified polyolefin resin particles in the aqueous dispersion have a total frequency of acid-modified polyolefin resin particles having a particle size twice or more of the median size of 10.0% or less in a volume cumulative particle size distribution measured by dynamic light scattering. Item 3. The aqueous polyolefin resin dispersion according to Item 1 or 2, wherein the acid-modified polyolefin resin contains structural units derived from at least one olefin selected from the group consisting of ethylene, isobutylene, 1-butene, and 2-butene. Item 4. The aqueous polyolefin resin dispersion according to any one of Items 1 to 3, wherein the acid-modified polyolefin resin contains 0.1 to 10 mass% of structural units derived from unsaturated carboxylic acid and / or structural units derived from unsaturated carboxylic acid anhydride, based on 100 mass% of all structural units. Item 5. Item 5. A method for producing the aqueous polyolefin resin dispersion according to any one of Items 1 to 4, comprising a step of stirring a composition containing at least the acid-modified polyolefin resin and an aqueous medium using a Max Blend impeller as a stirring impeller at a stirring rotation speed of 350 rpm or more. Item 6. A coating film obtained from the aqueous polyolefin resin dispersion according to any one of Items 1 to 4.

[0008] The aqueous polyolefin resin dispersion of the present invention has excellent stability in a high-temperature environment, can form a coating film with excellent surface smoothness, and can form a coating film with high transparency even when formed at a low temperature. Therefore, the aqueous polyolefin resin dispersion of the present invention is particularly suitable for use as an adhesive or a coating agent.

[0009] 1 is a schematic diagram of a paddle blade used when producing a conventional aqueous polyolefin resin dispersion. 2 is a schematic diagram of a propeller blade used when producing a conventional aqueous polyolefin resin dispersion. 3 is a schematic diagram of a Max Blend blade used when producing an aqueous polyolefin resin dispersion of the present invention.

[0010] 1. Aqueous Polyolefin Resin Dispersion In the present invention, the aqueous polyolefin resin dispersion refers to an emulsion in which acid-modified polyolefin resin particles are dispersed in an aqueous medium.

[0011] The polyolefin resin aqueous dispersion of the present invention (hereinafter also referred to simply as "aqueous dispersion") contains acid-modified polyolefin resin particles and an aqueous medium, and the acid-modified polyolefin resin constituting the acid-modified polyolefin resin particles contains 50% by mass or more of structural units derived from propylene, based on 100% by mass of all structural units. The acid-modified polyolefin resin particles in the aqueous dispersion are characterized in that, in a volume-cumulative particle size distribution obtained by measurement using a dynamic light scattering method, the median diameter (D50) is 0.03 to 1.00 μm, the 90% particle diameter (D90) is 2.00 times or less the median diameter, and the 95% particle diameter (D95) is 2.20 times or less the median diameter. The polyolefin resin aqueous dispersion of the present invention will be described in detail below.

[0012] [Acid-Modified Polyolefin Resin Particles] In the present invention, the acid-modified polyolefin resin particles refer to particles containing a polyolefin resin containing structural units derived from an unsaturated carboxylic acid and / or structural units derived from an unsaturated carboxylic acid anhydride.

[0013] The acid-modified polyolefin resin constituting the acid-modified polyolefin resin particles of the present invention contains 50% by mass or more of structural units derived from propylene based on 100% by mass of all structural units. From the viewpoint of further improving the stability of the aqueous dispersion in a high-temperature environment and from the viewpoint of forming a coating film with higher transparency even when formed at a low temperature, the content of the structural units derived from propylene is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0014] The acid-modified polyolefin resin may contain structural units derived from an olefin (unsaturated hydrocarbon) other than propylene. The olefin other than propylene is not particularly limited, and examples thereof include monoenes such as ethylene, isobutylene, 1-butene, 2-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, 3-methyl-1-pentene, and norbornene; and dienes such as butadiene and isoprene. One or more structural units derived from olefins other than propylene may be contained. Among these structural units derived from olefins, from the viewpoint of further improving the stability of the aqueous dispersion in a high-temperature environment, it is preferable to contain structural units derived from at least one olefin selected from the group consisting of ethylene, isobutylene, 1-butene, and 2-butene, and it is more preferable to contain structural units derived from ethylene and / or 1-butene. In the acid-modified polyolefin resin, the content of structural units derived from olefins other than propylene is usually about 1 to 45 mass% based on 100 mass% of all structural units, and from the viewpoint of improving the adhesion, flexibility, and heat resistance of a coating film formed from the aqueous dispersion, it is preferably 2 to 35 mass%, more preferably 3 to 30 mass%, and even more preferably 4 to 25 mass%.

[0015] The acid-modified polyolefin resin contains structural units derived from an unsaturated carboxylic acid and / or structural units derived from an unsaturated carboxylic acid anhydride. By including structural units derived from an unsaturated carboxylic acid and / or structural units derived from an unsaturated carboxylic acid anhydride in the acid-modified polyolefin resin, the aqueous nature and dispersibility of the acid-modified polyolefin resin particles in the aqueous dispersion are improved, and the stability of the aqueous dispersion in a high-temperature environment is improved.

[0016] The unsaturated carboxylic acid is not particularly limited, and examples thereof include unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, and isocrotonic acid; unsaturated dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid; and compounds in which one carboxy group of the unsaturated dicarboxylic acid is esterified or amidated. The unsaturated carboxylic acid anhydride is not particularly limited, and examples thereof include maleic anhydride and itaconic anhydride. One or more structural units derived from an unsaturated carboxylic acid may be contained. Furthermore, one or more structural units derived from an unsaturated carboxylic acid anhydride may be contained. Of these structural units, from the viewpoints of ease of incorporation into a polyolefin resin and excellent adhesion when formed into a coating film, preferably one or more structural units selected from structural units derived from acrylic acid, methacrylic acid, maleic acid, and maleic anhydride are used, more preferably structural units derived from acrylic acid and / or maleic anhydride, and even more preferably structural units derived from maleic anhydride.

[0017] In the acid-modified polyolefin resin, the content of structural units derived from unsaturated carboxylic acids and / or structural units derived from unsaturated carboxylic acid anhydrides (the total content when structural units derived from unsaturated carboxylic acids and structural units derived from unsaturated carboxylic acid anhydrides are contained) is usually about 0.05 to 20 mass% relative to 100 mass% of all structural units, and from the viewpoint of further improving the stability of the aqueous dispersion in a high-temperature environment, and from the viewpoint of improving the low water absorbency, water resistance, solvent resistance, and adhesion of a coating film formed from the aqueous dispersion, it is preferably 0.1 to 10 mass%, more preferably 1 to 8 mass%, and even more preferably 2 to 7 mass%.

[0018] The acid-modified polyolefin resin may contain structural units derived from other copolymerizable monomers, but preferably does not contain any. Examples of the copolymerizable monomers include unsaturated monocarboxylic acid esters, hydroxyl group-containing vinyl monomers, amino group-containing vinyl monomers, glycidyl group-containing vinyl monomers, cyano group-containing vinyl monomers, sulfonic acid group-containing vinyl monomers, acetoacetoxy group-containing vinyl monomers, phosphate group-containing vinyl monomers, amide group-containing vinyl monomers, aromatic vinyl monomers, and heterocyclic vinyl monomers. One or more structural units derived from the copolymerizable monomers may be contained. In the acid-modified polyolefin resin, the total content of the structural units derived from the copolymerizable monomers is usually about 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of all structural units.

[0019] The acid-modified polyolefin resin can be produced by copolymerizing at least propylene with an unsaturated carboxylic acid and / or an unsaturated carboxylic acid anhydride according to a known method. The form of the acid-modified polyolefin resin is not particularly limited, and examples thereof include a random copolymer, a block copolymer, and a graft copolymer.

[0020] When the acid-modified polyolefin resin is a graft copolymer, methods for introducing structural units derived from unsaturated carboxylic acids and / or unsaturated carboxylic anhydrides into the polyolefin resin include, for example, a method of reacting a polyolefin resin with an unsaturated carboxylic acid and / or an unsaturated carboxylic anhydride by heating to a temperature above the melting point of the polyolefin resin in the presence of a radical generator, or a method of dissolving the polyolefin resin in an organic solvent, adding the unsaturated carboxylic acid and / or an unsaturated carboxylic anhydride and a radical generator, and then heating to react. The polyolefin resin may also be a biomass-derived polyolefin resin. Biomass-derived polyolefin resins can be produced, for example, by fermenting biomass-derived components (biomass resources) such as starch and sugars obtained from corn, sugarcane, sweet potato, etc., with microorganisms to obtain bioethanol, dehydrating the bioethanol to obtain propylene, and polymerizing the propylene. The polyolefin resin may also be a recycled polyolefin resin containing a biomass-derived polyolefin resin.

[0021] Examples of radical generators used in the graft copolymerization include organic peroxides such as di-tert-butyl peroxide, dicumyl peroxide, tert-butyl hydroperoxide, tert-butylcumyl peroxide, benzoyl peroxide, dilauryl peroxide, cumene hydroperoxide, tert-butyl peroxybenzoate, ethyl ethyl ketone peroxide, and di-tert-butyl diperphthalate, and azonitriles such as azobisisobutyronitrile. These may be appropriately selected and used depending on the reaction temperature, etc.

[0022] The weight-average molecular weight of the acid-modified polyolefin resin is not particularly limited, but is usually about 5,000 to 150,000, and from the viewpoint of improving the adhesion between the substrate and the coating film formed from the aqueous dispersion and from the viewpoint of improving the flexibility of the coating film formed from the aqueous dispersion, it is preferably 20,000 to 120,000, more preferably 40,000 to 100,000, and even more preferably 60,000 to 90,000. In the present invention, the weight-average molecular weight of the acid-modified polyolefin resin is a value obtained by gel permeation chromatography (GPC) and converted into standard polystyrene.

[0023] In the aqueous dispersion of the present invention, the content of the acid-modified polyolefin resin particles is not particularly limited and may be appropriately adjusted depending on the film-forming conditions, the desired thickness and performance of the coating film, etc., but is usually about 1 to 60 mass %, and from the viewpoint of maintaining an appropriate viscosity of the aqueous dispersion and exhibiting good coating film formability, it is preferably 3 to 50 mass %, more preferably 5 to 40 mass %, and even more preferably 10 to 30 mass %.

[0024] [Aqueous Medium] In the present invention, the aqueous medium means a liquid containing water as a main component (containing 50% by mass or more).

[0025] The water is not particularly limited, and examples thereof include distilled water, ion-exchanged water, city water, industrial water, etc. Among these, distilled water and ion-exchanged water are preferred from the viewpoint of preventing the inclusion of impurities.

[0026] The aqueous medium preferably contains a hydrophilic organic solvent in order to promote the conversion of the acid-modified polyolefin resin to a water-soluble form and to reduce the dispersed particle size.

[0027] In the present invention, the hydrophilic organic solvent means an organic solvent having a solubility in water at 20° C. of 10 g / L or more.

[0028] From the viewpoint of facilitating removal from the coating film, the hydrophilic organic solvent preferably has a boiling point of 185°C or less at normal pressure, more preferably 50 to 150°C.

[0029] Examples of hydrophilic organic solvents include alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, n-amyl alcohol, isoamyl alcohol, sec-amyl alcohol, tert-amyl alcohol, 1-ethyl-1-propanol, 2-methyl-1-butanol, n-hexanol, and cyclohexanol; ketones such as methyl ethyl ketone, methyl isobutyl ketone, ethyl butyl ketone, and cyclohexanone; ethers such as tetrahydrofuran and dioxane; ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, 3-methoxybutyl acetate, and propyl acetate. Examples of the hydrophilic organic solvent include esters such as methyl pionate, ethyl propionate, diethyl carbonate, and dimethyl carbonate; glycol derivatives such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, ethylene glycol ethyl ether acetate, and diethylene glycol monomethyl ether; 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 3-methoxy-3-methyl-1-butanol, methoxybutanol, acetonitrile, dimethylformamide, dimethylacetamide, diacetone alcohol, ethyl acetoacetate, 1,2-dimethylglycerin, 1,3-dimethylglycerin, and trimethylglycerin. These hydrophilic organic solvents may be used alone or in combination of two or more. Of these hydrophilic organic solvents, from the viewpoint of further promoting the conversion of the acid-modified polyolefin resin to a water-based solution, at least one selected from the group consisting of alcohols, ketones, ethers, and glycol derivatives is preferred, and at least one selected from the group consisting of ethanol, n-propanol, isopropanol, n-butanol, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, dioxane, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, and diethylene glycol monomethyl ether is more preferred.

[0030] In the aqueous medium, the content of the hydrophilic organic solvent is 50% by mass or less, and from the viewpoints of accelerating the conversion of the acid-modified polyolefin resin to an aqueous solution, reducing the dispersed particle size, and reducing the amount of organic solvent used, the content is preferably 1 to 45% by mass, more preferably 2 to 40% by mass, and even more preferably 3 to 35% by mass.

[0031] The aqueous medium may contain a hydrophobic organic solvent in order to further promote the conversion of the acid-modified polyolefin resin to an aqueous solution and to further reduce the dispersed particle size.

[0032] In the present invention, the hydrophobic organic solvent means an organic solvent having a solubility in water at 20° C. of less than 10 g / L.

[0033] From the viewpoint of facilitating removal from the coating film, the hydrophobic organic solvent preferably has a boiling point of 185°C or less at normal pressure, more preferably 50 to 150°C.

[0034] Examples of hydrophobic organic solvents include aliphatic hydrocarbon solvents such as n-pentane, n-hexane, n-heptane, and petroleum ether; alicyclic hydrocarbon solvents such as cycloheptane and cyclohexane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; and halogenated solvents such as carbon tetrachloride, 1,2-dichloroethane, 1,1-dichloroethylene, trichloroethylene, 1,1,1-trichloroethane, and chloroform. These hydrophobic organic solvents may be used alone or in combination of two or more.

[0035] In the aqueous medium, the content of the hydrophobic organic solvent is usually 20% by mass or less, and from the viewpoints of accelerating the conversion of the acid-modified polyolefin resin to an aqueous solution, reducing the dispersed particle size, reducing the amount of organic solvent used, and suppressing gelation of the aqueous dispersion, it is preferably 1 to 15% by mass, more preferably 1 to 10% by mass, and even more preferably 1 to 5% by mass.

[0036] In the aqueous dispersion of the present invention, the content of the aqueous medium is not particularly limited and is usually about 40 to 99% by mass. From the viewpoint of maintaining an appropriate viscosity of the aqueous dispersion and exhibiting good coating film-forming ability, the content is preferably 45 to 95% by mass, more preferably 50 to 90% by mass, and even more preferably 55 to 85% by mass.

[0037] [Other Components] The aqueous dispersion of the present invention preferably further contains a basic compound. The basic compound neutralizes some or all of the carboxy groups in the acid-modified polyolefin resin, and the electrical repulsive force between the generated carboxy anions suppresses aggregation between the fine particles, thereby improving the stability of the aqueous dispersion. From the viewpoint of facilitating removal from the coating film, the basic compound preferably has a boiling point of 185°C or less at normal pressure.

[0038] Examples of basic compounds include ammonia, triethylamine, N,N-dimethylethanolamine, isopropylamine, aminoethanol, dimethylaminoethanol, diethylaminoethanol, ethylamine, diethylamine, isobutylamine, dipropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, sec-butylamine, propylamine, n-butylamine, 2-methoxyethylamine, 3-methoxypropylamine, 2,2-dimethoxyethylamine, monoethanolamine, morpholine, N-methylmorpholine, N-ethylmorpholine, pyrrole, and pyridine. These basic compounds may be used alone or in combination of two or more. Of these, N,N-dimethylethanolamine is preferred from the viewpoint of further improving the dispersibility of the acid-modified polyolefin resin particles.

[0039] In the aqueous dispersion of the present invention, the content of the basic compound is not particularly limited, and is usually about 0.3 to 12 times equivalent relative to the acid groups, such as carboxy groups, possessed by the acid-modified polyolefin resin. From the viewpoint of improving the stability of the aqueous dispersion and from the viewpoint of facilitating removal from the coating film, the content is preferably 0.5 to 10 times equivalent, more preferably 0.8 to 7 times equivalent, and even more preferably 0.9 to 5 times equivalent. Furthermore, in the aqueous dispersion of the present invention, the content of the basic compound is usually about 0.1 to 20% by mass, and from the above viewpoints, it is preferably 0.5 to 15% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 5% by mass.

[0040] The aqueous dispersion of the present invention may contain other polymers, crosslinking agents, other additives, and the like in appropriate amounts in order to further improve the performance of the coating film depending on the purpose.

[0041] The other polymers are not particularly limited and include, for example, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinyl chloride, polyvinylidene chloride, styrene-maleic acid resin, styrene-butadiene resin, butadiene resin, acrylonitrile-butadiene resin, polyacrylonitrile resin, polymethacrylonitrile resin, acrylamide resin, methacrylamide resin, chlorinated polyethylene resin, chlorinated polypropylene resin, modified nylon resin, urethane resin, phenolic resin, silicone resin, epoxy resin, tackifying resin such as rosin, etc. These polymers may be used alone or in combination of two or more.

[0042] The crosslinking agent is not particularly limited, and examples thereof include a crosslinking agent having self-crosslinking properties, a compound having multiple functional groups reactive with carboxy groups in the molecule, and a metal having polyvalent coordination sites. Specific examples of the crosslinking agent include an isocyanate compound, a melamine compound, a urea compound, an epoxy compound, a carbodiimide compound, an oxazoline group-containing compound, a zirconium salt compound, and a silane coupling agent. These crosslinking agents may be used alone or in combination of two or more.

[0043] Examples of other additives include leveling agents, antifoaming agents, anti-popping agents, pigments, pigment dispersants, ultraviolet absorbers, weather resistance agents, and flame retardants.

[0044] The aqueous dispersion of the present invention preferably does not substantially contain a non-volatile water-forming aid. Although the use of a non-volatile water-forming aid is not excluded in the aqueous dispersion of the present invention, the aqueous dispersion of the present invention can finely and stably disperse the acid-modified polyolefin resin particles in an aqueous medium without using a non-volatile water-forming aid.

[0045] In the present invention, the term "non-volatile aqueous dispersion aid" refers to a chemical or compound added for the purpose of promoting aqueous dispersion or stabilizing the aqueous dispersion, and "non-volatile" means that the chemical or compound has no boiling point at normal pressure or a boiling point of 300°C or higher at normal pressure.

[0046] In the present invention, "substantially free from non-volatile water-forming aids" means that no non-volatile water-forming aids are used during the production of the aqueous dispersion (during the aqueous dispersion of the acid-modified polyolefin resin), and the resulting aqueous dispersion consequently contains no non-volatile water-forming aids. In the aqueous dispersion of the present invention, the content of the non-volatile water-forming aids is preferably 5 parts by mass or less, more preferably 2 parts by mass or less, even more preferably 0.5 parts by mass or less, and most preferably 0 parts by mass per 100 parts by mass of the acid-modified polyolefin resin.

[0047] Examples of non-volatile water-soluble aids include surfactants, compounds having protective colloidal properties, modified waxes, acid-modified compounds with a high acid value, and water-soluble polymers.

[0048] Examples of surfactants include cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants, fluorine-based surfactants, and reactive surfactants, including those commonly used in emulsion polymerization as well as emulsifiers. Examples of anionic surfactants include sulfate ester salts of higher alcohols, higher alkyl sulfonic acids and their salts, higher carboxylic acids and their salts such as oleic acid, stearic acid, and palmitic acid, alkylbenzene sulfonic acids and their salts, polyoxyethylene alkyl sulfate salts, polyoxyethylene alkylphenyl ether sulfate salts, and vinyl sulfosuccinate. Examples of nonionic surfactants include compounds having a polyoxyethylene structure such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyethylene glycol fatty acid esters, ethylene oxide-propylene oxide block copolymers, and polyoxyethylene fatty acid amides, and sorbitan derivatives such as polyoxyethylene sorbitan fatty acid esters. Examples of amphoteric surfactants include lauryl betaine and lauryl dimethylamine oxide. Examples of reactive surfactants include compounds having a reactive double bond, such as alkylpropenylphenol polyethylene oxide adducts and sulfate ester salts thereof, allylalkylphenol polyethylene oxide adducts and sulfate ester salts thereof, and allyldialkylphenol polyethylene oxide adducts and sulfate ester salts thereof.

[0049] Examples of compounds having protective colloidal action, modified waxes, acid-modified compounds with a high acid value, and water-soluble polymers include polyvinyl alcohol, carboxy group-modified polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, modified starch, polyvinylpyrrolidone, polyacrylic acid and salts thereof, acid-modified polyolefin waxes and salts thereof, such as carboxy group-containing polyethylene wax, carboxy group-containing polypropylene wax, and carboxy group-containing polyethylene-propylene wax, acrylic acid-maleic anhydride copolymers and salts thereof, styrene-(meth)acrylic acid copolymers, ethylene-(meth)acrylic acid copolymers, isobutylene-maleic anhydride alternating copolymers, and carboxy group-containing polymers and salts thereof, such as (meth)acrylic acid-(meth)acrylic acid ester copolymers, polyitaconic acid and salts thereof, water-soluble acrylic copolymers having amino groups, gelatin, gum arabic, and casein, which are generally used as dispersion stabilizers for fine particles.

[0050] [Particle Size of Acid-Modified Polyolefin Resin Particles] In the aqueous dispersion of the present invention, the acid-modified polyolefin resin particles in the aqueous dispersion have a median diameter (D50) of 0.03 to 1.00 μm, a 90% particle diameter (D90) of 2.00 times or less the median diameter, and a 95% particle diameter (D95) of 2.20 times or less the median diameter, in a volume cumulative particle size distribution measured by dynamic light scattering. The acid-modified polyolefin resin particles in the aqueous dispersion have the specific particle size distribution, which results in an aqueous dispersion that is excellent in stability under high-temperature conditions, can form a coating film with excellent surface smoothness, and can form a highly transparent coating film even when formed at low temperatures. In the present invention, the median diameter refers to the particle diameter (D50) at which the cumulative frequency reaches 50% in a volume cumulative particle size distribution measured by dynamic light scattering using a particle size distribution analyzer. In the present invention, the 90% particle size refers to the particle size (D90) at which the cumulative frequency reaches 90% in a volume-accumulated particle size distribution measured by a dynamic light scattering method using a particle size distribution analyzer, and the 95% particle size refers to the particle size (D95) at which the cumulative frequency reaches 95% in a volume-accumulated particle size distribution measured by a dynamic light scattering method using a particle size distribution analyzer.

[0051] The median diameter (D50) of the acid-modified polyolefin resin particles in the aqueous dispersion is preferably 0.10 to 0.80 μm, more preferably 0.15 to 0.60 μm, and even more preferably 0.20 to 0.40 μm, from the viewpoint of obtaining an aqueous dispersion which has superior stability in a high-temperature environment, is capable of forming a coating film with superior surface smoothness, and is capable of forming a coating film with higher transparency even when formed at a low temperature.

[0052] From the viewpoint of obtaining an aqueous dispersion that is superior in stability under high-temperature environments, capable of forming a coating film with superior surface smoothness, and capable of forming a coating film with higher transparency even when formed at low temperatures, the 90% particle size (D90) of the acid-modified polyolefin resin particles in the aqueous dispersion is preferably 1.90 times or less the median size, more preferably 1.80 times or less the median size, even more preferably 1.75 times or less the median size, and even more preferably 1.65 times or less the median size. Furthermore, the 90% particle size (D90) of the acid-modified polyolefin resin particles in the aqueous dispersion is usually 1.20 times or more the median size.

[0053] From the viewpoint of obtaining an aqueous dispersion that is superior in stability under high temperature environments, capable of forming a coating film with superior surface smoothness, and capable of forming a coating film with higher transparency even when formed at low temperatures, the 95% particle size (D95) of the acid-modified polyolefin resin particles in the aqueous dispersion is preferably 2.10 times or less the median size, more preferably 2.00 times or less the median size, even more preferably 1.90 times or less the median size, and even more preferably 1.85 times or less the median size. Furthermore, the 95% particle size (D95) of the acid-modified polyolefin resin particles in the aqueous dispersion is usually 1.30 times or more the median size.

[0054] In one embodiment of the aqueous dispersion of the present invention, the 70% particle size (D70) of the acid-modified polyolefin resin particles in the aqueous dispersion is, for example, 1.05 to 1.30 times the median size, and may be 1.05 to 1.25 times, 1.05 to 1.23 times, 1.05 to 1.20 times, or 1.05 to 1.15 times the median size. In the present invention, the 70% particle size refers to the particle size (D70) at which the cumulative frequency reaches 70% in a volume-accumulated particle size distribution measured by dynamic light scattering using a particle size distribution analyzer.

[0055] In one embodiment of the aqueous dispersion of the present invention, the 20% particle size (D20) of the acid-modified polyolefin resin particles in the aqueous dispersion is, for example, 0.40 to 0.95 times the median size, and may be 0.45 to 0.90 times, 0.50 to 0.90 times, 0.55 to 0.90 times, 0.60 to 0.90 times, or 0.65 to 0.90 times the median size. In the present invention, the 20% particle size refers to the particle size (D20) at which the cumulative frequency reaches 20% in a volume-accumulated particle size distribution measured by dynamic light scattering using a particle size distribution analyzer.

[0056] In one embodiment of the aqueous dispersion of the present invention, the 10% particle size (D10) of the acid-modified polyolefin resin particles in the aqueous dispersion is, for example, 0.30 to 0.90 times the median size, and may be 0.35 to 0.85 times, 0.40 to 0.80 times, 0.45 to 0.80 times, or 0.50 to 0.80 times the median size. In the present invention, the 10% particle size refers to the particle size (D10) at which the cumulative frequency is 10% in a volume-accumulated particle size distribution measured by dynamic light scattering using a particle size distribution analyzer.

[0057] In the aqueous dispersion of the present invention, in the volume-accumulated particle size distribution measured by dynamic light scattering, the total frequency (distribution rate) of acid-modified polyolefin resin particles having a particle size twice or more of the median size is usually 10.0% or less, and from the viewpoint of obtaining an aqueous dispersion that is more stable in a high-temperature environment, can form a coating film with more excellent surface smoothness, and can form a coating film with higher transparency even when formed at low temperatures, it is preferably 9.0% or less, more preferably 8.0% or less, even more preferably 7.0% or less, and particularly preferably 6.0% or less. In addition, the total frequency (distribution rate) of acid-modified polyolefin resin particles having a particle size twice or more of the median size is usually 0.3% or more.

[0058] [Uses] The aqueous dispersion of the present invention has excellent adhesiveness or adhesion to various substrates, such as metal, glass, plastic molded bodies, films, synthetic paper, and paper, and is therefore suitable for use as a coating agent, primer, paint (ink), and adhesive for these substrates. The aqueous dispersion of the present invention has excellent stability in high-temperature environments, and can form a coating film with excellent surface smoothness and high transparency even when formed at low temperatures, making it particularly suitable for use as an adhesive and coating agent.

[0059] 2. Method for Producing Aqueous Polyolefin Resin Dispersion The method for producing the aqueous dispersion of the present invention is described in detail below. The method for producing the aqueous dispersion of the present invention is not particularly limited, but one specific example is a production method including a step of stirring a composition containing the components described above in "1. Aqueous Polyolefin Resin Dispersion," i.e., an acid-modified polyolefin resin, an aqueous medium, and other components that are blended as necessary (e.g., a basic compound, other polymers, a crosslinking agent, a leveling agent, an antifoaming agent, an anti-popping agent, a pigment, a pigment dispersant, an ultraviolet absorber, a weathering agent, and a flame retardant), in a container equipped with a stirring blade.

[0060] Regarding stirring blades, in the production of aqueous dispersions, stirring blades for low viscosity fluids, such as paddle blades as shown in Figure 1 and propeller blades as shown in Figure 2, have traditionally been used. However, in the production of the aqueous dispersion of the present invention, in order to form acid-modified polyolefin resin particles having a specific particle size distribution in which the median diameter (D50) is 0.03 to 1.00 μm, the 90% particle diameter (D90) is 2.00 times or less the median diameter, and the 95% particle diameter (D95) is 2.20 times or less the median diameter, it is preferable to use a Max Blend blade, which is a stirring blade for high viscosity fluids that is not normally used in the production of aqueous dispersions. The Max Blend blade is a stirring blade in which a bottom paddle and an upper grid are integrated as shown in Figure 3, and an example is Max Blend (registered trademark) manufactured by Sumitomo Heavy Industries Process Equipment Co., Ltd. Examples of max blend impellers include standard types, high viscosity types, deep liquid types, and aeration stirring types, and the high viscosity type is preferred from the viewpoint of facilitating the production of acid-modified polyolefin resin particles having the specific particle size distribution.

[0061] The container may be, for example, one that is generally used in a solid / liquid mixing device or emulsifying machine, and is preferably one that is sealable and capable of being pressurized to 0.1 MPa or more.

[0062] Regarding the stirring speed, conventionally, for the purpose of simplifying equipment and reducing costs, a low stirring speed (about 300 rpm) at which the polyolefin resin is suspended in the aqueous medium has generally been adopted. On the other hand, in the production of the aqueous dispersion of the present invention, high-speed stirring is preferred in order to facilitate the formation of acid-modified polyolefin resin particles having the specific particle size distribution. In the production of the aqueous dispersion of the present invention, the rotation speed of the stirring blade during stirring is usually 350 rpm or more, and from the viewpoint of making it easier to form acid-modified polyolefin resin particles having the specific particle size distribution, and from the viewpoint of preventing excessively strong shear force from causing aggregates to form, resulting in an increase in coarse particles, or conversely, from preventing excessively strong shear force from causing an increase in small particles, resulting in high viscosity, the rotation speed is preferably 350 to 1800 rpm, more preferably 450 to 1800 rpm, even more preferably 600 to 1700 rpm, still more preferably 800 to 1700 rpm, even more preferably 1000 to 1700 rpm, and particularly preferably 1200 to 1700 rpm.

[0063] The temperature inside the container during stirring (the temperature of the composition) is not particularly limited, but is usually about 60 to 220°C, and from the viewpoint of promoting the aqueous conversion of the acid-modified polyolefin resin and suppressing a decrease in the molecular weight of the acid-modified polyolefin resin, it is preferably 80 to 200°C, more preferably 100 to 180°C, and even more preferably 100 to 150°C.

[0064] The stirring time is not particularly limited, and stirring may be continued until coarse particles disappear, but is usually about 5 to 120 minutes, preferably 10 to 100 minutes, and more preferably 30 to 80 minutes.

[0065] After the acid-modified polyolefin resin has been sufficiently rendered aqueous and microparticulated by the above steps, the aqueous dispersion of the present invention is obtained by cooling the resin until the temperature inside the container is, for example, about 50°C or less, preferably about 40°C or less.

[0066] Thereafter, if necessary, a jet pulverization treatment may be carried out. Jet pulverization treatment is a treatment in which the aqueous dispersion is sprayed under high pressure from a nozzle or a fine hole such as a slit, and the resin particles are caused to collide with each other or with a collision plate or the like, thereby further reducing the resin particles by mechanical energy. Examples of devices used for the jet pulverization treatment include a homogenizer manufactured by A.P.V. Gaulin and a Microfluidizer M-110E / H manufactured by Mizuho Kogyo Co., Ltd.

[0067] The solids concentration of the resulting aqueous dispersion may be adjusted to a desired concentration. Examples of methods for adjusting the solids concentration of the aqueous dispersion include a method of distilling off the aqueous medium to a desired solids concentration, and a method of diluting with water.

[0068] The aqueous dispersion obtained by the production method of the present invention is a homogeneous liquid in which the acid-modified polyolefin resin is dispersed or dissolved in an aqueous medium. Here, "homogeneous liquid" refers to a state in which the aqueous dispersion does not appear to have any localized areas with different solid content concentrations from other areas, such as precipitation, phase separation, or skinning.

[0069] The aqueous dispersion obtained by the production method of the present invention has excellent stability in high-temperature environments because the acid-modified polyolefin resin particles have a specific particle size distribution. In addition, it is possible to form a coating film with excellent surface smoothness, and even when the coating film is formed at a low temperature, it is possible to form a coating film with high transparency.

[0070] [Coating Film] The coating film of the present invention is obtained from the aqueous dispersion of the present invention and is characterized by excellent surface smoothness and transparency. The coating film of the present invention can be obtained, for example, by applying the aqueous dispersion of the present invention to various substrates, drying the substrate, and optionally heating the substrate. The substrate is not particularly limited, and examples thereof include resin films such as thermoplastic resin films, metal foils such as aluminum foils, metal plates such as aluminum plates, glass, and paper.

[0071] The coating method is not particularly limited, and examples thereof include gravure roll coating, reverse roll coating, wire bar coating, lip coating, air knife coating, curtain flow coating, spray coating, dip coating, brush coating, etc. The amount of the aqueous dispersion to be applied may be appropriately adjusted depending on the application of the coating film.

[0072] As the drying device and heating device, a normal hot air circulation oven, an infrared heater, etc. may be used. The drying temperature, drying time, and heating temperature, heating time may be appropriately adjusted depending on the characteristics of the substrate, the type of aqueous medium, etc.

[0073] The surface roughness (Ra) of the coating film obtained from the aqueous dispersion of the present invention is usually 100 nm or less, preferably 80 nm or less, and is, for example, 30 nm or more, 40 nm or more, or 50 nm or more.

[0074] The coating film of the present invention has excellent surface smoothness and transparency, and is therefore particularly suitable for use as an adhesive layer, a primer layer, and a coating layer.

[0075] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Various measurement and evaluation methods are as follows.

[0076] (1) Weight-average molecular weight of polyolefin resin Using a GPC analyzer (HLC-8020, manufactured by Tosoh Corporation, column: TSK-GEL), the polyolefin resin was dissolved in tetrahydrofuran and measured at 40°C, and the weight-average molecular weight was determined from a calibration curve prepared using a polystyrene standard sample. When the resin was difficult to dissolve in tetrahydrofuran, orthodichlorobenzene was used.

[0077] (2) Structure of acid-modified polyolefin resin 1 Using a H-NMR analyzer (manufactured by JEOL Ltd., 500 MHz), the structure of each of the acid-modified polyolefin resins produced in Production Examples 1 to 6 was analyzed in orthodichlorobenzene (d4) at 120° C. The results are shown in Table 1.

[0078] (3) Particle Size Distribution of Acid-Modified Polyolefin Resin Particles The particle size of the acid-modified polyolefin resin particles in each aqueous dispersion produced in Examples 1 to 9 and Comparative Examples 1 to 4 was measured by dynamic light scattering using a particle size distribution analyzer (Nikkiso Co., Ltd., Nanotrac Wave-UZ152). The aqueous dispersion was diluted with water to a predetermined reflected light intensity to prepare a measurement sample, and the refractive index of the acid-modified polyolefin resin used in the analysis was 1.5. From the obtained volume-accumulated standard particle size distribution, the 10% particle size (D10), 20% particle size (D20), median size (D50), 70% particle size (D70), 90% particle size (D90), 95% particle size (D95), and the total frequency (distribution ratio) of acid-modified polyolefin resin particles having a particle size of at least twice the median size were obtained. The results are shown in Table 1.

[0079] (4) High-Temperature Stability Each aqueous dispersion produced in Examples 1 to 9 and Comparative Examples 1 to 4 was weighed between 1.0000 and 1.1000 g and heated at 150°C until the mass of the residual solids reached a constant weight. The solid concentration (weight after drying / weight before drying) was measured. The obtained solid concentration was then multiplied by 500 to determine the mass (g) of solids in 500 g of each aqueous dispersion produced in Examples 1 to 9 and Comparative Examples 1 to 4. Furthermore, 500 g of each aqueous dispersion produced in Examples 1 to 9 and Comparative Examples 1 to 4 was placed in a glass container (500 ml), capped, and allowed to stand at 50°C for 10 days. The aqueous dispersion was then filtered through a 300-mesh stainless steel filter, and the mass (g) of the solid residue (aggregates) remaining on the filter was measured. The solid residue percentage was then calculated using the following formula. High-temperature stability was also evaluated according to the following criteria. The results are shown in Table 1. Solid residue rate (mass%) = (mass (g) of solid residue remaining on filter / mass (g) of solid content in 500 g of aqueous dispersion) x 100 <Evaluation criteria> A: The solid residue rate is 0.5 mass% or less. B: The solid residue rate is more than 0.5 mass% and 0.9 mass% or less. C: The solid residue rate is more than 0.9 mass%.

[0080] (5) Low-Temperature Film Formability (Transparency During Low-Temperature Film Formation) Each of the aqueous dispersions produced in Examples 1 to 9 and Comparative Examples 1 to 4 was applied to the corona-treated surface of a PET film (manufactured by Unitika Ltd., thickness 25 μm) so that the coating thickness after drying would be 3 μm, and the film was dried at 40° C. for 1 minute to obtain a coating. The haze of the resulting coating film was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH4000 model). The low-temperature film formability was then evaluated according to the following criteria. The results are shown in Table 1. A: Haze is 2% or less. B: Haze is more than 2% and less than 6%. C: Haze is more than 6%.

[0081] (6) Surface Smoothness Each aqueous dispersion produced in Examples 1 to 9 and Comparative Examples 1 to 4 was applied to the corona-treated surface of a PET film (manufactured by Unitika Ltd., thickness 25 μm) so that the coating thickness after drying would be 3 μm, and the film was dried at 60° C. for 1 minute to obtain a coating. The surface roughness Ra (μm) of the resulting coating film was measured using a nano 3D optical interferometry system VS1800 (manufactured by Hitachi High-Technologies Corporation). The results are shown in Table 1.

[0082] (Production Example 1) Production of Acid-Modified Polyolefin Resin PP-1 In a four-necked flask, 200 g of a propylene-ethylene copolymer (propylene / ethylene=95 / 5 (mass ratio), weight average molecular weight 80,000) was heated and dissolved in 400 g of xylene under a nitrogen atmosphere. Thereafter, while maintaining the temperature in the system at 120°C, 20 g of maleic anhydride as an unsaturated carboxylic acid and 11 g of dicumyl peroxide as a radical generator were each added over 1 hour with stirring, and then reacted for 2 hours. After completion of the reaction, the resulting reaction product was poured into a large amount of acetone to precipitate a resin.

[0083] The precipitated resin was washed once with a solution of triethylamine in acetone (mass ratio: triethylamine / acetone = 1 / 4), and then washed with acetone to remove unreacted maleic anhydride. Thereafter, the resin was dried under reduced pressure in a vacuum dryer to obtain acid-modified polyolefin resin PP-1.

[0084] (Production Example 2) Production of Acid-Modified Polyolefin Resin PP-2 Using a propylene-ethylene copolymer (propylene / ethylene=80 / 20 (mass ratio), weight average molecular weight 60,000), an acid-modified polyolefin resin PP-2 was obtained in the same manner as in Production Example 1.

[0085] (Production Example 3) Production of Acid-Modified Polyolefin Resin PP-3 250 g of a propylene-ethylene copolymer (propylene / ethylene = 55 / 45 (mass ratio), weight average molecular weight 55,000) was dissolved in 500 g of xylene under heating in a four-neck flask under a nitrogen atmosphere. Thereafter, while maintaining the temperature in the system at 120°C, 20 g of maleic anhydride as an unsaturated carboxylic acid and 11 g of dicumyl peroxide as a radical generator were each added over 1 hour with stirring, and then the mixture was allowed to react for 2 hours. After completion of the reaction, the resulting reaction product was poured into a large amount of acetone to precipitate a resin.

[0086] The precipitated resin was washed once with an acetone solution of triethylamine (mass ratio: triethylamine / acetone = 1 / 4), and then washed with acetone to remove unreacted maleic anhydride. Thereafter, the resin was dried under reduced pressure in a vacuum dryer to obtain acid-modified polyolefin resin PP-3.

[0087] (Production Example 4) Production of acid-modified polyolefin resin PP-4 Using a propylene-ethylene copolymer (propylene / ethylene=25 / 75 (mass ratio), weight average molecular weight 50,000), acid-modified polyolefin resin PP-4 was obtained in the same manner as in Production Example 3.

[0088] (Production Example 5) Production of Acid-Modified Polyolefin Resin PP-5 280 g of propylene-butene-ethylene terpolymer (propylene / 1-butene / ethylene = 64.8 / 23.9 / 11.3 (mass ratio)) was heated and melted in 500 g of xylene in a four-neck flask under a nitrogen atmosphere. Thereafter, while maintaining the temperature in the system at 170°C, 32 g of maleic anhydride as an unsaturated carboxylic acid and 6 g of dicumyl peroxide as a radical generator were each added over 1 hour with stirring, and then the mixture was allowed to react for 1 hour. After completion of the reaction, the resulting reaction product was poured into a large amount of acetone to precipitate a resin.

[0089] The precipitated resin was further washed with acetone to remove unreacted maleic anhydride, and then dried under reduced pressure in a vacuum dryer to obtain acid-modified polyolefin resin PP-5.

[0090] (Production Example 6) Production of Acid-Modified Polyolefin Resin PP-6 280 g of propylene-butene copolymer (propylene / 1-butene=80 / 20 (mass ratio)) was heated and dissolved in 470 g of xylene in a four-neck flask under a nitrogen atmosphere. Thereafter, while maintaining the temperature in the system at 140°C, 40 g of maleic anhydride as an unsaturated carboxylic acid and 28 g of dicumyl peroxide as a radical generator were each added over 2 hours with stirring, and then the reaction was allowed to proceed for 6 hours. After completion of the reaction, the resulting reaction product was poured into a large amount of acetone to precipitate a resin.

[0091] The precipitated resin was washed once with a solution of triethylamine in acetone (mass ratio: triethylamine / acetone = 1 / 4), and then washed with acetone to remove unreacted maleic anhydride. Thereafter, the resin was dried under reduced pressure in a vacuum dryer to obtain acid-modified polyolefin resin PP-6.

[0092] Example 1 60 g of acid-modified polyolefin resin PP-1, 70 g of THF, 10 g of N,N-dimethylethanolamine (DMEA), and 160 g of distilled water were charged into a sealable pressure-resistant glass container (manufactured by Taiatsu Glass Industries Co., Ltd., TEM-U1000N) equipped with a heater and equipped with a Maxblend impeller (registered trademark) as an agitator. The mixture was heated with stirring at a rotation speed of 600 rpm, and the internal temperature of the system was maintained at 120°C, and stirring was continued for 60 minutes. The mixture was then cooled by air cooling until the internal temperature reached 40°C, after which 100 g of distilled water was added. Then, 100 g of the aqueous medium was distilled off under a reduced pressure atmosphere at 50°C. After cooling to room temperature, stirring was stopped, and the mixture was filtered through a 300-mesh stainless steel filter to obtain an aqueous dispersion.

[0093] (Examples 2 to 9, Comparative Examples 1 to 4) Aqueous dispersions were obtained in the same manner as in Example 1, except that the type of acid-modified polyolefin resin, the type of stirring blade, the stirring temperature, and the stirring rotation speed were changed to the conditions shown in Table 1.

[0094]

[0095] As shown in Table 1, the aqueous dispersions of Examples 1 to 9 had 90% particle diameters (D90) of the acid-modified polyolefin resin particles in the aqueous dispersions that were 2.00 times or less the median diameter (D50), and 95% particle diameters (D95) of the acid-modified polyolefin resin particles that were 2.20 times or less the median diameter (D50), and the acid-modified polyolefin resin particles had high particle diameter uniformity, so the aqueous dispersions had excellent stability in high-temperature environments and low-temperature film-forming properties. Furthermore, the coating films formed from the aqueous dispersions of Examples 1 to 9 had excellent surface smoothness.

[0096] In the aqueous dispersion of Comparative Example 1, the stirring rotation speed during production was low, so the 90% particle size (D90) of the acid-modified polyolefin resin particles in the aqueous dispersion was 2.10 times the median size (D50), and the 95% particle size (D95) was 2.45 times the median size (D50), resulting in low particle size uniformity for the acid-modified polyolefin resin particles. Therefore, the aqueous dispersion of Comparative Example 1 had insufficient stability in a high-temperature environment and poor low-temperature film-forming properties. Furthermore, the coating film formed from the aqueous dispersion of Comparative Example 1 had poor surface smoothness.

[0097] The aqueous dispersion of Comparative Example 2 used an acid-modified polyolefin resin having a content of structural units derived from propylene of less than 50% by mass, so the 90% particle size (D90) of the acid-modified polyolefin resin particles in the aqueous dispersion was 2.36 times the median size (D50), and the 95% particle size (D95) was 2.91 times the median size (D50), resulting in low particle size uniformity for the acid-modified polyolefin resin particles. Therefore, the aqueous dispersion of Comparative Example 2 was poor in stability under high temperature conditions and in low-temperature film-forming properties. Furthermore, the coating film formed from the aqueous dispersion of Comparative Example 2 had poor surface smoothness.

[0098] The aqueous dispersion of Comparative Example 3 was produced using a paddle impeller as the stirring blade, and the stirring rotation speed during stirring was low, so sufficient shear force could not be applied during stirring. Therefore, the 90% particle size (D90) of the acid-modified polyolefin resin particles in the aqueous dispersion was 2.60 times the median size (D50), and the 95% particle size (D95) was 2.90 times the median size (D50), resulting in low particle size uniformity of the acid-modified polyolefin resin particles. Therefore, the aqueous dispersion of Comparative Example 3 was poor in stability under high temperature conditions and low-temperature film-forming properties. Furthermore, the coating film formed from the aqueous dispersion of Comparative Example 3 had poor surface smoothness.

[0099] The aqueous dispersion of Comparative Example 4 was produced using a propeller impeller as the stirring blade, and the stirring rotation speed during stirring was low, so sufficient shear force could not be applied during stirring. Therefore, the 95% particle size (D95) in the aqueous dispersion was 2.30 times the median size (D50), and the particle size uniformity of the acid-modified polyolefin resin particles was low. Therefore, the aqueous dispersion of Comparative Example 4 was poor in stability under high temperature conditions and in low-temperature film-forming properties. Furthermore, the coating film formed from the aqueous dispersion of Comparative Example 4 was poor in surface smoothness.

Claims

1. A polyolefin resin aqueous dispersion containing acid-modified polyolefin resin particles and an aqueous medium, wherein the acid-modified polyolefin resin constituting the acid-modified polyolefin resin particles contains 50% by mass or more of structural units derived from propylene, out of 100% by mass of all structural units, and the acid-modified polyolefin resin particles in the aqueous dispersion have a median diameter (D50) of 0.03 to 1.00 μm, a 90% particle diameter (D90) of 2.00 times or less the median diameter, and a 95% particle diameter (D95) of 2.20 times or less the median diameter, in a volume cumulative standard particle size distribution obtained by measurement using a dynamic light scattering method.

2. The polyolefin resin aqueous dispersion described in claim 1, wherein the acid-modified polyolefin resin particles in the aqueous dispersion have a total frequency of acid-modified polyolefin resin particles having a particle diameter of at least twice the median diameter in a volume cumulative particle size distribution obtained by measurement using a dynamic light scattering method of 10.0% or less.

3. The polyolefin resin aqueous dispersion according to claim 1, wherein the acid-modified polyolefin resin contains structural units derived from at least one olefin selected from the group consisting of ethylene, isobutylene, 1-butene, and 2-butene.

4. The polyolefin resin aqueous dispersion according to claim 1, wherein the acid-modified polyolefin resin contains 0.1 to 10 mass% of structural units derived from unsaturated carboxylic acids and / or structural units derived from unsaturated carboxylic acid anhydrides, based on 100 mass% of all structural units.

5. A method for producing an aqueous polyolefin resin dispersion according to any one of claims 1 to 4, comprising the step of stirring a composition containing at least the acid-modified polyolefin resin and an aqueous medium at a stirring speed of 350 rpm or more using a Max Blend impeller as an impeller.

6. A coating film obtained from the aqueous dispersion of polyolefin resin according to any one of claims 1 to 4.

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