Crosslinked polymethyl methacrylate particle dispersion for the manufacture of separators

Abstract

To provide polymethyl methacrylate particles capable of obtaining a separator that has satisfactory properties by thermally decomposing at relatively low temperature while suppressing swelling in the solution medium used in the production of polyimide separators.SOLUTION: In the crosslinked polymethyl methacrylate particles for the production of separators, 5% weight loss temperature in thermogravimetric analysis is 190°C-300°C, and the volume average particle diameter measured by the dynamic light scattering method is 1.0 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to crosslinked polymethyl methacrylate particles for the manufacture of separators. [Background technology]

[0002] In recent years, lithium-ion secondary batteries have been widely used as power sources for electric vehicles, etc. Separators for lithium-ion secondary batteries used in these batteries are required to avoid breakage due to dendritic lithium growth, which can occur when metallic lithium is used as the anode. Separators for lithium-ion secondary batteries are also required to satisfy various properties, such as membrane thickness (thinness), mechanical strength, ionic conductivity (when containing electrolyte), electrical insulation, electrolyte resistance, shutdown effect, electrolyte retention, and wettability. Polyimide separators with a three-dimensionally ordered macropore (3DOM) structure, which have excellent mechanical strength, have been proposed as separators that satisfy these requirements.

[0003] Patent Document 1 discloses a method for producing a separator for a secondary battery made of a porous resin film in which pores have a three-dimensional stereoregular arrangement structure and the pores are connected to each other through interconnecting pores.

[0004] This method is a step of preparing a narrowly dispersed spherical microparticle dispersion slurry by uniformly dispersing narrowly dispersed spherical microparticles in a dispersion medium to prepare a microparticle dispersion slurry; a step of preparing a narrowly dispersed spherical microparticle dispersion film by drying the microparticle dispersion slurry to obtain a narrowly dispersed spherical microparticle dispersion film; a fine particle-resin film forming step of heat-treating the film to form a fine particle-resin film in which the fine particles are three-dimensionally stereoregularly arranged in a resin matrix; a porous resin film forming step of contacting the fine particle-resin film with an inorganic acid other than hydrofluoric acid, an organic acid, water, or an alkaline solution to dissolve and remove the fine particles, or heating the fine particle-resin film to remove the fine particles, thereby forming pores in the resin matrix that are interconnected by interconnecting pores and have a three-dimensional stereoregular array structure; The dispersion medium contains a precursor of a resin that constitutes the resin matrix, and the surfaces of the narrowly dispersed spherical fine particles are inactive to the dispersion medium.

[0005] Patent Document 2 discloses a method for producing a polyimide separator, which comprises mixing a mixture of polyamic acid, ethylene glycol, and a nonionic surfactant in N,N-dimethylacetamide with polymer particles that are thermally decomposable in a temperature range of more than 370°C and less than the decomposition temperature of the polyimide and have a glass transition point lower than that of the polyimide to prepare a slurry, forming a film from the slurry, and heat-treating the film at a temperature of more than 370°C and less than the decomposition temperature of the polyimide in an inert gas atmosphere with an oxygen concentration of 10 vol% or less to convert the polyamic acid into polyimide through a thermal imidization reaction, thermally decomposing and removing the polymer particles to form a 3DOM structure in which a plurality of macropores of uniform shape and size are regularly arranged in a three-dimensional direction, thereby obtaining a polyimide separator having the 3DOM structure. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2014 / 196656 [Patent Document 2] Japanese Patent Application Publication No. 2018-97915 Summary of the Invention [Problem to be solved by the invention]

[0007] From a safety perspective, obtaining a 3DOM structure by removing polymer particles through pyrolysis is preferable. However, the polymer particles used here, such as polymethyl methacrylate particles, are soluble in the liquid medium that dissolves polyamic acid. This causes the particles to swell in the liquid medium, which can result in a stable 3DOM structure being difficult to obtain.

[0008] Therefore, there is a need to provide polymethyl methacrylate particles that can be inhibited from swelling in the liquid medium used in the production of polyimide separators and can be pyrolyzed at relatively low temperatures, thereby providing separators with good properties. [Means for solving the problem]

[0009] The present inventors have conducted extensive research and found that the above problems can be solved by the following means, and have completed the present invention. That is, the present invention is as follows: <Aspect 1> Crosslinked polymethyl methacrylate particles for producing a separator, The 5% weight loss temperature in thermogravimetric analysis is 190°C to 300°C, The volume average particle size measured by dynamic light scattering is 1.0 μm or less. Crosslinked polymethyl methacrylate particles. <Aspect 2> The crosslinked polymethyl methacrylate particles, polyamic acid, an aprotic polar liquid medium, and Protic polar liquid media 2. The crosslinked polymethyl methacrylate particles according to claim 1, wherein the separator is produced using a dispersion containing: <Aspect 3> The crosslinked polymethyl methacrylate particles according to aspect 1 or 2, wherein the volume average particle diameter is 50 nm to 500 nm. <Aspect 4> The crosslinked polymethyl methacrylate particles according to any one of Aspects 1 to 3, wherein the crosslinked polymethyl methacrylate particles are crosslinked with a crosslinking agent having a carbon-carbon unsaturated double bond. <Aspect 5> Liquid medium, and Crosslinked polymethyl methacrylate particles according to any one of aspects 1 to 4, dispersed in the liquid medium. A crosslinked polymethyl methacrylate particle dispersion comprising: <Aspect 6> The dispersion according to aspect 5, wherein the liquid medium is a mixed liquid medium of an aprotic polar liquid medium and a protic polar liquid medium. <Embodiment 7> The dispersion according to embodiment 5 or 6, further comprising a polyamic acid. <Aspect 8> Applying the dispersion according to aspect 7 to a substrate to be coated; and The applied dispersion is subjected to a heat treatment to react the polyamic acid to form a polyimide matrix, and the crosslinked polymethyl methacrylate particles are eliminated to form interconnected pores. A method for producing a porous separator, comprising: [Effects of the Invention]

[0010] According to the present invention, it is possible to provide polymethyl methacrylate particles that are inhibited from swelling in a liquid medium used in the production of a polyimide separator and can be thermally decomposed at a relatively low temperature, thereby making it possible to obtain a separator with excellent properties. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Cross-linked polymethyl methacrylate particles> The crosslinked polymethyl methacrylate particles of the present invention for producing a separator are The 5% weight loss temperature in thermogravimetric analysis is 200°C to 300°C, and The volume average particle size measured by dynamic light scattering is 1.0 μm or less.

[0012] In general, when polymethyl methacrylate particles are dispersed in a liquid medium used in producing a polyamide separator, for example, an aprotic polar liquid medium such as N-methylpyrrolidone, or a mixed liquid medium containing such an aprotic liquid medium, they dissolve or swell, which can result in large variations in particle size.

[0013] In contrast, the crosslinked polymethyl methacrylate particles having the above-described structure have an appropriate amount of chemical bonding, which results in suppressed swelling even when dispersed in the above-described liquid medium, thereby enabling the separator to have controlled porosity. Furthermore, the crosslinked polymethyl methacrylate particles having the above-described structure can be thermally decomposed at a relatively low temperature because the chemical bonding is not excessive, thereby preventing the separator properties from being deteriorated by the heat generated during decomposition.

[0014] The crosslinked polymethyl methacrylate particles of the present invention are particularly useful when producing separators using a dispersion containing polyamic acid, an aprotic polar liquid medium, and a protic polar liquid medium. This is because the aprotic polar liquid medium, such as N-methylpyrrolidone, promotes dissolution of the polyamic acid, while the protic polar liquid medium inhibits swelling of the crosslinked polymethyl methacrylate particles. This effectively inhibits swelling, even for crosslinked polymethyl methacrylate particles of the present invention with a 5% weight loss temperature of 200°C to 300°C in thermogravimetric analysis, i.e., crosslinked polymethyl methacrylate particles with a moderate degree of crosslinking. Furthermore, such crosslinked polymethyl methacrylate particles with a moderate degree of crosslinking can be thermally decomposed at a relatively low temperature, thereby enabling the production of separators with excellent properties.

[0015] In the present invention, the "5% weight loss temperature" refers to the temperature at which a 20 mg sample loses 5% mass when heated in nitrogen at a heating rate of 20°C / min in thermogravimetric analysis (TGA). This 5% weight loss temperature may be 190°C or higher, 195°C or higher, 200°C or higher, 205°C or higher, 210°C or higher, 215°C or higher, 220°C or higher, 225°C or higher, 230°C or higher, 235°C or higher, 240°C or higher, 245°C or higher, 250°C or higher, 255°C or higher, 260°C or higher, 265°C or higher, 270°C or higher, or 275°C or higher; or may be 300°C or lower, less than 300°C, 295°C or lower, 290°C or lower, or 285°C or lower. This analysis can be performed using, for example, a Thermo plus EVO DSC8230 (Rigaku Corporation).

[0016] The volume-average particle diameter of the crosslinked polymethyl methacrylate particles is preferably 1.0 μm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, or 450 nm or less, from the viewpoint of well-controlling the porosity of the separator. This average particle diameter may be 50 nm or more, 70 nm or more, 100 nm or more, 120 nm or more, 150 nm or more, 170 nm or more, or 200 nm or more. This average particle diameter refers to the histogram mean particle diameter (D50) value based on the scattering intensity distribution measured by dynamic light scattering. Measurement by dynamic light scattering can be performed, for example, using a DelsaMax CORE (Beckman Coulter).

[0017] The coefficient of variation of the particle size of the crosslinked polymethyl methacrylate particles may be less than 20%. This coefficient of variation may be 18% or less, 15% or less, 13% or less, 10% or less, or 8% or less, or may be greater than 0%, 1% or more, 2% or more, 3% or more, 4% or more, 5% or more, 7% or more, 10% or more, 12% or more, 15% or more, or 17% or more. The uniformity of particle size provided by such a coefficient of variation allows the use of these particles in the manufacture of separators to produce separators with highly uniform interconnected pores. The coefficient of variation here is calculated based on the particle size distribution derived from the scattering intensity distribution measured by dynamic light scattering.

[0018] The crosslinking of the crosslinked polymethyl methacrylate particles is preferably performed by a crosslinking agent having a carbon-carbon unsaturated double bond, from the viewpoint of satisfying the above-mentioned 5% weight loss temperature, volume average particle size, and coefficient of variation. As such a crosslinking agent, for example, a triazine-based crosslinking agent or an acrylic-based crosslinking agent can be used.

[0019] Examples of triazine crosslinking agents that can be used include triallyl cyanurate, triallyl isocyanurate (TAIC), and trimethallyl isocyanurate.

[0020] Examples of acrylic crosslinking agents that can be used include acrylate crosslinking agents such as 1,4-butanediol diacrylate and trimethylolpropane triacrylate, and methacrylate crosslinking agents such as 1,4-butanediol dimethacrylate and trimethylolpropane trimethacrylate.

[0021] Among these, it is more preferable to use a triazine-based crosslinking agent from the viewpoint of satisfying the above-mentioned 5% weight loss temperature, volume average particle size, and coefficient of variation.

[0022] The crosslinked polymethyl methacrylate particles can be produced, for example, by emulsion polymerization, which comprises the steps of preparing an oil phase, preparing an aqueous phase, mixing the oil phase and the aqueous phase to emulsify the components of the oil phase, and then polymerizing the mixture.

[0023] The oil phase contains an acrylic monomer and a crosslinker.

[0024] Examples of acrylic monomers that can be used include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0025] As the crosslinking agent, the above-mentioned crosslinking agents can be used.

[0026] The aqueous phase may contain water, a surfactant, and a polymerization initiator.

[0027] As the water, ion-exchanged water, distilled water, etc. can be used.

[0028] The surfactant is not particularly limited, and cationic surfactants, anionic surfactants, nonionic surfactants, etc. can be used.

[0029] As the cationic surfactant, for example, ammonium salt surfactants such as tetramethylammonium chloride, alkylamine salt surfactants such as monomethylamine hydrochloride, etc. can be used.

[0030] Examples of anionic surfactants that can be used include carboxylic acid surfactants such as sodium octanoate, and sulfonic acid surfactants such as sodium dialkylsulfosuccinate.

[0031] As the nonionic surfactant, for example, an ester surfactant such as glycerin laurate, or an ether surfactant such as polyoxyethylene alkyl ether can be used.

[0032] The step of emulsifying the oil phase components and further polymerizing them can be carried out by adding the oil phase to the water phase and emulsifying and mixing them while heating to a predetermined temperature using a homogenizer or the like.

[0033] As the polymerization initiator, for example, ammonium persulfate or the like can be used.

[0034] <Crosslinked polymethyl methacrylate particle dispersion> The crosslinked polymethyl methacrylate particle dispersion of the present invention is a liquid medium, and Cross-linked polymethyl methacrylate particles dispersed in a liquid medium Contains:

[0035] The crosslinked polymethyl methacrylate particle dispersion of the present invention may further contain a polyamic acid as a polyimide precursor.

[0036] (liquid medium) The liquid medium may be, for example, a liquid medium commonly used in the production of polyimide separators. An aprotic polar liquid medium can be used as the liquid medium. Alternatively, a mixed liquid medium of an aprotic polar liquid medium and a protic polar liquid medium can be used as the liquid medium. These dispersion media can be used alone or in combination of two or more.

[0037] Aprotic polar liquid media are generally solvents that do not contain acidic hydrogen, and examples of aprotic polar liquid media that can be used include N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

[0038] The protic polar liquid medium is generally a solvent containing acidic hydrogen, and examples of the protic polar liquid medium include phenol-based liquid media, monohydric alcohol-based liquid media, and polyhydric alcohol-based liquid media.

[0039] As the phenol-based liquid medium, for example, cresols can be used.

[0040] Examples of monohydric alcohol liquid media that can be used include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, and n-hexanol.

[0041] Examples of polyhydric alcohol liquid media that can be used include glycol liquid media such as propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, and polyethylene glycols having a molecular weight of 600 or less, as well as glycerin.

[0042] In particular, it is preferable to use a combination of an aprotic polar liquid medium and a protic polar liquid medium, especially a polyhydric alcohol-based liquid medium, as the liquid medium, because the aprotic polar liquid medium promotes dissolution of the polyamic acid while the protic polar liquid medium suppresses swelling of the crosslinked polymethyl methacrylate particles. In this case, the content of the aprotic polar liquid medium may be 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more, or 80% by mass or less, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, or 45% by mass or less, based on the mass of the liquid medium.

[0043] (Crosslinked polymethyl methacrylate particles) As the crosslinked polymethyl methacrylate particles, for example, the above-mentioned crosslinked polymethyl methacrylate particles can be used.

[0044] The content of the crosslinked polymethyl methacrylate particles is preferably 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more, based on the total mass of the crosslinked polymethyl methacrylate particle dispersion, from the viewpoint of improving interconnectivity. This content may be 70% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, or 45% by mass or less, based on the total mass of the crosslinked polymethyl methacrylate particle dispersion.

[0045] (Polyamic acid) Polyamic acid is a polymer of tetracarboxylic acid and diamine, and is a precursor of polyimide obtained by equimolar polymerization of at least one tetracarboxylic acid and one diamine. Polyamic acid can be obtained by polymerizing an acid anhydride, particularly an acid dianhydride, of the following tetracarboxylic acid with a diamine.

[0046] The content of polyamic acid is preferably 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, or 30% by mass or more, based on the total mass of the crosslinked polymethyl methacrylate particle dispersion, from the viewpoint of suppressing the porosity of the separator and thereby suppressing breakthrough by lithium. This content is preferably 40% by mass or less, 35% by mass or less, or 33% by mass or less, based on the total mass of the crosslinked polymethyl methacrylate particle dispersion, from the viewpoint of favorably forming interconnected pores and thereby improving lithium ion permeability.

[0047] The content of polyamic acid is preferably 100 parts by mass or more, 150 parts by mass or more, 170 parts by mass or more, 200 parts by mass or more, or 220 parts by mass or more relative to 100 parts by mass of crosslinked polymethyl methacrylate particles in the crosslinked polymethyl methacrylate particle dispersion, from the viewpoint of suppressing the porosity of the separator and, as a result, suppressing breakthrough by lithium. The content is preferably 500 parts by mass or less, 450 parts by mass or less, 400 parts by mass or less, 350 parts by mass or less, 300 parts by mass or less, 280 parts by mass or less, or 250 parts by mass or less relative to 100 parts by mass of crosslinked polymethyl methacrylate particles in the crosslinked polymethyl methacrylate particle dispersion, from the viewpoint of favorable formation of communicating pores and, as a result, favorable lithium ion permeability.

[0048] (Polyamic acid: tetracarboxylic acid) The tetracarboxylic acids that make up polyamic acid are ethylene tetracarboxylic acid, butane tetracarboxylic acid, cyclopentane tetracarboxylic acid, cyclohexane tetracarboxylic acid, 1,2,4,5-cyclohexane tetracarboxylic acid, 1,2,3,4-cyclohexane tetracarboxylic acid, pyromellitic acid (1,2,4,5-benzene tetracarboxylic acid-1,2,4,5-), 1,1-bis(2,3-dicarboxyphenyl)ethane, bis(2,3-dicarboxyphenyl)methane, bis(3,4-dicarboxyphenyl)methane, 3,3',4,4'-biphenyl tetracarboxylic acid, 2,2,6,6-biphenyl tetracarboxylic acid, 2,2-bis(3,4-dicarboxyphenyl)propane, 2,2-bis(2,3-dicarboxyphenyl)propane, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane, and 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane. One or more of the following can be used: bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane, bis(3,4-dicarboxyphenyl)sulfone, bis(3,4-dicarboxyphenyl)ether, bis(2,3-dicarboxyphenyl)ether, 3,3',4,4'-benzophenonetetracarboxylic acid, 2,2',3,3'-benzophenonetetracarboxylic acid, 4,4-(p-phenylenedioxy)diphthalic acid, 4,4-(m-phenylenedioxy)diphthalic acid, 1,2,5,6-naphthalenetetracarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid, 2,3,6,7-naphthalenetetracarboxylic acid, 1,2,3,4-benzenetetracarboxylic acid, 3,4,9,10-perylenetetracarboxylic acid, 2,3,6,7-anthracenetetracarboxylic acid, 1,2,7,8-phenanthrenetetracarboxylic acid, and 9,9-fluorene bisphthalate.

[0049] (Polyamic acid: diamine) As the diamine constituting the polyamic acid, fatty acid diamines, aromatic diamines, etc. can be used alone or in combination.

[0050] As the aliphatic diamine, for example, one having about 2 to 15 carbon atoms can be preferably used, such as pentamethylenediamine, hexamethylenediamine, heptamethylenediamine, etc.

[0051] As the aromatic diamine, a diamino compound having one or about 2 to 10 phenyl groups bonded thereto can be preferably used, such as phenylenediamine and its derivatives, diaminodiphenyl compounds and their derivatives, diaminotriphenyl compounds and their derivatives, diaminonaphthalene and its derivatives, aminophenylaminoindan and its derivatives, diaminotetraphenyl compounds and their derivatives, diaminohexaphenyl compounds and their derivatives, and cardo-type fluorenediamine derivatives.

[0052] As the phenylenediamine, m-phenylenediamine, p-phenylenediamine, etc. can be used, and as the phenylenediamine derivative, a diamine having an alkyl group such as a methyl group or an ethyl group bonded thereto, such as 2,4-triphenylenediamine, can be used.

[0053] Diaminodiphenyl compounds are compounds in which two aminophenyl groups are bonded to each other via another group. The bond may be an ether bond, a sulfonyl bond, a thioether bond, a bond via an alkylene or its derivative group, an imino bond, an azo bond, a phosphine oxide bond, an amide bond, or a ureylene bond. The alkylene bond has about 1 to 6 carbon atoms, and the derivative group has one or more hydrogen atoms of the alkylene group substituted with a halogen atom or the like.

[0054] Examples of diaminodiphenyl compounds include 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl methane, 3,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ketone, 3,4'-diaminodiphenyl ketone, and 3,4'-diaminodiphenyl ketone. Examples of suitable bis(p-aminophenyl)amines include 4-methyl-2,4-bis(p-aminophenyl)pentane, 2,2-bis(p-aminophenyl)propane, 2,2'-bis(p-aminophenyl)hexafluoropropane, 4-methyl-2,4-bis(p-aminophenyl)-1-pentene, 4-methyl-2,4-bis(p-aminophenyl)-2-pentene, iminodianiline, 4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphine oxide, 4,4'-diaminoazobenzene, 4,4'-diaminodiphenylurea, and 4,4'-diaminodiphenylamide.

[0055] The diaminotriphenyl compound has two aminophenyl groups and one phenylene group bonded via other groups, and the other groups are selected from the same groups as those in the diaminodiphenyl compound.

[0056] Examples of diaminotriphenyl compounds include 1,3-bis(m-aminophenoxy)benzene, 1,3-bis(p-aminophenoxy)benzene, and 1,4-bis(p-aminophenoxy)benzene. Examples of diaminonaphthalenes include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene. Examples of aminophenylaminoindanes include 5- or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindan.

[0057] Examples of diaminotetraphenyl compounds that can be used include 4,4'-bis(p-aminophenoxy)biphenyl, 2,2'-bis[p-(p'-aminophenoxy)phenyl]propane, 2,2'-bis[p-(p'-aminophenoxy)biphenyl]propane, and 2,2'-bis[p-(m-aminophenoxy)phenyl]benzophenone.

[0058] As the cardo-type fluorene derivative, 9,9-bisanilinefluorene or the like can be used.

[0059] The aromatic diamine may be a compound in which the hydrogen atoms of these aromatic diamines are substituted with at least one substituent selected from the group consisting of halogen atoms, methyl groups, methoxy groups, cyano groups, phenyl groups, and the like.

[0060] <<Method for manufacturing porous separator>> The method of the present invention for producing a porous separator includes: Applying the crosslinked polymethyl methacrylate dispersion containing polyamic acid to a substrate; and The coated dispersion is subjected to a heat treatment to react the polyamic acid to form a polyimide matrix, and the crosslinked polymethyl methacrylate particles are eliminated to form interconnected pores. Includes:

[0061] The substrate to be coated can be any substrate that is inactive to the dispersion and has a flat surface from which the separator can be easily peeled off after drying. Examples of substrates that can be used include glass plates, polymer sheets such as polyethylene terephthalate, polypropylene, aramid, cellulose, and polytetrafluoroethylene, and metal sheets such as stainless steel.

[0062] The temperature of the heat treatment is not particularly limited as long as it is a temperature that can react the polyamic acid to form a polyimide matrix and can eliminate the crosslinked polymethyl methacrylate particles, and may be, for example, 300°C or higher, 320°C or higher, 330°C or higher, 340°C or higher, or 345°C or higher, or 500°C or lower, 480°C or lower, 450°C or lower, 400°C or lower, 380°C or lower, or 360°C or lower.

[0063] This heat treatment can be carried out in an air atmosphere or in an inert atmosphere with a low oxygen concentration, such as a nitrogen atmosphere. When the heat treatment is carried out in an inert atmosphere, the oxygen concentration may be 10 vol% or less, 5 vol% or less, 4 vol% or less, 3 vol% or less, 2 vol% or less, or 1 vol% or less, or may be 0 vol% or more. [Example]

[0064] The present invention will be specifically explained with reference to examples and comparative examples, but the present invention is not limited to these.

[0065] <<Preparation of polymethyl methacrylate particles>> Example 1 A 2-liter flask was equipped with a stirrer, reflux condenser, thermometer, nitrogen gas inlet tube, and 1000 ml separatory funnel for monomer introduction, and placed in a hot water bath. The flask was charged with 639.2 parts by mass of distilled water, 0.7 parts by mass of sodium dialkyl sulfosuccinate (referred to as "SDASS" in Table 1) as a surfactant, and 0.6 parts by mass of ammonium persulfate (APS) as an initiator to prepare an aqueous phase. While introducing nitrogen gas into the aqueous phase, the internal temperature was raised to 50°C.

[0066] Separately, an oil phase was prepared by mixing 339.2 parts by mass of methyl methacrylate (MMA) with 20 parts by mass of triallyl isocyanurate as a crosslinking agent.

[0067] This oil phase was added from the separatory funnel to the aqueous phase maintained at about 50°C over a period of 3 hours with stirring to carry out emulsion polymerization. The mixture was further aged for 5 hours to complete the polymerization, yielding 1,000 parts by mass of a particle dispersion.

[0068] Examples 2 to 4, Comparative Examples 1 to 3, and Reference Examples 1 to 5 Particles of Examples 2 to 4, Comparative Examples 1 to 3, and Reference Examples 1 to 5 were produced in the same manner as in Example 1, except that the production conditions were changed as shown in Table 1.

[0069] The crosslinkers shown in Table 1 are as follows: Acrylic A: Viscoat #195, Osaka Organic Chemical Industry Co., Ltd. (1,4-butanediol diacrylate) Acrylic B: Viscoat #295, Osaka Organic Chemical Industry Co., Ltd. (trimethylolpropane triacrylate) TAIC: (triallyl isocyanurate)

[0070] <Evaluation as polymethyl methacrylate particles> <Measurement of physical properties> (Volume average particle size) The volume average particle size of the obtained particles was measured by dynamic light scattering using DelsaMax CORE (Beckman Coulter).

[0071] (coefficient of variation) The coefficient of variation of the particle size of the obtained particles was calculated based on the particle size distribution derived from the scattering intensity distribution measured by dynamic light scattering.

[0072] (5% weight loss temperature) Using a thermogravimetric analyzer Thermo plus EVO DSC8230 (Rigaku Corporation), 20 mg of the obtained particles were heated in nitrogen at a heating rate of 20°C / min, and the temperature at which the mass had decreased by 5% was measured.

[0073] <Dispersibility> 0.5 g of the obtained particles was added to 1.0 g of a liquid medium mixed at the ratio shown in Table 1, and the mixture was stirred, and the appearance was observed visually. The evaluation criteria were as follows: A: The dispersion did not gel and the particles were well dispersed. B: The dispersion became gel-like.

[0074] <Decomposition temperature> The evaluation criteria are as follows: A: The 5% weight loss temperature of the particles is in the range of 190℃ to 300℃. B: The 5% weight loss temperature of the particles is less than 190°C or more than 300°C.

[0075] <<Making the separator>> 7.0 g of polyamic acid was dissolved in 13.3 g of a liquid medium mixed at the ratio shown in Table 1, and 3.0 g of the prepared polymethyl methacrylate particles was mixed therein to prepare a slurry. The solids concentration in the slurry was 43.0 mass%. This slurry was applied to a glass substrate and heat-treated at 350°C in a nitrogen atmosphere containing 2 vol% oxygen to obtain a porous separator with interconnected pores.

[0076] <Evaluation as a separator> <exterior> The obtained separator was visually observed and the evaluation criteria were as follows: A: A uniform separator was obtained. B: A uniform separator was not obtained.

[0077] <Uniformity of interconnected holes> The uniformity of the interconnected pores of the prepared separator was observed using an S-3400N electron microscope (Hitachi High-Technologies Corporation). The evaluation criteria were as follows: A: It had uniform interconnected pores. B: The material did not have uniform interconnected pores.

[0078] Table 1 shows the configurations and evaluation results of the examples, comparative examples, and reference examples.

[0079] [Table 1]

[0080] From Table 1, it can be seen that the particles of the examples having a 5% weight loss temperature of 190°C to 300°C had good dispersibility and polymer decomposition temperature, and the separators obtained using these particles had good film-forming properties and appearance.

Claims

1. a liquid medium, and Crosslinked polymethyl methacrylate particles dispersed in said liquid medium. It contains the liquid medium is a mixed liquid medium of an aprotic polar liquid medium and a protic polar liquid medium, the protic polar liquid medium is a phenol-based liquid medium, a monohydric alcohol-based liquid medium, or a polyhydric alcohol-based liquid medium; The crosslinked polymethyl methacrylate particles The 5% weight loss temperature in thermogravimetric analysis is 190°C to 300°C, The volume average particle size measured by dynamic light scattering is 1.0 μm or less, Crosslinked polymethyl methacrylate particle dispersion for the production of separators.

2. The dispersion according to claim 1, wherein the proton-based polar liquid medium is at least one selected from the group consisting of a phenol-based liquid medium selected from cresols, a monohydric alcohol-based liquid medium selected from ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, and n-hexanol, and a polyhydric alcohol-based liquid medium selected from propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, polyethylene glycol having a molecular weight of 600 or less, and glycerin.

3. A dispersion described in claim 1 or 2, wherein the volume average particle diameter of the polymethyl methacrylate particles is 50 nm to 500 nm.

4. A dispersion described in any one of claims 1 to 3, wherein the crosslinked polymethyl methacrylate particles are crosslinked by a crosslinking agent having a carbon-carbon unsaturated double bond.

5. The dispersion according to any one of claims 1 to 4, further comprising a polyamic acid.

6. Applying the dispersion according to claim 5 to a substrate to be coated; and The applied dispersion is subjected to a heat treatment to react the polyamic acid to form a polyimide matrix, and the crosslinked polymethyl methacrylate particles are eliminated to form interconnected pores. A method for producing a porous separator, comprising:

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