JP7764156B2 - Crosslinked polymethyl methacrylate particle dispersion for the manufacture of separators - Google Patents

Description

The present invention relates to crosslinked polymethyl methacrylate particles for use in the production of separators.

In recent years, lithium-ion secondary batteries have come into widespread use as power sources for electric vehicles, among other things. The separators used in these lithium-ion secondary batteries must be able to avoid breakage due to dendritic lithium growth, which can occur when metallic lithium is used in the anode. Separators for lithium-ion secondary batteries must also meet various requirements, including 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 offers excellent mechanical strength, have been proposed as separators that meet these requirements.

Patent Document 1 discloses a method for manufacturing a separator for a secondary battery, which is made of a porous resin film in which pores have a three-dimensional stereoregular arrangement structure and are interconnected by interconnecting pores.

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;
and a porous resin film formation step in which the fine particle-resin film is brought into contact with an inorganic acid other than hydrofluoric acid, an organic acid, water, or an alkaline solution to dissolve and remove the fine particles, or the fine particle-resin film is heated to remove the fine particles, thereby forming pores in the resin matrix that are interconnected by interconnecting pores and have a three-dimensional stereoregular arrangement structure, wherein the dispersion medium contains a precursor of the resin that constitutes the resin matrix, and the surfaces of the narrowly dispersed spherical fine particles are inactive to the dispersion medium.

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 at temperatures above 370°C and below the decomposition temperature of the polyimide and have a glass transition point below the glass transition point of the polyimide to prepare a slurry; forming a film from the slurry; heat-treating the film at a temperature above 370°C and below 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; forming a 3DOM structure in which multiple macropores of uniform shape and dimensions are regularly arranged in three dimensions; and obtaining a polyimide separator with the 3DOM structure.

International Publication No. 2014/196656 JP 2018-97915 A

Removing the polymer particles by thermal decomposition to obtain a 3DOM structure is preferable from a safety standpoint. 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, and as a result, a stable 3DOM structure cannot be obtained.

Therefore, there is a need to provide polymethyl methacrylate particles that suppress swelling in the liquid medium used in the production of polyimide separators and can be thermally decomposed at relatively low temperatures, thereby producing separators with good properties.

The present inventors have conducted extensive research and found that the above problems can be solved by the following means, and have thus 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 diameter measured by dynamic light scattering is 1.0 μm or less.
Crosslinked polymethyl methacrylate particles.
<Aspect 2> The crosslinked polymethyl methacrylate particles,
polyamic acid,
2. The crosslinked polymethyl methacrylate particles according to claim 1, wherein the separator is produced using a dispersion containing an aprotic polar liquid medium and a protic polar liquid medium.
<Embodiment 3> The crosslinked polymethyl methacrylate particles according to embodiment 1 or 2, wherein the volume average particle diameter is 50 nm to 500 nm.
<Embodiment 4> The crosslinked polymethyl methacrylate particles according to any one of embodiments 1 to 3, wherein the crosslinked polymethyl methacrylate particles are crosslinked with a crosslinking agent having a carbon-carbon unsaturated double bond.
<Aspect 5> A crosslinked polymethyl methacrylate particle dispersion comprising: a liquid medium; and the crosslinked polymethyl methacrylate particles according to any one of Aspects 1 to 4 dispersed in the liquid medium.
<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: A method for producing a porous separator, comprising: applying the dispersion according to Aspect 7 to a substrate to be coated; and subjecting the applied dispersion to a heat treatment, thereby reacting the polyamic acid to form a polyimide matrix and eliminating the crosslinked polymethyl methacrylate particles to form interconnected pores.

The present invention provides polymethyl methacrylate particles that suppress swelling in the liquid medium used in the production of polyimide separators and can be thermally decomposed at relatively low temperatures, thereby enabling the production of separators with excellent properties.

<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 diameter measured by dynamic light scattering is 1.0 μm or less.

Generally, when polymethyl methacrylate particles are dispersed in a liquid medium used in the production of polyamide separators, such as 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.

In contrast, crosslinked polymethyl methacrylate particles having the above configuration form appropriate chemical bonds, which results in suppressed swelling even when dispersed in the above liquid medium, thereby enabling the separator to have controlled porosity. Furthermore, because the crosslinked polymethyl methacrylate particles having the above configuration do not have excessive chemical bonds, they can be thermally decomposed at relatively low temperatures, thereby preventing the separator's properties from deteriorating due to the heat generated during decomposition.

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 the crosslinked polymethyl methacrylate particles of the present invention, which have 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 relatively low temperatures, thereby enabling the production of separators with excellent properties.

In this specification, the term "5% weight loss temperature" refers to the temperature at which a 20 mg sample loses 5% of its 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; it may also 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, for example, using a Thermo Plus EVO DSC8230 (Rigaku Corporation).

From the perspective of effectively controlling the porosity of the separator, 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. 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 average particle diameter (D50) value based on the scattering intensity distribution measured by dynamic light scattering. Measurement by dynamic light scattering can be performed using, for example, a DelsaMax CORE (Beckman Coulter, Inc.).

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.

The crosslinking of the crosslinked polymethyl methacrylate particles is preferably carried out with a crosslinking agent having a carbon-carbon unsaturated double bond, in order to satisfy the above-mentioned 5% weight loss temperature, volume average particle size, and coefficient of variation. Examples of such crosslinking agents that can be used include triazine-based crosslinking agents and acrylic-based crosslinking agents.

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

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.

Among these, the use of triazine-based crosslinking agents is more preferable from the viewpoint of satisfying the above-mentioned 5% weight loss temperature, volume average particle size, and coefficient of variation.

These crosslinked polymethyl methacrylate particles can be produced, for example, by emulsion polymerization. Emulsion polymerization is a method that involves preparing an oil phase, preparing an aqueous phase, mixing the oil phase and aqueous phase to emulsify the components of the oil phase, and then polymerizing the resulting mixture.

The oil phase contains an acrylic monomer and a crosslinker.

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.

The crosslinking agents listed above can be used as the crosslinking agent.

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

Ion-exchanged water, distilled water, etc. can be used as water.

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

Examples of cationic surfactants that can be used include ammonium salt surfactants such as tetramethylammonium chloride, and alkylamine salt surfactants such as monomethylamine hydrochloride.

Examples of anionic surfactants that can be used include carboxylic acid surfactants such as sodium octanoate, and sulfonic acid surfactants such as sodium dialkyl sulfosuccinate.

Examples of nonionic surfactants that can be used include ester surfactants such as glycerin laurate, and ether surfactants such as polyoxyethylene alkyl ether.

The process of emulsifying the oil phase components and then 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.

Examples of polymerization initiators that can be used include ammonium persulfate.

<Crosslinked polymethyl methacrylate particle dispersion>
The crosslinked polymethyl methacrylate particle dispersion of the present invention is
It contains a liquid medium, and cross-linked polymethyl methacrylate particles dispersed in the liquid medium.

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

(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 may be used as the liquid medium. Alternatively, a mixed liquid medium of an aprotic polar liquid medium and a protic polar liquid medium may be used as the liquid medium. These dispersion media may be used alone or in combination of two or more.

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

A protic polar liquid medium is generally a solvent containing acidic hydrogen. Examples of protic polar liquid media that can be used include phenol-based liquid media, monohydric alcohol-based liquid media, and polyhydric alcohol-based liquid media.

Examples of phenol-based liquid media that can be used include cresols.

Examples of monohydric alcohol-based 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.

Examples of polyhydric alcohol-based liquid media that can be used include glycol-based liquid media such as propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, and polyethylene glycol with a molecular weight of 600 or less, as well as glycerin.

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, relative to the mass of the liquid medium, and may be 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.

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

From the perspective of improving interconnectivity, the content of 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. 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.

(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.

The polyamic acid content 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 favorable formation of interconnected pores and therefore favorable lithium ion permeability.

The polyamic acid content 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 thereby suppressing breakthrough by lithium. This 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 interconnecting pores and, therefore, favorable lithium ion permeability.

(Polyamic acid: tetracarboxylic acid)
Tetracarboxylic acids constituting polyamic acids include 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-benzenetetracarboxylic 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, 9,9-bisphthalic acid fluorene, and the like.

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

Aliphatic diamines with a carbon number of approximately 2 to 15 are preferably used, such as pentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

Aromatic diamines that can be preferably used are diamino compounds having one or 2 to 10 phenyl groups bonded to them, 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.

Examples of phenylenediamines that can be used include m-phenylenediamine and p-phenylenediamine, and examples of phenylenediamine derivatives include diamines to which alkyl groups such as methyl groups and ethyl groups are bonded, such as 2,4-triphenylenediamine.

Diaminodiphenyl compounds are compounds in which two aminophenyl groups are bonded to each other via another group. The bond can be an ether bond, sulfonyl bond, thioether bond, bond via alkylene or its derivative group, imino bond, azo bond, phosphine oxide bond, amide bond, ureylene bond, etc. Alkylene bonds have approximately 1 to 6 carbon atoms, and their derivative groups have one or more hydrogen atoms in the alkylene group substituted with a halogen atom or the like.

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 Examples of suitable amines include 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.

Diaminotriphenyl compounds are compounds in which two aminophenyl groups and one phenylene group are bonded via other groups, and the other groups are selected from the same groups as in diaminodiphenyl compounds.

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.

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.

Examples of cardo-type fluorene derivatives that can be used include 9,9-bisanilinefluorene.

In addition, these aromatic diamines may be compounds in which the hydrogen atoms are substituted with at least one substituent selected from the group consisting of halogen atoms, methyl groups, methoxy groups, cyano groups, phenyl groups, etc.

<<Method for manufacturing porous separator>>
The method of the present invention for producing a porous separator includes:
The method includes applying the crosslinked polymethyl methacrylate dispersion containing polyamic acid to a substrate to be coated, and subjecting the applied dispersion to a heat treatment to react the polyamic acid to form a polyimide matrix and eliminate the crosslinked polymethyl methacrylate particles to form continuous pores.

The substrate to be coated can be any material that is inert to the dispersion and has a flat surface that allows the separator to be easily peeled off after drying. Examples of materials 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.

The heat treatment temperature is not particularly limited as long as it is a temperature that can react the polyamic acid to form a polyimide matrix and eliminate the crosslinked polymethyl methacrylate particles. For example, the temperature may be 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.

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 greater than 0 vol%.

The present invention will be explained in detail using examples and comparative examples, but the present invention is not limited to these.

<<Preparation of polymethyl methacrylate particles>>
Example 1
A 2-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1000-ml separatory funnel for monomer introduction, and placed in a hot water bath. 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 were charged into the flask to prepare an aqueous phase. While introducing nitrogen gas into the aqueous phase, the internal temperature was raised to 50°C.

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.

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

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.

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)

<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).

(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.

(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.

<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.

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

<<Making the separator>>
7.0 g of polyamic acid was dissolved in 13.3 g of a liquid medium mixed in 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 interconnecting pores.

<Evaluation as a separator>
<exterior>
The obtained separator was visually observed and evaluated based on the following criteria:
A: A uniform separator was obtained.
B: A uniform separator was not obtained.

<Uniformity of interconnected holes>
The uniformity of the interconnecting pores of the prepared separator was observed using an S-3400N electron microscope (Hitachi High-Technologies Corporation). The evaluation criteria were as follows:
A: Uniform interconnected pores were present.
B: No uniform interconnected pores were present.

The configurations and evaluation results of the Examples, Comparative Examples, and Reference Examples are shown in Table 1.

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

Claims (6)

a liquid medium; and crosslinked polymethyl methacrylate particles dispersed in the liquid medium ;
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 protic polar liquid medium is at least one selected from the group consisting of a phenolic liquid medium selected from cresols, a monohydric alcohol 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 liquid medium selected from propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, polyethylene glycol having a molecular weight of 600 or less, and glycerin. 3. The dispersion according to claim 1, wherein the volume average particle size of the polymethyl methacrylate particles is 50 nm to 500 nm. 4. The dispersion according to claim 1, wherein the crosslinked polymethyl methacrylate particles are crosslinked with a crosslinking agent having a carbon-carbon unsaturated double bond. The dispersion according to any one of claims 1 to 4 , further comprising a polyamic acid. 6. A method for producing a porous separator, comprising: applying the dispersion according to claim 5 to a substrate to be coated; and subjecting the applied dispersion to a heat treatment, thereby reacting the polyamic acid to form a polyimide matrix and eliminating the crosslinked polymethyl methacrylate particles to form interconnected pores.