Raw material for secondary battery separator coating material, secondary battery separator coating material, secondary battery separator, and secondary battery

Abstract

This raw material for a secondary battery separator coating material contains: a resin which is a reaction product of a polyvinyl alcohol and a modified methylolmelamine condensation resin having an acidic group; or a polyvinyl alcohol and a modified methylolmelamine condensation resin having an acidic group.

Description

Raw material for coating material for secondary battery separator, coating material for secondary battery separator, secondary battery separator, and secondary battery

[0001] The present invention relates to a coating material raw material for secondary battery separators, a coating material for secondary battery separators, a secondary battery separator, and a secondary battery, and more particularly to a coating material raw material for secondary battery separators, a coating material for secondary battery separators containing the coating material raw material for secondary battery separators, a secondary battery separator having a coating film of the coating material for secondary battery separators, and a secondary battery having the secondary battery separator.

[0002] Conventionally, a separator is provided in a secondary battery to separate the positive electrode from the negative electrode and to allow ions in the electrolyte to pass through.

[0003] As such a separator, for example, a polyolefin porous film is known.

[0004] On the other hand, a coating layer may be provided on the surface of the separator to impart various physical properties. Such a coating layer is formed, for example, by applying a coating material for a secondary battery separator to the surface of the separator and drying it.

[0005] As such a coating material for a secondary battery separator, for example, a coating material for a secondary battery separator has been proposed that includes a coating material raw material for a secondary battery separator that includes a water-soluble polymer obtained by polymerizing a water-soluble polymer raw material that includes methacrylamide and methacrylic acid, and an inorganic filler (see, for example, Patent Document 1).

[0006] Japanese Patent Application Laid-Open No. 2021-103676

[0007] On the other hand, if the separator changes shape due to heat shrinkage, it may cause a short circuit between the positive electrode and the negative electrode, so the coating layer is required to have heat resistance.

[0008] The present invention provides a coating material raw material for secondary battery separators that has excellent heat resistance and ion permeability, a coating material for secondary battery separators that includes the coating material raw material for secondary battery separators, a secondary battery separator that includes a coating film of the coating material for secondary battery separators, and a secondary battery that includes the secondary battery separator.

[0009] The present invention [1] is a raw material for a coating material for a secondary battery separator, which contains a resin that is a reaction product of a modified methylol melamine condensation resin having an acidic group and polyvinyl alcohol, or which contains the modified methylol melamine condensation resin having an acidic group and the polyvinyl alcohol.

[0010] The present invention [2] includes the raw material for a coating material for a secondary battery separator according to the above [1], which contains the resin and the acidic group is a sulfonic acid group.

[0011] The present invention [3] includes the coating material raw material for a secondary battery separator according to the above [1], which contains the modified methylol melamine condensation resin having the acidic group and the polyvinyl alcohol, and the content of the polyvinyl alcohol is 5 parts by mass or more and less than 50 parts by mass per 100 parts by mass of the modified methylol melamine condensation resin.

[0012] The present invention [4] includes a coating material for a secondary battery separator, which includes the raw material for a coating material for a secondary battery separator according to any one of the above [1] to [3] and inorganic particles.

[0013] The present invention [5] includes a secondary battery separator comprising a porous membrane and a coating film of the coating material for a secondary battery separator according to the above [4] arranged on at least one side of the porous membrane.

[0014] The present invention [6] includes a secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator described in the above [5] disposed between the positive electrode and the negative electrode.

[0015] The coating material raw material for secondary battery separators of the present invention contains a resin that is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, or contains a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, and therefore has excellent heat resistance and ion permeability.

[0016] The coating material for a secondary battery separator of the present invention contains the raw material for a coating material for a secondary battery separator of the present invention, and therefore has excellent heat resistance and ion permeability.

[0017] The secondary battery separator of the present invention has a coating film of the coating material for a secondary battery separator of the present invention, and therefore has excellent heat resistance and ion permeability.

[0018] The secondary battery of the present invention includes the secondary battery separator of the present invention, and therefore has excellent heat resistance and ion permeability.

[0019] The coating material raw material for secondary battery separators contains a resin that is a reaction product of a modified methylol melamine condensation resin having acidic groups and polyvinyl alcohol, or a modified methylol melamine condensation resin having acidic groups and polyvinyl alcohol. Below, we will describe in detail a first invention in which the coating material raw material for secondary battery separators contains a resin that is a reaction product of a modified methylol melamine condensation resin having acidic groups and polyvinyl alcohol, and a second invention in which the coating material raw material for secondary battery separators contains a modified methylol melamine condensation resin having acidic groups and polyvinyl alcohol.

[0020] 1. Raw Material for Coating Material for Secondary Battery Separator <<First Invention>> The raw material for coating material for secondary battery separator contains a resin that is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol.

[0021] The resin is the reaction product of a modified methylol melamine condensation resin having acidic groups and polyvinyl alcohol.

[0022] <Modified Methylolmelamine Condensation Resin Having Acidic Group> A modified methylolmelamine condensation resin having an acidic group (hereinafter, may be referred to as a modified methylolmelamine condensation resin) is a condensation polymer of modified methylolmelamine.

[0023] The modified methylol melamine is a reaction product of methylol melamine and an acid component.

[0024] [Methylolmelamine] Methylolmelamine is a reaction product of melamine with formaldehyde or paraformaldehyde. When paraformaldehyde is used, the paraformaldehyde is hydrolyzed to formaldehyde and then reacted with melamine.

[0025] To react melamine with formaldehyde, water, melamine, and formaldehyde are mixed and heated.

[0026] The amount of formaldehyde to be blended per mole of melamine will be described in detail below, but from the viewpoint of increasing the molecular weight, it is, for example, 3.0 or more, preferably more than 3.0, more preferably 3.2 or more, and even more preferably 3.3 or more, and for example, 5.0 or less, preferably 4.0 or less.

[0027] Regarding the heating conditions, the heating temperature is, for example, 40° C. or more, preferably 50° C. or more, and for example, 90° C. or less, preferably 80° C. or less. The heating time is, for example, 0.5 hours or more, preferably 2 hours or more, and for example, 6 hours or less.

[0028] In the above reaction, the pH is 3.0 or higher, preferably 4.0 or higher, more preferably 8.0 or higher, even more preferably 9.0 or higher, particularly preferably 10.0 or higher, and for example, 13.0 or lower, preferably 12.0 or lower, more preferably 11.8 or lower. The pH can be adjusted by adding an alkali (e.g., sodium hydroxide).

[0029] As a result, melamine (general formula (1-1) below) reacts with formaldehyde (general formula (1-2) below) to produce methylolmelamine (general formula (1-3) below).

[0030] Preferably, the methylol melamine (general formula (1-3)) is trimethylol melamine in which three of six hydrogen atoms in the amino groups of melamine have been modified to methylol groups, or the methylol melamine is tetramethylol melamine in which four of six hydrogen atoms in the amino groups of melamine have been modified to methylol groups.

[0031] [Modified Methylol Melamine] As described above, modified methylol melamine is a reaction product of methylol melamine and an acid component.

[0032] The acid component is a component capable of reacting with the methylol group of methylol melamine, and is a component for introducing an acidic group into the methylol melamine by reacting with the methylol group of the methylol melamine.

[0033] The acid component includes, for example, a carboxylic acid, a phosphoric acid, and a sulfurous acid.

[0034] Examples of carboxylic acids include maleic acid, succinic acid, phthalic acid, formylbenzoic acid, aminobenzoic acid, acrylic acid, methacrylic acid, and amino acids (e.g., glycine). Carboxylic acids also include their salts and anhydrides. Examples of carboxylic acid salts include sodium maleate and sodium succinate. Examples of carboxylic acid anhydrides include maleic anhydride, succinic anhydride, and phthalic anhydride.

[0035] Examples of phosphoric acid include phosphonobenzoic acid and aminophenylphosphonic acid. Phosphoric acid also includes salts thereof. Examples of phosphoric acid salts include sodium phosphonobenzoate.

[0036] Examples of sulfurous acid include sulfobenzoic acid, formylbenzenesulfonic acid, sulfanilic acid, pyrosulfite, and sulfurous acid. Sulfurous acid includes its salts. Examples of sulfurous acid salts include sodium bisulfite, sodium sulfite, and sodium pyrosulfite. Preferred examples of sulfurous acid include sodium bisulfite, sulfanilic acid, and sodium pyrosulfite. More preferred examples of sulfurous acid include sodium bisulfite.

[0037] The acid component preferably includes sulfurous acid.

[0038] The acid component can be used alone or in combination of two or more kinds.

[0039] The modified methylol melamine can be obtained by reacting methylol melamine with an acid component.

[0040] To react the methylolmelamine with the acid component, water, the methylolmelamine, and the acid component are mixed and heated.

[0041] The amount of the acid component is, per mole of methylolmelamine, for example, 0.44 moles or more, preferably 0.50 moles or more, more preferably 0.60 moles or more, and even more preferably 0.70 moles or more, and for example, 1.00 moles or less, preferably 0.90 moles or less, and more preferably 0.80 moles or less.

[0042] Regarding the heating conditions, the heating temperature is, for example, 50° C. or more, preferably 70° C. or more, and for example, 100° C. or less, preferably 90° C. or less. The heating time is, for example, 0.5 hours or more, preferably 1 hour or more, and for example, 6 hours or less.

[0043] This allows the methylol melamine to react with the acid component to obtain a modified methylol melamine. Specifically, one of the methylol groups in the methylol melamine can be modified by the acid component to an acidic group derived from the acid component.

[0044] That is, the modified methylol melamine has an acidic group derived from the acid component. Specifically, when the acid component is carboxylic acid, the modified methylol melamine has a carboxyl group as the acidic group. When the acid component is phosphoric acid, the modified methylol melamine has a phosphate group as the acidic group. When the acid component is sulfurous acid, the modified methylol melamine has a sulfonic acid group as the acidic group. A sulfonic acid group is preferably used as the acidic group.

[0045] More specifically, when the acid component is sodium hydrogen sulfite, methylol melamine (general formula (1-3) below) reacts with sodium hydrogen sulfite (general formula (2-1) below) to produce modified methylol melamine (general formula (2-2) below). The modified methylol melamine represented by general formula (2-2) below has a sulfonic acid group as the acidic group.

[0046] The modified methylol melamine is preferably the modified methylol melamine represented by the above general formula (2-2).

[0047] As described above, the modified methylolmelamine condensation resin is a condensation polymer of modified methylolmelamine (modified methylolmelamine having an acidic group). The method for producing the modified methylolmelamine condensation resin will be described later.

[0048] <Polyvinyl Alcohol> Polyvinyl alcohol is a component that imparts wettability and improves adhesion when this resin is used as a raw material for a coating material for a secondary battery separator.

[0049] Examples of polyvinyl alcohol include unmodified polyvinyl alcohol and modified polyvinyl alcohol.

[0050] Examples of modified polyvinyl alcohols include anionic group-modified polyvinyl alcohols (e.g., carboxyl group-modified polyvinyl alcohols, sulfo group-modified polyvinyl alcohols) and hydrophobic group-modified polyvinyl alcohols. Preferred examples of modified polyvinyl alcohols include anionic group-modified polyvinyl alcohols. More preferred examples of modified polyvinyl alcohols include carboxyl group-modified polyvinyl alcohols.

[0051] As the polyvinyl alcohol, preferably, modified polyvinyl alcohol is used.

[0052] The saponification degree of polyvinyl alcohol is, for example, 70 mol% or more, preferably 85 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more, and for example, 100 mol% or less, preferably 99 mol% or less.

[0053] The polyvinyl alcohol may be used alone or in combination of two or more kinds.

[0054] Polyvinyl alcohol can also be prepared as an aqueous solution of polyvinyl alcohol, where the solid content of polyvinyl alcohol in the aqueous solution is, for example, 5% by mass or more and, for example, 50% by mass or less.

[0055] <Method for Producing Resin> The resin is obtained by reacting a modified methylolmelamine condensation resin with polyvinyl alcohol.

[0056] Specifically, the resin is obtained by first producing a modified methylol melamine condensation resin by dehydration condensation of modified methylol melamine and then reacting this modified methylol melamine condensation resin with polyvinyl alcohol, which is different from a resin obtained by dehydration condensation of modified methylol melamine to obtain a modified methylol melamine condensation resin and then reacting it with vinyl alcohol.

[0057] Specifically, modified methylol melamine and polyvinyl alcohol are first mixed with water, and then the modified methylol melamine is subjected to dehydration condensation, and the resulting modified methylol melamine condensation resin is reacted with polyvinyl alcohol.

[0058] In the dehydration condensation, two methylol groups in the modified methylol melamine undergo dehydration condensation to obtain a modified methylol melamine condensation resin.

[0059] In addition, in the reaction between the modified methylol melamine condensation resin and polyvinyl alcohol, the methylol groups of the modified methylol melamine condensation resin react with the hydroxyl groups of the polyvinyl alcohol. The reaction of methylol groups with hydroxyl groups is described, for example, in Journal of Cleaner Production Volume 255, 10 May 2020, 120062, "Efficient removal of Congo red dye from aqueous solution by adsorbent films of polyvinyl alcohol / melamine-formaldehyde composite and bacterial effects."

[0060] In this reaction, for example, an acid (for example, sulfuric acid) is added and the mixture is heated in water at a pH of 5 or more but less than 8.

[0061] Regarding the heating conditions, the heating temperature is, for example, 50° C. or more, preferably 60° C. or more, and for example, 90° C. or less, preferably 80° C. or less. The heating time is, for example, 1 hour or more, preferably 2 hours or more, and for example, 6 hours or less.

[0062] The blending ratio of the modified methylol melamine is, relative to 100 parts by mass of the total amount of the modified methylol melamine and polyvinyl alcohol, for example, 70 parts by mass or more, preferably 80 parts by mass or more, more preferably 85 parts by mass or more, and for example, 99 parts by mass or less, preferably 95 parts by mass or less.

[0063] The blending ratio of polyvinyl alcohol is, for example, 1 part by mass or more, preferably 5 parts by mass or more, and for example, 30 parts by mass or less, preferably 20 parts by mass or less, more preferably 15 parts by mass or less, per 100 parts by mass of the total amount of modified methylol melamine and polyvinyl alcohol.

[0064] As a result of the above, the modified methylol melamine condensation resin reacts with the polyvinyl alcohol to obtain a resin (aqueous solution of the resin).

[0065] The reaction from melamine to resin can be carried out continuously or in portions.

[0066] In the aqueous solution of the resin, the solids concentration of the modified methylol melamine condensation resin is, for example, 10% by mass or more, preferably 15% by mass or more, and for example, 40% by mass or less.

[0067] Such resins have the same acidic groups as the modified methylol melamine.

[0068] In the above explanation, methylol melamine is reacted with an acid component to obtain a modified methylol melamine, and then the modified methylol melamine is subjected to dehydration condensation to obtain a modified methylol melamine condensation resin. However, depending on the amount of the acid component, not all of the methylol melamine may react with the acid component, and some methylol melamine may remain. In such cases, three methylol groups in the methylol melamine and two methylol groups in the modified methylol melamine condense.

[0069] The resin is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, and therefore has excellent heat resistance, ion permeability, and storage stability.

[0070] The raw material for the coating material for a secondary battery separator contains the above-mentioned resin (aqueous solution of the above-mentioned resin).

[0071] Furthermore, additives such as wetting agents, dispersants, hydrophilic resins, humectants, antifoaming agents, and pH adjusters can be blended into the coating material raw material for secondary battery separators in appropriate proportions as needed. In other words, the coating material raw material for secondary battery separators contains additives as needed. The additives can be blended into the resin during and / or after production.

[0072] On the other hand, since the coating material raw material for secondary battery separators contains a resin obtained using polyvinyl alcohol, it has excellent wettability. Therefore, the coating material raw material for secondary battery separators has excellent wettability even without containing a wetting agent. That is, the coating material raw material for secondary battery separators preferably contains the above resin but does not contain a wetting agent.

[0073] In the raw material for a coating material for a secondary battery separator, the solid content concentration of the resin is, for example, 10 mass % or more, or preferably 15 mass % or more, and for example, 40 mass % or less.

[0074] The raw material for the coating material for secondary battery separators contains the above resin, and therefore has excellent heat resistance, ion permeability, and storage stability.

[0075] <<Second Invention>> The raw material for a coating material for a secondary battery separator contains a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol.

[0076] <Modified methylol melamine condensation resin having an acidic group> The modified methylol melamine condensation resin having an acidic group is the same as the modified methylol melamine condensation resin having an acidic group in the first invention. That is, the modified methylol melamine condensation resin having an acidic group is a condensation polymer of modified methylol melamine. The modified methylol melamine is a reaction product of methylol melamine and an acid component.

[0077] Methylolmelamine is the reaction product of melamine and formaldehyde.

[0078] The amount of formaldehyde blended per mole of melamine is, for example, 2.4 moles or more, preferably 2.6 moles or more, more preferably 2.8 moles or more, even more preferably 2.9 moles or more, and for example, 5.0 moles or less, preferably 4.0 moles or less, more preferably 3.3 moles or less, even more preferably 3.1 moles or less.

[0079] When the amount of formaldehyde blended per mole of melamine is equal to or greater than the above lower limit and equal to or less than the above upper limit, almost all of the three amino groups in the melamine can be modified to methylol groups.

[0080] In addition, in the reaction between melamine and formaldehyde, the heating temperature, heating time and pH are the same as those in the first invention.

[0081] The methylol melamine preferably includes trimethylol melamine in which all three amino groups in melamine have been modified to methylol groups.

[0082] As described above, the modified methylol melamine is a reaction product of methylol melamine and an acid component.

[0083] Examples of the acid component include the acid components listed in the first invention. A preferred example of the acid component is sulfurous acid.

[0084] The acid component can be used alone or in combination of two or more kinds.

[0085] Similarly to the first invention, the modified methylol melamine can be obtained by reacting methylol melamine with an acid component. Specifically, to react the methylol melamine with the acid component, water, methylol melamine, and the acid component are mixed and heated.

[0086] The amount of the acid component is, per mole of methylolmelamine, for example, 0.10 moles or more, preferably 0.15 moles or more, more preferably 0.30 moles or more, even more preferably 0.40 moles or more, and particularly preferably 0.43 moles or more, and for example, less than 0.70 moles, preferably 0.65 moles or less, more preferably 0.60 moles or less, and even more preferably 0.50 moles or less.

[0087] The heating conditions are the same as those in the first invention.

[0088] This allows the methylol melamine to react with the acid component to produce a modified methylol melamine. Specifically, one of the methylol groups in the methylol melamine can be modified by the acid component to an acidic group derived from the acid component. The acidic group is preferably a sulfonic acid group.

[0089] As described above, the modified methylolmelamine condensation resin having an acidic group is a condensation polymer of modified methylolmelamine (modified methylolmelamine having an acidic group).

[0090] To condensation polymerize the modified methylol melamine, for example, the modified methylol melamine is heated in water at a pH of 5 or more but less than 8 by adding an acid (for example, sulfuric acid).

[0091] Regarding the heating conditions, the heating temperature is, for example, 50° C. or more, preferably 60° C. or more, and for example, 90° C. or less, preferably 80° C. or less. The heating time is, for example, 1 hour or more, preferably 2 hours or more, and for example, 6 hours or less.

[0092] Thereafter, an alkali (for example, sodium hydroxide) is added to adjust the pH to, for example, 11 or more and 13 or less, thereby terminating the condensation polymerization reaction.

[0093] As a result of the above, two methylol groups in the modified methylol melamine undergo dehydration condensation to obtain a modified methylol melamine condensation resin (aqueous dispersion of modified methylol melamine condensation resin).

[0094] The reaction from melamine to the modified methylolmelamine condensation resin can be carried out continuously or in portions.

[0095] In the aqueous dispersion of the modified methylol melamine condensation resin, the solids concentration of the modified methylol melamine condensation resin is, for example, 10% by mass or more, preferably 15% by mass or more, and for example, 40% by mass or less.

[0096] Such modified methylol melamine condensation resins have the same acidic groups as the modified methylol melamine.

[0097] The content of the modified methylol melamine condensation resin is, relative to 100 parts by mass of the total amount of the modified methylol melamine condensation resin and polyvinyl alcohol, for example, 60 parts by mass or more, preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and for example, 98 parts by mass or less, preferably 90 parts by mass or less.

[0098] In the above explanation, methylol melamine is reacted with an acid component to obtain a modified methylol melamine, and then the modified methylol melamine is subjected to dehydration condensation to obtain a modified methylol melamine condensation resin. However, depending on the amount of the acid component, not all of the methylol melamine may react with the acid component, and some methylol melamine may remain. In such cases, three methylol groups in the methylol melamine and two methylol groups in the modified methylol melamine condense.

[0099] <Polyvinyl alcohol> Examples of polyvinyl alcohol include the polyvinyl alcohols described in the first invention. Examples of polyvinyl alcohol include preferably anionic group-modified polyvinyl alcohol. Examples of modified polyvinyl alcohol include more preferably carboxyl group-modified polyvinyl alcohol.

[0100] The saponification degree of polyvinyl alcohol is the same as that mentioned in the first invention.

[0101] The polyvinyl alcohol may be used alone or in combination of two or more kinds.

[0102] The content of polyvinyl alcohol is, for example, 3 parts by mass or more, preferably 5 parts by mass or more from the viewpoint of improving heat resistance, more preferably 6 parts by mass or more, even more preferably 10 parts by mass or more, and for example, 60 parts by mass or less, preferably less than 50 parts by mass, more preferably 40 parts by mass or less, even more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less, from the viewpoint of improving heat resistance and ion permeability, relative to 100 parts by mass of the modified methylol melamine condensation resin.

[0103] The content of polyvinyl alcohol is, for example, 2 parts by mass or more, preferably 10 parts by mass or more from the viewpoint of improving heat resistance, and for example, 40 parts by mass or less, preferably 30 parts by mass or less, more preferably 20 parts by mass or less, from the viewpoint of improving heat resistance and ion permeability, relative to 100 parts by mass of the total amount of the modified methylol melamine condensation resin and polyvinyl alcohol.

[0104] Polyvinyl alcohol can also be prepared as an aqueous solution of polyvinyl alcohol, where the solid content of polyvinyl alcohol in the aqueous solution is, for example, 5% by mass or more and, for example, 50% by mass or less.

[0105] <Preparation of Raw Material for Coating Material for Secondary Battery Separator> The raw material for coating material for secondary battery separator is prepared by separately packaging a modified methylol melamine condensation resin having an acidic group (aqueous dispersion of the modified methylol melamine condensation resin) and polyvinyl alcohol, which are then mixed at the time of use.

[0106] In addition, additives such as wetting agents, dispersants, hydrophilic resins, humectants, antifoaming agents, and pH adjusters can be blended into the coating material raw material for secondary battery separators in appropriate proportions as needed. That is, the coating material raw material for secondary battery separators contains additives as needed. The additives can be blended into the modified methylol melamine condensation resin and / or polyvinyl alcohol having acidic groups.

[0107] On the other hand, this coating material raw material for secondary battery separators has excellent wettability because it contains a modified methylolmelamine condensation resin having acidic groups and polyvinyl alcohol. Therefore, the coating material raw material for secondary battery separators has excellent wettability even without containing a wetting agent. That is, the coating material raw material for secondary battery separators preferably contains a modified methylolmelamine condensation resin having acidic groups and polyvinyl alcohol, but does not contain a wetting agent.

[0108] The coating material raw material for secondary battery separators contains a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, and therefore has excellent heat resistance and ion permeability.

[0109] Such a coating material raw material for a secondary battery separator (first invention and second invention) can be suitably used particularly as a raw material for a coating material for a secondary battery separator.

[0110] Hereinafter, the coating material for secondary battery separators obtained using this raw material for coating material for secondary battery separators will be described in detail.

[0111] 2. Coating Material for Secondary Battery Separator The coating material for secondary battery separator contains the above-mentioned raw material for the coating material for secondary battery separator and inorganic particles.

[0112] Inorganic particles include, for example, oxides, nitrides, carbides, sulfates, hydroxides, and potassium titanate. Oxides include, for example, alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide. Nitrides include, for example, silicon nitride, titanium nitride, and boron nitride. Carbides include, for example, silicon carbide and calcium carbonate. Sulfates include, for example, magnesium sulfate and aluminum sulfate. Hydroxides include, for example, aluminum hydroxide and aluminum oxide hydroxide. Silicates include, for example, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand, and glass.

[0113] The inorganic particles are preferably hydroxide particles, and more preferably aluminum oxide hydroxide particles.

[0114] The inorganic particles have an average median diameter D50 of, for example, 0.1 μm or more, preferably 0.5 μm or more, and for example, 5 μm or less, preferably 1 μm or less.

[0115] The inorganic particles can be used alone or in combination of two or more kinds.

[0116] The blending ratio of the inorganic particles will be described later.

[0117] To produce a coating material for a secondary battery separator, first, inorganic particles and, if necessary, a dispersant are mixed with water to prepare an aqueous dispersion of inorganic particles. When a dispersant is mixed, the coating material for a secondary battery separator contains the dispersant.

[0118] Examples of the dispersant include ammonium polycarboxylate and sodium polycarboxylate, and preferably ammonium polycarboxylate.

[0119] The mixing ratio of the dispersant (solid content) relative to 100 parts by mass of the inorganic particles is, for example, 0.1 parts by mass or more, preferably 0.5 parts by mass or more, and for example, 10 parts by mass or less, preferably 3 parts by mass or less.

[0120] The dispersants can be used alone or in combination of two or more kinds.

[0121] Next, the raw material for a coating material for a secondary battery separator (aqueous dispersion of the raw material for a coating material for a secondary battery separator) is blended into the aqueous dispersion of inorganic particles and stirred.

[0122] The stirring method is not particularly limited, and examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, a stirrer, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and a stirring blade.

[0123] The coating material for a secondary battery separator may contain the above-mentioned additives in an appropriate ratio as needed. That is, the coating material for a secondary battery separator contains the above-mentioned additives as needed. The coating material for a secondary battery separator preferably does not contain a wetting agent.

[0124] These additives can be used alone or in combination of two or more kinds.

[0125] This allows the production of a coating material for a secondary battery separator. The coating material for a secondary battery separator is obtained as an aqueous dispersion in which the material is dispersed in water.

[0126] The solid content concentration of the aqueous dispersion of the coating material for a secondary battery separator is, for example, 10 mass % or more, preferably 20 mass % or more, more preferably 30 mass % or more, and for example, 50 mass % or less.

[0127] In the coating material (solid content) for a secondary battery separator, the content of the coating material raw material (solid content) for a secondary battery separator is, for example, 3.0 parts by mass or more, preferably 4.2 parts by mass or more, and 10.0 parts by mass or less, preferably 7.0 parts by mass or less, and more preferably 6.0 parts by mass or less, relative to 100 parts by mass of the total amount of the coating material raw material (solid content) for a secondary battery separator and the inorganic particles. The content of the inorganic particles is, for example, 90.0 parts by mass or more, preferably 93.0 parts by mass or more, and more preferably 94.0 parts by mass or more, and for example, 97.0 parts by mass or less, preferably 95.8 parts by mass or less, relative to 100 parts by mass of the total amount of the coating material raw material (solid content) for a secondary battery separator and the inorganic particles.

[0128] In addition, in the coating material (solid content) for secondary battery separators, the content of the raw material (solid content) for secondary battery separator coating material is, relative to 100 parts by mass of the total amount of the inorganic particles, for example, 3.0 parts by mass or more, preferably 4.0 parts by mass or more, more preferably 4.4 parts by mass or more, and for example, 10.0 parts by mass or less, preferably 7.0 parts by mass or less, more preferably 6.0 parts by mass or less, and even more preferably 5.0 parts by mass or less.

[0129] The coating material for a secondary battery separator contains a raw material for the coating material for a secondary battery separator, and therefore a secondary battery separator having a coating film obtained using the coating material for a secondary battery separator has excellent heat resistance and ion permeability.

[0130] Hereinafter, a secondary battery separator obtained by using this coating material for a secondary battery separator will be described in detail.

[0131] 3. Secondary Battery Separator A secondary battery separator includes a porous membrane and a coating film of a coating material for secondary battery separators that is disposed on at least one side of the porous membrane.

[0132] [Porous membrane] Examples of the porous membrane include polyolefin porous membranes and aromatic polyamide porous membranes. Examples of the polyolefin porous membrane include polyethylene porous membranes and polypropylene porous membranes. Preferred examples of the porous membrane include polyolefin porous membranes.

[0133] The thickness of the porous membrane is, for example, 1 μm or more, preferably 5 μm or more, and for example, 40 μm or less, preferably 20 μm or less.

[0134] [Coating Film] The coating film provides heat resistance to the porous film and is made of a coating material for a secondary battery separator.

[0135] The thickness of the coating film is, for example, 1 μm or more, and for example, 10 μm or less, preferably 8 μm or less.

[0136] [Method for Manufacturing Secondary Battery Separator] The method for manufacturing a secondary battery separator includes a first step of preparing a porous membrane, and a second step of applying a separator coating material to at least one surface of the porous membrane.

[0137] (First Step) In the first step, a porous membrane is prepared.

[0138] (Second Step) In the second step, a coating material for a secondary battery separator is applied to at least one surface of the porous membrane, and then dried as necessary, to obtain a coated membrane.

[0139] To apply the coating material for a secondary battery separator to at least one side of the porous membrane, first, if necessary, one side of the porous membrane is subjected to a surface treatment to form a surface treatment layer on one side of the porous membrane. That is, in such a case, the secondary battery separator comprises the porous membrane, the surface treatment layer, and the coating film of the coating material for a secondary battery separator.

[0140] Examples of surface treatments include corona discharge treatment, glow discharge treatment, plasma treatment, and ozone treatment. In this manufacturing method, preferably, no surface treatment is performed in order to improve ion permeability. In other words, the secondary battery separator preferably does not include a surface treatment layer.

[0141] The coating method for applying the coating material for secondary battery separators is not particularly limited, and examples thereof include a wire bar method, a gravure coater method, a small-diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, a microgravure coater method, a knife coater method, an air doctor coater method, a blade coater method, a rod coater method, a squeeze coater method, a cast coater method, a die coater method, a screen printing method, and a spray coating method. A preferred coating method is the wire bar method.

[0142] The drying temperature is, for example, 40°C or higher and, for example, 80°C or lower.

[0143] In this way, a secondary battery separator is manufactured, which includes a porous membrane and a coating film of the above-mentioned coating material for a secondary battery separator that is disposed on at least one surface of the porous membrane.

[0144] In the above description, the coating film of the coating material for a secondary battery separator is disposed on at least one side of the porous membrane, but the coating film can also be disposed on both sides of the porous membrane.

[0145] This secondary battery separator includes a coating film of the above-described coating material for a secondary battery separator. Therefore, the secondary battery separator has excellent heat resistance and ion permeability. Therefore, this secondary battery separator can be suitably used in the manufacture of secondary batteries.

[0146] 4. Secondary Battery A secondary battery includes a positive electrode, a negative electrode, the above-mentioned secondary battery separator disposed between the positive electrode and the negative electrode, and an electrolyte impregnated in the positive electrode, the negative electrode, and the above-mentioned secondary battery separator.

[0147] As the positive electrode, for example, a known electrode including a positive electrode current collector and a positive electrode active material laminated on the positive electrode current collector is used.

[0148] Examples of the positive electrode current collector include conductive materials such as aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, and conductive glass.

[0149] The positive electrode active material is not particularly limited, but examples thereof include known positive electrode active materials such as lithium-containing transition metal oxides, lithium-containing phosphates, and lithium-containing sulfates.

[0150] These positive electrode active materials can be used alone or in combination of two or more.

[0151] As the negative electrode, for example, a known electrode including a negative electrode current collector and a negative electrode active material laminated on the negative electrode current collector is used.

[0152] The negative electrode current collector may be made of a conductive material such as copper or nickel.

[0153] The negative electrode active material is not particularly limited, but may be a carbon active material, such as graphite, soft carbon, or hard carbon.

[0154] These negative electrode active materials can be used alone or in combination of two or more.

[0155] When a lithium ion battery is used as the secondary battery, the electrolyte may be, for example, a solution in which a lithium salt is dissolved in a carbonate compound such as ethylene carbonate (EC), propylene carbonate (PC), or ethyl methyl carbonate (EMC).

[0156] To manufacture a secondary battery, for example, a separator for the secondary battery is sandwiched between a positive electrode and a negative electrode, and these are housed in a battery casing (cell), and an electrolyte is injected into the battery casing. In this way, a secondary battery can be obtained.

[0157] The secondary battery includes the secondary battery separator, and therefore has excellent heat resistance and ion permeability.

[0158] <Effects> In the first aspect of the present invention, the raw material for coating a secondary battery separator contains a resin that is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, and therefore has excellent heat resistance and ion permeability.

[0159] Specifically, the resin is obtained using a modified methylolmelamine condensation resin having an acidic group, which has excellent heat resistance, and therefore has excellent heat resistance.

[0160] Furthermore, since the resin is obtained using polyvinyl alcohol, it has excellent wettability, which allows the coating film to be formed uniformly, thereby improving heat resistance and ion permeability.

[0161] In addition, since the resin has excellent wettability, for example, when a coating material raw material for a secondary battery separator containing this resin is applied to the surface of a porous membrane, a coating film can be formed without performing a surface treatment (for example, corona discharge treatment) on the surface of the porous membrane.

[0162] When the surface of the porous membrane is subjected to a surface treatment, the interaction between the coating material raw material for a secondary battery separator and the porous membrane becomes strong, and a part of the coating material raw material for a secondary battery separator may enter the pores of the porous membrane, which may result in a decrease in ion permeability.

[0163] On the other hand, according to this coating material raw material for secondary battery separators, a coating film can be formed on the surface of the porous membrane without performing surface treatment, so that a part of the coating material raw material for secondary battery separators can be prevented from penetrating into the pores of the porous membrane, thereby further improving ion permeability.

[0164] This resin also has excellent storage stability.

[0165] Specifically, from the viewpoint of improving wettability, it has been considered to first condense modified methylol melamine to obtain a modified methylol melamine condensation resin having an acidic group, and then add polyvinyl alcohol to the modified methylol melamine condensation resin. In other words, in such a case, polyvinyl alcohol is added later.

[0166] However, in such a case, if the resin is stored for a certain period of time after the addition of polyvinyl alcohol, the hydroxyl groups of the polyvinyl alcohol interact with the methylol groups of the modified methylol melamine resin, causing gelation.

[0167] On the other hand, this resin can be obtained by dehydrating and condensing modified methylol melamine and then reacting the resulting modified methylol melamine condensation resin having acidic groups with polyvinyl alcohol in advance. In other words, in this case, polyvinyl alcohol is added in advance.

[0168] The gelation can be suppressed by reacting the modified methylol melamine condensation resin with polyvinyl alcohol in advance. Although the reason for this is unclear, it is presumed that the reaction of the modified methylol melamine resin with polyvinyl alcohol in advance consumes hydroxyl groups in the polyvinyl alcohol in the reaction, reducing the hydroxyl groups in the polyvinyl alcohol during storage, thereby improving storage stability.

[0169] In the second aspect of the present invention, the raw material for the coating material for a secondary battery separator contains a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, and therefore has excellent heat resistance and ion permeability.

[0170] Specifically, the raw material for the coating material for secondary battery separators contains a modified methylolmelamine condensation resin having an acidic group, which has excellent heat resistance.

[0171] Furthermore, the coating material raw material for secondary battery separators contains polyvinyl alcohol, which provides excellent wettability, allowing for the formation of a uniform coating film, thereby improving heat resistance and ion permeability.

[0172] Furthermore, since the coating material raw material for secondary battery separators has excellent wettability, a coating film can be formed on the surface of the porous membrane without performing surface treatment (for example, corona discharge treatment).

[0173] When the surface of the porous membrane is subjected to a surface treatment, the interaction between the coating material raw material for a secondary battery separator and the porous membrane becomes strong, and a part of the coating material raw material for a secondary battery separator may enter the pores of the porous membrane, which may result in a decrease in ion permeability.

[0174] On the other hand, according to this coating material raw material for secondary battery separators, a coating film can be formed on the surface of the porous membrane without performing surface treatment, so that a part of the coating material raw material for secondary battery separators can be prevented from penetrating into the pores of the porous membrane, thereby further improving ion permeability.

[0175] The coating material for a secondary battery separator contains the above-mentioned raw material for a coating material for a secondary battery separator, and therefore has excellent heat resistance and ion permeability.

[0176] The secondary battery separator has a coating film of the above-mentioned coating material for secondary battery separators, and therefore has excellent heat resistance and ion permeability.

[0177] The secondary battery includes the above-mentioned secondary battery separator, and therefore has excellent heat resistance and ion permeability.

[0178] Specific numerical values ​​of the blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit values ​​(numerical values ​​defined as "not more than" or "less than") or lower limit values ​​(numerical values ​​defined as "not less than" or "exceeding") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the above "Modes for Carrying Out the Invention." Furthermore, unless otherwise specified in the following description, "parts" and "%" are based on mass.

[0179] <Details of Components> The trade names and abbreviations of the components used in each example and each comparative example are described in detail below. AF17: carboxyl group-modified polyvinyl alcohol, saponification degree exceeding 96 mol%, manufactured by Japan Vinyl Acetate & Poval Co., Ltd. Gohsenex CKS-50: sulfo group-modified polyvinyl alcohol, saponification degree 99 mol%, manufactured by Mitsubishi Chemical Corporation Gohsenex WO-320: hydrophilic group-modified polyvinyl alcohol, saponification degree 98.5 mol%, manufactured by Mitsubishi Chemical Corporation Kuraray Poval 5-98: unmodified polyvinyl alcohol, saponification degree 98 mol% to 99 mol%, manufactured by Kuraray Co., Ltd. Kuraray Poval 60-98: unmodified polyvinyl alcohol, saponification degree 98 mol% to 99 mol%, manufactured by Kuraray Co., Ltd. Kuraray Poval 44-88: unmodified polyvinyl alcohol, saponification degree 87 mol% to 89 mol%, manufactured by Kuraray Co., Ltd. Kuraray Poval 5-74: Unmodified polyvinyl alcohol, saponification degree 72.5 to 74.5 mol%, manufactured by Kuraray Co., Ltd. Wetting agent: Acetylene-based surfactant, Olfine E1010, manufactured by Nissin Chemical Industry Co., Ltd. Ammonium polycarboxylate: Dispersant, aqueous solution of ammonium polycarboxylate, trade name SN5468, manufactured by San Nopco Ltd.

[0180] <<First Invention>> <Preparation of Aqueous Solutions of Polyvinyl Alcohol> A separable flask equipped with a stirrer was charged with 100 parts by mass of water, and while stirring, 10 parts by mass of each polyvinyl alcohol (AF17, Gohsenex CKS-50, Gohsenex WO-320, and Kuraray Poval 5-98) was added little by little, and the temperature was raised to 95°C and maintained for 5 hours. After confirming that the polyvinyl alcohol had completely dissolved, the mixture was cooled, and an appropriate amount of water was added to prepare 10% aqueous solutions of polyvinyl alcohol for each polyvinyl alcohol.

[0181] <Resin Production> Production Example 1 33.0 parts by mass of water and 114.8 parts by mass of 37% formaldehyde were charged into a four-neck flask equipped with a stirrer, thermometer, and reflux condenser, and mixed with stirring. 51 parts of melamine were then added with stirring. The mixture was then heated to 60°C, adjusted to pH 11.0 with a 40% aqueous sodium hydroxide solution, and reacted at 67°C for 2 hours, then cooled to 50°C. Methylol melamine was thus obtained.

[0182] Next, 32 parts by weight of sodium hydrogen sulfite was added, and the pH was adjusted to 11.5 with 40% aqueous sodium hydroxide solution. The mixture was allowed to react for 4 hours at 80°C. This resulted in a modified methylol melamine. Next, 150 parts by weight of an aqueous solution of AF17 (10%) and 342 parts by weight of water were added, and the pH was adjusted to 6.6 with 40% sulfuric acid. The mixture was allowed to react for 8 hours at 72°C. The reaction was then stopped by adjusting the pH to 12.0 with 25% aqueous sodium hydroxide solution. An appropriate amount of water was then added. This resulted in a resin (aqueous resin solution) (solids concentration 16% by weight).

[0183] Production Examples 2 to 4 Resins (aqueous solutions of resins) (solid content concentration: 16% by mass) were obtained according to the same procedure as in Production Example 1. However, the formulation of each component was changed according to Table 1. The values ​​shown in Table 1 are parts by mass (solid content).

[0184] Comparative Production Example 1: 200.0 parts by mass of distilled water was placed in a separable flask equipped with a stirrer and reflux condenser, and the atmosphere was replaced with nitrogen gas. The temperature was then raised to 80°C. Next, 0.6 parts by mass of ammonium persulfate was added, and the following monomer composition was then continuously added over 3 hours. The mixture was then maintained for another 3 hours to complete the polymerization. Aqueous ammonia was added to adjust the pH to 9.0, and then an appropriate amount of water was added to obtain an aqueous solution of a water-soluble polymer with a solids content of 16.0%. {Monomer composition} Methacrylamide 95.0 parts by mass Methacrylic acid 5.0 parts by mass 25% aqueous ammonia 5.0 parts by mass Distilled water 300.0 parts by mass

[0185] Reference Production Example 1: 692.0 parts by mass of water and 277.8 parts by mass of 37% formaldehyde were charged into a four-neck flask equipped with a stirrer, thermometer, and reflux condenser, and mixed with stirring. 144.0 parts of melamine were then added with stirring. The mixture was heated to 60°C, adjusted to pH 11.0 with a 25% aqueous sodium hydroxide solution, and then reacted at 75°C for 3 hours, followed by cooling to 60°C. Methylol melamine was thus obtained.

[0186] Next, 50.9 parts by mass of sodium hydrogen sulfite was added as an acid component, and the mixture was reacted for 2 hours at 80° C. This gave a modified methylol melamine.

[0187] Next, after cooling to 40 ° C or less, water was added to adjust the solid content to 20 mass %. Further, the pH was adjusted to 6.8 with 40% sulfuric acid, and condensation was carried out at 70 ° C for 3 hours. After that, the pH was adjusted to 12.0 with 25% aqueous sodium hydroxide solution to terminate the reaction. Next, an appropriate amount of water was added to obtain a resin (aqueous resin solution) (solid content concentration 18 mass %).

[0188] <Production of a coating material raw material for a secondary battery separator, a coating material for a secondary battery separator, and a secondary battery separator> Example 1 (Production of a coating material raw material for a secondary battery separator) The resin of Production Example 1 was used as a coating material raw material for a secondary battery separator.

[0189] (Production of a coating material for a secondary battery separator) According to the formulation shown in Table 2, a dispersant was added to 123 parts by mass of water. Next, while stirring with a disperser (1000 rpm), 100 parts by mass of boehmite (aluminum hydroxide oxide, manufactured by Navaltec, trade name "Apilar AOH60", average median diameter D50: 0.9 μm) as inorganic particles was gradually added. After addition, the mixture was further stirred with a homogenizer (5000 rpm). This resulted in an aqueous dispersion of inorganic particles (solid concentration of inorganic particles: 45% by mass).

[0190] Next, the raw material for the coating material for a secondary battery separator was blended into the aqueous dispersion of inorganic particles, and water was added appropriately, followed by stirring.

[0191] The resulting mixture was then filtered through a 300 mesh (48 μm filtration particle size) filter to produce a coating material for a secondary battery separator (a dispersion of a coating material for a secondary battery separator). The solid content of the dispersion of the coating material for a secondary battery separator was 40 mass %.

[0192] (Production of Secondary Battery Separator) [First Step] A polyolefin porous membrane (without surface treatment (corona treatment)) was prepared as a porous membrane.

[0193] [Second step] Using a wire bar, the above-mentioned coating material for secondary battery separators (dispersion of coating material for secondary battery separators) was applied to one side of a polyolefin porous membrane, and then dried at 50°C. As a result, coating films (thicknesses of 4 µm and 2 µm) of the coating material for secondary battery separators were formed on one side of the polyolefin porous membrane. As a result, secondary battery separators were manufactured.

[0194] Examples 2 to 4, Comparative Example 1, and Reference Example 1: A secondary battery separator coating material raw material, a secondary battery separator coating material, and a secondary battery separator were obtained based on the same procedures as in Example 1. However, the formulation of each component was changed according to Table 2. The values ​​listed in Table 2 are parts by mass (solid content). In Comparative Example 1, a secondary battery separator coating material was produced by blending E1010 with an aqueous solution of the water-soluble polymer of Production Comparative Example 1, which is a secondary battery separator coating material raw material. In Reference Example 1, a secondary battery separator coating material was produced by blending the resin of Production Reference Example 1, which is a secondary battery separator coating material raw material, with an aqueous solution of AF (10%).

[0195] <Evaluation> [Heat Resistance] The secondary battery separators of each Example, Comparative Example, and Reference Example were cut into 5 cm x 5 cm pieces to serve as test specimens. These test specimens were left in an oven at 150°C for 1 hour. The length of each side of the test specimen was measured before and after leaving it there. The shrinkage rate was calculated from the length of each side before shrinkage and the length of each side after shrinkage according to the following formula (1). The results are shown in Table 2. Shrinkage rate (%) = {average length of one side before shrinkage (cm) - average length of one side after shrinkage (cm)} / average length of one side before shrinkage (cm) x 100 (1)

[0196] [Ion Permeability] The air permeability resistance of the secondary battery separators of each Example, Comparative Example, and Reference Example was measured in accordance with JIS-P-8117 using an Oken-type air permeability smoothness tester manufactured by Asahi Seiko Co., Ltd. The decrease in air permeability relative to the air permeability of the porous membrane itself was taken as Δ air permeability. Specifically, Δ air permeability was calculated based on the following formula (2). The smaller the Δ air permeability, the more excellent the ion permeability was evaluated. The results are shown in Table 2. Δ air permeability = measured air permeability - 180 (air permeability of the porous membrane itself) (2)

[0197] [Storage Stability] The viscosity change amount after storage for one month at 40°C was measured for the resins of each Production Example and each Production Reference Example, and for aqueous solutions of the water-soluble polymer of Comparative Example 1. The viscosity was measured using a viscometer manufactured by Toki Sangyo Co., Ltd. at 60 rpm and 25°C. The viscosity change amount was calculated based on the following formula (3). The results are shown in Table 2. Viscosity change amount = (viscosity after storage for one month at 40°C - viscosity immediately after polymerization) / viscosity immediately after polymerization (3)

[0198] <Discussion> Examples 1 to 4, which use a resin that is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, are found to have superior heat resistance compared to Comparative Example 1, which uses an acrylic water-soluble polymer.

[0199] It is clear that Examples 1 to 4, in which polyvinyl alcohol is added beforehand, have superior storage stability compared to Reference Example 1, in which polyvinyl alcohol is added later.

[0200] <<Second Invention>> <Production of Modified Methylol Melamine Condensation Resin Having Acidic Groups> Production Example 5 692.0 parts by mass of water and 277.8 parts by mass (3.43 mol) of 37% formaldehyde were charged into a four-neck flask equipped with a stirrer, thermometer, and reflux condenser and mixed with stirring. Furthermore, 144.0 parts by mass (1.14 mol) of melamine was added with stirring. The mixture was heated to 60°C, adjusted to pH 11.0 with a 25% aqueous sodium hydroxide solution, and then reacted at 75°C for 3 hours and cooled to 60°C. This yielded methylol melamine.

[0201] Next, 50.9 parts by mass (0.489 mol) of sodium hydrogen sulfite was added as an acid component, and the mixture was reacted for 2 hours at 80° C. This gave a modified methylol melamine.

[0202] Next, after cooling to below 40°C, water was added to adjust the solids concentration to 20% by mass. The pH was further adjusted to 6.8 with 40% sulfuric acid, and condensation was carried out at 70°C for 3 hours. The reaction was then stopped by adjusting the pH to 12.0 with a 25% aqueous sodium hydroxide solution. This yielded a modified methylol melamine condensation resin (aqueous dispersion of modified methylol melamine condensation resin). An appropriate amount of water was then added to adjust the solids concentration of the aqueous dispersion of modified methylol melamine condensation resin to 18% by mass.

[0203] Production Example 6: 692.0 parts by mass of water and 259.3 parts by mass (3.20 mol) of 37% formaldehyde were charged into a four-neck flask equipped with a stirrer, thermometer, and reflux condenser and mixed with stirring. Furthermore, 144.0 parts by mass (1.14 mol) of melamine was added with stirring. The mixture was heated to 60°C, adjusted to pH 11.0 with a 25% aqueous sodium hydroxide solution, and then reacted at 75°C for 3 hours and cooled to 60°C. This yielded methylol melamine.

[0204] Next, 33.9 parts by mass (0.426 mol) of sodium hydrogen sulfite was added as an acid component, and the mixture was reacted for 2 hours at 80° C. This gave a modified methylol melamine.

[0205] Next, after cooling to below 40°C, water was added to adjust the solids concentration to 20% by mass. The pH was further adjusted to 6.8 with 40% sulfuric acid, and condensation was carried out at 70°C for 3 hours. The reaction was then stopped by adjusting the pH to 12.0 with a 25% aqueous sodium hydroxide solution. This yielded a modified methylol melamine condensation resin (aqueous dispersion of modified methylol melamine condensation resin). An appropriate amount of water was then added to adjust the solids concentration of the aqueous dispersion of modified methylol melamine condensation resin to 18% by mass.

[0206] <Production of acrylic water-soluble polymer> Production Example 7 A separable flask equipped with a stirrer and reflux cooling was charged with 200.0 parts by mass of distilled water, and after purging with nitrogen gas, the temperature was raised to 80°C. Next, 0.6 parts by mass of ammonium persulfate was added, and then the following monomer composition was continuously added over 3 hours, and the mixture was maintained for another 3 hours to complete the polymerization. Aqueous ammonia was added to adjust the pH to 9.0, and then an appropriate amount of water was added to obtain an aqueous solution of a water-soluble polymer with a solids content of 20.0%. {Monomer composition} Methacrylamide 95.0 parts by mass Methacrylic acid 5.0 parts by mass 25% aqueous ammonia 5.0 parts by mass Distilled water 300.0 parts by mass

[0207] <Preparation of Polyvinyl Alcohol> Each polyvinyl alcohol (Kuraray Poval 60-98, AF17, Kuraray Poval 44-88, Kuraray Poval 5-74) was prepared as follows. Specifically, 100 parts by mass of water was charged into a separable flask equipped with a stirrer, and 10 parts by mass of polyvinyl alcohol was added little by little while stirring. The temperature was then raised to 95°C and maintained for 3 hours. After confirming that the polyvinyl alcohol was completely dissolved, the mixture was cooled, and an appropriate amount of water was added to prepare a 10% aqueous solution of polyvinyl alcohol.

[0208] <Production of Secondary Battery Separator Coating Material Raw Material, Secondary Battery Separator Coating Material, and Secondary Battery Separator> Examples 5 to 11 and Comparative Examples 2 to 4 (Production of Secondary Battery Separator Coating Material Raw Material) A modified methylolmelamine condensation resin having acidic groups and polyvinyl alcohol were separately prepared according to the formulations listed in Table 3. This produced a secondary battery separator coating material raw material. In Comparative Example 4, an acrylic water-soluble polymer was used instead of the modified methylolmelamine condensation resin having acidic groups. The values ​​listed in Table 3 are for the solid content.

[0209] (Production of a coating material for a secondary battery separator) According to the formulation shown in Table 3, a dispersant was added to 123 parts by mass of water. Next, while stirring with a disperser (1000 rpm), 100 parts by mass of boehmite (aluminum hydroxide oxide, manufactured by Navaltec, trade name "Apilar AOH60", average median diameter D50: 0.9 μm) as inorganic particles was gradually added. After addition, the mixture was further stirred with a homogenizer (5000 rpm). This resulted in an aqueous dispersion of inorganic particles (solid concentration of inorganic particles: 45% by mass).

[0210] Next, the raw material for the coating material for a secondary battery separator was blended into the aqueous dispersion of inorganic particles (specifically, a modified methylolmelamine condensation resin (or a water-soluble acrylic polymer) having an acidic group and polyvinyl alcohol, which had been prepared separately, were mixed and blended), and water was added appropriately, followed by stirring.

[0211] The resulting mixture was then filtered through a 300 mesh (48 μm filtration particle size) filter to produce a coating material for a secondary battery separator (a dispersion of a coating material for a secondary battery separator). The solid content of the dispersion of the coating material for a secondary battery separator was 40 mass %.

[0212] (Production of Secondary Battery Separator) [First Step] A polyolefin porous membrane (without surface treatment (corona treatment)) was prepared as a porous membrane.

[0213] [Second step] Using a wire bar, the above-mentioned coating material for a secondary battery separator (dispersion of the coating material for a secondary battery separator) was applied to one side of a polyolefin porous membrane, and then dried at 50°C. As a result, a coating film (thickness 4 µm) of the coating material for a secondary battery separator was formed on one side of the polyolefin porous membrane. As a result, a secondary battery separator was produced.

[0214] <Evaluation> [Heat Resistance] Heat resistance was evaluated in the same manner as in the first invention. The results are shown in Table 3.

[0215] [Ion Permeability] Ion permeability was evaluated in the same manner as in the first invention. The results are shown in Table 3.

[0216] <Discussion> It is clear that Examples 5 to 11, which contain a modified methylol melamine condensation resin having an acidic group and polyvinyl alcohol, have superior heat resistance and ion permeability compared to Comparative Example 2, which does not contain polyvinyl alcohol, and Comparative Examples 3 and 4, which do not contain a modified methylol melamine condensation resin having an acidic group.

[0217]

[0218]

[0219]

[0220] The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included in the scope of the following claims.

[0221] The coating material raw material for secondary battery separators, the coating material for secondary battery separators, and the secondary battery separators of the present invention are suitably used in the production of secondary batteries.

Claims

1. A coating material raw material for secondary battery separators, comprising a resin that is a reaction product of a modified methylolmelamine condensation resin having an acidic group and polyvinyl alcohol, or comprising the modified methylolmelamine condensation resin having an acidic group and the polyvinyl alcohol.

2. The coating material raw material for a secondary battery separator according to claim 1, comprising the resin, and the acidic group is a sulfonic acid group.

3. The coating material raw material for secondary battery separators according to claim 1, comprising the modified methylol melamine condensation resin having the acidic group and the polyvinyl alcohol, wherein the content of the polyvinyl alcohol is 5 parts by mass or more and less than 50 parts by mass per 100 parts by mass of the modified methylol melamine condensation resin.

4. A coating material for a secondary battery separator, comprising the coating material raw material for a secondary battery separator according to claim 1 and inorganic particles.

5. A secondary battery separator comprising: a porous membrane; and a coating film of the coating material for secondary battery separators according to claim 4, which is disposed on at least one surface of the porous membrane.

6. A secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator according to claim 5 disposed between the positive electrode and the negative electrode.

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