Composition for forming an insulating layer

Abstract

To provide a coating composition for forming an insulating film, which has stable quality and can be produced at a low cost.SOLUTION: An insulating layer forming composition includes boehmite, a binder, and an organic solvent, and the boehmite has a weight loss rate of 10.0 mass% or less in the range of 200 to 450°C and a weight loss rate of 5.0 or more and 13.5 mass% or less in the range of 450 to 600°C in a thermogravimetric analysis measured at a heating rate of 10°C / min under an air stream.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a composition for forming an insulating layer.

Background Art

[0002] In order to improve the safety of secondary batteries, an insulating layer may be provided on the electrodes. For example, in a secondary battery using a non-aqueous electrolyte, it is known to prevent short circuits by forming an insulating layer so as to cover the joint portion between the current collector constituting the electrode and the active material layer (Patent Document 1).

[0003] This insulating layer can be formed using a paint paste containing inorganic particles, a binder or its precursor, and a solvent. For example, Patent Document 1 describes a paint paste for forming an insulating layer containing γ-alumina particles, a binder resin, and an organic solvent. Patent Document 2 describes a paint solution for forming an insulating layer containing boehmite, bicarbonate, a crosslinked resin precursor, and an organic solvent.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Known paint pastes or paint solutions for forming an insulating layer may have problems such as deterioration in quality over time and change in paint viscosity. Alternatively, some contain additives for enhancing the stability of the paint and have cost problems.

[0006] An object of the present invention is to provide a coating composition for forming an insulating film that has stable quality and can be manufactured at low cost.

Means for Solving the Problems

[0007] The present invention is as follows. <<Aspect 1>> A composition for forming an insulating layer, containing boehmite, a binder, and an organic solvent, Regarding the boehmite, in thermogravimetric analysis measured at a heating rate of 10 ° C. / min under an air stream, the weight loss rate in the range of 200 to 450 ° C. is 10.0% by mass or less, and the weight loss rate in the range of 450 to 600 ° C. is 5.0% by mass or more and 13.5% by mass or less, a composition for forming an insulating layer. <<Aspect 2>> The composition for forming an insulating layer according to Aspect 1, wherein the crystallite diameter (020) of the boehmite is 100 nm or more and 750 nm or less. <<Aspect 3>> The composition for forming an insulating layer according to Aspect 1 or 2, wherein the average particle diameter D50 of the boehmite is 0.1 μm or more and 5.0 μm or less. <<Aspect 4>> The composition for forming an insulating layer according to any one of Aspects 1 to 3, wherein the binder is at least one selected from the group consisting of a fluororesin, a polyimide, and a polyamideimide. <<Aspect 5>> The composition for forming an insulating layer according to Aspect 4, wherein the binder contains polyvinylidene fluoride (PVDF). <<Aspect 6>> The composition for forming an insulating layer according to any one of Aspects 1 to 5, wherein the organic solvent is an aprotic polar solvent. <<Aspect 7>> The composition for forming an insulating layer according to Aspect 6, wherein the organic solvent contains N-methyl-2-pyrrolidone (NMP). <<Aspect 8>> The composition for forming an insulating layer according to any one of Aspects 1 to 7, wherein the ratio of the mass of the binder to the total mass of the boehmite and the binder is 1% by mass or more and 45% by mass or less. "Aspect 9: The composition for forming an insulating layer according to any one of Aspects 1 to 8, wherein the amount of the organic solvent in the composition for forming an insulating layer is 50 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass in total of the boehmite and the binder." "Aspect 10: The composition for forming an insulating layer according to any one of Aspects 1 to 9, which is a composition for forming an insulating layer of a battery." "Aspect 11: A method for producing boehmite for use in the composition for forming an insulating layer according to any one of Aspects 1 to 10, comprising:" "in thermogravimetric analysis measured at a heating rate of 10 °C / min under an air stream," "the weight loss rate in the range of 200 to 450 °C is more than 10% by mass, or" "the weight loss rate in the range of 450 to 600 °C is more than 13.5% by mass," "heating the boehmite at a temperature of 200 °C or higher and 600 °C or lower." "A method for producing boehmite." "Aspect 12: The method for producing boehmite according to Aspect 11, wherein the temperature of the heat treatment is 400 °C or higher and 500 °C or lower." "[Advantages of the Invention]"

[0008] "According to the present invention, there is provided a coating composition for forming an insulating film with stable quality, for example, with suppressed viscosity change over time. The coating composition of the present invention does not need to contain components other than boehmite, a binder, and an organic solvent for stabilizing the quality. Therefore, the coating composition of the present invention has a low manufacturing cost." "[Modes for Carrying Out the Invention]"

[0009] "Composition for Forming an Insulating Layer" "The composition for forming an insulating layer of the present invention is" "a composition for forming an insulating layer containing boehmite, a binder, and an organic solvent, wherein" "for the boehmite, in thermogravimetric analysis measured at a heating rate of 10 °C / min under an air stream," "the weight loss rate in the range of 200 to 450 °C is 10.0% by mass or less, and" The weight loss rate in the range of 450 to 600 °C is 5.0% by mass or more and 13.5% by mass or less.

[0010] 〈Boehmite〉 Boehmite is generally an alumina monohydrate represented by the composition formula AlOOH. However, the boehmite in the present invention is a concept that includes those with a higher degree of hydration and those with a lower degree of hydration than the composition of AlOOH.

[0011] However, the boehmite contained in the composition for forming an insulating layer of the present invention requires that, in thermogravimetric analysis measured at a heating rate of 10 °C / min under an air stream, the weight loss rate in the range of 200 to 450 °C is 10.0% by mass or less, and the weight loss rate in the range of 450 to 600 °C is 5.0% by mass or more and 13.5% by mass or less.

[0012] (Weight loss rate in thermogravimetric analysis) Regarding the boehmite in the present invention, the weight loss rate in the range of 200 to 450 °C being 10.0% by mass or less means that there is little water of hydration that is easily desorbed in the boehmite. By satisfying this requirement, it is considered that the components in the composition for forming an insulating layer are suppressed from reacting with free water and deteriorating.

[0013] Also, the weight loss rate in the range of 450 to 600 °C being 5.0% by mass or more is considered to mean that there is a certain amount of water of hydration that is difficult to desorb to some extent in the boehmite. By satisfying this requirement, the boehmite particles in the composition for forming an insulating layer are likely to solvate with a solvent that is preferably an aprotic polar compound, and it is considered that a decrease in the viscosity of the composition for forming an insulating layer due to aggregation of the boehmite particles can be suppressed.

[0014] In this regard, for example, when the weight loss rate in the range of 450 to 600 °C is substantially 0 as in the case of alumina, etc., it is considered that aggregation between particles is likely to occur, and the composition for forming an insulating layer will have a decrease in viscosity.

[0015] On the other hand, it is considered that a weight loss rate in the range of 450 to 600 °C being 13.5 mass% or less means that the amount of functional groups that are difficult to desorb in boehmite is limited. By satisfying this requirement, the reaction between the functional groups and the binder is restricted, and it is considered that the deterioration of the composition due to the reaction can be suppressed.

[0016] However, the present invention is not bound by a specific theory.

[0017] From the viewpoint of reducing the amount of easily desorbable water of hydration, the weight loss rate in the range of 200 to 450 °C in the thermogravimetric analysis of boehmite is preferably lower, and may be 8.0 mass% or less, 5.0 mass% or less, 3.0 mass% or less, 2.5 mass% or less, 2.0 mass% or less, 1.5 mass% or less, 1.0 mass% or less, 0.5 mass% or less, 0.3 mass% or less, or 0.1 mass% or less, or may be 0.0 mass%.

[0018] On the other hand, from the viewpoint of ensuring a certain amount of water of hydration that is somewhat difficult to desorb, the weight loss rate in the range of 450 to 600 °C in the thermogravimetric analysis of boehmite may be 6.0 mass% or more, 7.0 mass% or more, 8.0 mass% or more, 9.0 mass% or more, 10.0 mass% or more, 11.0 mass% or more, or 12.0 mass% or more, and may be 13.0 mass% or less, 12.5 mass% or less, 12.0 mass% or less, 11.5 mass% or less, or 11.0 mass% or less.

[0019] The thermogravimetric analysis of boehmite may be performed in the range of room temperature to 700 °C using a commercially available thermogravimetric analyzer, accurately weighing about 10 mg of boehmite, filling it into a platinum pan as a sample, and heating it at a rate of 10 °C / min under an air flow of 200 mL / min. Then, from the obtained TG chart, the weight loss rate in the range of 200 to 450 °C and the weight loss rate in the range of 450 to 600 °C may be calculated respectively.

[0020] (Crystallite size) The crystallite size (020) of boehmite contained in the composition for forming an insulating layer of the present invention may be 100 nm or more and 750 nm or less.

[0021] The composition for forming an insulating film of the present invention exhibits excellent stability over time. Here, when the crystallite size (020) of boehmite is 100 nm or more and 750 nm or less, the stability over time of the composition for forming an insulating film is further improved, and in particular, the decrease in the viscosity of the composition over time is effectively suppressed.

[0022] The crystallite size (020) of boehmite may be 200 nm or more, 300 nm or more, 400 nm or more, 500 nm or more, or 600 nm or more, and may be 700 nm or less, 650 nm or less, 600 nm or less, 550 nm or less, or 500 nm or less.

[0023] The crystallite size of boehmite may be calculated from the Scherrer's equation shown below with K = 0.94 using the half-value width β of the peak at 2θ = 14.48° corresponding to the (020) plane of boehmite obtained from XRD analysis. D = Kλ / βcosθ {In the formula, D is the crystallite size, K is the Scherrer constant, λ is the wavelength of X-rays, B is the half-value width, and θ is the Bragg angle.}

[0024] XRD analysis may be performed, for example, under the following conditions using a commercially available X-ray diffractometer. X-ray source: CuKα (wavelength 1.5418 Å) Tube voltage: 40 kV Tube current: 250 mA Scanning angle: 2θ = 5 to 85° Scanning speed: 4° / min

[0025] (Average particle size D50) From the viewpoint of increasing the electrical insulation of the obtained insulating layer, the average particle size D50 of boehmite contained in the composition for forming an insulating layer of the present invention is preferably larger, and from the viewpoint of increasing the dispersibility of boehmite in the composition for forming an insulating layer, it is preferably smaller.

[0026] Considering the above two viewpoints, the average particle size D50 of boehmite in the insulating layer-forming composition may be 0.1 μm or more, 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, or 0.8 μm or more, and may be 4.0 μm or less, 3.0 μm or less, 2.5 μm or less, 2.0 μm or less, 1.5 μm or less, or 1.0 μm or less.

[0027] In this specification, the average particle size D50 may be determined as the particle size at which the cumulative volume fraction is 50% in the particle size distribution obtained by the light scattering method using laser light. The insulating layer-forming composition may be diluted with an appropriate solvent (e.g., NMP, etc.) and then subjected to this particle size distribution measurement.

[0028] (Specific surface area) The specific surface area of boehmite contained in the insulating layer-forming composition of the present invention may be appropriately set from the viewpoint of achieving both the electrical insulation of the resulting insulating layer and the dispersibility of boehmite in the insulating layer-forming composition. From such a viewpoint, the specific surface area of boehmite in the insulating layer-forming composition of the present invention is 1 m 2 / g or more, 3 m 2 / g or more, 5 m 2 / g or more, 7 m 2 / g or more, 10 m 2 / g or more, 15 m 2 / g or more, or 20 m 2 / g or more, and may be 150 m 2 / g or less, 100 m 2 / g or less, 80 m 2 / g or less, 50 m 2 / g or less, 30 m 2 / g or less, 20 m 2 / g or less, 15 m 2 / g or less, 10 m 2 / g or less, or 8 m 2 / g or less.

[0029] The specific surface area of boehmite contained in the insulating layer-forming composition of the present invention may be a value measured by the BET method using nitrogen as the adsorbate.

[0030] <Binder> The composition for forming an insulating layer of the present invention contains a binder.

[0031] Examples of the binder contained in the composition for forming an insulating layer of the present invention include fluororesins, polyimides, polyamide-imides, etc., and may be one or more selected from the group consisting of these.

[0032] Examples of the fluororesin may be selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene·perfluoroalkoxyethylene copolymer (PFA), tetrafluoroethylene·hexafluoropropylene copolymer (FEP), tetrafluoroethylene·ethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene·chlorotrifluoroethylene copolymer (ECTFE), etc.

[0033] The binder may contain a fluororesin, may contain PVDF, and in particular, may consist of PVDF.

[0034] The ratio of the binder in the composition for forming an insulating layer of the present invention may be appropriately set in consideration of the stability of the composition over time, the electrical insulation of the obtained insulating layer, the mechanical strength of the obtained insulating layer, etc. The ratio of the binder in the composition for forming an insulating layer may be 1% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, or 20% by mass or more, and may be 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, or 25% by mass or less, as the ratio of the mass of the binder to the total mass of boehmite and the binder.

[0035] <Organic solvent> The solvent contained in the composition for forming an insulating layer of the present invention is an organic solvent. When the solvent of the composition for forming an insulating layer is an organic solvent, the amount of adsorbed water of the binder can be maintained without increasing, which is advantageous in terms of the storage stability of the composition for forming an insulating layer.

[0036] The solvent contained in the composition for forming an insulating layer of the present invention may be an aprotic polar solvent from the viewpoint of enhancing the dispersibility of the binder. The aprotic polar solvent may be selected from, for example, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, hexamethylphosphoric triamide, dimethyl sulfoxide, acetonitrile, methylacetamide, tetrahydrofuran, etc., may contain NMP, or may consist of NMP.

[0037] The amount of the organic solvent in the composition for forming an insulating layer of the present invention may be appropriately set in consideration of the coatability and storage stability of the composition for forming an insulating layer.

[0038] The amount of the organic solvent in the composition for forming an insulating layer may be 50 parts by mass or more, 100 parts by mass or more, 150 parts by mass or more, 200 parts by mass or more, 250 parts by mass or more, or 300 parts by mass or more, and may be 500 parts by mass or less, 450 parts by mass or less, 400 parts by mass or less, 350 parts by mass or less, or 300 parts by mass or less with respect to a total of 100 parts by mass of boehmite and the binder.

[0039] 〈Optional Component〉 The composition for forming an insulating layer of the present invention may be composed only of the above-mentioned boehmite, binder, and organic solvent, or may contain optional components other than these. Examples of such optional components include surfactants, viscosity modifiers, dispersants, colorants, defoamers, etc.

[0040] However, even if the composition for forming an insulating layer of the present invention does not contain such optional components, the effects intended by the present invention can be achieved. Therefore, the composition for forming an insulating layer of the present invention may substantially not contain any optional components other than boehmite, a binder, and an organic solvent. That the composition for forming an insulating layer substantially does not contain any optional components means that the mass ratio of the optional components to the total mass of the composition for forming an insulating layer is 5% by mass or less, 3% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less, or this value may be 0% by mass.

[0041] 〈Use〉 The composition for forming an insulating layer of the present invention is suitable for forming insulating layers of printed circuit boards, multilayer wiring boards, semiconductor devices, display devices, batteries, etc. The composition for forming an insulating layer of the present invention is particularly suitable as a composition for forming an insulating layer of a battery, and is most suitable as a composition for forming an insulating layer of a secondary battery.

[0042] 《Method for Producing Composition for Forming Insulating Layer》 The composition for forming an insulating layer of the present invention may be produced, for example, by mixing a predetermined boehmite, a predetermined binder, and a predetermined organic solvent and mixing them while performing wet dispersion using an appropriate disperser.

[0043] The disperser for wet dispersion may be appropriately selected from known ones and used. As the disperser, for example, a ball mill, a bead mill, a planetary mixer, etc. may be used.

[0044] By mixing boehmite, a binder, and an organic solvent and performing wet dispersion, a composition for forming an insulating layer with good dispersibility can be obtained without substantially changing the crystal system, crystallite size, and specific surface area of the boehmite.

[0045] However, the average particle size D50 of boehmite will slightly decrease even by wet dispersion. Therefore, it is appropriate to adjust the average particle size D50 of the raw boehmite to be used for dispersion to a value slightly larger than the desired value of the average particle size D50 of boehmite in the insulating layer-forming composition.

[0046] 《Method for Producing Boehmite》 In the present invention, as the boehmite, that which shows a predetermined weight loss in thermogravimetric analysis is used.

[0047] As the boehmite in the present invention, among those available, those conforming to the requirements of the present invention may be selected and used, or those not conforming to the requirements of the present invention may be used as raw materials after adjusting the mode of weight loss in thermogravimetric analysis to a desired mode.

[0048] From another aspect, the present invention provides a method for producing boehmite for use in the insulating layer-forming composition of the present invention.

[0049] The method for producing boehmite of the present invention is in thermogravimetric analysis measured at a heating rate of 10 °C / min under an air stream, the weight loss rate in the range of 200 to 450 °C is more than 10% by mass, or the weight loss rate in the range of 450 to 600 °C is more than 13.5% by mass and includes heat-treating the boehmite at a temperature of 200 °C or higher and 600 °C or lower.

[0050] From the viewpoint of reducing the weight loss rate of the obtained boehmite in the range of 200 to 450 °C, the heat treatment temperature may be higher than 200 °C, and may be 250 °C or higher, 300 °C or higher, 350 °C or higher, or 400 °C or higher. On the other hand, from the viewpoint of maintaining the weight loss rate of the obtained boehmite in the range of 450 to 600 °C at 5.0% by mass or more, the heat treatment temperature may be 460 °C or lower. The heat treatment temperature may typically be 400 °C or higher and 500 °C or lower.

[0051] The heat treatment time may be appropriately set in consideration of the mode of weight loss of the raw material boehmite, the desired mode of weight loss of the obtained boehmite, the temperature of the heat treatment, and the like. The heat treatment time may be 10 minutes or more, 20 minutes or more, 30 minutes or more, 45 minutes or more, or 1 hour or more, and may be 12 hours or less, 8 hours or less, 6 hours or less, 4 hours or less, 3 hours or less, 2 hours or less, or 1.5 hours or less.

[0052] The atmosphere of the heat treatment may be any of an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere. The heat treatment may typically be performed in air or under a nitrogen atmosphere.

Examples

[0053] 《Thermogravimetric analysis》 The thermogravimetric analysis of boehmite was performed using a commercially available thermogravimetric analyzer (manufactured by Rigaku Corporation, model "Thermo plus EVO2"). Approximately 10 mg of boehmite was precisely weighed and filled into a platinum pan as a sample, and measurements were taken in the range of room temperature to 700 °C at a heating rate of 10 °C / min under an air flow rate of 200 mL / min. From the obtained TG chart, the weight loss rates in the ranges of 200 to 450 °C and 450 to 600 °C were calculated respectively.

[0054] 《XRD analysis》 The crystal system of boehmite was determined from XRD analysis.

[0055] The crystallite size D of boehmite was calculated from the Scherrer's equation shown below using the half-width β of the peak at 2θ = 14.48° corresponding to the (020) plane of boehmite determined from XRD analysis, with K = 0.94. D = Kλ / βcosθ {In the formula, D is the crystallite size, K is the Scherrer constant, λ is the wavelength of X-rays, B is the half-width, and θ is the Bragg angle.}

[0056] The XRD analysis for determining the crystal system and crystallite size of boehmite was performed under the following conditions. Measuring device: Rigaku Corporation, model name "RINT TTR III" X-ray source: CuKα (wavelength 1.5418 Å) Tube voltage: 40 kV Tube current: 250 mA Scanning angle: 2θ = 5 - 85° Scanning speed: 4° / min

[0057] 《Specific surface area》 The specific surface area of boehmite was measured by the BET method using nitrogen as the adsorbate.

[0058] 《Particle size measurement》 The average particle size (D50) of the boehmite (or alumina) used in the preparation of the paint paste (composition for forming an insulating layer) was determined as the particle size at a cumulative volume fraction of 50% in the particle size distribution obtained by the light scattering method using laser light, with the powder dispersed in NMP as the sample. As the measuring device, the model name "LA-960" manufactured by Horiba, Ltd. was used, and as the refractive index, 1.660 was adopted for boehmite and alumina, and 1.468 was adopted for NMP.

[0059] The average particle size (D50) of boehmite in the paint paste was measured by the same method as above, with the paint paste diluted with NMP as the sample.

[0060] 《Viscosity measurement》 The viscosity of the paint paste was measured by an E-type viscometer for both the initial viscosity immediately after paste preparation and the viscosity after storage of the obtained paint paste in a sealed state at a storage temperature of 60°C for 4 days. The measurement conditions were as follows. Note that the storage at 60°C for 4 days is an accelerated test equivalent to storage at room temperature for 90 days. Measuring device: manufactured by Toki Sangyo Co., Ltd., model name "TVE-33H" Type of cone and rotor: 1°34’×R24 Paste input amount: approximately 1 mL Shear rate: 21.5 S -1 Measurement temperature: 20°C

[0061] The viscosity retention rate is the value obtained by dividing the viscosity after storage by the initial viscosity and expressing it as a percentage. In this example, when the viscosity retention rate is low, it indicates that the quality of the paint paste has deteriorated.

[0062] 《Example 1》 80 parts by mass of commercially available boehmite B1 as boehmite, 20 parts by mass of polyvinylidene fluoride resin (PVDF), and 317 parts by mass of N-methylpyrrolidone (NMP) were mixed, and a large-flow circulation type bead mill (circulation mill) was used as a disperser, and wet dispersion was carried out at a circulation flow rate of 10 L / min to obtain a paint paste (composition for forming an insulating layer).

[0063] The results of various evaluations performed on the boehmite used here and the obtained paint paste by the above method are shown in Table 1.

[0064] 《Example 2》 The blending amount of NMP was changed to 257 parts by mass, and as a disperser, instead of the large-flow circulation type bead mill, a batch type bead mill (batch mill) was used, the charging amount was 100 mL, and wet dispersion was carried out for 25 minutes. Otherwise, in the same manner as in Example 1, a paint paste was obtained. The results of various evaluations are shown in Table 1.

[0065] 《Example 3》 As boehmite, a commercially available boehmite B1 heat-treated in air at 450 °C for 1 hour was used, and otherwise, in the same manner as in Example 2, a paint paste was obtained. The results of various evaluations are shown in Table 1.

[0066] 《Comparative Example 1》 80 parts by mass of commercially available boehmite B2 was used instead of boehmite B1 as boehmite, and otherwise, in the same manner as in Example 1, a paint paste was obtained. The results of various evaluations are shown in Table 1.

[0067] 《Example 4》 As the boehmite, commercially available boehmite B2 heat-treated in air at 475°C for 1 hour was used. The blending amount of NMP was changed to 257 parts by mass, and as the disperser, a planetary mixer (P mixer) was used. With a charging amount of approximately 500 mL, a paint paste was obtained in the same manner as in Comparative Example 1 except for performing wet dispersion for 3 hours. The results of various evaluations are shown in Table 1.

[0068] 《Comparative Example 2》 As the boehmite, commercially available boehmite B3 heat-treated in air at 110°C for 20 hours was used. Except that the blending amount of NMP was changed to 317 parts by mass, a paint paste was obtained in the same manner as in Example 4. The results of various evaluations are shown in Table 1.

[0069] 《Comparative Example 3》 Except that commercially available alumina A1 (α-alumina) was used instead of boehmite, a paint paste was obtained in the same manner as in Example 4. The results of various evaluations are shown in Table 1.

[0070]

Table 1

[0071] From the above Examples and Comparative Examples, the following can be understood.

[0072] First, in Comparative Example 2 using boehmite with a weight loss rate of 13.1 mass% in the range of 200 to 450 °C, the viscosity retention rate of the paint paste after storage was as low as 64.7%. In contrast, in Examples 1 to 4 and Comparative Example 1 using boehmite with a weight loss rate of 10 mass% or less in the range of 200 to 450 °C, and in Comparative Example 3 using α-alumina with this value being 0, the viscosity retention rate of the paint paste showed a higher value than that of Comparative Example 2. Among these, Examples 1 to 4 using boehmite with a weight loss rate of 5.0 mass% or more and 13.5 mass% or less in the range of 450 to 600 °C showed an even higher viscosity retention rate of 78% or more for the paint paste. In particular, in Examples 3 and 4 using boehmite with a weight loss rate of 10.0 mass% or more and 12.5 mass% or less in the range of 450 to 600 °C, the viscosity retention rate of the paint paste exceeded 90%, showing an extremely high value.

[0073] From the comparison between Example 2 and Example 3, and the comparison between Comparative Example 1 and Example 4, it was verified that by heating boehmite to adjust the weight loss rate in the above temperature range, the viscosity retention rate of the paint paste can be improved.

[0074] However, in Comparative Example 3 using α-alumina with a weight loss rate of 0 in the range of 450 to 600 °C, the viscosity retention rate of the paint paste after storage was 66.4%, showing only a slight improvement from 64.7% of Comparative Example 2. From this, it is understood that in order to improve the viscosity retention rate of the paint paste, the weight loss rate in the range of 450 to 600 °C needs to be 5.0 mass% or more.

[0075] Furthermore, it was verified that the paint paste prepared using boehmite with a crystallite size (020) of 100 nm or more and 750 nm or less, and boehmite with an average particle size D50 of 0.1 μm or more and 5.0 μm or less showed excellent viscosity retention.

[0076] 《Analysis Example》 The average particle size D50 of boehmite in the paint pastes prepared in the above Examples 1 and 2 and Comparative Example 1, respectively, was measured by the above-described method. The results are shown in Table 2 together with the thermogravimetric reduction rate, the average particle size D50 in the powder form, and the viscosity retention rate of the paint paste.

[0077]

Table 2

Claims

1. An insulating layer forming composition containing boehmite, a binder, and an organic solvent, For the boehmite, in thermogravimetric analysis measured at a heating rate of 10 °C / min under an air stream, The weight loss rate in the range of 200 to 450 °C is 5.0 mass% or less, and The weight loss rate in the range of 450 to 600 °C is 5.0 mass% or more and 12.5 mass% or less, An insulating layer forming composition.

2. The crystallite diameter (020) of the boehmite is 100 nm or more and 750 nm or less, and the insulating layer forming composition according to Claim 1.

3. The average particle diameter D50 of the boehmite is 0.1 µm or more and 5.0 µm or less, and the insulating layer forming composition according to Claim 1 or 2.

4. The binder is one or more selected from the group consisting of fluororesin, polyimide, and polyamideimide, and the insulating layer forming composition according to any one of Claims 1 to 3.

5. The binder contains polyvinylidene fluoride (PVDF), and the insulating layer forming composition according to Claim 4.

6. The organic solvent is an aprotic polar solvent, and the insulating layer forming composition according to any one of Claims 1 to 5.

7. The organic solvent contains N-methyl-2-pyrrolidone (NMP), and the insulating layer forming composition according to Claim 6.

8. The ratio of the mass of the binder to the total mass of the boehmite and the binder is 1 mass% or more and 45 mass% or less, and the insulating layer forming composition according to any one of Claims 1 to 7.

9. The composition for forming an insulating layer according to any one of claims 1 to 8, wherein the amount of the organic solvent in the composition for forming an insulating layer is 50 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass in total of the boehmite and the binder.

10. The composition for forming an insulating layer according to any one of claims 1 to 9, which is a composition for forming an insulating layer of a battery.

11. A method for producing boehmite for use in a composition for forming an insulating layer containing boehmite, a binder, and an organic solvent, for the boehmite, in thermogravimetric analysis measured at a heating rate of 10 ° C. / min under an air stream, the weight loss rate in the range of 200 to 450 ° C. is 5.0% by mass or less, and the weight loss rate in the range of 450 to 600 ° C. is 5.0% by mass or more and 13.5% by mass or less, the production method is, in thermogravimetric analysis measured at a heating rate of 10 ° C. / min under an air stream, the weight loss rate in the range of 200 to 450 ° C. exceeds 10% by mass, or the weight loss rate in the range of 450 to 600 ° C. exceeds 13.5% by mass including heat-treating the boehmite at a temperature of 300 ° C. or higher and 600 ° C. or lower, A method for producing boehmite.

12. The method for producing boehmite according to claim 11, wherein the temperature of the heat treatment is 400 ° C. or higher and 500 ° C. or lower.

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